Ac Corroosion Phd Thesis

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Politecnico di Milano

Dipartimento di Chimica, Materiali e Ingegneria Chimica, G. Natta

INFLUENCE OF ALTERNATING CURRENT

ON METALS CORROSION

Sara Goidanich

Advisor: Prof.Luciano Lazzari

Tutor: Prof. Pietro Pedeferri

Coordinator: Prof. Giuseppe Zerbi

Ph.D. Thesis in Materials Engineering XVII Course

2002 2005


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0 INTRODUCTION ..................................................................................................................1

1

AC-INDUCED CORROSION................................................................................................3

1.1 DEFINITIONS .......................................................................................................................3

1.2 DIRECT CURRENT (DC) INTERFERENCE...............................................................................4

1.2.1 Metallic Structures Interfered by DC........................................................................6

1.2.2 Maximum Allowable Interference.............................................................................8

1.3 CASE HISTORIES OFAC-INDUCED CORROSION ....................................................................8

1.4 MORPHOLOGY CHARACTERISTICS OFAC CORROSION .......................................................10

1.5 ALTERNATING CURRENT INTERFERENCE ............................................................................11

1.5.1 Conductive interference.........................................................................................12

1.5.2 Magnetic induction (coupling) ................................................................................13

Evaluation of induced peak voltage .................................................................................14

1.5.3 Steady-State or Load Conditions...........................................................................15

1.5.4 Fault Conditions.....................................................................................................16

Conductive coupling.........................................................................................................16

Inductive coupling.............................................................................................................16

Combined effect ...............................................................................................................16

Transferred voltages ........................................................................................................17

1.6 CHARACTERISATION OFAC-INDUCED CORROSION .............................................................171.7 CORROSION RATE .............................................................................................................18

1.8 EVALUATION OF CORROSION RISKS AND CATHODIC PROTECTION MONITORING ...................20

1.8.1 DC Potential Measurements in the Presence of AC..............................................21

1.8.2 AC voltage .............................................................................................................24

1.8.3 AC current densities...............................................................................................25

1.8.4 AC Density/DC Density Ratio ................................................................................25

1.8.5 Cathodic Protection Criteria Based on AC and DC Densities Measured by

Coupons 26

1.8.6 Size of the Coating Defects ...................................................................................27

Probabilistic approach......................................................................................................27

1.8.7 ER corrosivity probes.............................................................................................30

ER based AC corrosivity probe ........................................................................................30

1.9 DAMAGE TO PIPELINE EXTERNAL COATING.........................................................................31

1.10 SAFETY FOR PERSONNEL..............................................................................................32

1.11 AC INFLUENCE ON CATHODIC PROTECTION...................................................................32

1.12 HOWAC INFLUENCES CORROSION PROCESSES ............................................................33

1.12.1

Formation of a Non Protective Oxide Layer...........................................................34

1.12.2 Faradic Rectification Effect ....................................................................................34

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Limitations of the faradic rectification models...................................................................38

A model including double layer capacitance ....................................................................38

1.12.3 Effect on Overpotentials.........................................................................................39

1.12.4 Effect of the Anodic and Cathodic Half-Cycles......................................................40

Positive half-cycle rectified AV signal...............................................................................41Negative half-cycle rectified AV signal .............................................................................42

Full-wave sinusoidal AV ...................................................................................................43

1.12.5 AC-Induced Fluid Flow...........................................................................................43

1.13 EFFECT OF THEAC FREQUENCY ...................................................................................44

Effect of the waveform......................................................................................................45

1.14 AC EFFECTS ON THE CORROSION OF THE DIFFERENT METALS .......................................46

1.14.1 DC Potential Variation in the Presence of AC .......................................................46

1.15 PASSIVE METALS..........................................................................................................47

1.15.1 Influence of Stray Currents on Corrosion of Steel in Concrete..............................47

1.16 EFFECT OFAC ON CORROSION IN SOILS .......................................................................48

1.17 INTERFERENCE MITIGATION...........................................................................................49

1.17.1 DC Mitigation..........................................................................................................49

Stationary interference .....................................................................................................49

Non stationary interference ..............................................................................................50

1.17.2 Mitigation of AC Interference .................................................................................51

Lumped grounding............................................................................................................51

Cancellation wires ............................................................................................................51Gradient control wires.......................................................................................................52

2 EXPERIMENTAL ................................................................................................................53

2.1 SETTING UP OF THE ELECTRICAL CIRCUIT ..........................................................................54

2.1.1 AC mesh ................................................................................................................55

Characteristics of the capacitor ........................................................................................55

2.1.2 DC mesh ................................................................................................................56

Characteristics of the inductor (L) ....................................................................................56

2.1.3 Common Branch ....................................................................................................562.1.4 Efficiency of the Circuit ..........................................................................................56

2.2 TESTS ON GALVANIC CORROSION ......................................................................................57

2.2.1 Experimental Conditions........................................................................................57

Solution Composition........................................................................................................57

Cell, samples and Electrodes Characteristics..................................................................57

2.2.2 Experimental Procedure ........................................................................................58

2.3 TESTS WITH SEPARATED CIRCUITS: ACAND DC OVERLAPPED ON THE SAME SAMPLE ........59

2.3.1 Experimental Conditions........................................................................................59

Solution Composition........................................................................................................59

Cell, Samples and Electrodes Characteristics .................................................................59

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2.4 DETERMINATION OF THEANODIC AND CATHODIC POLARISATION CURVES............................61

2.4.1 Measurement Procedure .......................................................................................61

2.4.2 Experimental conditions.........................................................................................62

Solution Composition .......................................................................................................64Cell and Electrodes ..........................................................................................................65


2.5 WEIGHT LOSS TESTS ........................................................................................................67

2.5.1 Weight Loss Determination....................................................................................68

2.5.2 Test in Simulated Seawater Chlorinity: 35 g/L NaCl..............................................69

2.5.3 Test in Simulated Soil Solution ..............................................................................69

Specimens........................................................................................................................69

2.6 CHARACTERISATION OF THE CORROSION MORPHOLOGY OF THEAC CORROSION ................71

2.7 EFFECT OFAC ON PASSIVE METALS .................................................................................72

2.7.1 Samples geometry.................................................................................................72

2.7.2 Solution composition..............................................................................................72

2.7.3 Cell and Electrodes................................................................................................73

Potentiodynamic tests ......................................................................................................73

Immersion Tests...............................................................................................................73

2.7.4 Potentiodynamic Tests Procedure.........................................................................73

Polarisation Curve............................................................................................................73

Experimental Procedure...................................................................................................742.7.5 Immersion Tests Procedure...................................................................................75

Preliminary tests...............................................................................................................75

Second set of tests...........................................................................................................75

2.8 CATHODIC PROTECTION MONITORING................................................................................75

2.8.1 First set of measurements .....................................................................................76

2.8.2 Second set of measurements ................................................................................77

2.9 OSCILLOSCOPE MEASUREMENTS.......................................................................................79

3 RESULTS ...........................................................................................................................81

3.1 TESTS ON GALVANIC CORROSION ......................................................................................81

3.2 TESTS WITH SEPARATED CIRCUITS: ACAND DC OVERLAPPED ON THE SAME SAMPLE ........84

3.2.1 Cathodic Polarisation (CP) Tests...........................................................................84

3.2.2 Anodic Polarisation (AP) Tests ..............................................................................86

3.2.3 Free Corrosion Tests .............................................................................................87

3.3 ANODIC AND CATHODIC POLARISATION CURVES.................................................................89

3.3.1 Tafel Equation........................................................................................................89

3.3.2 Carbon Steel ..........................................................................................................90

0.4 g/L Sodium Sulphate Solution....................................................................................90

Simulated soil solution......................................................................................................92

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Simulated seawater solution: 35 g/L NaCl .......................................................................95

