METALLURGICAL INVESTIGATION OF PREMATURE FAILURES FOR 316L

AUSTENITIC STAINLESS STEEL PIPES

by

Mohammed Al Muaisub

B.Sc., King Fahd University of Petroleum and Minerals, 2007

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF APPLIED SCIENCE

in

THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES

(Materials Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA

(Vancouver)

May 2018

© Mohammed Al Muaisub, 2018

ii

The following individuals certify that they have read, and recommend to the Faculty of Graduate

and Postdoctoral Studies for acceptance, the thesis entitled:

Metallurgical investigation of premature failures for 316L austenitic stainless steel pipes

submitted by Mohammed Al Muaisub in partial fulfillment of the requirements for

the degree of Master of Applied Science

in Materials Engineering

Examining Committee:

Edouard Asselin, Materials Engineering

Supervisor

Tom Troczynski, Materials Engineering

Supervisory Committee Member

Daan Maijer, Materials Engineering

Supervisory Committee Member

iii

ABSTRACT

This thesis analyzed and investigated the premature failures of pipes made from type

316L austenitic stainless steels. Multiple leaks were observed in scattered locations of a

piping network of around 10 km after only 4 months in service transferring ammonium

sulfate solution. The initial investigation indicated that the piping network was constructed

3 years earlier. After the construction, the stainless steel pipes were hydrotested to

ensure the joints integrity. However, the piping network was not properly drained and

dried after the hydrotest which resulted in water stagnation for the complete idle period

between construction and commissioning. Therefore, an electrochemical, chemical,

mechanical and metallurgical testing and analyses were conducted to determine the

damage mechanism which consequently caused these failures. I have conducted

electrochemical tests on a 316L stainless steel electrode in chloridized ammonium sulfate

solution to determine its corrosivity. The electrochemical tests showed that the corrosion

rates of 316L SS in ammonium sulfate solution is very low. This conclusion was supported

by other laboratory studies at higher temperature and by the industrial corrosion tables

published online. Also, two spools from the piping network that experienced the failures

were analyzed using stereoscope, optical microscope, scanning electron

microscopy/energy dispersive spectrometry, X-ray fluorescence and carbon/sulfur

analyzer, tensile testing and microhardness testing. The results of these tests indicated

that the pipes were leaking at the 6 O’clock position near the weld and heat affected zone

areas. The morphology of the attack illustrated a narrow opening with large sub-surface

cavity and tunneling initiated from the internal surface of the pipes. The weld joints

displayed weld defects in terms of root concavity and lack of penetration. The

iv

metallurgical investigation strongly suggests that the pipes failed due to Microbiologically

Influenced Corrosion (MIC). During the idle period of 3 years, the stagnant untreated

water in the closed system was an appropriate environment for bacterial growth leading

to severe damage at the welding joints and the base metal.

v

LAY SUMMARY

This thesis discussed and investigated an interesting type of failure that many industrial

plants, as well as commercial and residential buildings, might experience in their water

system network. Pipes, or even equipment, made of stainless steel, might fail in an

accelerated manner due to bacteria. The scientific term for this type of failure is

Microbiologically Influenced Corrosion (MIC). These bacteria do not “eat” the metal per

se, but they are directly or indirectly involved in chemical and electrochemical reactions

that increase the rate of the attack on the steel components. It is reported that MIC is a

rather common type of corrosion damage that might represent up to 20% of the total

corrosion failures in stainless steel systems.

vi

PREFACE

This thesis is original, independent work by the author, Mohammed Al Muaisub. The

electrochemical tests and analysis that were presented in Chapter 5 is based on my work

in the Corrosion Lab in UBC. The metallurgical, chemical and mechanical tests in Chapter

7 were conducted in the laboratory of the sponsoring company. The following was

extracted from the analytical and research work which is presented in the thesis.

Conference:

Mohammed Al Muaisub, Akram Alfantazi, "Premature Failure of 316L Stainless

Steel Pipes Due to MIC", 2nd NACE European Area Conference, 27-29 May 2018,

Genoa, Italy (Poster Presentation - Submitted and Accepted)

vii

TABLE OF CONTENTS

ABSTRACT ..................................................................................................................... iii

LAY SUMMARY .............................................................................................................. v

PREFACE .......................................................................................................................vi

TABLE OF CONTENTS ................................................................................................. vii

LIST OF TABLES ............................................................................................................xi

LIST OF FIGURES ......................................................................................................... xii

LIST OF SYMBOLS, ABBREVIATIONS, AND NOMENCLATURE ............................... xvi

ACKNOWLEDGEMENTS ........................................................................................... xviii

1 INTRODUCTION ...................................................................................................... 1

2 BACKGROUND ........................................................................................................ 3

3 LITERATURE REVIEW .......................................................................................... 10

3.1 Austenitic Stainless Steels ............................................................................... 10

3.2 Pitting Corrosion in Type 316L SS ................................................................... 13

3.2.1 Critical Factors in Pitting Corrosion ........................................................... 14

3.2.1.1 Surface Environment ................................................................................. 15

3.2.1.2 Potential ..................................................................................................... 16

3.2.1.3 Alloy Composition ...................................................................................... 16

3.2.1.4 Temperature .............................................................................................. 17

3.2.1.5 Surface Condition ...................................................................................... 18

viii

3.2.2 Corrosion in Ammonium Sulfate Solution .................................................. 18

3.2.3 Pitting Corrosion in Potable and Sea Water .............................................. 20

3.3 Microbiologically Influenced Corrosion (MIC) ................................................... 22

3.3.1 Susceptible Materials to MIC ..................................................................... 23

3.3.2 Mechanisms of MIC Bacteria Types .......................................................... 24

3.3.2.1 Sulfate Reducing Bacteria (SRB) .............................................................. 25

3.3.2.2 Iron Oxidizing Bacteria (IOB) ..................................................................... 27

3.3.3 Morphology of the MIC Attack ................................................................... 27

3.3.4 Locations of MIC Failures .......................................................................... 28

3.3.5 Effect of Water Quality on MIC .................................................................. 31

4 OBJECTIVES ......................................................................................................... 33

5 ELECTROCHEMICAL TESTING ON 316L SS IN AMMONIUM SULFATE

SOLUTION .................................................................................................................... 34

5.1 Specimen Preparation ...................................................................................... 34

5.2 Electrolyte Composition ................................................................................... 35

5.3 Corrosion Cell Setup ........................................................................................ 35

5.4 Electrochemical Techniques ............................................................................ 36

5.4.1 Open Circuit Potential (OCP) ..................................................................... 36

5.4.2 Potentiodynamic Polarization Test ............................................................ 36

5.5 Results and Discussion of the Electrochemical Tests on 316L SS Electrode .. 37

ix

5.5.1 Effect of Increasing Chloride Concentration .............................................. 37

5.5.2 Effect of Increasing Ammonium Sulfate Concentration ............................. 42

5.5.3 Effect of Increasing the Scan Rate of the Anodic Polarization Test ........... 43

6 TESTING PROCEDURE FOR THE FAILED 316L SS PIPES ................................ 45

6.1 Visual Examination ........................................................................................... 45

6.2 Chemical Testing ............................................................................................. 46

6.2.1 Chemical Analysis of the Process Sample ................................................ 46

6.2.2 X-Ray Fluorescence and Carbon/Sulfur Analyses ..................................... 46

6.3 Mechanical Testing .......................................................................................... 48

6.3.1 Tensile Tests ............................................................................................. 48

6.3.2 Hardness Tests ......................................................................................... 50

6.4 Metallurgical Testing ........................................................................................ 51

6.4.1 Samples Preparation for Metallography Tests ........................................... 51

6.4.2 Stereoscope and Optical Microscope Analyses ......................................... 52

6.4.3 Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy

Analyses ................................................................................................................. 53

7 RESULTS AND DISCUSSION FOR THE FAILED 316L SS PIPES ....................... 55

7.1 Visual Examination ........................................................................................... 55

7.2 Chemical Testing ............................................................................................. 64

7.2.1 Chemical Analysis of the Process Samples ............................................... 64

x

7.2.2 X-Ray Fluorescence and Carbon/Sulfur Analyses ..................................... 64

7.3 Mechanical Testing .......................................................................................... 65

7.3.1 Tensile Tests ............................................................................................. 65

7.3.2 Hardness Tests ......................................................................................... 66

7.4 Metallurgical Testing ........................................................................................ 68

7.4.1 Stereoscope and Optical Microscope Analyses ......................................... 68

7.4.2 Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy

Analyses ................................................................................................................. 72

7.5 Effect of Welding on MIC Initiation ................................................................... 88

7.6 Effect of Hydrostatic testing procedure on MIC Initiation .................................. 89

7.7 Effect of the Idle period between construction and commissioning on MIC

propagation ................................................................................................................ 90

8 CONCLUSIONS AND RECOMMENDATIONS ....................................................... 92

8.1 Summary of the Findings from the Conducted Laboratory Tests ..................... 92

8.2 Summary of Key Observations from the Investigation ..................................... 95

8.3 Recommendations to Avoid the Reoccurrence of the Failure .......................... 96

8.4 Suggested Future Work ................................................................................... 97

BIBLIOGRAPHY ........................................................................................................... 99

APPENDIX .................................................................................................................. 114

xi

LIST OF TABLES

Table 12.1: Process and Operation Parameters ............................................................. 5

Table 22.2: Findings of the Conducted NDT Methods by the Plant Inspectors ............... 6

Table 33.1: Chemical Composition of Type 304L and 316L SS (ASTM A312, 2016). .. 12

Table 43.2: Mechanical Properties of 304L and 316L Austenitic Stainless Steels. (ASTM

A312/A312M, 2016) ...................................................................................................... 12

Table 53.3: Pitting Resistance Equivalent Number for type 304L and 316L Stainless

Steels ............................................................................................................................ 17

Table 65.1: Testing Environment and Conditions .......................................................... 35

Table 76.1: The Dimensions of the Specimens Used in the Tensile Tests .................... 49

Table 87.1: pH and chloride concentrations in 3 process samples ............................... 64

Table 97.2: XRF and carbon-sulfur analysis for the two failed pipes ............................. 65

Table 107.3: Tensile tests results illustrate that both samples meet the mechanical

properties requirements for the type 316L SS. .............................................................. 66

Table 117.4: Microhardness values for the failed sample including the weld area, HAZ

and parent metal. .......................................................................................................... 67

Table 127.5: Qualitative EDS analysis for the S1 spot. Note the high carbon content

suggesting the presence of bacteria.............................................................................. 80

Table 137.6: Qualitative EDS analysis for the S2 spot. ................................................. 81

Table 147.7: Qualitative EDS analysis for deposit Area. ............................................... 83

Table 157.8: Qualitative EDS analysis for the area specified in Figure 7.31. ................ 86

Table 167.9: Qualitative EDS analysis for the area specified in the following Fig 7.32. 86

xii

LIST OF FIGURES

Figure 12.1: Photographs of the leaking pipes. Note that the pinholes located at or near

the weld joints. Note the white flaky deposits from the leaked ammonium sulfate

solution. ........................................................................................................................... 7

Figure 22.2: Photographs of the leaking pipes. Note that the pinholes here are located

at the parent metal. ......................................................................................................... 8

Figure 32.3: Photographs of two X-Ray films generated using RT inspection method at

the weld joints pinholes’ locations. Note the rounded indications alongside the weld

area. ................................................................................................................................ 9

Figure 55.1: Open Circuit Potential of 316L SS in 2 – 4 g/l of chloride in 100 g/l

ammonium sulfate solution ............................................................................................ 38

Figure 65.2: Open Circuit Potential of 316L SS in 6 – 8 g/l of chloride in 100 g/l

ammonium sulfate solution ............................................................................................ 38

Figure 75.3: Open Circuit Potential of 316L SS in 400 g/l ammonium sulfate solution .. 39

Figure 85.4: The effect of increasing chloride content from 2 - 4 g/l in 100 g/l ammonium

sulfate on the 316L working electrode ........................................................................... 40

Figure 95.5: The effect of increasing chloride content from 6 - 8 g/l in 100 g/l ammonium

sulfate on the 316L working electrode ........................................................................... 41

Figure 105.6: The effect of presence of the chloride ions on the corrosion behaviour of

the 316L working electrode in 400 g/l ammonium sulfate solution ................................ 41

Figure 115.7: Open Circuit Potential of 316L SS in chloridised ammonium sulfate

solution at 1 g/l of NaCl ................................................................................................. 42

xiii

Figure 125.8: The effect of increasing the ammonium sulfate concentration from 100 to

400 g/l on the 316L working electrode........................................................................... 43

Figure 135.9: Anodic polarization curves show the effect of increasing the scan rate in

the experiment on the measured potentials and current densities. ............................... 44

Figure 146.1: The Tensile Test Sample. Note the actual dimensions in Table 6.1. ....... 49

Figure 157.1: Photograph of the failed spool in as received condition. Note that the

pinhole located at the weldment area. ........................................................................... 56

Figure 167.2: Photograph of the other failed spool in as-received condition. Note that

the pinhole here is located at the parent metal. ............................................................. 56

Figure 177.3: A Close-up view of the failed spool showing the pinhole at the fusion line.

