Crevice corrosion testing of C276 material in chlorinated seawater...

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Crevice corrosion testing of C276 material in chlorinated seawater at high

temperature for a heat exchanger system

Maria Eleni MITZITHRA1

1TWI Ltd, Cambridge, ,United Kingdom, [email protected]

Abstract : This paper describes a laboratory scale flow system that TWI built to simulate the dynamic

complex in-service environment of a shell-and-tube cooler, and presents the results obtained from

crevice corrosion testing of C-276 material using this specific set-up. The test environment consisted of

artificial seawater (ASTM D1141), with 0.8-1ppm free chlorine and 7ppm dissolved oxygen (DO) at

80°C. The DO of 7 ppm represents the oxygen content in the seawater when it enters the heat exchanger.

Specimens from different locations of a shell-and-tube cooler were extracted, including the weld

overlay, the parent metal of the tubes and associated welds. The crevice assembly was made with two

overlapping metal coupons, employing different configurations, to provide both a metal-to-metal crevice

and a typical PTFE crevice. A high torque of 16N.m was applied. The test was proven feasible and the

specimens taken from the weld overlay suffered from corrosion. The weld and parent tube specimens

did not exhibit signs of corrosion. This suggested that the increased susceptibility of the weld overlay to

localized corrosion at that elevated temperature is influenced by the complexity of the structure’s

geometry and coarser microstructure compared to that of the welds. It is recommended that alloy C-276

should not be used in equipment exposed to chlorinated seawater at 80°C. Still, well-established welding

procedures/practices and surface passivation techniques may allow the C-276’s safe temperature limit

to be maximized. Finally, it is suggested that safe operating temperature limits are established for

materials selected for service in offshore heat exchangers/coolers via appropriate testing.

Keywords : “C276”, “chlorine”, “dissolved oxygen”, “seawater”

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Introduction

On a typical oil and gas platform, heat exchangers are critical components for the proper

operation of any hydrocarbon separation process plant. After initial processing in the separators,

the oil and gas streams are generally heated or cooled in heat exchangers (depending on the

process design of the topside system). These heat exchangers can be shell and tube type, plate

type or printed circuit type with the first 2 types being most common. In shell and tube

exchangers, the process fluid and the heating/cooling medium passes through tubes or around

the tubes inside a cylindrical shell. The heat exchangers heat or cool down the process fluid

either by direct or by indirect heat transfer. In indirect heat transfer the process fluid is heated

or cooled against a heating or cooling medium which in turn is heated or cooled in subsequent

heat exchangers against hot flue gas or sea water respectively. In direct heat transfer, the process

fluid is directly heated or cooled against hot flue gas or sea water respectively [1]. It is common

practice for seawater to be used untreated apart from dosing with hypochlorite in cooling

systems (‘a once through seawater system’). Free chlorine can be continuously dosed at a

concentration of less than 1ppm into the seawater in order to prevent biofouling. There are

cases, especially in relatively warm waters, where the chlorinated seawater enters and leaves

the coolers at approximately 20°C and 40°C respectively. However, it is likely, for example in

the case of a shell-and-tube type cooler, the water temperature where the hot fluid enters the

tube bundle can reach higher values as a consequence of the process design. This may mean

that the local water temperature may approach to the gas inlet temperature which could exceed

80°C. As can be understood, the operation of the cooler presents a dynamic system, where the

structural materials of the component can experience very harsh conditions. Thus selection of

materials must be met to ensure both economical design and reliable performance.

This paper describes a laboratory scale flow system that TWI built to simulate the complex in-

service environment of a shell-and-tube cooler. It also presents the results of the testing carried

out, employing this specific set up, of a Ni-based alloy (C276), typically used for this

component, in order to assess its resistance to crevice corrosion in the simulated in service

conditions.

Experimental procedure

Material and specimen preparation prior to exposure

The material subject to testing in this study was taken from a shell-and-tube cooler. Specimens

were taken from parent tubes made from N01276 alloy and from weld overlay (placed on

carbon steel substrate as part of the tubesheet) made from ER NiCrMo-4 alloy. The tubes were

welded on the tubesheet by an autogeneous welding procedure (Tungsten Inert Gas/TIG

procedure). The welds were also subjected to testing.