3.3.3 Galvanised Steel....................................................................................................96

3.3.4 Carbon Steel, Zinc and Copper in 1 M Solution of their Ions ................................98

3.4 WEIGHT LOSS TESTS ......................................................................................................104

3.4.1 Simulated Seawater Solution: 35 g/L NaCl..........................................................1043.4.2 Simulated Soil Solution ........................................................................................105

Aerated solution without thermostating..........................................................................105

Aerated solution - thermostated at 20C ........................................................................110

De-aerated solution - thermostated at 20C...................................................................111

3.5 CHARACTERISATION OF THE CORROSION MORPHOLOGY IN THE PRESENCE OFAC

INTERFERENCE ............................................................................................................................113

Neither AC nor DC interference .....................................................................................113

AC = 100 and 300 A/m2..................................................................................................115

0.3 A/m2DC anodic interference....................................................................................119

Potential measurements.................................................................................................120

3.6 EFFECT OFAC ON CARBON STEEL REBAR IN CONCRETE .................................................121

Second set of measurements.........................................................................................124

3.7 EFFECT ON CATHODIC PROTECTION MONITORING ............................................................127

3.7.1 First set of measurements ...................................................................................127

3.7.2 Second set of measurements ..............................................................................129

3.8 MEASUREMENTS BY OSCILLOSCOPE ................................................................................131

4 DISCUSSION....................................................................................................................133

4.1 AC DENSITY THRESHOLD ................................................................................................133

4.2 AC INFLUENCE ON CORROSION PROCESS........................................................................134

4.2.1 Corrosion Rate and AC Current Efficiency (CE%)...............................................134

4.2.2 Influence of AC on Corrosion Kinetics.................................................................136

4.2.3 Corrosion Potential...............................................................................................137

4.2.4 Analysis of the Faradaic Rectification Models .....................................................138

Oscilloscope measurements ..........................................................................................140

Results of the models.....................................................................................................1404.2.5 Proposed Mechanism..........................................................................................145

4.3 CORROSION MORPHOLOGY IN THE PRESENCE OFAC.......................................................146

4.4 CARBON STEEL INALKALINE SOLUTION............................................................................146

4.5 CATHODIC PROTECTION MONITORING..............................................................................148

4.5.1 Field Monitoring Approach ...................................................................................148

5 CONCLUSIONS................................................................................................................151

AC density threshold ......................................................................................................151

Corrosion rate and AC current efficiency (CE%)............................................................151Influence of AC on Corrosion Kinetics............................................................................152

IV


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Proposed mechanism.....................................................................................................152

Carbon steel in alkaline solution.....................................................................................153

Cathodic protection monitoring.......................................................................................153

6 ACKNOWLEDGMENTS...................................................................................................155

7 REFERENCES .................................................................................................................157

V


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VI


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0 INTRODUCTION

Alternating current enhanced corrosion has been considered since the beginning of the XX

century[ ]1 . However, for many years, corrosion experts did not recognise corrosion due to

alternating currents (AC) on metallic structures as an important phenomenon as corrosion has

been normally attributed to direct current.

During the last 20 years pipelines failures in the USA, Canada and Europe have been ascribed toAC interference. The presence of AC caused serious damages on interfered metallic structures

even when cathodic protection was applied. In some cases, failures also occurred on pipelines

with OFFpotential lower than 850 mV CSE, that is normally considered a safe condition. A

number of studies carried out to investigate the effect of AC on corrosion have concluded that

metallic structures corrode at an accelerated corrosion rate in the presence of stray AC [2-9]. The

scientific community has slowly recognised that, under certain conditions, alternating currents can

cause corrosion although at a lower rate than direct current. The number of scientific publications

on this topic has therefore increased during the last 15 years.[ , , , , , , , ]2 3 4 5 6 7 8 9

What is clear is that the increased number of AC related failures is associated to the more and

more frequent parallelism between buried pipelines and AC high tension transmission lines which

use the same right of way. A particularly harmful situation is when parallelism is associated with

the use of high dielectric coatings, like extruded polyethylene or polypropylene, which leads to high

induced AC current densities at coating defects.

In the near future, at least in Italy, a new interference condition is expected because of the

overlapping of high current AC traction systems (25 kV and 50 Hz) and the traditional direct current

(DC) traction systems that have been operating for about a century.

However, while for DC interference corrosion on buried structures is a well known problem and

there is a large agreement on protection criteria to be used for corrosion mitigation and

international standards are available since many years[ ]10 ,AC induced corrosion continues to be a

controversial subject and many aspects need to be clarified, especially the relationship between

AC density and corrosion rate.

Different theories have been proposed about the mechanism by which AC produces and

enhances corrosion, although no one is able to fully explain the phenomenon. Furthermore it is not

yet clear how to estimate the risk of AC-induced corrosion by means of AC density or alternating


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Introduction

2

voltage. Another important aspect that needs further investigations is related to cathodic protection

monitoring in the presence of AC.

The experimental work dealt with the following:

design of an electric circuit to allow independent control and measurement of AC and DC

contributions in order to guarantee reliable measurements;

study of the effect of AC current on galvanic corrosion;

study of AC and DC overlapping through the designed circuit;

weight loss test in order to quantify the damage caused by AC and estimation of AC

corrosion threshold parameters;

determination of AC effects on passive metals;

cathodic protection monitoring in the presence of AC and DC stray currents;

determination of anodic and cathodic polarisation curves in the presence of AC. The aim

of these experiments was the evaluation of AC on the corrosion mechanism as far as

corrosion kinetics is concerned;

analysis of the mathematical models proposed in literature and comparison with

experimental data.

The analysis of the experimental data allowed to draw conclusions about the following topics:

AC density threshold;

mechanism of AC-induced corrosion;

corrosion morphology in the presence of AC

AC induced corrosion of carbon steel in alkaline solution (concrete);

Cathodic protection monitoring.

The first two years of this experimental research were co-fund by MIUR (Italian Ministry of

School, University and Research).


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1 AC-INDUCED CORROSION

Before discussing the AC induced corrosion, it is important to emphasise that very often

corrosion attacks are carelessly attributed to non-existent interference corrosion conditions, simply

because the corrosion conditions are ignored. This is typical for internal corrosion attacks in water

heaters or leakage of piping embedded in apartment floors, most often attributed to ghost stray

current, through nobody knows its origins.

Interference corrosion greatly worries owners of buried structures, because of the severe

damages it causes. The interference can be the result of DC or AC, flowing predominantly with a

frequency of 50 Hz (public electricity supply).

A pipeline which shares a common path with AC transmission lines becomes energized by the

magnetic and electric fields surrounding the power system in the air and soil. This AC interference

can result in an electrical shock hazard for people touching the pipeline or metallic structures

connected to the pipeline; furthermore, corrosion of the pipeline, damages to the pipelines coating,

insulating flanges and rectifiers can occur. The induced AC-voltages can lead to an increase of thecorrosion rate, the breakdown of a passive layer or increasing pitting corrosion.

In this chapter a brief summary of the DC interference characteristics and the state of art of AC

induced corrosion phenomena are reported.

1.1 DEFINITIONSAs a general definition, interference is any alteration in the electric field caused by a foreign

structure[ ]11 . Figure 1.1 illustrates a general scheme of interference in an electrodic system where

two electrodes exchange current in the presence of a foreign body: when it is a conductor, the

current is intercepted, when it is an insulator, the current is withdrawn. In both cases, there is a

redistribution of current and equi-potential surfaces within the altered electric field.

The first instance, when the interfered structure has ionic or electronic conductivity, leads to the

classic and most well known interference conditions, the so-called stray current. When the

structure is far from the electrodes, the influence on the electric field is usually negligible[11].