...................................................................................................................................... 58

Figure 187.4: A Close-up view of the failed spool showing the pinhole at the parent

metal. ............................................................................................................................ 58

Figure 197.5: Close-up view of the internal surface of the failed spool. ........................ 59

Figure 207.6: Photograph shows the pinhole and the subsurface tunneling after

sectioning the spool circumferentially. Note the close-up views of the pinhole mouth at

the ID. ............................................................................................................................ 60

Figure 217.7: A Close-up view of the sectioned spool near the pinhole. ....................... 61

Figure 227.8: Photograph shows the internal surface of the failed spool. ..................... 62

Figure 237.9: Close-up view shows the multiple pits initiated at the incomplete root

penetration area of the weld as indicated in Figure 5.8. ................................................ 62

Figure 247.10: Tensile test results show the tensile and yield strength of the two

samples. ........................................................................................................................ 66

xiv

Figure 257.11: Photomacrograph showing the exact location of the indentations for the

microhardness testing. .................................................................................................. 68

Figure 267.12: Photograph shows a prepared and hot mounted sample of the localized

damage at the weld area. .............................................................................................. 69

Figure 277.13: Photomicrograph shows the localized attack in as polished-condition. . 70

Figure 287.14: Photomicrograph shows the localized attack in as etched-condition. .... 70

Figure 297.15: Photomicrograph of the 316L SS spool sample away from the pinhole in

as-polished condition. .................................................................................................... 71

Figure 307.16: Photomicrograph of the 316L SS sample away from the pinhole in as-

etched condition. ........................................................................................................... 72

Figure 317.17: SEM image at low magnification showing the cross-section photograph

of the subsurface cavity and tunneling. 20x Magnification. ........................................... 73

Figure 327.18: SEM image showing the cross-section photograph of the subsurface

cavity. Note the preferential attack mechanism. 70x Magnification ............................... 74

Figure 337.19: SEM image showing the cross-section photograph of the subsurface

cavity. Note the preferential attack mechanism. 100x Magnification ............................. 74

Figure 347.20: SEM photomicrographs showing the cross-section of the subsurface

cavity. Note the preferential attack mechanism. 250x Magnification ............................. 75

Figure 357.21: Photographs show the morphology of MIC attack on the fusion line of SS

weldment. Note the dendritic attack as pointed by (D) compared to nondendrite area on

(A). The Image on the right shows the MIC attack of the fusion line and the weld area.

(Jack, 2002) .................................................................................................................. 76

xv

Figure 367.22: SEM image shows the pinhole in as received condition. Magnification

25x. ............................................................................................................................... 77

Figure 377.23: A Magnified SEM image of the same location of Figure 7.9. Note the

dimension of the pits openings. Magnification 100x. ..................................................... 77

Figure 387.24: SEM image shows the same previous pinhole at higher magnification

(200x). Note the clear preferential attack mechanism at the edge of the pit. ................. 78

Figure 397.25: SEM images of the cross-section of the pinhole in as-received condition.

Note the irregular shape suggesting bacteria presence. Mag. 1200x and 2000x. ......... 79

Figure 407.26: A form of bacterial cells in a colony at a magnification of 2700x. (Dexter,

2003) ............................................................................................................................. 80

Figure 417.27: EDS analysis for the S1 spot in Figure 7.12. Note the high carbon peak.

...................................................................................................................................... 80

Figure 427.28: EDS analysis for the S2 spot in Figure 7.12. Notice the high iron peak

and the low carbon peak. .............................................................................................. 81

Figure 437.29: SEM Image shows deposits on the internal surface of the pipe. ........... 82

Figure 447.30: EDS analysis of the doposits as shown in Figure 7.29. ......................... 83

Figure 457.31: EDS analysis (Above) for the pinhole in as-received condition (Below). 85

Figure 467.32: EDS analysis (Above) for the pinhole in another location in as-received

condition (Below). .......................................................................................................... 87

xvi

LIST OF SYMBOLS, ABBREVIATIONS, AND NOMENCLATURE

T Temperature

icorr Corrosion Current Density

Ecorr Corrosion Potential

Ep Pitting Potential

Er Repassivation Potential

MAWP Maximum Allowable Working Pressure

RT Radiographic Testing

UT Ultrasonic Testing

NDT Non-Destructive Testing

MIC Microbiologically Influenced Corrosion

UDC Under-Deposit Corrosion

NPS Nominal Pipe Size

ID Inner Diameter

OD Outer Diameter

(NH4)2SO4 Ammonium Sulfate Solution

NaCl Sodium Chloride

NACE National Association of Corrosion Engineers

API American Petroleum Institute

ASTM American Society For Testing Materials

XRF X-Ray Fluorescence

SEM Scanning Electron Microscopy

EDS Energy Dispersive X-Ray Spectrometer

NH3 Ammonia

H2SO4 Sulfuric Acid

Ø Diameter

xvii

SS Stainless Steel

ppm Parts Per Million

PREN Pitting Resistance Equivalent Number

CPT Critical Pitting Temperature

Cl SCC Chloride Induced Stress Corrosion Cracking

SRB Sulfate Reducing Bacteria

H2S Hydrogen Sulfide

IOB Iron Oxidizing Bacteria

CMIC Chemical Microbiologically Influenced Corrosion

EMIC Electrical Microbiologically Influenced Corrosion

HAZ Heat Affected Zone

WPS Welding Procedure Specification

OCP Open Circuit Potential

PDP Potentiodynamic Polarization

ACS American Chemical Society

Ag/AgCl The Silver/Silver Chloride Reference Electrode

LVDT Linear Variable Differential Transformer

PT Dye Penetrant Test

OM Optical Microscope

ISO International Organization for Standardization

ASM American Society for Metals

HRB Rockwell Hardness Scale B

HV Vickers Hardness

TDS Total Dissolved Solids

TSS Total Suspended Solids

xviii

ACKNOWLEDGEMENTS

I offer my continuing gratefulness to my supervisor Dr. Alfantazi for his guidance and

support that helped me in my graduate study. His feedback and patience helped me to

complete successfully my thesis and thus my Master Degree in Materials Engineering.

I would like also to thank Dr. Asselin, Dr. Maijer, Dr. Poole and Dr. Troczynski who served

as the examining committee members for my thesis. Their constructive feedback

tremendously helped me to improve my thesis. My thanks goes to the faculty, staff and

my fellow students in materials engineering department who I learned a lot from them all.

Special thanks to my colleagues Matlub, Ibrahim and Mohammed who assisted and

motivated me on a continuous basis.

Also, thanks to my former manager in SABIC Mr. Ali Al-Hazemi who provided the support

and encouragement to continue my higher education. Finally, thanks to Saudi Basic

Industries Corporation (SABIC) who supported me financially to continue my graduate

studies abroad in the University of British Columbia (UBC).

1

1 INTRODUCTION

In petrochemical industries, one of the major roles of metallurgical and corrosion

engineers is to investigate and analyze different types of failures that were experienced

in piping and pipe fittings, tanks, vessels, and rotating equipment in the plants. One of the

main objectives of such investigations is to clearly define and identify the possible

damage mechanisms that contributed to the failure. Also, an equally important objective

of any such investigation is to propose mitigation methods or remedial actions in order to

avoid the reoccurrence of the failure in the future.

Conducting a failure analysis is a complex process that requires broad knowledge of

different technical disciplines and the use of different techniques. For example, it is

essential to know the difference between an indicator, a cause, a damage mechanism

and a consequence. Although the previously mentioned terminologies might overlap, it

is critical to be able to recognize them during the failure analysis process in order to

determine the suitable course of action that will help prevent the repetition of the failure

in the future. (Scutti, 2002)

Recently, a petrochemical plant experienced a major incident in one of the main pipes

that carries ammonium sulfate solution. There were multiple leaks due to localized attack

in different locations along the pipe. This localized attack was in the form of numerous

pits in the heat-affected zone and base metal. The pit morphologies and locations, as well

as the history of the implemented hydrotesting procedure and water flushing activities,

2

suggested that the probable damage mechanism was Microbiologically Influenced

Corrosion (MIC). Since the total length of the pipes is around 10 kilometers and its

material of construction is type 316L austenitic stainless steel (SS), the financial impact

of this failure was several million dollars. In addition, the piping network was just 4 months

in service at the time of failure. All of these facts indicated that this was a high-value failure

analysis project. Moreover, because of the widespread leaks, such an investigation

became a high priority due to the adverse effects on maintaining the continuous operation

of the plant.

So, the question here is why did a newly commissioned 316L stainless steel pipe system

leak in multiple locations after only 4 months in service? In addition, how did it happen?

Therefore, the aim of this study is to investigate and analyze the damage mechanism of

the newly installed 316L stainless steel pipe system. Moreover, a thorough review of the

literature was presented on the localized damage mechanisms of austenitic stainless

steels with a specialized focus on Microbiologically Influenced Corrosion (MIC). In

addition, the literature review chapter highlights the adverse effect of the weldment and

prolonged water stagnation on the corrosion behaviour. Also, a detailed collection of data

and discussion were presented in the background chapter about the pre-commissioning

and commissioning procedures, the process and operating parameters and the

conducted Non-Destructive Tests (NDT) conducted by the plant inspectors.

3

2 BACKGROUND

It is crucial, as a part of the metallurgical failure investigation process, to collect all the

information and documents that are related to the failure in question. This important step

is required since the failure may not be a direct consequence of a single cause.

Understanding this point will broaden the horizon of the failure analyst trying to unearth

all possible causes that might contribute to the failure at hand. For example, when a tube

failed due to stress corrosion cracking, two conditions should be present in order to have

this damage mechanism. The first condition is the presence of an environment where

there should be chemical or electrochemical reactions take place on the surface of the

material i.e. corrosion mechanism. The second condition is residual or applied tensile

stresses. Without these conditions, stress corrosion cracking is not possible.

Therefore, in this chapter, all related information about the failed pipes was presented.

This information and data includes the material of construction of the failed pipes, the

location of the failure in the plant site, Non-Destructive Testing (NDT) results conducted

on the site, construction and commissioning related data, and finally, process and

operating parameters.

The pipes material of construction, which are intended to transfer ammonium sulfate

solution, are made from type 316L austenitic stainless steel. The nominal pipe size “NPS”

is 3 inch schedule 10, i.e. the pipe outer diameter is 3.5 inch and the pipe thickness is

0.12 inch (3.05 mm). The pipes were in service for only 4 months. However, the piping

4

was first constructed 3 years earlier. The pipes were joined together by welding using

ER316L as the filler material which has similar chemical composition to the base metal.

The industrial standard used for the pipe installation required that upon completion of

construction, and to ensure the integrity of the pipes, hydrostatic testing should be carried

out as part of the pre-commissioning procedure. The main purpose of the hydrostatic

testing, or in short hydrotesting, is to ensure that the pipe system can be operated at the

Maximum Allowable Working Pressure (MAWP). Another purpose is to ensure that there

are no major defects in both the material and the welded joints. After hydrotesting, the

pipes should be drained, dried, purged with nitrogen and kept in this preserved condition

for the entire extended idle period.

Plant documents showed that hydrotesting was conducted. However, the water quality

and proper draining and drying were not controlled and monitored. In addition, during the

3 year idle period, purging with an inert gas like nitrogen was not performed.

The quality of the water used for the hydrotesting should follow the specified guideline.

Seawater and raw/well waters should not be used. Even potable water could only be used

if was treated to ensure that chloride, oxygen and microorganisms were minimal. During

the review of the background information on this failure, no document was found

pertaining to the type or the quality of the water used.

5

Table 2.1 shows the process media and the operating parameters for the solution

transferred through the pipes after commissioning the plant.

Table 12.1: Process and Operation Parameters

Description Value

1 Process concentration 10% ± 1.0 wt. ammonium sulfate and 0.2% Urea

2 Ammonium sulfate pH 3.5 as a set value with performance limit between 3-4

3 Ammonium sulfate

density

1050 Kg/m3 normal/set value with performance limit of

1000-1100 Kg/m3

4 Temperature 45oC – 50oC

5 Pressure 3 – 4 Kg/cm2g (42.7 – 56.9 psi)

In this plant, ammonium sulfate is produced from a chemical reaction between ammonia

and sulfuric acid, as shown in the reaction below:

2𝑁𝐻3 + 𝐻2𝑆𝑂4 = (𝑁𝐻4)2𝑆𝑂4

When the concentration of the ammonium sulfate solution reached 10% wt., the solution

is pumped to an ammonium sulfate crystallization unit through a number of pipes. In that

unit, ammonium sulfate crystals are produced. The failed pipes were part of the piping

circuits that carried the produced solution to the ammonium crystallization unit. As part of

evaluating the criticality of the leakage, the plant inspectors conducted a series of NDT

methods on the leaking pipes in situ as shown in Table 2.2.

6

Table 22.2: Findings of the Conducted NDT Methods by the Plant Inspectors

NDT Method Findings

1

External

Visual

Inspection

Multiple pinholes leaks were observed in the weldments and in

the parent pipe. All the leaks were located at the bottom of the

pipe, 6 O’clock position. No jetting or dripping of fluid was noticed

from the leak locations and only minor wetness & solid deposit

(white flaky deposit) was observed around the leak location. Also,

all the weld joints on the line were found to be with minor rust.

However, no other abnormality noticed on the line. See Figures

2.1 and 2.2.

2 Ultrasonic

Testing (UT)

Ultrasonic Testing (UT) to measure the pipe thickness was

carried out and all the readings were found to be acceptable; for

3” Pipe Ø: UT readings are between 2.8 mm to 3.20 mm against

3.05 mm Nominal Thickness.

3 Radiographic

Testing (RT)

Rounded indications (of size 3~6 mm dia) and elongated

indications (up to 10 mm long) were observed in many weld joints

along the circumference. No other abnormality noticed on the

line. See Figure 2.3.

7

Figure 12.1: Photographs of the leaking pipes. Note that the pinholes located at or near the weld joints. Note the white flaky deposits from the leaked ammonium sulfate solution.

Weld Joint

Weld Joint

Pinhole

Pinhole

The white buildup deposits are from

the leaked ammonium sulfate

The white buildup deposits are from

the leaked ammonium sulfate

8

Figure 22.2: Photographs of the leaking pipes. Note that the pinholes here are located at the parent metal.