Table 1 provides with the chemical composition of the material. The chemical compositions of

the parent tube, tube to tubesheet welds and weld overlay are close to identical. The weld

(overlay) microstructure was of dendritic pattern, with coarse grains and evidence of micro-

segregation, as second phase particles or possibly carbides rich in alloy elements i.e. Mo and

Cr, were found to be uniformly distributed along the grain boundaries.

Table 2 summarizes the oxygen and nitrogen content of the tube to tubesheet welds. Within the

frame of work carried out by Q.Lu, [2], [3], 625 alloy welds, generated using the same

autogenous welding procedure as the ones under testing, were used as a benchmark for an

‘acceptable’ weld. In particular, the nitrogen content is considered normal and falls within the

range measured for the 625 alloy welds (0.010-0.025% wt). The oxygen content is slightly

higher than the value of 0.01% recorded for the 625 welds. Energy Dispersive X-ray (EDX)

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semi-quantitative analysis indicated an average dilution of 9.5% for the weld overlays. This

level of iron dilution is typical for the first layer of weld overlay on the carbon steel substrate.

The locations from which specimens were extracted for crevice corrosion testing are shown on

Figure 1.

Table 1: Elemental composition of the heat exchanger’s material

Element,

% (m/m)

C Si Mn P S Cr Mo Ni Cu

Parent

tube 0.009 0.22 0.39 0.010 0.004 16.37 17.1 58.4 0.03

Tube to

tubesheet

weld

0.006 0.30 0.53 0.016 0.004 16.69 16.6 56.6 0.04

Weld

overlay

surface

0.011 0.27 0.60 0.014 0.006 16.63 17.0 56.4 0.04

Weld

overlay

(middle)

0.011 0.51 0.66 0.013 0.005 16.11 17.0 57.2 0.04

Table 1 (continued): Elemental composition of the heat exchanger’s material

Element, %

(m/m)

Ti V Co W Fe

Parent tube 0.01 0.17 0.23 1.50 5.37

Tube to

tubesheet weld 0.01 0.15 0.24 3.00 5.59

Weld overlay

surface 0.01 0.15 0.24 2.59 5.93

Weld overlay

(middle) 0.01 0.17 0.23 1.50 5.37

Table 2: Average nitrogen and oxygen content in tube to tubesheet welds

Element, % (m/m) N O

Tube to tubesheet weld 0.015 0.049

Figure 1: Position of the different types of specimens taken across a tube-weld-tubesheet

section.

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Figure 2: View of one square specimen extracted from the surface of the weld overlay, of one

full ring specimen from parent tube material and one full ring specimen of tube to tubesheet

weld.

Square shaped specimens, 20mm long x 20mm wide x 4mm thick, were extracted from three

positions in the weld overlay: surface, middle and above carbon steel/weld overlay interface

(example in Figure 2). Full ring specimens (4mm thick) were extracted from the parent tubes

and tube to tubesheet welds (examples in Figure 2). The exposed surfaces of the specimens

were ground to a 120 grit finish (in order to represent as closely as possible the in-service

condition) and cleaned thoroughly with acetone and ethanol.

The specimens were mounted in crevice assemblies as depicted in Figures 3 and 4. The crevice

assembly was made with two overlapping metal coupons, to provide the following

combinations: weld overlay - weld overlay specimens, parent tube - parent tube specimens and

finally, tube to tubesheet weld - tube to tubesheet weld specimens. The crevice assembly was

completed using two glass reinforced PTFE castellated crevice washers and Ti Gr2 bolts and

nuts. In order to avoid any undesirable corrosion of the bolts and nuts (Ti Gr2 can be susceptible

to corrosion at the test temperature), adhesive tape was used to isolate exposed surfaces from

the test environment. The applied bolt torque was 16N.m. This configuration provided both a

metal to metal crevice and typical PTFE crevice.

Figure 3: Schematic representation of the crevice assembly.

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Figure 4: Crevice assembly of overlapped weld ring and overlapped weld overlay specimens

An additional test specimen was extracted from the tube sheet weld overlay and was exposed

to the test environment without use of a crevice assembly, to serve as a control sample. Prior

to immersion, all test specimens were weighed.