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AC-Induced Corrosion

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IsolatorConductor

Equipotential lines

Current lines

M N M N

a) b)

Figure 1.1 - General scheme of electrical interference between two electrodes on a bodya) conductor; b) isolator[11]

1.2 DIRECT CURRENT DC) INTERFERENCEInterference can be of two types: stationaryand non-stationary.

Stationary interference is verified when the structure is immersed in a stationary electric field

generated, for example, by a cathodic protection (CP) system, and the effect is greater as the

structure is closer to the groundbed. In Figures 1.2 and 1.3,two general cases are reported. In the

first case, the interfered pipeline crosses the protected one and the zone close to the groundbed

tends to gather current from the soil, which is released at the crossing zone, causing localcorrosion. In the second case, the two pipelines are almost parallel and the releasing current zone

is usually more extensive than in the previous case, and typically occurs in low resistivity soil. In

both cases, if the interfered structure is provided with an integral coating, interference cannot take

place, but when the coating has a number of faults, corrosion is very severe since, current

concentrates in them.

Non-stationary interferencetakes place when the electric field is variable, as in the typical case

of stray currents dispersed by traction systems, as illustrated in Figure 1.4. Interference takes

place only during the trains transit, and often, in spite of the limited duration, a few minutes, theeffects may be severe due to high circulating current.

The mechanism of direct stray current corrosion is simple: the DC traction system has a positive

aerial conductor and, for safety reasons, a grounded negative pole, so the return current path is

double: track and soil, with over 50% in soil even when all precautions to reduce current dispersion

in the ground have been taken[11]. A pipeline, parallel to rail, buried between the track and the

substation is easily interfered, with corrosion taking place where current leaves the structure near

the substation.


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5

Groundbed

Interfered Pipeline

T/R

Corrosion attacks

Interference current

Figure 1.2 - Scheme of stationary interference between two crossing pipelines[11]

T/R

Low resistivity

Groundbed

Interfered pipeline Corrosion attacks

Interference current

soil

Figure 1.3 - Scheme of stationary interference between two almost parallel pipelines[11]

E

protection potential

time

Substation

cathodic zone anodic zone

Figure 1.4 - Scheme of non-stationary interference caused by stray current dispersed by a DC transit

system[11]

Rail

Pipeline

Substation

R

R

R Ri iI

i

1 2rail

pipe

Figure 1.5 - Electrical scheme of non-stationary interference


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From an electrical point of view, the interference currentmay be evaluated from the balance of

the ohmic drop through the circuit, when overpotential contributions on both pipeline and rails are

neglected.

With reference to Figure 1.5,where Iis the current that passes through the rail (estimated to be

about 50% of the total current tension) and I*is the interference current that passes through the

pipeline, then:

2pipe1

r

RRR

IR*I

++= Eq. 1.1

where, Rr, Rpipe indicate the track and pipe resistance respectively, R1 and R2 the ground

resistance. Corrosion damages, quantified through I*, decrease as the track resistance Rr

decreases (for instance by using welding joints to reduce the ohmic drop to about 1 mV/m) and by

increasing the parallel path resistance by applying protective coatings, especially in the cathodic

zone (increase in R1+ R2) and by installing insulating joints on the pipelines (increase in Rpipe).

1.2 1 Metallic Structures Interfered by DCDC interference occurs when a metallic structure is placed in a DC electric field. The reaction

that takes place over the surfaces receiving the current (cathodic zones) is primarily oxygen

reduction and then hydrogen evolution at more negative potentials, by the following reactions:

O2+ 2 H2O + 4e-= 4 OH- Eq. 1.2

2H2O + 4e-= H2+ 2OH- Eq. 1.3

On anodic zones, where DC current leaves the metallic structures, the anodic reaction is metal

(Me) dissolution when the metal is active:

Me = Mez++ ze- Eq. 1.4

When the metal is passive, the anodic reaction depends on the type of metal and environment. In

alkaline media, such as pristine concrete where both stainless steel and carbon steel rebars are in

passive condition, the anodic reaction is oxygen evolution, by the following reaction:

2 H2O = O2+ 4 H++ 4e- Eq. 1.5

Because acidity is produced, passivity may be destroyed at high currents and long time of

interference, still provoking iron dissolution. For stainless steel in a neutral environment, the

corrosion reaction is mainly metal dissolution, so corrosion effects are as harmful as in the case of

carbon steel.

According to Faradays law, the amount of metal that dissolves by reaction 1.4 is directly

proportional to current and time. A flow of 1 A dissolves about 9 kg of iron in a year, equal to about

25 g/day. A current density of 1 A/m2provokes a metal thickness loss at a rate of 1 mm/year.


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Figure 1.6 - Interference on metallic structures[11]

With reference to Figure 1.6, ignoring the electrode overpotential at M and N surfaces, path 1

includes only the electrolyte ohmic drop, while path 2 includes the overpotential of the cathodic

oxygen reduction and the anodic iron dissolution on the interfered structure. Therefore, introducingthe IRdrop within the structure, we obtain[11]:

++

++

=

S

LI

S

LI

S

LI

S

LI 2a2strc21 Eq. 1.6

or when the structures ohmic drop is ignored:

ES

LI

S

LI

S

LI ac221 +

Eq. 1.7

where indicates the electrode overpotential with respect to the free corrosion potential(= E Ecorr) localised at anodic (a) and cathodic (c) surfaces of the interfered structure. Terms

and str refer to environment and metal resistivity, respectively. Assuming negligible the IRdrop

in the interfered metallic structure the following result is achieved: the overpotential sum E, which

represents the driving voltage for current circulation in the interfered structure, is equal to the

ohmic drop saving within the environment through the interference path. The term in brackets

can be approximated to:

strerfint LICE Eq. 1.8

where C is a geometric constant, Iinterf is interference current, Lstr is structure length. It follows

that, for a fixed geometry, interference effects, determined by the available E, are increasing as

structure length increases.

The available driving voltage, E, is also given by:

strLEEdLE = Eq. 1.9

where E is the average electric field gradient. From Eq. 1.8 and 1.9 it follows:


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E

'CI erfint Eq. 1.10

where C is a geometric constant. Then, Iinterfincreases as electric field gradient E increases

and environment resistivity decreases.

The interference current Iwhich gives the corrosion rate, is generally not easy to calculate since

the electric field solution is required.

1.2.2 Maximum Allowable InterferenceInterference is as negligible as the interference current density is low. From a general viewpoint,

it may be reasonable to assume the value at which carbon steel passivates in concrete, that is

1 mA/m2, or even double that, as an engineering compromise [11]. Unfortunately, because in

practice the circulating current is not measurable, interference acceptance criteria refer to thepotential measurement, to distinguish between freely corroding structures and those under CP. For

the latter, potential variation should remain within the protection interval, while for the former, the

maximum potential variation in the positive direction should not exceed 20-50 mV with respect to

the free corrosion potential.

1.3 CASE HISTORIES OF AC-INDUCED CORROSIONHigh alternating current density can cause corrosion attacks on steel in soil, despite the steel

surface being provided with proper cathodic protection[ , ]1213 .

Actual cases of AC corrosion on buried steel pipelines have been reported since 1986[ ]14 . The

cause of two corrosion anomalies on polyethylene coated pipelines in Germany, that were installed

in 1980 paralleling a 15 kV AC powered rail transit system operating at the frequency of 16-2/3 Hz,

was attributed to AC corrosion, although the instant-OFF potentials were more negative than

1.0 V CSE at those sites.

Several cases of AC corrosion were reported in France where ONpotentials were at an

adequately low level between 2.0 V CSE and 2.5 V CSE[ ]15 Five cases of AC corrosion in North

America were reported even if ONpotentials were sufficiently negative in all cases[ ]16 .

