Corrosion Science 51 (2009) 186–190

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Corrosion Science

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Characterization of corrosion of X70 pipeline steel in thin electrolyte layerunder disbonded coating by scanning Kelvin probe

A.Q. Fu a, X. Tang a,b, Y.F. Cheng a,*

a Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4b College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, China

a r t i c l e i n f o

Article history:Received 30 July 2008Accepted 19 October 2008Available online 29 October 2008

Keywords:A. Organic coatingA. SteelB. PolarizationB. Scanning Kelvin probeC. Passivity

0010-938X/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.10.018

* Corresponding author. Tel.: +1 403 220 3693; faxE-mail address: fcheng@ucalgary.ca (Y.F. Cheng).

a b s t r a c t

Scanning Kelvin probe technique was used to characterize the electrochemical corrosion behavior of X70steel in a thin layer of near-neutral pH and high pH solutions, respectively. Results demonstrate that pas-sivity can be developed on steel in the near-neutral pH solution layer as thin as 60 lm, which is attrib-uted to the fact that Fe2+ concentration in aqueous phase could reach saturation in the thin solution layer.The solubility of FeCO3 is reached to drop out of solution as a precipitate. With the increase of solutionlayer thickness, it becomes more difficult for Fe2+ concentration to reach saturation. Consequently, thepassivity cannot be maintained, and the steel shows an active dissolution state. Anodic dissolution rateof steel increases with the immersion time. The electrochemical polarization behavior of X70 steel in highpH solution is approximately independent of the solution layer thickness and immersion time. In thinsolution layer, diffusion and reduction of oxygen dominate the cathodic process, as demonstrated bythe presence of cathodic limiting diffusive current. In the bulk solution, the absence of limiting diffusivecurrent density in cathodic polarization curve indicates that the main cathodic reaction is reduction ofH2CO3 and HCO�3 , and the formed film is thus mainly FeCO3.

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1. Introduction

It has been acknowledged [1] that corrosion and stress corro-sion cracking (SCC) of pipelines are dependent on the coating prop-erties and degradation modes. For example, high pH SCC ofpipeline occurs under permeable coating, such as asphalt, wherecathodic protection (CP), corrosive species and oxygen could pen-etrate through coating to reach the pipeline steel surface to forma concentrated carbonate/bicarbonate/chloride environment [2–7].Occurrence of near-neutral pH SCC is usually associated with animpermeable coating, such as polyethylene tape, where CP isblocked and an anaerobic, near-neutral pH environment is devel-oped under the disbonded coating [8–14].

Generally, the trapped electrolyte between coating and the pipe-line steel is very thin, especially during the initial stage of develop-ment of an electrochemical environment for corrosion reaction.Electrochemical corrosion of steel in a thin electrolyte layer is dis-tinctly different from that in the bulk solution. For example, thesmall ohmic potential drop and non-uniform current distributionin the thin electrolyte layer are expected to affect significantly theelectrochemical mechanism of corrosion reaction of the steel [15].

To date, there have been a number of research conducted toinvestigate corrosion of pipeline steel in carbonate–bicarbonate–

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: +1 403 282 8406.

chloride solutions [8,10,11,13,16–18]. For example, Parkins andco-workers [19–21] have done a systematic study of pipeline steelcorrosion in the concentrated carbonate–bicarbonate solutions.The research found that the presence of Fe3O4, a major constituentof mill scale, on the steel surface can promote potentials within therange where SCC is possible. Furthermore, they demonstrated thatselective dissolution at grain boundaries of the steel begins atpotentials approximating the lower bound of the cracking domain.It was supposed that high pH pipeline SCC is attributed to anodicdissolution at the grain boundaries and repeated rupture of passivefilms that form over the crack tip. Moreover, Cheng’s group[12,22,23] has focused on corrosion of pipeline steels in dilutedcarbonate–bicarbonate solutions. The results demonstrated thatthe steel is in an active dissolution state. The presence of corrosionproduct deposit is capable of promoting desorption of hydrogenatoms on the steel surface. Upon hydrogen permeation into steel,the dissolution rate of steel is enhanced. However, none of themwere conducted under a thin aqueous layer to reproduce the actualelectrochemical condition developed under the disbonded coating.