Experimental set up and solution preparation

The testing was performed in a Pyrex glass vessel with a capacity of 10L. The aqueous test

solution was artificial seawater (ASTM D1141), with 0.8-1ppm free chlorine and 7ppm

dissolved oxygen (DO) (as determined in the flow-cell, see below) at 80°C. 7 ppm is the

dissolved oxygen content in the seawater the moment it enters the heat exchanger. The

operation of the cooler presents a dynamic system that allows insufficient time for the dissolved

oxygen concentration to decay to the significantly lower equilibrium concentration (~1-2 ppm)

at 80°C. Full test conditions are given in Table 3.

Table 3: Test conditions

Test conditions Parameter

Temperature (°C) 80

Pressure (bara) 1

Dissolved Oxygen (ppm) ~7

Solution Aerated artificial seawater ASTM D1141

with 0.8-1ppm free chlorine

The test was performed using a flow loop system as shown in Figures 5 and 6. The total duration

of the crevice corrosion test was 28 days.

Figure 5: Schematic representation of the set up

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Figure 6: Image of the experimental set up

The flow loop system consisted of:

Test vessel containing the test solution at 80°C, with appropriate inlets and outlets

Hotplate and band heating tapes to provide the desired test temperature in the vessel

Temperature feedback and control system using thermocouples to monitor and maintain the temperature of the vessel throughout the test duration

Electric pump for the transfer of the solution from the test vessel to the measurement flow cell at an approximately flow rate of 0.2ml/min

Flow cell containing oxygen and chlorine sensors for continuous automatic monitoring of the levels of dissolved oxygen in the test solution at 30oC.

Cooling coils to lower the temperature of the sampled test solution to within the operational envelope of the monitoring probes prior to analytical

measurements.

Species analyser with continuous automatic display of dissolved oxygen in the test solution

Heating coil to raise the temperature of the analysed solution prior to return to the test solution in vessel

Test glass cell containing stock concentrated hypochlorite solution (10,000ppm hypochlorite solution continuously fed into the test solution).

Peristatic pump to allow for controlled transfer of small volumes of concentrated hypochlorite solution to the test vessel at a flow rate less than

0.1ml/min.

A gas mixture of 70% O2-30% N2 was continuously purged through the test solution at ambient

pressure. Trial tests were performed to establish a suitable gas purge rate in order to meet and

sustain the dissolved oxygen level at approximately 7ppm at elevated temperature.

Optimisation trials were also performed to identify a suitable hypochlorite concentration and

pump rate to maintain the desired level of free chlorine in the test solution.

The level of the DO was continuously monitored using an Oxysense Pro-X oxygen sensor

and recorded via a CRIUS® Analyser over the entire test period. The software applied a

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salinity correction factor (3.5% wt) to the raw data for dissolved oxygen obtained from the

sensor. The free chlorine levels were monitored manually using a portable chlorine monitor

EXTECH CL 500. Two measurements were taken per day for the duration of each test in

order to assess and maintain the level of the free chlorine in the test solution to the desired

value. Figures 7 show the crevice assemblies mounted on appropriate holder prior and after

immersion in the test solution.

Figure 7: Crevice assembly of the test specimens

Post test examination

Following a period of 14 days, half of the crevice test assemblies and the control specimen were

removed from the test vessel and all specimens inspected. The control specimen was

subsequently re-immersed in the test solution. The remaining crevice test assemblies and

control were removed and characterised after 28 days. Following removal from the test

environment, specimens were rinsed with de-ionised (DI) water and dried in air before initial

post-test characterisation. Specimens were weighed and the final mass recorded, prior to

examination by light microscopy. Pertinent areas of interest were also examined and analysed

using SEM and EDX. The exposed surfaces of the specimens were then ultrasonically cleaned

and re-examined by light microscopy. The surfaces of the specimens that appeared to be

noticeably affected by corrosion were scanned using the surface profilometry method (Alicona

apparatus) in order to assess the extent of surface damage those areas with significant corrosion.

Results

Figure 8 reveals the evolution of free chlorine and dissolved oxygen in the test vessel

throughout the duration of the test.