A field survey using coupons on a pipeline buried parallel to a 66 kV overhead AC power

transmission line showed that a high level of AC current can cause corrosion on coupons even

though the instantOFF potentials are more negative than 1.0 V CSE[ ]17 .

In Oregon (USA) the failure of a pipeline that had been laid in high voltage AC utility corridors

was investigated. Severe AC corrosion occurred resulting in four holes in the pipeline after only

five months life and before operation had begun. The pipeline was left buried in place withoutcathodic protection while the construction of the plant was completed. There appeared to be some


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coating disbondment. The pits typically had a small aerial extent and were very deep. It was

verified that there was no DC interference at the leak locations[ ]18 .

In Figure 1.7 the measured AC voltages taken during a survey of a pipeline which had reported

AC-induced corrosion are shown[18].

0

20

40

60

80

100

0 2 4 6 8 10 12 1

Distance - Miles

ACVoltage

4

Figure 1.7 - Induced voltage on a AC-interfered pipeline [18]

Also in Texas a careful investigation of a pipeline failure was carried out. The review of the

annual survey data, bimonthly rectifier readings and an ON/OFF close interval survey data showed

apparent satisfaction of industry criteria for effective corrosion mitigation. Nothing in the review of

the cathodic protection history suggested a plausible explanation for the rapid rate of corrosion

experienced on this pipeline[ ]19 . The pipe-to-soil AC potentials and the number of corrosion

anomalies as a function of the location were evaluated. The plot of the AC potentials and defects

are shown in Figure 1.8. This data indicated that the highest population of corrosion anomalies

were located in the first five miles where the AC potentials typically exceeded 4 V. The excessive

AC current densities observed on the coupon test stations and the physical and chemical analysis

indicated that the likely cause of the observed corrosion anomalies was AC corrosion. This

conclusion was supported by the correlation of higher defect occurrences within areas of higher

AC potentials.

It can be observed by the analysis of the reported case histories that AC corrosion is possible

despite satisfying the conventional cathodic protection criteria such as the 0.85 V CSE potential

criterion.


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10

0

2

4

6

8

10

12

14

290 295 300 305 310

Mile Post

ACPotentia

l(VCSE)

0.000

0.003

0.006

0.009

0.012

0.015

0.018

0.021

Defectspe

rFoot

AC Potential

Anomalies Per Foot

Figure 1.8 - AC potential and defects distribution [19]

1.4 MORPHOLOGY CHARACTERISTICS OF AC CORROSIONIn DC stray current interference, the corrosion products are soluble due to the low pH at the

discharge location and the corroded area is generally found to be free of corrosion products. In

case of AC interference the corrosion products are generally present in the pits.

Measurements performed on cathodically protected coupons and on corroded pipelines

suggested a mechanism of AC induced corrosion which involves local alkalization of the soil

environment close to a coating defect[7,19,20,[ , ]2122 . This has probably to be related to the enhanced

cathodic reaction during the AC interference.

The enhanced corrosion due to the influence of AC voltages tends to be of a localised nature.

The corrosion morphology is reported to be of a shallow saucer-shaped pit of a several millimetre

diameter that is overlaid by a layer of hard encrusted earth[ ]23 .

AC corrosion can take on the appearance of generalised corrosion when the defect surface is

large. These large areas are in fact several pits that have coalesced over a period of time. InFigure 1.9 the corrosion of a coupon which was placed in an Italian situ and was subjected to AC

interference is shown.


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Figure 1.9 Corroded coupon placed in an area in Italy characterised by high AC interference

The appearance of the corrosion may therefore vary depending on the duration of the

interference from small point-shaped attacks distributed across the steel surface to large and deeplocal attacks with smooth round corrosion morphology.

Generally the coating in the area surrounding the pits is damaged, often brittle and has poor

adhesion[19].Coating disbondment is often associated to AC interference.

A typical aspect of failures occurred on cathodically protected pipelines is summarised in

Figure 1.10.

Corrosion product placed on top of an hard oxide layerand corrosion diameter is larger than the coating

defect diameter ( = a few cm )Coating

Possibility of coatingdisbondment

Metal

Corrosion product placed on top of an hard oxide layerand corrosion diameter is larger than the coating

defect diameter ( = a few cm )Coating

Possibility of coatingdisbondment

Metal

Figure 1.10 - Specific characteristics of AC corrosion [12]

1.5 ALTERNATING CURRENT INTERFERENCEAs the DC interference, also the AC interference can be either stationary or non-stationary.

Alternating current interference is generated by AC traction systems or high voltage transmission

lines, through conduction and induction mechanisms. The effects of power system interference on

pipelines are due to the relative voltage difference which is created between the pipeline metal and

the local soil.

Where a pipeline runs closely parallel to and/or crosses an AC power line, steady state and fault

AC voltages can be induced on the pipeline. Induced AC voltages, which are respect to groundpotential, arise as a result of electromagnetic capacitive and inductive effects. The magnitude of


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the induced voltage depends on the AC transmission line voltage/current, the pylon dimensions,

the separation distance of the pipeline from the AC transmission line, the distance over which the

pipeline are in parallel with and crosses the AC transmission line, soil resistivity, coating

conductivity of the pipeline and whether the AC transmission line is experiencing a fault or large

transient surge associated with a switch on[23]

.

The AC power lines induce a secondary alternating current in the steel pipes buried nearby: field

measurements indicated that this secondary AC field has a voltage drop on the order of

kilovolts/km along insulated pipes[ ]24 . The potential drop between the bare pipe and the

surrounding soil was reported in the range of 5 to 70 V.

An example of voltage profile induced along a pipeline by a 380kV50 Hz power line carrying

balanced currents of 630 A is shown in Figure 1.11.

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Distance (km)

ACVoltage(V)

Figure 1.11 - Example of voltage profile induced along a pipeline by a nearby 380kV-50Hz power l ine[ ]25

1.5 1 Conductive interferenceConductive interference due to currents flowing in the soil is of particular concern at locations

where the pipeline is close to transmission line structures which may inject large currents into the

soil during power line fault conditions. Such structures include transmission line tower or pile

foundations and substation grounding systems[ ]26 .

Interference by conduction takes place when a buried structure spreads AC current. This is the

case of grounding networks and high voltage transmission lines with a phase grounded. Stray

currents choose the path of least resistance, therefore they affect buried metallic structures such

as buried steel pipelines and tanks[11].

There are two typical cases of conductive interference: dispersion from a grounding network

(plant earthing systems) and from AC traction systems (rails and their grounding conductor). In


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13

these cases, AC voltage on interfered structures in soil can be calculated when geometry, soil

resistivity and AC voltage of dispersing sources are known.

1.5.2 Magnetic induction (coupling)Magnetic induction (coupling) acts along the entire length of pipeline which is approximately

parallel to the power line and can result in significant pipeline potentials even at relatively large

separation distances. Inductive coupling takes place when an high voltage transmission line with

its neutral grounded is parallel to a buried well coated pipeline. In such a way, an alternating

magnetic flux is linked between high voltage transmission line and soil, which induces an

alternating current in the coated pipeline. In fact, the pipeline and the soil behave as the second

coil of a transformer, where the first coil is the high voltage transmission line and the soil. In other

words, this is an auto-transformer, with a 1:1 transformation ratio. [11].

Figure 1.12 is a schematic representation of the inductive coupling on a steel pipelines located

near a high voltage AC transmission systems. The voltages are induced in the pipeline due to

imbalances in the distance of the various phase wires and in the current carried in each phase.

The resulting voltages in the pipeline are a function of the distance of the pipeline from the phase

wires, the voltage in the system, the current carried by the system and the phase imbalance in the

current.