Pinhole

Pinhole

The white buildup deposits are from

the leaked ammonium sulfate

9

Figure 32.3: Photographs of two X-Ray films generated using RT inspection method at the weld joints pinholes’ locations. Note the rounded indications alongside the weld area.

The light-colored region is the

circumferential weld joining the pipes

Indications of weld defects

Indications of weld defects

10

3 LITERATURE REVIEW

Based on the background of this failure, the 316L stainless steel pipes experienced

localized corrosion attack. Also, the abnormal idle time between construction and

commissioning is anticipated to play a significant role in the failure mechanisms.

Moreover, since the pinholes were located at or near the welded area, this indicates that

welding procedure/parameters could also contribute to this failure.

In this chapter, the published literature was reviewed in the following areas: localized

attack of 316L SS in the form of pitting corrosion, the corrosion behaviour of 316L SS in

ammonium sulfate solution, the corrosion of weldments in stainless steel, the MIC attack

of 316L SS and its common relation with hydrostatic testing in terms of the water quality,

the procedure, and the aftermath of the extended idle period.

3.1 Austenitic Stainless Steels

Since petrochemical industrial equipment and piping deal with different chemicals and

service environments with different operating parameters i.e. temperature, pressure, pH,

flow rate, etc., it became necessary to select suitable materials of construction that can

maintain such conditions. Austenitic stainless steels can withstand processes with

temperatures varying between cryogenic levels up to 600 oC. In addition, they are suitable

to be used in severe corrosive environments (Davis, 2006).

11

The family of austenitic stainless steels comprises many alloys with different chemical

compositions. Generally, they are classified as a family of iron based alloys that have

more than 50% of Iron, between 16% to 26% of chromium and less than 35% of nickel

(Demeri, 2013). The two most common alloys in this category are 304L SS and 316L SS,

where the “L” stands for low carbon content. The advantage of using the L grades is to

maintain the corrosion resistance of the material by reducing its susceptibility to

sensitization (AWS D10.4, 1986). The sensitization occurs due to the precipitation of the

chromium carbide along the austenite grain boundaries which results in depleting the

adjacent boundaries from chromium that will increase the susceptibility to corrosion (Kutz,

2002).

The 304L and 316L SS differ in the chromium and nickel contents, which increase the

corrosion resistance and the hardenability of the materials, respectively. Yet, the major

difference between the two alloys is the addition of molybdenum in the composition for

the resistance to halogen acids. Molybdenum increases the pitting resistance of the

material from oxidizing chlorides. Table 3.1 shows the chemical composition of the two

alloys. (Kelly, 2002)

12

Table 33.1: Chemical Composition of Type 304L and 316L SS (ASTM A312/A312M, 2016).

Grade

Weight, %, maximum, unless otherwise indicated

C Mn Si P S Cr Ni Mo

TP 304L 0.035 2.00 1.00 0.045 0.030 18.0-20.0 8.0-13.0 ---

TP 316L 0.035 2.00 1.00 0.045 0.030 16.0-18.0 10.0-14.0 2.00-3.00

As for the mechanical properties, austenitic stainless steels normally have superior

toughness, higher rates of work hardening and low yield strength with high ductility when

compared against ferritic or martensitic stainless steels (Cramer, 2005). Table 3.2 shows

the mechanical properties of the 304L and 316L austenitic stainless steels. Notice that

the minimum requirements for the tensile and yield strengths are affected by the carbon

content.

Table 43.2: Mechanical Properties of 304L and 316L Austenitic Stainless Steels. (ASTM A312/A312M, 2016)

Grade UNS Designation. Tensile

strength, min ksi [MPa]

Yield strength, min

ksi [MPa]

Elongation in 2 in. or 50 mm (or 4D), min, %

Longitudinal Transverse

TP304 S30400 75 [515] 30 [205] 35 25

TP304L S30403 70 [485] 25 [170] 35 25

TP316 S31600 75 [515] 30 [205] 35 25

TP316L S31603 70 [485] 25 [170] 35 25

As mentioned earlier, one of the main purposes for the use of austenitic stainless steel is

its ability to resist a corrosive attack in certain environments. Olsson (2003) stated that

the presence of the passive layers on stainless steel surfaces gives such material its

13

superior corrosion resistance characteristic (Olsson, 2003). It is worth noting that these

passive layers on the stainless steel surfaces are influenced by the alloying elements in

the chemical composition (Olsson, 2003). Once the bare surface is exposed to the

environment, the chemical reaction between them results in a thin and resistant film made

of chromium oxide that drives the corrosion rate to very low level (Olefjord, 1980).

Generally, various damage mechanisms could take place on piping and equipment made

from type 316L austenitic stainless steels. In rare cases, stainless steel could experience

uniform corrosion. However, the most common forms of corrosion are as follows: pitting,

crevice corrosion, intergranular corrosion, galvanic corrosion, microbiologically influenced

corrosion, high-temperature corrosion, stress corrosion cracking, and erosion (Dillon,

2015).

In the subsequent sections, a greater focus was placed on one of the major forms of

localized corrosion, which is pitting corrosion. Then, in a separate section, a detailed

literature review was presented on Microbiologically Influenced Corrosion (MIC).

3.2 Pitting Corrosion in Type 316L SS

In the beginning, it is critical to correctly characterize the localized corrosion and pitting

as to whether they are corrosion forms or distinctive damage mechanisms. In order to do

so, localized corrosion and pitting should be defined first. Localized corrosion is generally

a loss of metal that occurs in isolated areas on surfaces that are generally uncorroded.

14

On the other hand, pitting could be classified as an extreme form of localized corrosion

and can be defined as loss of metal with a common morphology of a V shape, i.e.

penetrates deeply with a narrow surface opening. Therefore, pitting should be considered

as a corrosion form rather than separate damage mechanism. This is mainly due to the

various damage mechanisms that exhibit pitting as the morphology of the attack (Dillon,

2015).

As mentioned previously, stainless steels are selected to be used in many static and

rotating equipment due to a thin oxide layer called the passive film in nanometer-scale

which is formed on the surface and significantly lower the corrosion rate. However, the

passive layers can breakdown in localized locations. When this happens, it results in an

accelerated corrosion of the underlying metal which then causes pitting. The criticality of

these pits could be understood by their consequences. For example, main structural

components could fail due to major localized thickness loss that adversely affect the

minimum required thickness to maintain the working pressure. Also, pits could lead to

major components failure by acting as initiation sites for different cracking mechanisms.

(Frankel, 2003)

3.2.1 Critical Factors in Pitting Corrosion

There are various factors influencing pitting corrosion. Such factors include surface

environment, surface condition, metal composition, potential and temperature. Critical

surface environment factors include ion concentrations, inhibitor concentrations, and pH.

15

Other fundamental aspects of the pitting corrosion are the stochastic nature of the

processes and the different stages of the localized attack. Therefore, in this section, a

thorough review was presented on these critical factors. (Frankel, 2003)

3.2.1.1 Surface Environment

Generally, pitting corrosion is a result of the attack of aggressive anion species on the

oxide passive layer (Frankel, 2003). Such attack is exhibited as localized damage due to

the passive layer breakdown (Frankel, 2003). Moreover, it was well established in the

early stages of studying the pitting corrosion that the severity of this type of corrosion is

strongly correlated with the bulk chloride concentrations (Leckie, 1966). This is due to the

fact that chloride is an anion of a strong acid where a number of metal cations show

significant solubility in chloride solutions (Galvele, 1981). Generally, type 316L stainless

steel is suitable for use at ambient or near ambient temperature for low concentrations of

chloride, i.e. in the range of 200 – 1000 ppm (Grubb, 2005).

The corrosion inhibitor needs to be introduced first prior to the initiation of any pits.

Controlling the pitting using corrosion inhibitor once the pit started might not solve the

problem. Pitting is thought to be autocatalytic in nature which means that once the pit

initiated and started growing, the localized environment is changed in a way that promotes

further growth of the pit (Frankel, 2003).

16

3.2.1.2 Potential

All metals and alloys have characteristic potentials. Generally, for a typical austenitic

stainless steel potentiodynamic polarization test result., the scan starts from lower

potentials to higher potentials where a pit will initiate when the potential reaches the pitting

potential (EP). The pit will carry on to propagate with high current density until the potential

lowers to reach the repassivation potential (ER) where the passive film will reproduce,

thus significantly lowering the current density. (ASTM International Standard G61 - 86,

2014)

3.2.1.3 Alloy Composition

As highlighted by Szklarska-Smialowska (2004), the composition of the alloy and the

microstructure could be controlled to decrease the susceptibility of the material for pitting.

Moreover, the physical and chemical imperfections of the materials which are formed

during the production of most engineering alloys could lead to the pits development.

Various elements like molybdenum, chromium, nitrogen and nickel were stated to have a

significant effect on improving the pitting resistance of stainless steels. (Szklarska-

Smialowska, 2004)

It is worth noting that a simplified yet effective equation was introduced to determine the

pitting resistance of various materials based on the alloying elements composition. The

Pitting Resistance Equivalent Number (PREN) could be calculated based on the following

equation: (Cleland, 1996)

𝑃𝑅𝐸𝑁 = 𝐶𝑟 (𝑤𝑡. %) + 3.3𝑀𝑜 (𝑤𝑡. %) + 16𝑁 (𝑤𝑡. %)

17

Lorenz first introduced the basic form of this equation in 1969 as follows: (Lorenz, 1969)

𝑃𝑅𝐸𝑁 = 𝐶𝑟 + 3.3𝑀𝑜

However, it was noticed that nitrogen plays a significant role in the pitting resistance when

added as an alloying element. Therefore, Truman in 1987 fine-tuned the PREN equation

to be the currently most used format as highlighted earlier with the addition of 16N

(Truman, 1987). Table 3.3 illustrates the PREN values for the materials listed in Table

3.1.

Table 53.3: Pitting Resistance Equivalent Number for type 304L and 316L Stainless Steels

Grade UNS Designation Cr Mo PREN

TP 304L S30403 18.0 – 20.0 --- 18 – 20

TP 316L S31603 16.0 – 18.0 2.0 – 3.0 22.6 – 27.9

3.2.1.4 Temperature

Temperature role in determining the pitting resistance is vital. Based on many studies, a

new correlation with pitting potential was introduced which was labeled as Critical Pitting

Temperature (CPT) where it derived from the stipulated experimental procedure for each

metal and alloy. It was found that in artificial seawater, for example, the pitting potential

decreases when the solution temperature increases. Therefore, materials that

demonstrate higher CPT tend to have higher resistance to pitting corrosion. (Ovarfort,

1989; Arnvig, 1996)

18

3.2.1.5 Surface Condition

Another mostly unconsidered critical factor for pitting corrosion is the material surface

condition where materials with rougher surface exhibit higher susceptibility and rate for

pitting corrosion. It was reported that a stainless steel sample of type 302 that was finished

to 120-grit had a pitting potential of about 150 mV lower than a sample that was finished

to 1200-grit in chloride containing environment. (Laycock, 1997)

Moreover, Sedriks (1996) demonstrated in his publication “Corrosion of Stainless Steels”

that multiple applied treatments like heating, grinding and abrasive blasting could

detrimentally affect the pitting resistance. As for the heat treatment, if applied improperly,

it could result in a chromium-depleted region that introduced a preferential site for pitting

due to the lower corrosion resistance. Furthermore, another side effect of improper heat

treatment is the heat tint oxide that also becomes a preferred site for pitting to initiate.

(Sedriks, 1996)

3.2.2 Corrosion in Ammonium Sulfate Solution

A limited number of papers and studies were found discussing and investigating the

electrochemical and corrosion behaviours of 316L stainless steels in ammonium sulfate

solutions at low temperature environment. One of the earliest published studies on the

corrosion behaviour in ammonium sulfate solution was conducted by Sugibayashi in

1954. Although the article was published in the Japanese language, the summary was

translated into English. It was found that the corrosion rate of 18-8 steel, i.e. 304 SS,

19

decreases at temperatures below 80 oC and significantly increases at temperatures

higher than 90 oC. They also reported that when the 18-8 material was modified by the

addition of molybdenum, the corrosion rate decreased. In particular, with the addition of

2.65 % Mo, the corrosion rate decreased to less than 5% of that measured for 18-8 based

on a 7-day immersion test, i.e. corrosion loss in weight of 0.11 mg/cm2 vs. 2.6 mg/cm2

(Sugibayashi, 1954).

Another early study on the corrosion behaviour of ammonium sulfate solution at

evaporative conditions was conducted by Lebedev (1974). The laboratory tests were

conducted in a harsh environment to simulate the worst-case scenario in the industrial

setting where the ammonium sulfate solution was at a boiling temperature and at pH of

5. The Kh18N10T and Kh17N13M2T materials, equivalent to 304 and 316 stainless steels

were exposed to such conditions for up to 4.5 months. It was found that the corrosion rate

of Kh18N10T is 0.11 mm/year whereas for Kh17N13M2T, the corrosion rate was 0.07

mm/year. Such findings coincide with the previously mentioned study that shows the

better corrosion resistance of 316 SS comparing to 304 SS. (Lebedev, 1974)

On another note, one of the closely related studies was conducted by Ghahreman (2012),

on 316L and 317L SS alloys in chloride containing ammonium sulfate (3.56 M) solution

at 100 oC. It was found that in naturally aerated condition, 316L is adequate material in

neutral and slightly acidic environments i.e. pH between 5.4 and 7.4 with the presence of

chloride at 0.12 M. (Ghahreman, 2012)

20

The same goes while reviewing the industrial guidelines, Outokumpu, a large corrosion

resistant alloy manufacturer, has published online its own corrosion tables for materials

exposed to different solutions at different concentrations and temperatures. The corrosion

tables showed that at any ammonium sulfate concentration, the corrosion rates of 304L,

316L and 317L is lower than 0.1 mm/year when the temperature is between 20 oC and

the boiling point.