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Figure 8: Evolution of free chlorine and dissolved oxygen in the test vessel throughout the total

duration of the test (28 days)

It is clear that free chlorine was generally sustained, with a small number of fluctuations, similar

to those observed/recorded in the field, within the desired range of 0.8-1ppm. Similarly, the

content of the dissolved oxygen was maintained at around 7.5ppm. Figure 9 shows an example

of the surface of the specimens under the castellated washers after 15 days of exposure. No

signs of crevice corrosion were noticed. These observations were accompanied by mass

changes (

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black material on the surfaces of a few weld overlay specimens (i.e. overlapping specimens)

was noted.

Table 4: Weight changes of specimens after 15 days of testing

Assemblies Type of

specimen

Initial weight

(g)

Weight after

15 days of

exposure (g)

Change in

weight (g)

Assembly 1 Weld ring 5.2285 5.2286 0.0001

Weld ring 5.5980 5.5975 -0.0005

Assembly 3 Parent ring 4.8670 4.8663 -0.0007

Parent ring 5.0365 5.0364 -0.0001

Assembly 5 Weld overlay 12.5355 12.5340 -0.0015

Weld overlay 12.4330 12.4323 -0.0007


Weld overlay 12.3753 12.3735 -0.0018

Control sample Weld overlay 12.5305 12.5294 -0.0011

Figure 10: Examples of surface of weld overlay specimens after 28 days of testing

Figure 11 shows examples of internal surface of parent and weld rings. Areas presenting

roughened surfaces and regions with spots of orange/white and black colouration were recorded

on the internal surfaces of the specimens. Evidence of weld sputtering on the internal surface

of weld ring specimen was collected. Table 5 summarizes the mass changes in specimens after

28 days of exposure. The mass changes for all specimens are considered to be low (

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Table 5: Weight changes of specimens after 28 days of testing

Assemblies Type of

specimen

Initial weight

(g)

Weight after

28 days of

exposure (g)

Change in

weight (g)

Assembly 2 Weld ring 5.1540 5.1544 0.0004

Weld ring 5.7692 5.7698 0.0006

Assembly 4 Parent ring 5.2990 5.2993 0.0003

Parent ring 5.1335 5.1339 0.0004


Weld overlay 12.5150 12.5141 -0.0009


Weld overlay 12.4554 12.4564 0.0010

Control sample Weld overlay 12.5294 12.5294 0.0000

No signs of crevice corrosion were noted and no mass change was recorded for the control

specimen.

SEM images of the internal surface of ring specimens is given in Figure 12. EDX analysis

revealed the presence of iron, silicon, magnesium and oxygen. Silicon may have been

introduced during the welding process.

An SEM image of the black area noticed on the surface of the weld overlay specimen (Figure

10) is given in Figure 13. This black area appears to be the product of the Mo oxidation, as

EDX analysis detected high amounts of Mo, ascribed as evidence of segregation of the Mo-rich

phases in the solidification structure of the weld overlay. Figure 13 also presents an SEM image

of an area of the surface of the same specimen. A notable Cr peak was recorded for the EDX

spectrum of the corroded area, attributed to the precipitation of Cr-rich phases on the grain

boundaries. The elemental constituents detected on unaffected areas were consistent with the

chemical composition of the material.

Figure 11: Examples of surface and internal surface of full ring specimens after 28 days of

testing.

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Figure 12: SEM imaging and EDX analysis of the annotated areas on the internal surface of

weld full ring specimens

Figure 13: SEM imaging and EDX analysis of the annotated areas on the surface of the weld

overlay specimen (Figure 10)

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Figure 13 (continued): SEM imaging and EDX analysis of the annotated areas on the surface

of the weld overlay specimen (Figure 10)

Figure 14 show maps of the surfaces of the weld overlay specimens that exhibited signs of

crevice corrosion (i.e. the surface under castellation) as characterised by surface profilometry.

Depth contrasts were measured. As shown in Figure 14 the affected areas are mainly located

around the hole in the middle of the specimens. In these affected areas, maximum crevice

corrosion depth values of 13 and 17µm at two different locations were measured.