Da = Db = Dc

Da Db Dc

Da

= Db = Dc

Da Db Dc

Figure 1.12 - Induced AC interference on a pipeline [18]

Inductive coupling causes voltage peaks on the pipeline at locations where the pipeline and

power line separation changes, such as when the pipeline approaches or recedes from the power

line right-of way. The crossing of the pipeline from one side of the power line to the other will also

cause an induced voltage peak. For example if a pipeline and transmission line are parallel to each

other and one of the two changes direction, the voltages will build-up at the point of divergence.Pipelines that pass under a transmission line at an angle of 90 do not have any AC current or


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14

voltage induced. Although these potential and current changes are physical in nature, they are

generally considered as electrical discontinuitiesbecause they cause a change in the longitudinal

electric field impressed upon the pipeline[18, ]27 .

The AC-induced voltages can even reach values that can cause serious danger to people

maintaining pipelines that are subjected to AC-induced corrosion [ ]28 .

The AC circulating within the pipeline is exchanged with soil at coating defects.

Evaluation of induced peak voltage

The following formula has been proposed to evaluate the induced pick voltage in

correspondence of discontinuities[27]

I2

fpEV

= Eq. 1.11

where, V = induced voltage at discontinuity (V);

E = coupling coefficient (V/km/A);

p = voltage coupling coefficient (km);

f = multiplying factor;

I = power line load current (A).

Equation 1.11 and the coupling parameters (Tab. 1.1)are based upon a nominal 10-meter power

line to pipeline separation or less. At greater distances, the induced voltage will be reduced by

approximately the inverse ratio of the pipeline distance relative to the 10-meter reference[27].


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15

Tab. 1.1 Values of the coefficients in Equation 1.11[27]

Steady state powerline coupling

coefficientPipeline induced voltage coefficient

Geometrical/electricalfactors

Power lineconfiguration

CouplingcoefficientE (V/km/A)

Coatingdescription

Resistance

(m2)

Voltagecoefficient

p (km)

Type ofdiscontinuity

Multiplyingfactor f

Single circuithorizontal

0.05 Excellent(thick)

100000 10.6 Pipelineapproach/recede

1

Single circuitvertical

0.05Excellent

(thin


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16

effect of a fault on the pipe. Induced steady-state pipeline potentials are more severe when the

pipeline coating has a high electrical resistance. However, a high coating resistance is desirable

from a cathodic protection standpoint, so reducing the coating resistance is not usually considered

a viable solution.

In spite of the relatively low magnetic field levels during steady-state conditions, induced voltages

on an unprotected pipeline can reach hundreds of volts[26]. Even with extensive grounding systems

connected to the pipeline, pipeline potentials can be on the order of tens of volts. This constitutes

primarily a shock hazard, which can be transferred miles away from the parallel corridor.

1.5.4 Fault ConditionsIn cases where the overhead line has an earthed neutral, unbalanced or fault conditions can

result in substantial currents flowing in the soil. For an unbalanced single-phase supply, e.g. an

electrified railway, or earth faults on the AC transmission line, hazardous voltages may be induced

even for short parallel lengths[23]. Also a fault at a pylon or a lightning discharge or where a

severed AC phase wire comes into contact with the ground may induce high voltages.

Conductive coupling

When a single-phase-to-ground fault occurs at a power line structure, the large fault current

injected into the soil by the structure raises the local soil potential. A pipeline located nearby,

however, will typically remain at a relatively low potential, due to the resistance of its coating and

grounding at points distant from the fault location. The pipeline potential rise will be particularly

small for a pipeline with a high resistance coating. Therefore, the earth around the pipeline will be

at a relatively high potential with respect to the pipeline steel potential[26].

During a fault, the worst affected part of a pipeline will usually be that part which is the closest to

the faulted power line[23]. However, for a well-coated pipeline, considerable voltages can be

transferred many kilometres beyond the end of a shared corridor.

Inductive coupling

During single-phase-to-ground fault conditions on the power line, induced potentials in a pipeline

with no mitigation system can reach thousands of volts, due to the intense magnetic field causedby the large current which may flow in the faulted wire[26].

Combined effect

Unless the pipeline is perpendicular to the power line, the pipeline will be simultaneously subject

to both inductive and conductive interference. In most cases the inductive and conductive

components are addictive and this may give rise to a very large voltage difference. In fact the

change in pipeline steel potential due to induction is essentially opposite in sign to the soil potential

change due to conduction, therefore inductive and conductive effects add, producing even more

severe coating stress voltages and touch voltages[23,26].


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17

Therefore for fault conditions, the AC voltages are a combination of both the inductive and

conductive voltages arising from fault currents flowing in the phase wires and into the soil from

each faulted pylon. In general, during a fault condition, the induced voltage will tend to increase

with [23,26]:

an increase in soil resistivity;

a decrease in the distance between the faulted structure and the pipeline;

a decrease in the conductivity of the pipeline external coating;

close proximity to a pylon earthing assembly.

The effect of a fault on the predicted induced AC voltage can extend several kilometres away

from he crossing point.

Transferred voltagesDue to inductive and conductive coupling, considerable voltages can be transferred many

kilometres beyond the end of a common corridor. As a result, it is necessary to properly study the

entire pipeline system, as mitigation may be required, for instance, to protect sites which are far

away from the transmission line exposure location[26].

1.6 CHARACTERISATION OF AC-INDUCED CORROSIONIn the corrosion field it is fundamental to ascertain the causes of corrosive phenomena. This may

be done by in situ measurements or by using samples that can be subsequently analysed in the

laboratory.

The following are some aspects that can be considered in order to identify a corrosion failure as

induced by AC interference[ ]29 :

presence of AC voltage on the pipeline;

protection voltage on the pipeline (negative OFF-potential greater than -850 mV CSE);

presence of a defect in the coating (usually an open small defect) often associated withcoating disbondment and degradation;

elevated pH values of the soil in the vicinity of the corrosion (pH >10);

the corrosion form is usually rounded (advanced stage) or penetrating pits (initial stage);

the corroded area is larger than the coating defect;

corrosion products are very bulky and could be removed as a block;

high AC and high DC cathodic protection densities.


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18

1.7 CORROSION RATEThe principal factors that affect AC corrosion can be summarised as follows[18,12,29, , ],3031

corrosion increases with increased AC density or alternating voltage (AV) (Figure 1.13);

cathodic protection decreases AC corrosion, but the magnitude of the DC currents

required is high;

corrosion decreases with increased AC frequency;

the chemical composition and resistivity of the soil have a strong influence on the

corrosion rate: corrosion increases in the presence of carbonates or bicarbonates and

decreases in the presence of chlorides;

corrosion increases with increased temperature;

the average corrosion rate decreases rapidly with time, then less slowly with the

exposure time and finally stabilizes to a steady value (Figure 1.14);

corrosion rates are higher when the area of exposed steel is approximately 1 cm2.

0.0

0.2

0.4

0.6

0.8

1.0

0 300 600 900 1200 1500 1800

Peak cell voltage (mV)

Corrosionrate(g/cm

2y)

Figure 1.13 - Corrosion rate as a function of peak alternating voltage. Experiments were conducted at room

temperature under nitrogen purge for 24 h. The 60 Hz AV was applied between a Cu-Ni alloy and acounter platinum mesh[31]


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19

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80 100 120 140 160

Time (h)

Corrosionrate(g/cm

2y

)

Figure 1.14 - Corrosion rate as a function of exposure time (AV = 1000 mV)[31]

Studies on carbon steel specimens exposed to soil under the influence of AC and CP

polarisation showed that corrosion rates due to AC corrosion are not necessarily high, but that the

rates are certainly multiples of the normal corrosion rates of steel in soil in the absence of AC[ ]32 .