Moreover, Sandvik, which is a well-known Swedish company manufacturing stainless

steel products among others, has also published its own corrosion tables in the public

domain. Their laboratory corrosion tests were performed with pure chemicals and water

almost saturated with air. They have also concluded that at any ammonium sulfate

concentration, the corrosion rates of 3R12, 3R60, and 18Cr13Ni3Mo, which are

equivalent to 304L, 316L and 317L, respectively, are lower than 0.1 mm/year when the

temperature is between 20 oC and the boiling point.

3.2.3 Pitting Corrosion in Potable and Sea Water

It is critical to review the literature regarding pitting corrosion in potable and seawater.

This is required since the pipes were exposed to water during the hydrotesting and with

improper draining and drying following the testing, residue water was stagnant for an

extended period. Also, based on the background info, it was not clear which type of water

was used. Therefore, it is important to briefly review the literature on this subject.

21

There are three main factors that should be considered when studying pitting corrosion

in potable and seawater systems. These factors are chloride concentrations, temperature,

and velocity. As for the chloride concentrations, it was briefly highlighted earlier that 316L

stainless steels could be used for a range of 200 – 1000 ppm at around ambient

temperature (Grubb, 2005). However, at a higher temperature, the required chloride

concentrations to cause chloride induced pitting corrosion or chloride Induced Stress

Corrosion Cracking (Cl SCC) is lower (Grubb, 2005). It is worth noting that for Cl SCC,

the industrial practice is that no practical minimum limit of chloride is required to initiate

such a damage mechanism. This is due to the well-established knowledge that there are

circumstances where chloride concentrates such as during alternating exposure to dry-

wet conditions. When this happens, Cl SCC could occur at elevated temperatures

exceeding 140 oF (60 oC) (API 571, 2011).

Moreover, the second factor, temperature, should be considered and analyzed prior to

recommending materials in seawater applications. Similar to the critical pitting

temperature (CPT), critical crevice corrosion temperature (CCT) must be studied and

measured using ASTM G 48 – 11 “Standard Test Methods for Pitting and Crevice

Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride

Solution”. Such test methods are used to compare and rank the different types of stainless

steels and their resistance to pitting and crevice corrosion. (Kovack, 1997)

22

As for the third factor, velocity, it is one of the most critical factors that should not be

overlooked during the design, construction and commissioning stages. It was reported

that at stagnant, intermittent or low water flow i.e. the velocity is less than 1.5 m/s,

stainless steels would develop localized corrosion (Grubb, 2005). One of the causes for

the development of localized corrosion is the formation of a microbial slime layer that

causes biofouling (John Grubb, 2005). Another reason is the precipitation of deposits due

to low/stagnant flow which creates Under Deposit Corrosion (UDC).

3.3 Microbiologically Influenced Corrosion (MIC)

Microbiologically Influenced Corrosion (MIC) was first officially recognized by the National

Association of Corrosion Engineers (NACE) to be the designated terminology to describe

this type of failure (Javaherdashti, 2008). Yet, in literature, various terms were used to

describe it, such as microbiologically induced corrosion, microbial corrosion, biocorrosion

and biodeterioration. It may seem more appropriate to address it as Biologically

Influenced Corrosion since the culprit here is not only microorganisms but also algae that

can be seen by the unaided-eye which can have detrimental effect resulting in under-

deposit corrosion. However, it is advisable to follow NACE and the literature norm. So, in

this thesis, this type of corrosion is referred to as Microbiologically Influenced Corrosion

(MIC).

23

3.3.1 Susceptible Materials to MIC

MIC attack was reported to be the cause of failure in many types of materials. However,

greater focus was directed towards materials that are extensively used in industrial

applications. Therefore, it is reported that iron based alloys, copper based alloys, nickel

based alloys and aluminum based alloys were among the most reported, studied and

investigated materials in regard to their susceptibility to MIC. Moreover, in general,

studies show that titanium and titanium based alloys are almost immune. (Jack, 2002)

Filip found that some types of bacteria even attack plastics (Filip, 1992). This highlights

the fallacy in the common impression that non-metallic materials are superior substitutes

to metallic materials.

Javaherdashti explained that the widespread reports of MIC could be a result of some

kind of a chemical response (Javaherdashti, 2008). However, it seems that trying to

correlate the MIC attack to some materials is not a practical approach since focusing the

research to understand the attachment of the planktonic cells to any material’s surface to

be sessile cells and the following microbial colonization is more logical. In other words, if

the bacteria are present in an environment that provides nutrients to colonization growth,

then the surface underneath this slime or biofilm will provide a localized environment or

a concentration cell that is significantly different from the bulk stream, irrespective of the

chemical composition of the base material. As such, the different bacteria that have

oxidizing, reducing or producing mechanisms, provide a wide range of localized corrosive

24

media that attack different types of materials. Such an explanation is supported by

Groysman (2010) who stated that the surface material has almost no effect on the biofilm

attachment and growth since microorganisms attach to different metallic and non-metallic

materials within 30 seconds of exposure with almost the same rate (Groysman, 2010).

The next section highlights in detail the specific damage mechanisms of different types

of bacteria.

3.3.2 Mechanisms of MIC Bacteria Types

During the research for industrial case studies where the damage mechanism was

attributed to MIC, many stipulated that Sulfate Reducing Bacteria (SRB) or Iron Oxidizing

Bacteria (IOB) were the responsible types of bacteria. However, as highlighted by

Geissler in the paper “Do not Just Blame the SRBs and APBs for MIC”, it stated the long

common belief of blaming the bacteria which generate hydrogen sulfide (H2S) or organic

acids as part of their metabolism to be responsible for MIC attacks. However, it is not only

these two types that could cause MIC (Geissler, 2015). A NACE publication about the

selection, application and evaluation of biocides in the oil and gas industry lists eight (8)

different types of bacteria in oilfield environments: acid producers, iron oxidizers, iron

reducers, general heterotrophs, methanogens, nitrate reducers, sulfur oxidizers, and

sulfur reducers (NACE, 2006). The reason behind the mainstream focus on the two

aforementioned types of bacteria to be studied, analyzed and linked to MIC was the fact

that laboratory culturing of such types are easier than the rest (Geissler, 2015).

25

Nevertheless, in this thesis literature review, the focus was on studying the mechanisms

of Sulfate Reducing Bacteria (SRB) and Iron Oxidizing Bacteria (IOB). This is due to the

significant similarities between this failure in question and other failures that occurred in

the industry with similar environment and materials of construction.

3.3.2.1 Sulfate Reducing Bacteria (SRB)

Sulfate Reducing Bacteria (SRB) were comprehensively and frequently studied and

analyzed to understand their corrosive effect on many metallic materials under different

environmental conditions. Yet, other groups like thiosulfate-reducing bacteria, nitrate-

reducing bacteria, acetogenic bacteria and methanogenic archaea were also suspected

to be influencing the corrosion behaviour of iron and iron based alloys. Nevertheless, SRB

was normally identified as the primary cause of failures attributed to MIC. This conception

was based on the following three aspects, as highlighted and quoted below by Enning

(2014):

“First, anoxic sulfate-rich environments (e.g., anoxic seawater) are particularly

corrosive.”

“Second, SRB and their characteristic corrosion product iron sulfide are

ubiquitously associated with anaerobic corrosion damage.”

“Third, no other physiological group produces comparably severe corrosion

damage in laboratory-grown pure cultures.” (Enning, 2014)

26

Therefore, a hypothesis of MIC SRB corrosion should be corroborated by comparing the

SRB mechanism and the other previously mentioned types of bacteria’s mechanism

based on these three aspects.

For SRB to take place, there are two different scenarios that should be highlighted. These

scenarios are Chemical Microbiologically Influenced Corrosion (CMIC) and Electrical

Microbiologically Influenced Corrosion (EMIC). The former starts with the attack on iron

by hydrogen sulfide which is the CMIC. The latter is when sulfate reducing bacteria attack

the iron by the direct utilization of the metal itself which happens by the direct electron

uptake, i.e. EMIC. It is worth mentioning that such scenario where noticed only on few

strains of SRB that were studied in the last few years (Enning, 2014).

Enning in an earlier publication assumed that EMIC is wide spread and has significant

technical relevance (Enning, 2012). Thus, CMIC and EMIC are considered the main

processes for the corrosion of iron by sulfate reducing bacteria (Enning, 2014).

Nevertheless, SRB attack can be further aggravated when oxygen ingresses in the

system (Lee, 1995). Oxygen ingress could result in the formation of corrosive sulfur

species due to the partial oxidation of dissolved hydrogen sulfide and iron sulfide deposits

at the surface of the steel component (Nielsen, 1993).

27

3.3.2.2 Iron Oxidizing Bacteria (IOB)

Iron oxidizing bacteria, or iron bacteria for short, utilize Fe(II) as an energy source, which

oxidizes ferrous iron to ferric iron. The main feature of iron oxidizing bacteria like

Gallionella is their vital part in concentrating the chloride ions under the tubercules

because of the reaction of the chloride ions with the produced ferric ions by the iron

oxidizing bacteria. As such, the localized environment under the tubercules consequently

become very acidic. (Javaherdashti, 2013)

3.3.3 Morphology of the MIC Attack

The morphology of MIC attack is one of the most discussed, studied, argued and also

debated in the academic and industrial research communities. Some authors confirmed

their failure investigation to be due to MIC solely based on the morphology of the attack.

Others have supported their hypothesis of MIC failure by conducting bacteria cell count

or other methods to analyze microbial presence and growth. Such methods are briefly

described in a later section.

Borenstein (2002) analyzed a failure case of austenitic stainless steel pipe in contact with

untreated stagnant water for 9 months. The through-wall pitting was sectioned and they

found a large subsurface cavity with a small opening from the surface. In order to ensure

that this type of attack was due to nothing but MIC, the authors used low chloride water

since the presence of chloride will most likely trigger a localized attack in the form of pitting

or crevice corrosion. (Borenstein, 2002)

28

It should be noted that the morphology of the MIC attack might be different when

comparing carbon steel to stainless steels. In a laboratory investigation of carbon steel

failed by MIC due to hydrotest water remaining in the pipes for an extended period of

time, the pit morphology was different. Unlike the narrow opening and tunneling, pits

caused by MIC in carbon steel are rather shallow and large. Sometimes exhibiting a

morphology called cup-shaped pits. (Huang, 2012)

It is worth mentioning that the above two references are not the only ones. Such

observations became well accepted identification of MIC pits such that many industrial

standards, guidelines and reports highlighting MIC morphology of the attack in stainless

steel is a wide subsurface cavity with a narrow opening whereas for carbon steel is cup-

shaped pits within pits. This was highlighted in the American Petroleum Institute

Recommended Practice (API 571) titled “Damage Mechanisms Affecting Fixed

Equipment in the Refining Industry”.

3.3.4 Locations of MIC Failures

Recognizing the probable or potential locations of MIC attacks are extremely beneficial

to understand and readily identify any future damage due to MIC in order to better prevent

or minimize their criticality and adverse consequences. As such, this section highlights

the abundant industrial and academic studies about MIC attack locations.

29

Prior to reviewing the literature on this subject, it is imperative to define the word

“weldment”. The term “weldment” includes the weld area, the heat affected zone (HAZ)

and the adjunct parent metal, as noted by NACE/ASTM G193 “Standards Terminology

and Acronyms Relating to Corrosion”. Therefore, with this in mind, this section reviews

the publications based on the aforementioned definition. (NACE/ASTM, 2012)

Davis in his book “Corrosion of Weldments” explained that MIC usually occurs at or near

the weld area and the heat affected zone. Kobrin in his article “Microbiologically

Influenced Corrosion of Stainless Steels by Water Used for Cooling and The Hydrostatic

Testing” identified that for weld defects like lack of penetration and root concavity, such

defects are normally the preferred sites for attachment and growth of microbial colonies.

(Kobrin, 1997)

As a general concept, MIC is not the only damage mechanism that attacks stainless steels

at the weld area. Cramer and Covino have listed other damage mechanisms such as

pitting corrosion, crevice corrosion, sensitization, Chloride Stress Corrosion Cracking (Cl

SCC) and caustic embrittlement. There are many causes of the stainless steel weldment

being the target of many damage mechanisms. Such reasons are:

During the welding process, the material exhibits a heating and cooling cycle

which consequently affects the weldment microstructure and chemical

composition. In this case, the corrosion resistance of non-filler welds will be

lower than of the base metal. This is also true when using filler material for

30

welding that is the same in chemical composition to the base metal. (Cramer,

2003)

The selected heat input and the welder workmanship will change how the

weldment solidifies, which, if done improperly, may reduce the corrosion

resistance. (Streicher, 1978)

The corrosion of stainless steel weldments based on the aforementioned causes could

be abated through the proper material selection of the base metal, by quality monitoring

the welding practices and workmanship, and by using a suitable filler material. (Cramer,

2003)

Furthermore, Planktonic bacteria are presumably attracted to the weld area, fusion line

or the heat affected zone (HAZ). Therefore, improper welding practices increase the

chances of MIC initiation either by introducing heat oxide tent or by root concavity or lack

of weld filler penetration which result in welding defect and a preferred zone for bacteria

to attach to the surface and form the biofilm layer that later causes the MIC. (Cramer,

2003)

As highlighted earlier, it is critical to shed some light on why microorganisms prefer to

attach themselves and colonize in the weldment area. One explanation for this issue is

that due to welding, the surface roughness and even the chemical composition are

different in a way that attracts the bacteria, or microorganisms in general, to colonize the

31

pipe internal surface that is in contact with water. Another explanation suggests that even

if the weld area and base metal have the same chemical composition and there is no

welding defect, bacteria tend to also attach to the weldment area because of residual

stresses introduced by the welding. (Cramer, 2003)

3.3.5 Effect of Water Quality on MIC

The quality of water which is used in hydrostatic testing is critical to the prevention of MIC

at the equipment and piping commissioning stages. To ensure the suitability of water for

the intended hydrostatic testing, a number of parameters should be controlled including

chloride content, bacteria count, oxygen concentration, and pH. Therefore, this section

discusses, in brief, the two main parameters: chloride concentration and the bacteria

count.