Scanning Kelvin probe (SKP) technique has been developed tomeasure potentiodynamic polarization curves [24–26] of steels inthin electrolyte. In this work, SKP was used to characterize theelectrochemical corrosion behavior of X70 pipeline steel in a thinlayer of carbonate/bicarbonate electrolyte to simulate both near-neural pH and high pH conditions developed under the disbondedcoating. As a comparison, the polarization curves of X70 steel in

Fig. 1. Schematic diagram of the apparatus for measuring polarization curve of steel in a thin aqueous solution layer by scanning Kelvin probe.

A.Q. Fu et al. / Corrosion Science 51 (2009) 186–190 187

bulk test solutions were measured. It is anticipated that a mecha-nistic insight would be developed to advance the understanding ofelectrochemical corrosion of pipeline steel under disbondedcoating.

2. Experimental

2.1. Electrode and solution

The working electrodes were fabricated from a sheet of API X70pipe steel with a chemical composition (wt.%): 0.06 C, 1.44 Mn,0.31 Si, 0.004 S, 0.01 P, 0.034 Ni, 0.16 Cr, 0.25 Mo, 0.005 V, 0.015Cu, 0.01 Ti, 0.002 B, 0.029 Al and Fe balance. The microstructureof the steel contains ferrite and pearlite. The working surface wassubsequently ground with 800 grit and 1000 grit emery papers,cleaned by distilled water and acetone.

The test solutions contained a NS4 solution and a concentratedcarbonate/bicarbonate solution to reproduce near-neutral pH andhigh pH conditions, respectively. NS4 solution had been widelyused to simulate the dilute electrolyte trapped between coatingand the pipeline steel, with the chemical composition: 0.483 g/lNaHCO3, 0.122 g/l KCl, 0.181 g/l CaCl2 � 2H2O and 0.131 g/lMgSO4 � 7H2O. Prior to test, NS4 solution was purged with 5%CO2/N2 gas for 1 h to achieve an anaerobic and near-neutral pHcondition (pH 6.8). The gas flow was maintained throughout thetest. The concentrated carbonate/bicarbonate solution was com-posed of 0.05 M Na2CO3, 0.1 M NaHCO3 and 0.1 M NaCl, with apH of about 9.6 (high pH solution). Both solutions were made fromanalytic grade reagents (Fisher Scientific) and ultra-pure water(18 MX cm in resistivity). All tests were conducted at room tem-perature (�22 �C).

2.2. Polarization curve measurements

The schematic diagram of test apparatus for polarization curvemeasurements in the thin aqueous solution layer trapped at steel/coating interface was shown in Fig. 1. The X70 steel working elec-trode was covered with a solution layer, whose thickness wasdetermined by a three-dimensional imaging device. A fusionbonded epoxy (FBE) membrane in 100 lm thickness was placedabove the solution. The Kelvin probe with a 500 lm tungsten tipwas set directly above the coating and the distance between theprobe-tip and coating surface was controlled at 250 lm. All theKelvin potentials were converted to corrosion potential versusstandard hydrogen electrode (SHE) in this work. The test chamberis sealed so that the solution evaporation is ignorable. It is worthypointing out that, generally, it is impossible to have exactly thesame potential everywhere on the working electrode under a thinelectrolyte layer as an ohmic potential drop caused by the currentflow to the counter electrode cannot be avoided. This ohmic drop

can be minimized by using a circular counter electrode surround-ing the working electrode [27]. However, the circular wire elec-trode with a small area may generate a large current densitypassing the counter electrode, enhancing the polarization of theelectrode. Since the measured currents were low in this workand the ohmic potential drop would not be significant. Therefore,the Pt foil electrode was used.

During the experiments, the current was applied by a PAR 263 Apotentiostat, and the resulting Volta potential difference betweensample and tip was measured through a M370 scanning electro-chemical workstation. The polarization curve was then measuredstarting from corrosion potential, cathodically first and then anodicportion by altering the current.