The results of this work are summarised in Figure 1.15.It can be noticed that AC current density in

the order of 20 A/m2could significantly increase corrosion; CP can be effective at "low" AC current

densities but not at "high" AC current densities. The data also suggest that even at "Low AC + No

CP" conditions, at AC densities which are considered 'safe' by many publications (20 A/m 2), the

observed increase in corrosion rate, relative to a control, is by a factor of 2. For "High AC + HighCP" results showed that protection could not be achieved. This last result is quite unexpected. It

does not seems logical to have higher corrosion rates whit higher cathodic protection. However

such behaviour has been reported also by other authors [29].In any case more analysis are needed

in order to verify such experimental evidence and to explain the reason of it. Most probably the

high corrosion rate reported can be explained considering the enhancement of the cathodic

process that can lead to coating disbondment[32].


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20

0

100

200

300

400

500

600

Low AC +No CP

High AC +No CP

Low AC +Low CP

Low AC +High CP

High AC +Low CP

High AC +High CP

Test category

R

elative(tocontrolspecimens)cor

rosion

rate >150>300High AC + High CP

300High AC + Low CP

>150150


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21

The European approach is the specification of the maximum allowable AC density to be

measured by means of corrosion coupons: the threshold value so far proposed is 30 A/m2[ ]35 .

The German standard, DIN 50 925 states that, for coated steel pipelines, AC current density

should be less than about 30 A/m2while DC current density should be maintained about 1 A/m2[ ]36 .

Others criteria are based on the maximum AC voltage. The NACE RP0177-2000[ ]37 suggests

15 V, but this value has been chosen mostly considering safety criteria for the persons than AC-

induced corrosion. The minimum voltage level below which AC enhanced corrosion cannot occur

has not yet been defined. It has been reported that it may occur at induced AC voltage levels of

only a few volts[23], therefore mitigation of AC potentials to values below 15 V may not be sufficient

with respect to AC current densities at the coating holidays. Application of increased cathodic

protection current may be required to control AC corrosion [32].

Evaluation of corrosion risk is achieved by several measurements. The association of several of

them makes it possible to consider a situation as potentially at risk or not.

The parameters influencing AC corrosion risks are[12,33]:

the real level of polarisation of the structure by cathodic protection;

the level of AC current density;

the extent of the metal surface exposed at coating defects;

the electrolyte (soil composition, resistivity, etc.). Soil composition strongly affects the

AC-corrosion risk. Identical electrical parameters can result in significantly different

ACcurrent densities;

climatic conditions, such as surface temperature, seem strongly affect the AC-current

density on coupons.

AC corrosion is most likely to occur only on well-coated structures where the AC current is shown

to transfer from very small defects in the coating.

1.8.1 DC Potential Measurements in the Presence of ACNatural gas transmission pipelines are usually supplied with cathodic protection (CP) to protect

coating defects against corrosion. The potential measurement is easy to carry out, but its

interpretation may present some concerns when stray currents cause huge ohmic drop

contributions which may lead to misinterpretation[11].

The scope of the cathodic protection monitoring is to verify the protection level of the structure by

checking that the potential matches the protection criteria. In the presence of DC or AC stray

currents the potential measurement may lead to misleading interpretations.

The potential reading is a function of the reference electrode position with respect to the structure

and is the sum of three terms (Figure 1.16):


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22

E = Eeq+ + IR Eq. 1.12

Eeqis the thermodynamic contribution and represents the potential value of the electrode versus

the standard hydrogen electrode; is the overpotential contribution, i.e. the polarisation due to

cathodic protection or interfering currents; IRis the ohmic drop in the electrolyte, depending upon

the reference electrode position with respect to the structure, on the electrolyte resistivity and on

the circulating current within the electrolyte. E, which includes all terms, is often called

ONpotential.

The term EIR= ET+ = Etrueis called true potentialor OFFpotentialand gives the value of

the overpotential due to the current flux.

E

Soil (or electrolyte)

Voltmeter

RI

I

I'

Eeq

Reference electrode

Pipeline

Reference electrode

Figure 1.16 - Potential measurement

Therefore stray currents strongly influence the potential reading because ohmic drop contribution

is often high. In case of DC stray currents, the measured potential is more negative than the true

potential at cathodic zones and more positive at anodic ones. To know the true polarisation level

the ohmic drop contribution has to be eliminated.

The ON/OFF technique is commonly used to determine the true potential of protected structures

provided a correct interpretation. For continuous monitoring, a corrosion coupon is often used with

a reference electrode located close to the coupon. This device is also called true potential probe.

The case of AC interference seems to be particularly critical: depolarisation time lower than 1 ms

and corrosion attacks on samples with instantaneous OFFpotential below of 0.85 V CSE have

been reported[33]. In literature there is lack of agreement about the correct procedure to measure

the true potential in the presence of AC interference [33,39,21, ,38 39]. It has also been claimed that the

potential measurement misinterpretations caused by the AC could be also more severe than those

related to the DC. In the specific case of coating defects in cathodically protected buried pipelines

influenced by induced AC current, the CP evaluation becomes complicated, and the usual

potential criteria are no longer necessarily valid.


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23

It has been claimed that the instantaneous OFF-potential measurement is a non suitable

technique for the characterisation of the AC-corrosion risk of cathodically protected structures

because the obtained data are strongly influenced by the soil composition causing a very fast

depolarisation of the metal surface[33].

In Figure 1.17 the instantaneous potential of a sample interfered by AC is represented. The curve

arepresent the case of a non protected pipeline, while band crepresent cathodic protection. The

form and the amplitude of the sinusoidal curves remain, but their position with respect to the

reference axis is changed[12].

Time (s)

E SCE a

c

b

-850

(mV)

Figure 1.17 - Evolution of the potentials in the absence (a) and the presence (b and c) of cathodic protection[

The samples potential curve is a sinusoidal curve whose displacement towards the more

electronegative potentials is proportional to the level of cathodic protection. Consequently, if the

level of cathodic protection is increased in such a way that there is no more positive current the

potential becomes more and more electronegative[12]. The case c is when total compensation of

the positive AC. However, to achieve this shift in potential high DC current are required, which

would result in the production of large amount of hydrogen which may be disadvantageous

because of all the problems relate to overprotection as hydrogen stress cracking or coating

disbondment.

Unfortunately the measurement of the true potential do not give information about the

instantaneous IR free pick potential.

Recently a measuring technique was developed to measure the AC induced effects on metallic

structures. The system, with a switch OFF method on a coupon, provide IR-free potential, the

current density and the phase angle at every instant of the AC period [39]. Using this measuring

technique, the pipeline is considered protected from AC corrosion if the coupon-to-soil

EOFFpotential is at any moment more negative than the protective potential range.


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24

1.8.2 AC voltageAC in soil or AC intercepted by buried metallic structures is surveyed by measuring the potential

with an electronic multimeter that measures AC voltage for frequencies between 15 and 100 Hz. A

mobile oscilloscope is useful for verifying the type of waves, their amplitude and frequency.

For safety reason, it is generally considered that the AC voltage should be lower than 15 V rms

(rms = root mean square)[37].

Unfortunately, unlike the case of DC stray currents, there is no correlation between corrosion rate

and AC voltage and therefore, even today there are no criteria based on AC voltage

measurements. It is however possible to estimate AC density flowing through a coating defect of a

certain size by means of the following equation:

r

4

Vi F = Eq. 1.13

when soil resistivity () , defect radius (r), and voltage drop to remote earth (VF) are known[11,16, 40].

The AC voltage level alone does not allow the evaluation of AC corrosion risk. Two separate

pipelines exposed to the same AC voltage may or may not be subject to AC corrosion.

Nonetheless, pipelines exposed to low AC voltage seem to be subject to very little risk[12].