In general, the variations of the previously mentioned parameters could be explained by

the type of water used and the subsequent treatment that was applied, if implemented.

For example, potable water does have lower chloride concentration and bacteria count

than seawater. As for the aquifer and well water, it depends on the geographical location,

as in near industrial or marine environment, and the depth of the wells. Depending on the

type of water used in hydrotesting, the severity of potential MIC could be correlated.

Dexter (2003) stressed that the first encounter of material to the possible microorganisms

is during the hydrostatic testing right after the piping fabrication in the shop. He continued

32

that, usually, the type of water used is untreated fresh well waters that have a

considerable amount of bacteria, like Gallionella, which is blamed for causing MIC attacks

on stainless steels. Therefore, the best line of defense is to treat the type of water used

for hydrotesting by biocides. (Dexter, 2003)

As for which type of biocides should be used to prevent the microbial presence in the

water, the oxidizing biocides are generally considered for such use. In particular, chlorine

and ozone are the most chemicals used in the cooling water systems (Rice, 1991).

However, due to regulation pertaining to the use of chlorine, ozone is increasingly used

over it since only minimal rates of dosing are required compared to chlorine (Videla,

1995). As a common industrial practice highlighted by Javeherdashti, it was stipulated to

control the bacteria count to 1000 cells/mL prior to using the water for hydrotesting

purposes. As such, the water is classified as a low corrosive type (Javeherdashti, 2013).

33

4 OBJECTIVES

Based on the review of the literature and the presented background information about the

failure of 316L SS pipes in ammonium sulfate solution in a premature manner, the

objectives in this thesis are:

Conduct electrochemical tests i.e. open circuit potential and potentiodynamic

polarization tests on 316L Stainless Steel electrodes in ammonium sulfate solution

at various chloride concentrations and different ammonium sulfate concentrations.

The objective of these tests is to prove that the material is suitable under conditions

that are similar to the service of the failed pipes and to add evidence to the theory

that the corrosion occurred before the pipes were put into service.

Confirm that the pipe material meets the nominal chemical composition of 316L

stainless steel and that it is not mechanically different from standard 316L.

Perform a detailed failure analysis.

34

5 ELECTROCHEMICAL TESTING ON 316L SS IN AMMONIUM

SULFATE SOLUTION

Several electrochemical tests in ammonium sulfate solution were conducted to simulate

the actual conditions of the process of the failed piping. Open Circuit Potential (OCP)

and Potentiodynamic Polarization (PDP) were conducted at different ammonium sulfate

concentrations and various chloride concentrations. The purpose was to evaluate if the

ammonium sulfate solution which was transported inside the piping was corrosive to the

piping material of construction, 316L SS. Thus, this chapter includes the testing method

and the discussion of the results on 316L SS electrodes.

5.1 Specimen Preparation

A cylindrical sample was cut into coin-shape from a 316L SS rod. A Teflon insulated wire

was attached to the back of the sample using conductive epoxy. The sample was then

mounted in cold-cure epoxy.

Grinding was performed to the mounted sample to remove first the thin layer of epoxy,

that sometimes exist due to imperfect handling of the resin, in order to have a uniform

and planar surface. Then, further grinding/polishing was done to 1200 grit silicon carbide

paper to remove any course scratches. Polishing was then done with 6 um and then 1

um diamond suspension. Finally, the specimen was washed with methanol, rinsed with

demineralized water and then dried with an air jet.

35

5.2 Electrolyte Composition

Ammonium sulfate solution (NH4)2SO4 was prepared using white granular (99% min.)

Ammonium sulfate ACS grade and demineralized water. Sodium chloride (NaCl) was

added to the solution at different concentrations. Table 5.1 show the prepared and tested

electrolyte composition. Note that all the tests conducted in naturally aerated solutions at

room temperature.

Table 65.1: Testing Environment and Conditions

Testing

Material

Conducted Electrochemical

Tests

Testing Conditions

(NH4)2SO4 NaCl

1 316L SS

Open Circuit Potential and

Potentiondynamic Polarization 100 g/l

2 – 4 g/l

(at 1 g/l increment)

2 316L SS

Open Circuit Potential and

Potentiondynamic Polarization 100 g/l

6 – 8 g/l

(at 1 g/l increment)

3 316L SS

Open Circuit Potential and

Potentiondynamic Polarization 400 g/l 0 and 1 g/l

5.3 Corrosion Cell Setup

A standard three-electrode corrosion cell was used to conduct the electrochemical tests.

A 1-liter of the ammonium sulfate solution was added to the cell followed by immersing

the prepared sample of 316L SS as the working electrode opposite to the counter

electrode made from graphite. Also, a silver chloride (Ag-AgCl) electrode was installed

36

acting as a reference electrode. A “VersaSTAT-4” potentiostat/galvanostat of Princeton

Applied Science was used to perform the open circuit potential and the potentiodynamic

polarization tests in order to analyze the corrosion behaviour of the 316L SS in the

intended service.

5.4 Electrochemical Techniques

5.4.1 Open Circuit Potential (OCP)

The OCP, sometimes called the corrosion potential, is defined as the potential difference

of the working electrode compared to the reference electrode when no current is applied.

The OCP tests of ammonium sulfate solution of different concentrations, 100 g/l and 400

g/l, and various chloride concentrations, 0 to 10 g/l of NaCl, were carried out at room

temperature. The measured values of the voltage in reference to the Silver-Silver chloride

electrode were plotted against the time in seconds.

5.4.2 Potentiodynamic Polarization Test

The method of potentiodynamic polarization is to change the working electrode potential

and measure the corresponding current as a function of potential/voltage. Based on the

test findings, the corrosion current density can be determined. The potentiodynamic

polarization tests were conducted following the OCP tests under the same conditions as

highlighted in the previous Table 5.1. Only anodic polarization tests were conducted. The

scan rate was set at 1 mV per 6 seconds i.e. 0.167 mV/s. Additionally, at exactly the same

concentrations and testing parameters, two anodic polarization tests on 316L SS

electrode were conducted while only varying the scan rate from 0.167 mV/s to 1 mV/s.

37

5.5 Results and Discussion of the Electrochemical Tests on 316L SS

Electrode

5.5.1 Effect of Increasing Chloride Concentration

Figures 5.1 – 5.3 illustrate the OCP test results for the 316L Stainless Steel working

electrode in naturally aerated ammonium sulfate solution at pH of 5.4 with various chloride

concentrations as a function of time. Figure 5.1 illustrates the noticeable increase in the

open circuit potential when increasing the chloride content from 2 g/l to 4 g/l in 10%

ammonium sulfate solution. Figure 5.2 tells a slightly different story when the chloride

content was increased from 6 g/l to 8 g/l while maintaining the other parameters at the

same values. Although the measured potential at the beginning of the test showed a

significant difference between the different chloride concentrations, when the time passed

and reached the 3600 s mark, the open circuit potential of 316L stainless steel for the

different chloride concentrations almost converged. Looking closer, the open circuit

potentials increased at a millivolt (mV) level with the increase of chloride content. As for

Figure 5.3, the open circuit potentials in 40% ammonium sulfate solution were measured

to compare the effect of adding 1 g/l of chloride with the same solution without the

presence of chloride. The results showed that the OCP increased with the addition of

chloride, which is consistent with the previous OCP tests.

38

Figure 45.1: Open Circuit Potential of 316L SS in 2 – 4 g/l of chloride in 100 g/l ammonium sulfate solution

Figure 55.2: Open Circuit Potential of 316L SS in 6 – 8 g/l of chloride in 100 g/l ammonium sulfate solution

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 500 1000 1500 2000 2500 3000 3500 4000

Op

en C

ircu

it P

ote

nti

al (

V v

s. A

g/A

gCl

Elec

tro

de)

Time (s)

2 g/l NaCl

3 g/l NaCl

4 g/l NaCl

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0 500 1000 1500 2000 2500 3000 3500 4000

Op

en C

ircu

it P

ote

nti

al (

V v

s. A

g/A

gCl

Elec

tro

de)

Time (s)

6 g/l NaCl

7 g/l NaCl

8 g/l NaCl

39

Figure 65.3: Open Circuit Potential of 316L SS in 400 g/l ammonium sulfate solution

Figures 5.4 – 5.6 illustrate the potentiodynamic polarization results on 316L SS while

increasing the chloride concentration. Figure 5.4 shows that in 10 wt.% of ammonium

sulfate solution, increasing the chloride from 2 g/l to 4 g/l causes a noticeable shift to

higher corrosion potentials and corrosion current densities. In Figure 5.5, with the same

ammonium sulfate concentration, further increasing the chloride content from 6 g/l to 8 g/l

increased the corrosion current densities on the 316L SS electrode. Moreover, at 400 g/l

of ammonium sulfate solution, Figure 5.6, two tests were conducted to see the effect of a

chloride-free solution and chloridized solution with 1 g/l NaCl. The results illustrated an

increase in the corrosion potentials and corrosion current densities while adding the

chloride to the solution. These tests showed that with the addition of chloride in the

solution, the corrosion current density increases. Specifically, the corrosion current

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 1000 2000 3000 4000

Op

en C

ircu

it P

ote

nti

al (

V v

s. A

g/A

gCl

Elec

tro

de)

Time (s)

No Chloride Added

1 g/l NaCl

40

density for the 316L SS electrode in chloridized solution with 1 g/l NaCl was 1.74 µA/cm2.

Therefore, the corrosion rate was 0.8 mpy i.e. 0.02 mm/year. This very low corrosion rate

agrees with the previously conducted studies highlighted in the literature review chapter.

Figure 75.4: The effect of increasing chloride Content from 2 - 4 g/l in 100 g/l ammonium sulfate on the 316L working electrode

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04

Po

ten

tial

(V

vs.

Silv

er C

hlo

rid

e El

ectr

od

e)

Current Density (A/cm2)

100 + 2

100 + 3

100 + 4

41

Figure 85.5: The effect of increasing chloride content from 6 - 8 g/l in 100 g/l ammonium sulfate on the 316L working electrode

Figure 95.6: The effect of presence of the chloride ions on the corrosion behaviour of the 316L working electrode in 400 g/l ammonium sulfate solution

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03

Po

ten

tial

(V

vs.

Ag/

AgC

l Ele

ctro

de)

Current Density (A/cm2)

100 + 6

100 + 7

100 + 8

-0.5

0

0.5

1

1.5

1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02

Po

ten

tial

(V

vs.

Ag/

AgC

l Ele

ctro

de)

Current Density (A/cm2)

0 g/l of Cl

1 g/l of Cl

42

5.5.2 Effect of Increasing Ammonium Sulfate Concentration

OCP experiments to measure the effect of the increase of ammonium sulfate

concentration rather than the chloride concentration on 316L SS were also conducted.

Figure 5.7 shows the open circuit potential of 316L SS in chloridised ammonium sulfate

solution at 1 g/l of NaCl. When the ammonium sulfate concentration increased from 10%

to 40%, the open circuit potential was significantly increased. The potentiodynamic

polarization tests, Figure 5.8, showed that when increasing the ammonium sulfate

concentration from 100 g/l to 400 g/l in the test solution, the corrosion potentials and the

corrosion current densities increased.

Figure 105.7: Open Circuit Potential of 316L SS in chloridised ammonium sulfate solution at 1 g/l of NaCl

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0 500 1000 1500 2000 2500 3000 3500 4000

Op

en C

ircu

it P

ote

nti

al (

V v

s. A

g/A

gCl

Elec

tro

de)

Time (s)

400 g/l AS

100 g/l AS

43

Figure 115.8: The effect of increasing the ammonium sulfate concentration from 100 to 400 g/l on the 316L working electrode

5.5.3 Effect of Increasing the Scan Rate of the Anodic Polarization

Test

At exactly the same concentrations and testing parameters, two anodic polarization tests

on 316L SS electrode were carried out while only varying the scan rate from 0.167 mV/s

to 1 mV/s. The ammonium sulfate concentration was 100 g/l with 5 g/l of NaCl. Figure 5.9

illustrates the effect of increasing the scan rate six times. The test results shows that

increasing the scan rate altered the measured results as the passive film had more time

to form at the slower scan rate. In an industrial setting, the passive film would have ample

time to form so the corrosion rate shown here (0.8 mpy) may actually be an overestimate.

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03

Po

ten

tial

(V

vs.

Ag/

AgC

l Ele

ctro

de)

Current Density (A/cm2)

100 g/l AS with 1 g/l NaCl400 g/l AS with 1 g/l NaCl

44

Figure 125.9: Anodic Polarization Curves show the effect of increasing the scan rate in the experiment on the measured potentials and current densities.

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03

Po

ten

tial

(V

vs.