Conventional polarization curve measurements were per-formed on X70 steel electrode in the bulk solution through a Solar-tron 1280C electrochemical system. A saturated calomel electrode(SCE) was used as reference electrode and a platinum foil as coun-ter electrode. The potential scanning rate was 0.5 mV/s. The mea-sured potentials were converted relative to SHE.

3. Results

3.1. Polarization curve measurements on X70 steel in a thin NS4solution layer

Fig. 2 shows the polarization curves of X70 steel in NS4 solutionlayer with various thicknesses after 1 h and 24 h of immersion,respectively. Since the current density was quite low, the differ-ence between passive and active behavior measured at differentelectrolyte layer thicknesses would not be very apparent. However,the different polarization behavior is still distinguishable fromFig. 2. When electrode was under a 60 lm NS4 solution layer,the steel would be passivated with a stable passive potential rangefrom approximately 0 to 0.5 V (SHE). This feature is not observed inthe polarization curves measured at 90 and 140 lm solution layers.When the solution layer thickness increased to 90 lm, passivity ofsteel became less stable after 1 h of immersion compared to thatmeasured under a 60 lm solution layer. After 24 h of immersion,passivity could not be maintained on the steel. Under a 140 lmsolution layer, passivity cannot be observed after both 1 h and24 h of immersion, and the steel showed an active dissolutionstate. Furthermore, the anodic current density increased whenthe immersion time increased from 1 to 24 h for all curves.

3.2. Polarization curve measurements on X70 steel in a thin layer ofconcentrated carbonate/bicarbonate solution

Fig. 3 shows the polarization curves of X70 steel in the concen-trated carbonate/bicarbonate solution layer with various

1E-8 1E-7 1E-6 1E-5 1E-4 1E-3

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1.5

after 1 hour after 1 day

E (

V, S

HE

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i (A/cm2)

a: 60 m

1E-8 1E-7 1E-6 1E-5 1E-4 1E-3-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0b: 90 m

i (A/cm2)

E (

V, S

HE

)

after 1 hour after 1 day

1E-7 1E-6 1E-5 1E-4-2.5

-2.0

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0.0

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1.0c: 140 m

i (A/cm2)

E (

V, S

HE

)

After 1 hour After 1 day

μ

μ

μ

Fig. 2. Polarization curves of X70 steel in thin NS4 solution layer after immersion of1 and 24 h: (a) 60 lm; (b) 90 lm; and (c) 140 lm.

1E-7 1E-6 1E-5 1E-4 1E-3-2.5

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1.0b: 90 m

E (

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After 1 hour After 1 day

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-2.0

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E (

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HE

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i (A/cm2)

After 1 hour After 1 day

μ

μ

μ

Fig. 3. Polarization curves of X70 steel in thin carbonate/bicarbonate solution layerafter immersion of 1 and 24 h: (a) 60 lm; (b) 90 lm; and (c) 140 lm.

188 A.Q. Fu et al. / Corrosion Science 51 (2009) 186–190

thicknesses after 1 and 24 h of immersion, respectively. It is seenthat passivity could be developed on steel in all solution layerthicknesses and immersion times. Furthermore, an apparent limit-

ing diffusive current density was observed in cathodic polarizationcurves. With the increase of solution layer thickness and immer-sion time, there was little change of the polarization behavior.

1E-7 1E-6 1E-5 1E-4 1E-3 0.01

-1.0

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0.0

0.2

E (

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HE

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i (A/cm2)

Fig. 4. Polarization curve of X70 steel measured in bulk NS4 solution.

1.5

A.Q. Fu et al. / Corrosion Science 51 (2009) 186–190 189

3.3. Polarization curve measurements in bulk solution

Fig. 4 shows the polarization curve of X70 steel measured in abulk NS4 solution. It is seen that there was a similar feature to thatmeasured in the 140 lm NS4 solution layer. No passivity was ob-served, and the steel was in an active dissolution state.

Fig. 5 shows the polarization curve of X70 steel measured in abulk carbonate/bicarbonate solution. It is seen that the steel waspassivated over a stable potential range. However, there was nolimiting diffusive current density phenomenon observed in thecathodic curve.