1

10

100

1000

10000

0 5 10 15 20 25 30VAC

Vcorr(m/year)

Initial Vcorr

Steady State Vcorr

Figure 1.18 - Corrosion rate measured by mean of ER corrosivity probes as a function of AC voltage (UAC) ona Danish site[7]

Field data showed that a relatively small-sized defect (6 to13 cm2) in the coating on the pipe in a

relatively low-resistivity soil could have AC current density in excess of 100 A/m2, while the AC

voltage could be as low as 6 V[32]. Others field surveys[7,13]showed that very high corrosion rates

were associated to AC potentials lower than 10 V (Figure 1.18). It has also been reported[13]that

coupons exposed to the same alternating voltage showed AC density which varied with time and


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25

location. This was probably due to some changes in soil resistivity and in chemical conditions

closest to the steel surfaces.

1.8.3 AC current densitiesThe determination of AC current densities is a very important parameter in order to evaluate AC

corrosion risks: the higher the density, the greater the risks. Current measurements on steel

coupons must be carried out with a shunt whose resistance is less than that of the pipeline, of the

potential coating defects and of the coupon. If this is not the case, the values will be significantly

erroneous.

As previously said the threshold value suggested by standards and laboratory tests[34,35,36]ranges

between 20 and 30 A/m2. Though it has been reported that an AC current density of 20 A/m2could

increase corrosion nearly two-fold relative to a control specimen; suggesting a threshold current

density lower than 20 A/m2[32].

It seems that there is not a theoretical 'safe' AC current density, i.e., a threshold below which AC

does not enhance corrosion; however, a practical one exists for which the increase in corrosion

due to AC is not appreciably greater than the free-corrosion rate for a particular soil condition [32].

1.8.4 AC Density/DC Density RatioGenerally higher is the AC higher is the DC cathodic protection required[32, ]41 . Therefore recently

in France theAC/DC current densityratiowas proposed as AC induced corrosion risk indicator[12]

:when the ratio is above 10, AC induced corrosion is almost certain; above 3 the risk is serious;

below 2, the risk is less, although not necessarily totally absent.

This approach has been proposed considering the following experimental data and field

measurement[12]:

measurements of DC and AC current carried on field showed that, at the corrosion sites,

the ratios were always higher than in those areas where no corrosion had been

observed;

laboratory measurements showed that in the presence of AC currents was necessary a

quantity of DC current greater than that is normally applied in order to obtain 850 mV;

through the calculation of the AC/DC current density ratio, errors due to individual

measurements of DC and AC intensities are partially eliminated; therefore the most

significant errors of measurement with regard to the internal resistance of

instrumentation and measurement circuits are eliminated;

even if the current densities increase in magnitude as the defects surface areas

decrease, the ratio remains constant whatever the surface areas of the coupons used to

perform the measurements;


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26

this ratio also makes it possible to adjust the level of DC current with regard to the AC

current, allowing therefore the definition of the quantity of DC current sufficient for

improved polarisation of a defect compared to a level of AC influence.

It should be noted that the value of the ratio provides no means of quantifying AC corrosion and

that this approach alone cannot be applied since there is no mention on the potential value. In fact

even if DC true potential measurements have been proven to be of difficult interpretation in the

presence of AC, their values remain a fundamental parameter during corrosion monitoring and risk

evaluations.

Furthermore considering the experimental evidence that high alternate current with high cathodic

protection levels may be associated with very high corrosion rates[32](Figure 1.15), it is possible to

have a not too high AC/DC ratio whereas the actual corrosion rate is very high.

1.8.5 Cathodic Protection Criteria Based on AC and DC DensitiesMeasured by Coupons

New cathodic protection (CP) criteria have been developed based on AC and DC current

densities measured using coupons[17]. The numerical expression of the criteria is reported in

Tab. 1.2, while the graphic expressions of the current density-based criteria are illustrated in

Figure 1.7.DC current density is determined as the average current density on a coupon (i.e. CP

conditions). Positive values of DC current density indicate that the current is flowing from the soil to

the coupon. Cathodic protection area is indicated in grey. Corrosion activity can be divided roughly

into three areas corresponding to the possible main risks of DC corrosion, AC corrosion andoverprotection [17].

DC corrosion is defined as the corrosion due to the lack of CP current, stray currents from DC

powered transit systems or other DC interference. The lowest limit of DC current density to prevent

DC corrosion was determined at 0.1 A/m2[17].On the other side the excessive DC current density

may lead to hydrogen stress cracking or coating disbondment [17].

The AC corrosion area corresponds either to low DC cathodic protection and low AC densities or

to high AC densities.

Tab. 1.2 - Numerical expression of the cathodic protection criteria for buried steel pipelines based on DCand AC current densities measured using coupons[17]

0.1 A/m2iDC< 1.0 A/m2then iAC< 25 iDC

1.0 A/m2iDC< 20 A/m2then iAC< 70 Arms/m

2

where:

iDC= DC density (A/m2)

iAC= AC density (A/m2)


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27

AC Corr osion

DC Corros ion

Overprotection

10070

25

10

25

1

0.10.01 0.1 1 10 20 100

Protection

iDC (A/m2)

iAC

(Arms

/m2)

AC Corr osion

DC Corros ion

Overprotection

10070

25

10

25

1

0.10.01 0.1 1 10 20 100

Protection

iDC (A/m2)

iAC

(Arms

/m2)

Figure 1.19 - Graphic illustration of the cathodic protection criteria for buried steel pipelines based on DC andAC current densities measured using coupons[17]

1.8.6 Size of the Coating DefectsSmall rather than large coating defects are susceptible to AC corrosion since the spread

resistance(in m2) associated with the defects increases with increasing area:

Rs= K sd (m2) Eq. 1.14

where Kis a constant depending on the geometry of the defect, dis a measure of the extensionof the defect, and sis the specific resistivity of the local soil environment adjacent to the defect

[7].

Experience[12] showed that the diameter of coating defects on a polyethylene coating is often

closer to a few millimetres than it is to a centimetre. In addition, cases of corrosion have been

observed on defects of very small areas (around 1 mm2).

Probabilistic approach

The idea that the current density flowing through an holiday in the insulating coating is a

meaningful parameter related to the risk of corrosion is commonly accepted within the community

of experts in AC corrosion; on the other side, the assessment of corrosion conditions is only

possible when the pipeline has already been laid down in the trench and the current density can be

really measured on simulated holidays. These measurements in the field are affected by a

possible wide variation of the spread resistance of these simulated holidays, which is connected to

complex and not yet completely understood electrochemical reactions deriving from DC and AC

current effects and the chemical composition of the soil in contact with the bare steel. It has been

demonstrated by laboratory tests that this spread resistance may, during time, increase by as

much as 100 times or decrease by as much 60 times [25].Simulation tools able to assess the level

of current density exchanged between pipe and soil through the insulating coating holidays havebeen proposed. The method consists in the integration of deterministiccalculations, necessary for


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28

the determination of the induced voltage profile along the pipeline route, with probabilistic

calculations necessary for the evaluation of the holidays size and consequently of the current

density.

The parameters and the characteristics relevant to the random quantities (i.e. the holidays area

distribution, the number of holidays per unit length) have been inferred starting from experimental

data coming from the field. The probability levels are directly related to:

the levels of induced voltage on the pipeline;

the number of holidays per unit length besides the other statistical characteristics of the

holidays distribution;

the length of the pipeline section considered.

Deterministic approaches are based on specific assumptions concerning the holiday size and

location and are normally focused on the worst case study, whereas probabilistic approaches takeinto account of the random nature of some significant parameters like the holidays size and

location.

The proposed algorithm[25]allows the assessment of the probability associated to the exceeding

of a certain threshold current density value (J*) in a generic section of pipeline; the advantage of

such an approach is the possibility to individuate those pipeline sections which are more exposed

to the AC corrosion risk. The algorithm is still deterministic as far as the calculation of electric

quantities (voltage, current, current density) is concerned while it is statistic as far as the holidays

data are concerned; therefore the results of the algorithm are given in terms of probability ofexceeding a certain value of current density for a generic section (J*), of given length, along the

pipeline.