Ag/

AgC

l Ele

ctro

de)

Current Density (A/cm2)

Slow Scan Rate

High Scan Rate

45

6 TESTING PROCEDURE FOR THE FAILED 316L SS PIPES

Conducting a proper investigation of the failed equipment usually results in significant

information about the critical factors that led to the failure. This wealth of knowledge aids

in avoiding the recurrence of the failure. Hence, in order to properly organize a thorough

failure analysis, it is essential to have a basic understanding of the various procedural

steps which are utilized during the metallurgical laboratory investigation. Therefore, in this

chapter, an in-depth explanation is presented for the conducted tests on the failed pipes

through metallographic, chemical and mechanical testing analyses.

6.1 Visual Examination

One of the first and foremost steps in any metallurgical failure investigation is the visual

examination. Visual examination can help to identify, in broader terms, the

fractured/damaged surface characteristics and any visible surface defects. During this

step, no destructive test is used. All of the following tests were conducted at the

sponsoring Company analytical laboratories.

Although dye penetrant test (PT) is considered a non-destructive testing (NDT) method

and is usually conducted during the visual examination step of the failed sample, such a

test was not conducted during this examination due to two reasons. First, PT is normally

used to clearly show all the locations of pits and surface cracks, which is not necessary

for this failure at hand since the failure and pits are visible by the unaided eye. The second

reason is that the PT may contaminate the surface if an EDS analysis, as part of the

46

scanning electron microscope, is carried out in an as-received condition, which is the

case in our investigation.

Therefore, prior to destructive sampling, many photographic records of key observations

were taken. Such records are referred to as macrophotographs, which usually utilize

magnifications between 1x and 50x. These photographs were taken using digital camera

and stereomicroscope, i.e. a low-power binocular optical microscope, with the proper light

sources.

6.2 Chemical Testing

6.2.1 Chemical Analysis of the Process Sample

Three samples from the ammonium sulfate process were obtained from the same unit of

the failed pipes. The samples were analyzed to determine the pH and the chloride

contents. The chloride concentration in the samples was analyzed using Ion

Chromatography, Dionex model DX-500. The analysis was conducted at the facilities of

the sponsoring Company.

6.2.2 X-Ray Fluorescence and Carbon/Sulfur Analyses

One of the essential starting points in the metallurgical investigation of the failures is the

identification and confirmation of the chemical composition of the failed sample. As in the

case of the failure at hand, the background information showed that the material of

47

construction was in fact 316L stainless steel. Therefore, X-Ray Fluorescence (XRF) and

Carbon/Sulfur analyses were conducted. All of the following tests were conducted at the

sponsoring Company analytical laboratories.

XRF is an analytical technique which is used for solid samples to illustrate the

concentrations of the elements. XRF spectrometers operate by irradiating the solid

sample with an X-ray beam of a high energy in order to excite the characteristic X-rays

from the elements that are present in the solid sample. In this investigation, the Bruker

S4 Pioneer XRF spectrometer was utilized.

For Carbon/Sulfur analysis, the LECO CS-600 Automated Analyzer was utilized. This

analyzer is designed for the prompt determination of carbon and sulfur in various

materials such as ferrous and non-ferrous alloys, and some non-metallic materials.

The analyzer operation concept is straightforward. Since the majority of metals and their

alloys burns in oxygen when heated to significantly elevated temperatures, the carbon

oxidizes to carbon dioxide (CO2) and sulfur converts to sulfur dioxide (SO2). The installed

Infrared detectors then measure the CO2 and SO2. Moreover, in order to increase the

speed and the accuracy, high-frequency induction heated furnaces are used.

48

6.3 Mechanical Testing

In order to find if the failed pipe meets the stipulated mechanical properties as per ASTM

A312 “Standard Specification for Seamless, Welded, and Heavily Cold Worked Austenitic

Stainless Steel Pipes”, it is necessary to conduct the required mechanical testing.

Therefore, this section covers the most commonly used mechanical tests: tensile test and

micro-hardness test. All of the following tests were conducted at the sponsoring Company

analytical laboratories.

6.3.1 Tensile Tests

Tensile testing is utilized to supplement microscopy and confirm its results in the

investigation of the components’ failures. The tensile test indicates the level of the ductility

and the strength.

Two pipe samples were cut and prepared to determine the tensile strength and

elongation. The specimens’ dimensions were as stipulated in the Figure 6.1 and Table

6.1 below.

49

Figure 136.1: The Tensile Test Sample. Note the actual dimensions in Table 6.1.

Table 76.1: The Dimensions of the Specimens Used in the Tensile Tests

Parameters Measured Values

1 The Width of Reduced Section 12.5 mm

2 Thickness 3.0 mm

3 Gauge Length 50.0 mm

4 The Width of Grip Section 20.0 mm

5 Grip Section 50.0 mm

6 Overall Length 200.0 mm

The tensile tests were conducted at two rates. The first rate purpose was to identify the

yield properties where the strain rate of 0.015 mm/mm/min up to 2.0% of tensile strain.

The second rate was used to identify the tensile properties where the strain rate of 0.402

mm/mm/min up until the complete and final fracture.

The tensile tests were conducted using a 50kN Instron Universal Tester at room

temperature of 21.7°C. The strain was calculated by 50 mm extensometer up to 2% strain

50

followed by Linear Variable Differential Transformer (LVDT) of the test machine. In the

next chapter, Results and Discussion, the test results are presented and discussed.

6.3.2 Hardness Tests

Hardness testing is one of the fastest, most effective and inexpensive tests used in failure

investigations. Hardness is the measure of the resistance of a material to the indentation.

Although there are several types of hardness test, Vickers Microhardness test was utilized

in this investigation. The main reason behind selecting this type is due to its very narrow

indenter which determines the localized hardness measurements and differentiates

between local hard spots and the surrounding area.

The purpose of a microhardness test is to determine the hardness of the material in very

localized areas. As in this current investigation, the microhardness tests were conducted

across the pipe cross-sectional area which includes: weld area, Heat Affected Zone (HAZ)

and base metal.

The principles governing the hardness testing is basically the same whether Rockwell

Testing or Vickers Microhardness was used. An indenter is pressed into the material by

known weights on a level system. The depth of penetration and the covered diamond-

shaped area are then measured and the hardness value of the material can be calculated.

51

The Vickers indentor is a standard square-based diamond pyramid with 136o angle α

between opposite faces and 146o angle between opposite edge of the pyramid. The type

of the utilized machine is Buehler MMT 7 Microhardness Tester.

The tests were conducted at room temperature (18.4°C) and the load was 300 gf. The

tests were conducted according to the following methods:

1. Standard Guide for Preparation of Metallographic Specimens – ASTM E3

2. Standard Test Method for Microindentation Hardness of Materials – ASTM E384

6.4 Metallurgical Testing

6.4.1 Samples Preparation for Metallography Tests

As stated previously, the first step in the metallographic analysis is to select a sample that

is representative of the materials to be tested. Then, the second step is to prepare the

metallographic specimens. Generally, five major operations are involved in preparing the

metallographic specimens: sectioning, mounting, grinding, polishing, etching. It is vital to

carefully implement such operations in order to ensure that the tested specimens were

not altered due to improper preparation practices. Incorrect preparation techniques may

negatively affect the microstructure and could result in inaccurate findings which points

to an incorrect cause of failure.

52

After sectioning and grinding, polishing was conducted. The goal of polishing is to remove

the scratches and the heavily deformed layer that was caused by grinding. Polishing is

the last step for metallographic specimens’ preparation prior to the microscopic

examination. After polishing, the specimen is cleaned by solvents for two purposes:

examining the polished surface using optical microscope followed by SEM/EDS and

etching the polished surface in order to examine under the optical microscope in as-

etched condition.

Therefore, in order to analyze the failed samples at hand, two samples were cut and

mounted by compression molding, followed by grit silicon carbide grinding from 120 µm

to 600 µm. Successively, the samples surfaces’ were polished using diamond abrasive

(6 µm).

6.4.2 Stereoscope and Optical Microscope Analyses

Optical metallography is the examination of materials using visible light to illustrate

magnified images of the micro and macrostructures. Microscopy is basically the

microstructural examination at magnifications of approximately 50x or higher. However,

macroscopy, i.e. macro-structural examination or simply stereoscope, involves

magnifications of 50x or lower. Microscopy and Macroscopy can both be utilized for as

received, as polished or as etched specimen evaluation.

53

6.4.3 Scanning Electron Microscopy and Energy-Dispersive X-Ray

Spectroscopy Analyses

SEM/EDS was used for the characterization of the metal structures and chemical

compositions, as well as in analyzing the fracture surfaces. An Oxford INCA 250 EDS

installed on a Quanta 200 SEM system was utilized for the analysis of the failure.

Magnifications from 25x to 2000x were employed in the analysis of multiple specimens.

Both types of specimens, i.e. the mounted and polished specimens and the as-received

samples were analyzed. The as-received samples without polishing or even cleaning

were studied intensively for two purposes: to illustrate the damaged surface morphology

and to analyze the chemical composition of any deposits or matters that may be

discovered during the process. Utilizing the capabilities of SEM, the surface morphology

was revealed in order to identify defects’ characteristics due to the high resolution and

depth of field of the SEM.

It is important to note that the x-ray peak energies for different elements might overlap.

Such an issue might lead to incorrect identification of elements, especially for 316L SS.

An example of peak overlap is between molybdenum and sulfur. Since 316L SS contain

2-3% of Mo, the EDS quantitative analysis might show sulfur around this weight

percentage. Missing this critical understanding of the peak overlaps might derail the

investigation to totally different direction by assuming high amount of sulfur present in the

material, for example. Another common peak overlaps are for Mo and Nb, Ti and V, Cr

and Mn, and Cr and oxygen. See Annex B of ISO 22309 “Microbeam Analysis -

54

Quantitative Analysis using Energy-Dispersive Spectrometry (EDS) for Elements with an

Atomic Number of 11 (Na) or Above”. (ISO 22309, 2011)

55

7 RESULTS AND DISCUSSION FOR THE FAILED 316L SS PIPES:

FAILURE ANALYSIS

In this chapter, all data and information derived from the failure analysis and laboratory

tests are presented. Moreover, a detailed analysis and careful interpretation of the test

results is described.

7.1 Visual Examination

Two samples from the 3-inch diameter failed pipe (spools) where received for the

metallurgical examination. A general overview of the received spools is shown in Figures

7.1 & 7.2. Meshed-like marking was observed covering part of the external surface of the

spools. The pinhole leak location is almost at the center of the meshed-like marking. After

inquiring from the plant inspectors, they highlighted that such markings are due to quick

fixes, i.e. using metallic clamps with rubber pads, in order to stop the leak while the line

was in operation to avoid interrupting the plant production by isolating the damaged pipe

which requires an unplanned shutdown.

56

Figure 147.1: Photograph of the failed spool in as received condition. Note that the

pinhole located at the weldment area.

Figure 157.2: Photograph of the other failed spool in as-received condition. Note that

the pinhole here is located at the parent metal.

Pinhole Location

Tensile Test Sample

Tensile Test Sample

57

Although the pinholes located at or near the 6 o’clock position in both spools, the vicinity

of the pinholes are not consistent. The pinhole in one of the spools is located near the

weldment while the other is in the parent metal, Figures 7.3 & 7.4. Such observation would

indicate that such failures are not only due to welding defects. On the other hand, this

observation may also indicate that the causes of the pinholes might not be the same.

58

Figure 167.3: A Close-up view of the failed spool showing the pinhole at the fusion line.

Figure 177.4: A Close-up view of the failed spool showing the pinhole at the parent metal.

2.8 cm pipe section

was cut - see Fig. 12

2.6 cm pipe

section was

cut - see

Fig. 10

59

Figure 7.5 shows the internal surface of the failed spool. The close-up view illustrates

discoloration and staining on the surface. The white deposits are from the ammonium

sulfate solution.

Figure 187.5: Close-up view of the internal surface of the failed spool.

Furthermore, a closer view of the lower portion of the spool indicated an interesting

finding, Figures 7.6 & 7.7. The spool was cut near the pinhole location and a subsurface

cavity, sometimes called tunneling, was observed. The tunneling indication coupled with

the extended idle period of 3 years where water was stagnant suggest that the probable

60

cause of failure is Microbiological Influenced Corrosion “MIC” as documented in many

published reports.

Figure 197.6: Photograph shows the pinhole and the subsurface tunneling after sectioning the spool circumferentially. Note the close-up views of the pinhole mouth at

the ID.

61

Figure 207.7: A Close-up view of the sectioned spool near the pinhole.

Moreover, the visual examination of the internal surface showed the morphology of the

weld, see Figures 7.8 & 7.9. A closer view of the weld illustrated weld defects like a lack

of penetration and root concavity. This is an indication of poor weld quality which could

be attributed to improper welding procedure and/or poor workmanship.

62

Figure 217.8: Photograph shows the internal surface of the failed spool.

Figure 227.9: Close-up view shows the multiple pits initiated at the incomplete root penetration area of the weld as indicated in Figure 7.8.

Incomplete Root Penetration (Weld Defect)

Complete Root Penetration (Proper Weld)

Pits

Pits at the weld - see Fig. 7.9

63

Another observation from the visual examination is related to the origin of the pinholes. It

is essential to determine whether the pinholes initiated from the internal surface or the

external surface. A closer view of both surfaces highlighted that there are multiple

separated pinholes found on the inner surface but only one from the outer side. Such

finding indicates that the pitting was initiated from the inner surface.