Figure 14: Mapping of depth across the surface of a weld overlay specimen. As a reference

depth a flat unaffected area was taken. The areas exhibiting higher corrosion depth vs. the

reference depth are indicated by blue, purple and red colour.

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Discussion

As mentioned above, it is a common practice the use of seawater continuously treated with

hypochlorite (or with 0.8-1ppm free chlorine) as a method for preventing biofouling in the

cooling system. Although, in relative warm geographical regions, the water can enter and leave

the heat exchanger at approximately 20oC and 40oC respectively, certain parts of the coolers

may operate at elevated temperatures. The water temperature at those locations (i.e. in the tube

sheet area where the hot gas enters the tube bundle) may be higher and close to the gas inlet

temperatures, e.g. 80oC or above. It is known that chlorine is highly unstable at 80°C.

Macdonald, 1977 has produced a phase diagram of the system at ambient pressure, relating to

the ‘closed system’ at equilibrium. In a dynamic system, the kinetics of decomposition of

hypochlorite are accelerated by elevated temperatures and the presence of some seawater

constituents (including bromide) [4]. Chlorine is known to be strong oxidant (in the form of

dissolved ClO- in seawater), thus, the corrosion potential of C276 alloy is expected to shift to

more noble potentials when exposed to chlorinated water (compared to those for C276 exposed

to unchlorinated water). On that basis, the increased susceptibility of the material to crevice

corrosion is expected.

The crevice corrosion test results indicate that following exposure of specimens to oxygenated

chlorinated seawater for a period of 28 days at 80°C, weld overlay specimens showed less

resistance to crevice corrosion compared to the parent tube and weld. Tube to tubesheet welds

were of finer microstructure than that of weld overlays. This difference in dendritic structure

could have also contributed to the presence of the localised corrosion in the weld overlay

specimens, as corrosion has a tendency to occur in coarser grains, especially when the boundary

of the corrosion resistance of the alloy approaches that of the operating environment, as is this

case in this system. Still, although it is generally understood that iron dilution of the weld

overlay affects the corrosion resistance of the material, in this particular case, the observed iron

dilution was at an acceptable level and thus it would not be predicted to have had a significant

impact on the performance of the material.

Damage was mainly located around the hole in the middle of the specimens, where the bolt of

the assembly was placed. As a high torque was applied, efficient contact was formed between

the surface of the specimens and the castellation, limiting the access of solution in the crevice

formed around the castellation. By contrast, the crevice formed between the bolt and the

specimen allowed limited access of solution and thus progressive development of stagnant

conditions in the crevice. These conditions would be predicated to lead to the creation of a

localised low pH environment in the crevice and thus localised corrosion. Finally, some modest

corrosion was observed in mechanical crevices formed between two overlapping specimens.

The pattern of results obtained in this work is in general agreement with similar tests performed

by Haynes International, according to which crevice corrosion was recorded for C-276

weldments exposed to wet chlorinated environment at 80°C [5].

According to the NORSOK standard, the minimum pitting resistance equivalent number

(PREN) of alloys for application in seawater is 40. PREN can be in general calculated according

to the following equation [6]:

PREN= %Cr+3.3%Mo +16%N (1)

The PREN for the investigated material is 67.78, falling within the range expected for Hastelloy

C-276 (64.0-73.8) and satisfying the NORSOK requirements. It has been reported that

chlorination lowers the critical temperatures for localised (crevice attack) although it is known

that nickel base alloys are somewhat less affected than other materials. Lefebvre, 1998 states

that at room temperature, alloy C-276 is not at risk of localised attack for levels of dissolved

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chlorine below 5ppm [7]. Determining a maximum safe residual chlorine level is considered to

be quite problematic due to the complexity of the parameters influencing crevice corrosion and

the different test procedures used by various researchers. Hack, 1998 has suggested “safe”

values with the range of 0.3-0.5mg/l [8], whereas Emerson Process Management, 2010 state

that Hastelloy C-276 is resistant to corrosion in the presence of ‘wet’ chlorine to temperatures

up of 80oC [9].