4. Discussion

4.1. Electrochemical corrosion behavior of steel in thin NS4 solutionlayer

It has been demonstrated [12,14] that the anodic and cathodicreactions of X70 steel in deoxygenated NS4 solution are oxidationof iron and the water reduction, respectively

Fe! Fe2þ þ 2e ð1ÞH2Oþ e! Hþ OH� ð2Þ

A layer of loose, porous corrosion product, Fe(OH)2, may form onelectrode surface, without changing the active dissolution state of

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Current density (A/cm2)

Pot

entia

l (V

, SH

E)

Fig. 5. Polarization curve of X70 steel measured in bulk high pH solution.

the steel. Furthermore, due to the diluted nature of the solution, alayer of FeCO3 deposit is not formed on the surface of steel in thebulk solution.

The present work shows that, under a thin layer of NS4 solution,such as 60 and 90 lm, the steel can be passivated stably. It isattributed to the fact that the Fe2+ concentration in aqueous phasecould reach a saturation state in the thin solution layer. Althoughthe amount of CO2�

3 generated by Reactions (3)–(5) is very small(diluted solution)

H2Oþ CO2 $ H2CO3 ð3Þ2H2CO3 þ 2e! H2 þ 2HCO�3 ð4Þ2HCO�3 þ 2e! H2 þ 2CO2�

3 ð5Þ

the solubility of FeCO3 can be reached to drop out of solution as aprecipitate once the Fe2+ concentration achieves saturation or evensuper-saturation by

Fe2þ þ CO2�3 ! FeCO3 ð6Þ

With the increase of the solution layer thickness, it becomesmore difficult for Fe2+ to reach saturation. It is apparent fromFig. 2 that, when the solution layer increases to 140 lm, the steelcan not be passivated any more. The measured polarization curveis quite similar to that measured in the bulk solution (Fig. 4).

When the steel electrode was immersed in solution for 24 h, themeasured polarization curves show significant differences fromthose measured after 1 h of immersion. As seen in Fig. 2, all curvesare shifted to a higher current density. Apparently, anodic dissolu-tion rate of steel increases with the immersion time. The enhanceddissolution of steel is ascribed to the fact that intermediate species,such as Fe(I)ad or Fe(II)ad, could be formed on electrode surface dur-ing deposit of carbonate corrosion product [28,29]. These adsorbedspecies play a ‘‘self-catalytic” role in corrosion of steel

FeðIÞ þ Fe! FeðIÞads þ FeðIIÞsol þ 2e ð7ÞFeðIIÞ þ Fe! FeðIIÞads þ FeðIIÞsol þ 2e ð8Þ

It is pointed out that the intermediate species principle is usedto illustrate the enhancement of dissolution rate of steel because itis associated with the formation carbonate product on the elec-trode surface in carbonate–bicarbonate solution, which has beenproven in the refereed work. Although there are other potentialfactors, such as a change of solution pH, which could also resultin the change of dissolution rate of steel, a further discussion isnot performed since there is no direct supportive evidence.

1E-8 1E-7 1E-6 1E-5 1E-4 1E-3-2.5

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E (

V, S

HE

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60 m 90 m 140 m

i (A/cm2)

μμ

μ

Fig. 6. Comparison of polarization curves of X70 steel after immersion in thin highpH solution layers with various thicknesses for 1 h.

190 A.Q. Fu et al. / Corrosion Science 51 (2009) 186–190

4.2. Electrochemical corrosion behavior of steel in a thin carbonate/bicarbonate solution layer

The present work shows that the electrochemical polarizationbehavior of X70 steel is approximately independent of the solutionlayer thickness (Fig. 6) and immersion time, i.e., all curves are fea-tured with an anodic passivation and a cathodic limiting diffusivecurrent density, as seen in Fig. 3. It has been accepted [30] that,accompanying reduction of H2CO3 and HCO�3 (Reactions (4) and(5)), a layer of stable iron carbonate can be formed by Reaction(6) and the following pathways:

Fe2þ þ 2HCO�3 ! FeðHCO3Þ2 ð9ÞFeðHCO3Þ2 ! FeCO3 þH2Oþ CO2 ð10Þ

Moreover, the oxygen reduction (Reaction (11)), which resultsin the presence of limiting diffusive current density in cathodicpolarization curves, could promote the oxidation of Fe(OH)2 toFe3O4

O2 þ 2H2Oþ 4e! 4OH� ð11Þ3FeðOHÞ2 þ 1=2O2 ! Fe3O4 þ 3H2O ð12Þ

Therefore, the formed film on electrode surface in a concentratedcarbonate/bicarbonate solution usually contains a mixture of ironcarbonate and iron oxide [19], contributing to passivity of the steel.