Finally, the photographs clearly illustrated multiple pits located few millimeters away from

the weld, in the Heat Affected Zone (HAZ). Also, other pits located at the weld in the area

that was affected by lack of penetration. Davis (2006) explained that MIC usually occurs

at or near the weld and the heat affected zone. This investigation revealed that most of

the pinhole leaks were located at the weld and HAZ. Kobrin, in his article

“Microbiologically Influenced Corrosion of Stainless Steels by Water Used for Cooling and

The Hydrostatic Testing” identified that for weld defects like lack of penetration and root

concavity, such defects are normally the preferred sites for attachment and growth of

microbial colonies. This was clearly illustrated as the pits located in such areas in Figure

23. (Davis, 2006; Kobrin, 1997)

64

7.2 Chemical Testing

7.2.1 Chemical Analysis of the Process Samples

Three samples from the ammonium sulfate process were obtained from the same unit of

the failed pipes. The samples were analyzed to determine the pH and the chloride

contents. Table 7.1 below highlights the findings.

Table 87.1: pH and chloride concentrations in 3 process samples

Parameter Unit Sample 1 Sample 2 Sample 3

pH --- 3.0 3.80 3.2

Chloride ppm 0.06 0.03 0.15

These results illustrate that chloride is in a very low amount the in the ammonium sulfate

process. When compared with the electrochemical tests that I have conducted in UBC

corrosion lab, the conducted tests were at 10,000 ppm of chloride. Moreover, even at

10,000 ppm of Cl, the 316L stainless steel is adequate for the service. Therefore, the

ammonium sulfate process with low chloride content did not cause the corrosion failure

at hand.

7.2.2 X-Ray Fluorescence and Carbon/Sulfur Analyses

The provided failed samples were analyzed using XRF and C/S analyses. The results

confirmed that the material of the pipe is SS 316L, see Table 7.2 below. Note that the

carbon and sulfur were analyzed using LECO CS-600 Automated Analyzer.

65

Table 97.2: XRF and carbon-sulfur analysis for the two failed pipes

Sample # 1 Sample # 2 Nominal Composition wt. %

Elements Conc. % Conc. % Max. unless otherwise indicated

C* 0.019 0.018 0.035

Al 0.033 0.045

Si 0.528 0.474 1.00

P 0.016 0.019 0.045

S* 0.016 0.016 0.030

Ti 0.006 0.018

V 0.059 0.058

Cr 17.236 17.313 16.0 – 18.0

Mn 1.381 1.555 2.00

Fe Balance Balance

Ni 10.294 10.200 10.0 – 14.0

Cu 0.111 0.112

Nb 0.008 0.008

Mo 2.074 2.110 2.00 – 3.00

7.3 Mechanical Testing

7.3.1 Tensile Tests

As stipulated in the previous chapter regarding the steps to properly conduct the tensile

testing, Figure 7.10 and Table 7.3 illustrate the test results. The two samples both show

that the tensile strength, yield strength, and elongation are all exceeding the minimum

requirements. Thus, the material of the examined spool is in compliance with the

mechanical properties stipulated in ASTM A312 Type 316L. Therefore, it is safe to

assume that the mechanical properties of the pipe are not compromised and that the

material in itself did not contribute to the failure at hand.

66

Figure 237.10: Tensile test results show the tensile and yield strength of the two samples.

Table 107.3: Tensile tests results illustrate that both samples meet the mechanical properties requirements for the type 316L SS.

Grade UNS

Designation

Tensile strength, min

MPa

Yield strength, min

MPa

Elongation in 2 in. or 50 mm (or 4D), min,

%

TP316L S31603 485 170 35

Sample 1 - 645 405 54.9

Sample 2 - 640 407 53.4

7.3.2 Hardness Tests

Microhardness test was conducted and the results are stated in Table 7.4 and Figure

7.11. The results of the microhardness tests showed values ranging between 169 and

187 HV. These values are not exceeding the maximum requirement of the hardness

67

values of austenitic stainless steel type 316L as stipulated in ASM International Handbook

“Minimum Room-Temperature Mechanical Properties of 304L and 316L Austenitic

Stainless Steels” (ASM International Handbook, 1990). The maximum hardness values

of 316L SS is 95 HRB, which is equivalent to 213 HV. As stated previously, all the

hardness values are below the maximum hardness value of the pipes.

Table 117.4: Microhardness values for the failed sample including the weld area, HAZ and parent metal.

Readings, µm

Indentation No. D1 D2 Hardness (HV)

8 @ 1 mm Interval 54.2 55.1 186.2

7 54.2 55.5 184.9

6 53.8 55.1 187.6

5 54.1 55.5 185.2

4 57 57.6 169.4

3 57.2 56.5 172.1

2 56.2 55.7 177.7

1 56.7 57 172.1

0 (Center of the Weld) 57.1 57 170.9

1 55.7 55.6 179.6

2 (Pinhole Area) -- -- --

3 56 55.5 178.9

4 56.3 56.6 174.5

5 55.3 56.8 177

6 55.5 55.1 181.9

7 55.8 55.2 180.6

68

Figure 247.11: Photomacrograph showing the exact location of the indentations for the microhardness testing.

7.4 Metallurgical Testing

7.4.1 Stereoscope and Optical Microscope Analyses

Two samples were cut from the failed spools and prepared through grinding, mounting,

and polishing to be examined using the optical microscope. Figures 7.12 -7.14 showed a

large subsurface cavity with only a small opening to the internal surface. This finding

indicates that the pinhole was originally initiated from the internal surface. Also, it supports

the evidence that this failure is due to MIC. As it was stated in the literature review chapter,

the pit morphology caused by MIC tends to be a narrow opening with a wide subsurface

cavity and tunneling at the pits sites. Although this is not a definite way to identify MIC as

pits with similar morphology could be formed by other damage mechanisms as well, the

100

110

120

130

140

150

160

170

180

190

200

-9 -7 -5 -3 -1 1 3 5 7 9

Har

dn

ess

(HV

)

Location in the Weldment (mm)

Hardness Readings Across the Weldment

69

metallurgical findings along with the piping commissioning history suggest that the

damage mechanism here is Microbiologically Influenced Corrosion (MIC). Further tests

such as biological and chemical analyses could be conducted to strengthen this

conclusion. However, the objective here is to focus on the metallurgical examination and

pinpoint to MIC failures based on the conducted tests in this regard.

Figure 257.12: Photograph shows a prepared and hot mounted sample of the localized damage at the weld area.

OD

ID

70

Figure 267.13: Photomicrograph shows the localized attack in as polished-condition.

Figure 277.14: Photomicrograph shows the localized attack in as etched-condition.

71

Another sample which was prepared for metallography was selected away from the

location of the pits to examine the microstructure of the base material. Figures 7.15 &

7.16 show the photomicrographs in as-polished and as-etched conditions, respectively.

The photomicrographs indicated normal microstructure without any abnormal inclusions.

Such findings indicate that the base metal is in sound condition and did not contribute to

this failure.

Figure 287.15: Photomicrograph of the 316L SS spool sample away from the pinhole in as-polished condition.

72

Figure 297.16: Photomicrograph of the 316L SS sample away from the pinhole in as-etched condition.

7.4.2 Scanning Electron Microscopy and Energy-Dispersive X-Ray

Spectroscopy Analyses

A Scanning Electron Microscope equipped with EDS elemental analyzer was utilized in

order to identify the morphology of the pitting in as received condition as well as the cross-

sectional profile through the pit.

The sample in Figure 7.8 with the pinhole located in the weldment was further investigated

using SEM. No cleaning was performed prior to placing the sample inside the SEM

chamber in order to avoid losing any vital evidence that helps further in the investigation.

73

Thus, Figure 7.17 shows the cross-section photograph of the subsurface cavity and

tunneling at 20x magnification. At higher magnifications, 70x, 100x, and 250x, as in

Figures 7.18, 7.19, and 7.20, respectively, the SEM photographs show an interesting

damage called preferential attack mechanism in the weld metal precipitates. As

highlighted by Jack (2002), the morphology of the MIC attack in stainless steel weldments

is preferential phase attack where he described it as “interdendritic attack”, see Figure

7.21. Such description fits the examined photomicrograph in my analysis of the failure

under SEM at the fusion line. This type of failure is usually related to preferential attacks

on either ferrite or austenite. Figures 7.18 - 7.20 illustrate the interdendritic attack at the

fusion line of the stainless steel weldment which show similar to Jack analyzed failure.

Figure 307.17: SEM image at low magnification showing the cross-section photograph of the subsurface cavity and tunneling. 20x Magnification.

74

Figure 317.18: SEM image showing the cross-section photograph of the subsurface cavity. Note the preferential attack mechanism. 70x Magnification

Figure 327.19: SEM image showing the cross-section photograph of the subsurface cavity. Note the preferential attack mechanism. 100x Magnification

75

Figure 337.20: SEM photomicrographs showing the cross-section of the subsurface

cavity. Note the preferential attack mechanism. 250x Magnification

A

B

76

Figure 347.21 has been removed due to copyright restrictions. It was a picture that shows the morphology of MIC attack on the fusion line of SS weldment. Jack compared between the dendritic attack to the nondendrite area. Original source: Thomas R. Jack,

Biological Corrosion Failures, ASM Handbook, Volume 11, 2002.

This preferential attack was also found during the analysis of the internal surface of the

other failed sample in as-received condition. Figure 7.22, illustrates a photograph of a pit

that is located at the fusion line. At higher magnification, Figures 7.23 and 7.24 show

again the preferential attack in the direction of the rolling.

77

Figure 357.22: SEM image shows the pinhole in as received condition. Magnification 25x.

Figure 367.23: A Magnified SEM image of the same location of Figure 7.22. Note the dimension of the pits openings. Magnification 100x.

Fusion Line

Weld Area HAZ

78

Figure 377.24: SEM image shows the same previous pinhole at higher magnification (200x). Note the clear preferential attack mechanism at the edge of the pit.

Furthermore, the second sample was analyzed using EDS to reveal the elemental

analysis at the critical locations. Figure 7.25 shows a high magnification of the same

location as in Figure 7.20-A. As shown in Figure 7.25-B, two spot analyses, S1 and S2,

were conducted at a distinctive feature. The EDS results are demonstrated in Figures

7.27 & 7.28 and Tables 7.5 & 7.6. One unanticipated finding was that the analysis of spot

S1 revealed a high amount of carbon around 60%, while S2 spot shows a lower amount

of Carbon. It is encouraging to compare this Figure with that found by Dexter who

highlighted that this is the shape of bacteria, Figure 7.26 (Dexter, 2003). Therefore, the

high carbon content coupled with the distinct shape suggest the presence of bacteria that

could cause this failure. While such a shape could simply be atmospheric carbon-

containing dust, the shape and location provide some evidence that this is bacterial

remnants.

79

Figure 387.25: SEM images of the cross-section of the pinhole in as-received condition. Note the irregular shape suggesting bacteria presence. Mag. 1200x and 2000x.

A

B

S1

S2

80

Figure 397.26 has been removed due to copyright restrictions. It was a photomicrograph of a form of bacterial cells in a colony at a magnification of 2700x.

Original source: Stephen C. Dexter, Microbiologically Influenced Corrosion, ASM Handbook, Volume 13A, ASM International, 2003.

Figure 407.27: EDS analysis for the S1 spot in Figure 7.25. Note the high Carbon peak.

Table 127.5: Qualitative EDS analysis for the S1 spot. Note the high Carbon content suggesting the presence of bacteria.

Element Wt% At%

C 59.92 74.10

O 21.50 19.96

Si 00.36 00.19

Mo 01.42 00.22

Ca 09.57 03.55

Cr 03.10 00.89

Fe 04.13 01.10

Matrix Correction ZAF

81

Figure 417.28: EDS analysis for the S2 spot in Figure 7.25. Notice the high Iron peak and the low Carbon peak.

Table 137.6: Qualitative EDS analysis for the S2 spot.

Element Wt% At%

C 08.75 24.27

O 14.57 30.35

Si 00.58 00.69

Mo 06.26 02.18

Cr 20.94 13.42

Fe 46.12 27.52

Ni 02.78 01.58

Matrix Correction ZAF

82

Figure 7.30 and Table 7.7 illustrate the EDS analysis of a deposit on the internal surface

of the pipe as shown in Figure 7.29. The elemental analysis showed high sulfur content

in the deposit. The presence of sulfur-containing deposit suggests that the type of bacteria

involved in the Microbiologically Influenced Corrosion (MIC) is Sulfate Reducing Bacteria

(SRB). As discussed in the literature chapter, MIC failures in the stainless steel pipes

were commonly attributed to SRB.

Figure 427.29: SEM Image shows deposits on the internal surface of the pipe.

83

Figure 437.30: EDS analysis of the deposits as shown in Figure 7.29.

Table 147.7: Qualitative EDS analysis for deposit Area.

Element Wt% At%

N 21.23 29.15

O 41.91 50.39

Al 00.72 00.51

Si 00.61 00.42

S 28.51 17.11

Fe 07.02 02.42

Matrix Correction ZAF

The conducted metallurgical analysis tests showed that the failure was caused by

Microbiologically Influenced Corrosion (MIC) based on the pit morphology, the

preferential attack mechanism in the weld and the presence of bacteria/bacteria product.

Yet, it could be argued that localized attack in this form of pitting could also result from

84

chloride induced pitting corrosion. Such argument can be contested by analyzing the

samples to look for any chloride concentration present in the as-received samples. The

EDS analysis illustrated in Figures 41 and 42 did not show any indication of chloride

presences. However, these were spot elemental analyses at high magnification at around

2000x. So, in order to clearly refute this argument, multiple EDS area analyses were

conducted for the location illustrated in Figure 7.20, which is at a lower magnification i.e.

around 250x. As shown in Figures 7.31 & 7.32 and Tables 7.8 & 7.9, no chloride was

detected in the elemental analysis by EDS. Therefore, this finding suggests that chloride

did not play a role in the failure at hand. In comparison, for failures caused by chloride

ions, as reported by Subai (2014), they detected chloride using EDS at the bottom surface

of the pipe at lower magnification in different locations at 4.16 %wt., 2.19 %wt. and 0.14

%wt. (Subai, 2014). Also, they cross-sectioned the pit and conducted EDS analysis in as-

polished condition and found 4.76%wt. of chloride at the bottom surface of the pit (Subai,

2014). Based on Subai study, if the failure at hand was due to chloride induced pitting

corrosion, then the conducted EDS analysis should detect it.