Apart from PREN, the flow regime (velocity and type) and the duration of the exposure should

also be taken into consideration in the evaluation of the material’s resistance to localised

corrosion. Recent data on crevice corrosion tests of Hastelloy C-276 for 180 days at 30°C, in

static and quiescent seawater, showed that the material suffered from crevice corrosion (120µm

corrosion depth) in the static environment while no signs of corrosion were noted in flowing

environments [5].

As is understood, alloy C-276 is not suitable for use in equipment exposed to chlorinated

seawater at 80oC. However, alloy C-276 may successfully provide service at lower

temperatures, when optimal welding procedures and practices (e.g. techniques that reduce the

segregation effect and susceptibility of the weld material to localized corrosion, absence of

flaws) are applied and consideration is given regarding the component’s design and complex

geometry. Crevice geometry should neither be neglected as the tighter the crevice the greater

the propensity for crevice corrosion. Another factor that should be also taken into account is

the passivation technique that may be followed, as the surface roughness/finish affects the

material’s corrosion resistance. As can be understood, achieving the optimal design and

geometry along with well-established welding procedures/practices and surface passivation

techniques would allow the C-276’s safe temperature limit to be maximised.

In general, it is recommended that safe operating temperature limits are established for materials

selected for service in offshore heat exchangers/coolers via appropriate testing that reflects the

dynamic conditions within the heat exchanger.

Conclusions

The following are the main conclusions drawn from this study on the crevice corrosion testing

of C276 material in chlorinated seawater for the case of a shell-and-tube cooler:

• A lab scale experimental set up was developed in order to be able to simulate a dynamic system as that of a shell-and-tube cooler and monitor its complex service environment. The

test was proven feasible in an environment consisting of seawater with high DO

concentration and free chlorine at 80°C.

• Specimens taken from the weld overlay suffered from corrosion. The autogenous weld and parent tube specimens did not exhibit signs of corrosion. This suggested that the increased

susceptibility of the weld overlay to localised corrosion at that elevated temperature is

influenced by the complexity of the structure’s geometry and the coarser microstructure

compared to that of the autogenous welds.

It is recommended that alloy C-276 should not be used in equipment exposed to chlorinated

seawater at 80oC. Alloy C-276 may successfully provide service at lower temperatures in

combination with the selection of appropriate welding procedures (and surface passivation

techniques). Safe operating temperature limits should be established for materials selected for

service in offshore heat exchangers/coolers via appropriate testing.

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Acknowledgements

The author would like to thank TWI staff, especially Qing Lu, David Smith Boyle and

Shiladitya Paul for their technical input to this work and, Mike Bennett, Ryan Bellward and

Ben Robinson for their contribution to the experimental work.

References

1. M. Bandopadhyay, 2014: “MSc thesis: Design optimization of heat exchangers in topside

systems for offshore oil and gas processing”, Norwegian University of Science and Technology

(NTNU), Department of energy and process engineering, Trondheim, Norway

2. Q. Lu et al, 2016: “Effect of iron content on corrosion resistance of Ni-Cr-Mo alloy weld

overlays in corrosive environments”, CORROSION 2016, paper no.7629, (Vancouver, CN

NACE)

3 Q. Lu et al, 2016: “Effect of iron content on localised corrosion resistance of Ni-Cr-Mo alloy

weld overlays in chloride-ion containing environment”, CORROSION 2016, paper no.7630

(Vancouver, CN NACE)

4. R.W. Macdonald et al, 1977, “The interaction of chlorine and seawater”, Pacific Marine

Science Report 77-6, Institute of Ocean Sciences, Patricia Bay, Victoria, B.C.

5.http://www.haynesintl.com/alloys/alloy-portfolio_/Corrosion-resistant-

Alloys/HASTELLOY-C-276-Alloy/resistance-to-seawater-crevice-corrosion (19/07/2016)

6. NORSOK M-001 Standard, rev. 2002: “Materials Selection”

7. Y.Lefebvre, 1998, “Seawater circuits: Treatments and Materials”, Editions Technip, Paris,

p.251

8. H.P. Hack, 1998, “Crevice Corrosion Behaviour of Molybdenum-Containing Stainless Steel

in Seawater”, Materials Performance, Vol. 22, No. 6, p. 24

9. Emerson Process Management, 2010 “Chemical Resistance Chart”, PN41-6018/rev. C

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