Thus, if the pipe under the electromagnetic influence of an AC power or of an electrified railway

line assumes an alternating voltage

V at the holiday location, the current density J exchanged

between pipe and soil through the holiday itself is given by:

A

4

d

V

AR

VJ

sh

h

+== Eq. 1.15

where Rh is the resistance to remote earth of the pipeline through the holiday, known as spread

resistance. The spread resistance is given by the sum of two contributions (Figure 1.20): Rch,

which represents the resistance relevant to the small cylinder inside the coating, and Reh

, which

represents the earth resistance (with respect to the remote earth) of the holiday; for points inside

the holiday the resistivity is h while in all the other points is s; A is the area of the holiday.


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29

d

soil s

pipeline axis

insulating coating

pipeline wall

holiday hA

Rch

Reh

= Rh

d

soil s

pipeline axis

insulating coating

pipeline wall

holiday hA

Rch

Reh

= Rh

Figure 1.20 Spread resistance or holiday resistance Rhto remote earth

Once the threshold current density value (J*) is established the upper limit (in function of the pipe

voltage V) for the holiday area A in order to have the exceeding of the limit value J *

for the current

density through the holiday itself can be evaluated.

2

h2s

d

*J

V16*A

Eq. 1.16

where A*(V) is the limit area.

The probability (P%) of exceeding a certain value for the current density is mainly related to two

important quantities[25]:

the length of the pipeline section considered (the longer is the section, the higher is the

probability);

the mean value of the voltage assumed by the pipeline in the same section (the higher is

the mean voltage the higher is the probability).

It is therefore possible to obtain set of curves relating the probability of exceeding the limit value

of current density J* versus the pipeline section length for a given value of the mean voltage V. In

Figure 1.21 an example is shown by considering a limit value of J*=30 A/m2.


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30

0

10

20

30

40

50

10 100 1000

Pipeline section length (m)

P%(J>J

*)

0

10

20

30

40

50

10 100 1000

Pipeline section length (m)

P%(J>J

*)

Figure 1.21 - Per cent probability (P%) of exceeding the current density limit J*=30A/m

2versus the pipeline

section length for different values of the mean voltage; the curves from top to bottom correspond toV = 50, 40, 30, 20, 10V respectively; n

h=0.7 holidays/km

[25]

1.8.7 ER corrosivity probesIn the specific case of CP of coating defects in buried pipelines influenced by induced AC

current, the CP evaluation becomes complicated, and the usual potential criteria are no longer

necessarily valid. In Denmark, the connection of high sensitivity electrical resistance (ER) sensor

for evaluation of CP and corrosion rate measurements has become usual practice, particularly

when studying risks of AC corrosion[7,20,21].

ER based AC corrosivity probe

The AC corrosivity probe with associated instrumentation is based on the high-resolution

differential electrical resistance. The technique utilises the fact that the electrical resistance of a

metal probe element increases when corrosion diminishes the thickness of the element.

Considering a metal element with dimensions L (length), W (width), and (thickness), the

electrical resistance of the element can be expressed by[7,20,21]

:

( )

=W

LTR Eq. 1.17

where (T) is the specific resistivity of the element material, which depend on the temperature.

Re-arranging the equation, the thickness of element can be obtained as a function of element

resistance[7,20,21]

:

( )R

1

W

LT = Eq. 1.18

Differentiation of the equation gives the corrosion rate (vcorr) of the element as:


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( )TLW

dt

dR

dt

dv

2

corr

=

= Eq. 1.19

Hence, once the time change of the electrical resistance of the element, the element dimensions

and resistivity are known the corrosion rate can be evaluated. The period of time needed to

quantify a certain corrosion rate can be calculated as follows [7,20,21]:

( )TLW

v

Rt

2

corr

= Eq. 1.20

The equation predicts that the necessary period of time increase with increasing W/L ratio, and

increases with the square of the element thickness. Hence, high sensitivity ER probes can be

produced by using thin elements, but this decreases probe lifetime as well. R can be regarded as

the minimum resistance change needed to provide a reliable measurement[7,20,21].

In Figure 1.22 an examples of corrosion rate measured by mean of ER corrosivity is given. It can

be noticed that, as previously said, the corrosion rate can be very high even when the AC potential

is lower than 10 V.

Figure 1.22 - Corrosion rate measured by mean of ER corrosivity probes and AC voltage throughout time ona Danish site[7]

1.9 DAMAGE TO PIPELINE EXTERNAL COATINGDuring an AC power fault condition, current flowing from the AC power lines to earth can give

rise to instantaneous very high conductive and inductive AC voltage and current on a pipeline.

Fault duration and magnitude are controlled by the AC circuit protection equipment installed by the

local AC power authority. Voltage levels of several thousand volts are possible and can damageexternal coating. To prevent undue damage to an external coating, a maximum short-term


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cumulative AC voltage level of 3 kV to 5 kV [23] is usually considered acceptable for fusion bonded

epoxy and polyethylene coated pipelines that have high integrity field joint coating. For tape-wrap,

coal tar enamel and asphalt coating systems 2 kV is suggested as threshold value.

Also in case of AC-induced corrosion occurred during steady-state conditions, damages of the

coating in the area surrounding the pits are often reported[19]. Coating disbondment is often

associated to AC interference due to the enhanced cathodic process.

1.10 SAFETY FOR PERSONNELIn terms of safety for personnel and the public, a potential shock hazard exists when someone

touches an exposed part of the pipeline (such as a valve) while standing on soil which is at a

different potential. The touch voltage to which a person would be subjected when standing nearthe pipeline and touching an exposed part of the pipeline is defined as the difference in potential

between the pipeline metal and the earth surface above the pipeline. Similarly, the step voltageis

the difference in potential between a persons feet, and is defined as the difference in earth surface

potential between two points spaced 1 m apart[23,26].

The following AC touch voltage levels are typically considered acceptable

Less than 50 V AC steady-state for buried pipeline sections that may be exposed

periodically by operator personnel, who are aware of the possible voltage levels and are

suitably prepared for the problem;

Less than 15 V AC steady-state for aboveground appurtenances to minimize the

danger to personnel and livestock.

The 50 V level is based on the fibrillation current threshold (50 mA) for an infinitely long fault

duration[ ]42 , for a body resistance of 1000 and assuming a person is bare-foot while touching the

pipe with his/her bare hand. The 15 V level is as set by NACE RP0177-2000[37]and has become

the standard value.

1.11 AC INFLUENCE ON CATHODIC PROTECTIONAs previously said the AC-interference causes perturbation of cathodic protection and AC

corrosion risks. One of the most serious consequences of steady state induced AC is that

corrosion can occur even if cathodic protection levels satisfy the standards and despite the

induced AC has been reduced to less than 15V as required by standards.

The presence of alternating voltage and current on buried metallic pipelines can cause

malfunctioning or even damage to the Transformer/Rectifier (T/R) units of impressed current


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cathodic protection. This is particularly valid for T/R units that are not equipped with suitable AC

filter and surge protection.

1.12 HOW AC INFLUENCES CORROSION PROCESSESThe mechanism by which AC affects the corrosion process is not yet understood: different

theories and hypotheses have been proposed, but none of them is able to fully explain and

describe such a phenomenon.

An important aspect to consider is whether at the frequency of the interfering signal, the

electrochemical processes are reversible or not[ ]43 . In fact in the presence of alternating voltage the

reversibility of the electrochemical processes is related to the frequency of the signal: if the

frequency is not high enough and the chemical reaction is not reversed, then the metal coulddegrade, for example, because the metal ions migrate and precipitate into the solution. The

irreversibility of the chemical reactions occurring at the interface may causes changes on the

double layer compo

Ac Corroosion Phd Thesis

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