85

Figure 447.31: EDS analysis (Above) for the pinhole in as-received condition (Below).

86

Table 157.8: Qualitative EDS analysis for the area specified in Figure 7.31.

Element Wt% At%

C 05.92 18.12

O 11.11 25.54

Si 01.00 01.31

Mo 03.77 01.45

S 01.65 01.90

Cr 27.94 19.76

Fe 45.62 30.05

Ni 02.99 01.87

Matrix Correction ZAF

Table 167.9: Qualitative EDS analysis for the area specified in the following Figure 7.32.

Element Wt% At%

C 11.94 32.97

O 09.09 18.84

Si 01.51 01.78

Mo 03.14 01.09

S 00.53 00.55

Cr 23.47 14.97

Fe 47.30 28.09

Ni 03.03 01.71

Matrix Correction ZAF

87

Figure 457.32: EDS analysis (Above) for the pinhole in another location in as-received condition (Below).

88

7.5 Effect of Welding on MIC Initiation

As pointed out in the background chapter of this thesis, the conducted radiographic

testing (RT) showed rounded and elongated indications which were observed in the weld

joints for the full circumference. This non-destructive testing results suggest improper

welding performed for the pipes joints. These indications could be caused by inadequate

Welding Procedure Specification (WPS) or poor workmanship which is more likely. These

defects adversely affect the pipe joint integrity since the material will be susceptible to

MIC at the weldment.

As previously noted in the literature review, planktonic bacteria are presumably attracted

to the weld area, fusion line or the heat affected zone (HAZ). Therefore, improper welding

practices increase the chances of MIC initiation either by introducing heat oxide tent or

by root concavity or lack of weld filler penetration which result in welding defect and a

preferred zone for bacteria to attach to the surface and form the biofilm layer that later

causes the MIC.

As highlighted earlier, it is critical to shed some light on why microorganisms prefer to

attach themselves and colonize in the weldment area. One explanation for this issue is

that due to welding, the surface roughness and even the chemical composition are

different in a way that attracts the bacteria, or microorganisms in general, to colonize the

pipe internal surface that is in contact with water. Another explanation suggests that even

if the weld area and base metal have the same chemical composition and there is no

89

welding defect, bacteria tend to also attach in the weldment area because of residual

stresses introduced by the welding. (Cramer, 2003)

7.6 Effect of Hydrostatic testing procedure on MIC Initiation

As described earlier in the background chapter, there are improper practices occurred

during the construction which could adversely affect the integrity of the piping. This

section discussed the potential issues of the water quality that might be used during the

construction phase for the hydrotesting.

It is important to know which type of water was used and how it was treated. The current

investigation was limited in this regard. The plant inspectors claimed that the used water

was in compliance with their plant standards. Such standards stipulated the use of

demineralized water, boiler condensate water, or potable water that was treated

effectively to limit the chloride and oxygen concentrations and the total bacteria count.

However, they failed to provide any documentation supporting their claim. Therefore, with

the failure at hand coupled with the lack of proof, the used water for hydrotest was at best

untreated or poorly treated potable water. A possible explanation is that during the

construction phase, the project inspectors may delegate the responsibility of the

hydrotesting to the contractor with little or even no quality monitoring or supervision from

their side. Such practices are not unusual in large project construction with tight

deadlines.

90

Therefore, it is likely that the water had a high bacterial count due to poor water treatment.

With this in mind, and in order to reduce the possibility of MIC initiation, the water should

have a bacteria count of less than 1000 (Javeherdashti, 2013). This can be achieved by

using biocide treatment to effectively reduce the planktonic and therefore sessile bacteria

in the system.

7.7 Effect of the Idle period between construction and commissioning

on MIC propagation

This section discusses one of the main contributing factors in this failure, i.e. the extended

contact time of the hydrotest water inside the piping. It is basically the idle time between

conducting the hydrotesting as a final phase of the construction and the time of

commissioning the new plant.

As discussed in the background chapter, the idle time lasts for 3 years. This is alarming

because constructing the piping, hydrotesting it and then leaving it in closed condition

without proper lay-up makes the pipeline prone to different undesirable outcomes.

In the literature review chapter, many references were cited about the time limit upon

which the MIC start to initiate and/or propagate. The cited intervals are as follows: 3 to 5

days, 14 days and 1 months. If the idle time planned to be higher than such interval, then

dry or even wet lay-up should be conducted. Nevertheless, the failure at hand crossed all

these intervals and reached more than 1000 days. Yet, they have not protected their

91

assets by conducting preservation method like purging with an inert gas in order to

prevent the piping from corrosion.

Simply put, the more time the bacteria are living in their ideal environment, the more

bacteria growth is expected, assuming the availability of the nutrient in the environment.

In general, when the used water for hydrotesting is less clean, such as having high total

dissolved solids (TDS), the more nutrient is available in the water for bacteria to consume

and grow.

It is worth mentioning that during the investigation of the failure at hand, other piping in

the same system which was constructed a year earlier were also experiencing pinholes.

Although these new failures were not investigated thoroughly, it is safe to assume that

the longer the idle period where water is in contact with the steel surface, the more

pinholes and deeper pits will be developed. This is based on that the time from

commissioning the pipes to the observed leaking pinhole is different. For the pipes with

3-year idle period, the first leak was noticed after only 4 months in service. However, for

the 1-year idle period, the first leak was noticed after 8 months. The other failures with

the one-year idle period were not included in this investigation.

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8 CONCLUSIONS AND RECOMMENDATIONS

8.1 Summary of the Findings from the Conducted Laboratory Tests

In this investigation, one of the main objectives was to identify the responsible damage

mechanism that consequently caused this failure and address, in details, all the

contributed factors that played significant roles in this failure. The investigation of the

pinhole defects in the 316L austenitic stainless steel pipes in ammonium sulfate solutions

suggested that the failure was caused by Microbiologically Influenced Corrosion (MIC).

The following highlights the main conclusions:

1. The Low Corrosivity of Chloridized Ammonium Sulfate Solution to 316L SS

The conducted electrochemical tests illustrated the low corrosivity of the chloridized

ammonium sulfate solution to 316L stainless steel. The corrosion rate based on the

anodic polarization test of 316L SS electrode in chloridized solution with 1 g/l NaCl would

not have exceeded 0.02 mm/year. This conclusion is supported by other studies

conducted on 316L stainless steel in chloridized ammonium sulfate at a higher

temperature reaching 100 oC. These studies and the published industrial corrosion tables

concluded the 316L SS is suitable for ammonium sulfate solution at any concentration

with a corrosion rate less than 0.1 mm/year.

2. Pitting Morphology

The most obvious finding to emerge from this investigation is the classical pits

morphology of MIC. It was numerously reported that a pit with a small opening “mouth”

93

and a large subsurface cavity with tunneling in stainless steels is likely to be a “signature”

of an attack by MIC. The visual and optical microscope examinations revealed the pitting

morphology that is similar to dozens of reported industrial cases with MIC identified as

the damage mechanism.

3. Pitting Location

Another important finding of this investigation is the location of the pinhole and the other

pits. It was evident that almost all the pits are located at or near the fusion line, i.e. the

heat affected zone (HAZ). This result was explained that the microorganisms prefer to

attach to the surface of the weldment. Such attachments could be due to different surface

roughness or different chemical composition between the weld area and the base metal.

Also, the attachment might be due to welding defects or residual stresses from the

welding.

4. Preferential Attack Mechanism

One of the most interesting findings is the preferential attack mechanism. The SEM

photomicrographs showed the preferential attack of ferrite stringers at the fusion line

which resulted in a dendritic surface at the pit. It was repeatedly highlighted in the

literature that the morphology of the MIC attacks in stainless steel weldments is a

preferential attack of a single phase.

94

5. The Presence of Bacteria

Surprisingly, one of the unanticipated findings during the SEM examination was that

bacteria may have been identified. There were two indications that suggest the existence

of bacteria inside the pit. The first indication was the shape. When compared with a Figure

by Dexter (2003), it illustrated the same shape of a bacterial cell in a colony. The second

indication was the elemental analysis. The EDS elemental spot analysis at the bacterial

cell showed a spike of carbon whereas in the adjacent area, the spot analysis did not

reveal the high carbon content.

Such finding strengthens the hypothesis that the MIC is the damage mechanism that led

to the piping multiple failures. As mentioned in the results and discussion chapter, the

analyzed sample was in as-received condition. Moreover, the pipe was sectioned through

the pit using dry cutting to reveal the subsurface cavity and tunnelling. Therefore, no

contamination was expected from the external environment.

6. The Absence of chloride

Generally, when pitting occur in austenitic stainless steel materials, it is widely believed

that chloride might be the main cause of the corrosion attack. Therefore, during the

investigation, the metallurgical examination of the failed sample in the as-received

condition did not show the presence of chloride. Thus, the absence of chloride means

that the failure was not due to chloride induced pitting corrosion which in turns strengthen

the conclusion that the failure occurred due to MIC considering the other vital findings.

95

8.2 Summary of Key Observations from the Investigation

If we take one step away from all the detailed findings which were revealed by the

electrochemical, metallurgical, chemical and mechanical tests, the bigger picture could

be visualized. As such, MIC attack could not occur without the below deficiencies that

paved the way to the failure.

1. Improper Hydrotesting Procedure

As highlighted in the background, the plant inspectors did not provide documentation that

proof the completion of all the required steps of the hydrotesting and the following draining

and drying. The residual water after the hydrotest was not properly drained and dried.

Therefore, a thorough draining and drying followed by effective inspection are critical to

ensure the dryness of the pipes immediately after the hydrotest which help to prevent

MIC in the first place.

2. The Welding Quality

The investigation revealed that the examined weld was performed improperly. Visual

examination showed welding defects in the form of root concavity and lack of penetration.

Such welding defects may favor the planktonic bacteria to attach to these areas and

colonize. Therefore, ensuring the proper workmanship and avoiding any welding defects

will safeguard the integrity of the pipe joint and reduce the likelihood for planktonic to be

sessile.

96

3. Idle Time “Contact Time”

A 3-year idle time between hydrotesting the piping during the construction phase and the

commissioning is very alarming. It was reported that a maximum of 30 days should be

between these two phases. If the period is expected to exceed this 30-day interval, the

line should be mothballed by purging inert gas to avoid active corrosion. However, the

plant professional reported that after hydrotesting, the line was closed and purging with

an inert gas was simply not performed.

8.3 Recommendations to Avoid the Reoccurrence of the Failure

Based on all the presented findings from the metallurgical examination and the

highlighted observations from the investigation in general, below highlights the main

recommendations:

1. It is recommended to replace the failed pipes with the in-kind material, i.e. 316L

Stainless Steel. The failure of the pipes was not attributed to improper material

selection.

2. It is strongly recommended to use treated water for the hydrostatic testing. Using

seawater, well water or even untreated potable water which have high bacteria

count will increase the possibility of damaging the pipes by Microbiologically

Influenced Corrosion (MIC).

3. It is recommended to completely drain and dry the pipes after the hydrotesting.

Failure to do so will result in water accumulation and stagnation which will increase

the likelihood of MIC attack on the pipes.

97

4. It is recommended to minimize the idle period between the construction of the

pipes and the commissioning of the unit to be preferably less than 14 days. If the

anticipated idle period will be longer than a month, then the pipes should be

preserved and protected by purging the pipes internally with nitrogen.

8.4 Suggested Future Work

This thesis presented and discussed in greater details the metallurgical investigation of

the failed 316L SS pipes, which includes determining the causes of failures and the

associated contributing factors that led to promptly end the useful life in a premature

manner. Nevertheless, such work could be expanded in future to include conducting the

following:

1. Electrochemical Testing:

Although the thesis discussed and studied several aspects of the failure at hand including

conducted OCO and Anodic Polarization tests, it deemed beneficial to expand the

conducting electrochemical tests to study the corrosivity of the ammonium sulfate solution

at simulated conditions to the industrial plant that experienced the failure. The simulated

tests should include the addition of Urea at concentrations of 0.2 wt.% with a pH value of

3.0. Also, the tests should include measuring the corrosion current density at different

solution states: stagnant and flowing conditions. Such study will highlight if in a stagnant

condition the 316L SS material would experience higher corrosion rate and by what

magnitude.

98

Moreover, besides ammonium sulfate solution, the electrochemical tests could also be

conducted for different types of water, i.e. seawater, well water and potable water. The

objective here is to identify the corrosion rate of each type of water in order to possibly

narrow-down the water type which was used in the hydrotest that resulted in such failure.

2. MIC Testing

This thesis investigated the failure based on the metallurgical analysis. However, such

investigation could be expanded to include conducting various microbial tests that are

usually used in MIC laboratory investigations. Such tests could include analyzing

different types of water and measure the bacteria count and the types of bacteria

present in the analyzed samples. The analysis could also include treating the water

samples with different types of biocides at different concentrations to measure what

would be the more effective method for MIC prevention.

Such MIC testing could also be coupled with chemical analysis of the water samples.

Analyzing the concentrations of TDS, TSS, chloride, oxygen, and pH, in relation to the

bacteria count and the performance of the biocides could be deemed valuable for

establishing significant and fundamental correlations.

99

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APPENDIX

Tensile Testing Results for the first sample

115

Tensile testing result for the second sample

116

More samples from the piping network experienced the same failure morphology.

117

Another sample from the failed piping network experienced the same failure morphology