This work shows that, in the thin solution layer, diffusion andreduction of oxygen dominate the cathodic process, as demon-strated by the presence of cathodic limiting diffusive current den-sity. It is thus expected that the dominant film formed on the steelsurface in this solution layer is Fe3O4. In the bulk solution, the ab-sence of limiting diffusive current density in cathodic polarizationcurve indicates that the main cathodic reaction is reduction ofH2CO3 and HCO�3 , and the formed film is thus mainly FeCO3.

4.3. Implications on pipeline SCC

Previous works [12,22] have shown that pipeline steel cannotbe passivated under near-neutral pH condition, but in an activedissolution state. Therefore, the dissolution-based mechanism thatis associated with the film rupture and repair does not apply fornear-neutral pH SCC of pipelines. It is thus proposed [1] thathydrogen is involved in near-neutral pH SCC of pipelines, accompa-nying anodic dissolution at crack tip. It is worthy pointing out thatprevious works about the electrochemical behavior of pipelinesteels in near-neutral pH solution, such as NS4 solution, have beenconducted in the bulk aqueous solution.

This work demonstrated that, when a thin layer of near-neutralpH electrolyte forms on the steel surface, a stable passivity wouldbe developed on steel. During development of electrochemicalenvironment under the disbonded coating, especially the earlystage, the trapped electrolyte between coating and the pipelinesteel is usually very thin. Moreover, the space under the coatingdisbondment is often very limited, contributing to generation ofa thin layer of electrolyte. It is thus reasonable to assume that pipe-line steel is actually under a thin layer of electrolyte over a quitelong time period, during which the steel would be in a passivestate, rather than an active dissolution state. Apparently, the con-ventional acknowledgement that pipeline steel is in an active stateunder near-neutral pH environment is questionable.

Furthermore, once crack initiates when steel is under a thinelectrolyte and thus in passivity, the role of hydrogen would be-come less important due to the potential inhibition of hydrogenpermeation into steel by the surface film [31]. As a consequence,the well-accepted proposition about the hydrogen involvementin pipeline SCC would be re-checked for its reliability.

5. Conclusions

Passivity can be developed on X70 steel in a NS4 solution layeras thin as 60 lm. It is attributed to the fact that Fe2+ concentrationin aqueous phase could reach saturation in the thin solution film.The solubility of FeCO3 is reached to drop out of solution as a pre-cipitate. With the increase in solution layer thickness, it becomesmore difficult for Fe2+ concentration to reach saturation. Conse-quently, passivity of steel cannot be maintained, and the steelshows an active dissolution state.

Anodic dissolution rate of steel increases with the immersiontime. The enhanced dissolution of steel can be ascribed to the factthat intermediate species, such as Fe(I)ad or Fe(II)ad, could beformed on electrode surface during deposit of carbonate corrosionproduct. These adsorbed species play a ‘‘self-catalytic” role in cor-rosion of steel.

The electrochemical polarization behavior of X70 steel in a con-centrated carbonate/bicarbonate solution is approximately inde-pendent of the solution layer thickness and immersion time, i.e.,all curves are featured with an anodic passivation and a cathodiclimiting diffusive current density. In thin solution layer, diffusionand reduction of oxygen dominates the cathodic process, as dem-onstrated by the presence of cathodic limiting diffusive current.In the bulk solution, the absence of limiting diffusive current den-sity in cathodic polarization curve indicates that the main cathodicreaction is reduction of H2CO3 and HCO�3 , and the formed film isthus mainly FeCO3.

Acknowledgements

This work was supported by Canada Research Chairs Programand Natural Science and Engineering Research Council of Canada(NSERC).

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