Investigation of Alternating Current Corrosion of ...profs.hsu.ac.ir/.../Investigation-of-Alternating-Current-Corrosion.pdfInvestigation of Alternating Current Corrosion ... KEY WORDS:

CORROSION SCIENCE SECTION

578 CORROSION—SEPTEMBER 2009

Submitted for publication June 2008; in revised form, April 2009. ‡ Corresponding author. E-mail: [email protected]. * SGK Swiss Society for Corrosion Protection, Technoparkstr. 1,

CH-8005 Zürich, Switzerland. ** E.ON-Ruhrgas AG, Huttropstrasse 60, DE-45138 Essen, Germany.

Investigation of Alternating Current Corrosion of Cathodically Protected Pipelines: Development of a Detection Method, Mitigation Measures, and a Model for the Mechanism

M. Büchler‡,* and H.-G. Schöneich**

AbstrAct

Investigations performed in recent years regarding alternat-ing current (AC) corrosion on cathodically protected pipelines are reported and summarized. A model for the corrosion mech-anism is shown and the formation of a rust layer is experi-mentally demonstrated. The use of the coulometric oxidation for detecting the degree of AC corrosion on coupons by oxidiz-ing the Fe(II) in the rust layer is introduced and experimen-tal results from field investigations are presented. The effect of the cathodic protection level on the corrosion rate is shown. In order to obtain data for optimizing the cathodic protec-tion parameters, the corrosion rate was investigated at vari-ous on-potentials and interfering AC voltages. Based on the obtained results it was possible to demonstrate that the cor-rosion rate can significantly be decreased if the off-potential is more negative than –0.85 V vs. copper/copper sulfate (Cu/CuSO4) electrode (CSE) and the on-potential is in the range of –1.2 VCSE. Moreover, it was possible to demonstrate that the high AC corrosion rate on coupons can readily be decreased if the cathodic protection level is adjusted. To obtain an under-standing of the processes involved and to clarify the influence of alkalinity on the corrosion rate, the effect of cathodic cur-rent density on the pH value on the metal’s surface of coupons were analyzed with a new in situ pH measurement tech-nique. The effects of pH, spread resistance, and electrochemi-cal reduction are discussed with respect to the experimentally observed corrosion rate.

KEY WORDS: alternating current corrosion, cathodic protec-tion, corrosion mechanism, mitigation measures, pH value

IntroductIon

The phenomenon “alternating current corrosion,” or “AC corrosion,” has been investigated in detail since the observation of the first corrosion damages induced by AC corrosion on cathodically protected pipelines in 1988.1-2 Despite all these investigations performed since then, the involved mechanisms are still not com-pletely understood. In recent years, extensive work has been performed to obtain a more profound under-standing of the involved processes. The results were published in German or in European conference pro-ceedings.3-4 To make these data accessible to a wider public, they are summarized in this article.

In general, it is well established that AC corro -sion can only occur if the AC current density exceeds 30 A/m2.5-6 However, it was found that higher current densities do not necessarily lead to corrosion attack. While the AC current densities determined on cou-pons do provide information on the corrosion state, the AC potential measured on pipelines was found to be merely an indicator rather than a criterion. In extended field investigations, corrosion attack was found on samples that had a less than 5-V AC poten-tial.7 The development of the coulometric oxidation turned out to be a promising technique to determine the amount of AC corrosion taking place on coupons in the field.3,8-11 The characterization and analysis of coupons subjected to AC interference in field applications allowed the evaluation of the reliability of the coulometric oxidation in determining the occur-rence of weight loss as a result of AC corrosion on coupons.

ISSN 0010-9312 (print), 1938-159X (online)09/000101/$5.00+$0.50/0 © 2009, NACE International


CORROSION—Vol. 65, No. 9 579

It was demonstrated that the operation param-eters of the cathodic protection strongly influence the corrosion rate.12 These results were confirmed by recent investigations.3,13-18 This effect was found in artificial soil solutions, in soils, but also in sodium hydroxide (NaOH) solutions as reported by Leutner, et al.,12 and Nielsen, et al.13 By running laboratory tests in artificial soil solutions at various cathodic protection levels and evaluating the corrosion rate, it was possible to determine their influence on the corrosion rate. To obtain a more profound under-standing of the involved mechanisms, measurements of the pH at the steel surface during cathodic protec-tion and measurements of the corrosion rate in highly alkaline solutions at high AC current densities were performed.

ExpErImEntAl procEdurEs

All solutions were prepared from reagent-grade chemicals and deionized water. For the simulation of the behavior in soil, coupons were exposed to artifi-cial soil solution in quartz sand. The composition of the electrolyte is given in Table 1. All laboratory inves-tigations of corrosion rates were run for 3 months. The investigated surface was circular with an area of 1 cm2 in all tests. The corrosion rate was either deter-mined by mass loss or by determination of the change of electrical resistivity of the coupon due to corro-sion (ER coupons). The AC and direct current (DC) voltages were applied by transformers connected in series on the coupons and a counter electrode made of activated titanium or stainless steel 1.4435 (UNS N08904)(1). Any interference of DC and AC currents was avoided by shortening the DC transformer with a capacitor of 10 mF. The frequency of the AC voltage was 50 Hz in all cases.

For the electrochemical characterization and cou-lometric oxidation of coupons, a potentiostat/galva-nostat in combination with a computer was used. The coulometric oxidation was run with an anodic current of 50 A/m2. In the case of field studies the coupons were excavated after coulometric oxidation and the corrosion was determined using mass-loss measure-ment. Exposure times varied between 2 years and 15 years.

The pH in front of the coupon was determined by means of a pH sensor.8,16 The pH sensor was placed in the center of the steel surface (Figure 1). The pH value was determined based on the oxygen evolution poten-tial on the sensor. Additionally, the spread resistance of the sensor was used to estimate the pH value. The confined geometry resulted in a homogeneous cur-rent distribution on the steel surface, linear diffusion, and linear migration. The experiment was performed

in quartz sand and artificial soil solution according to Table 1.

Cathodic DC current densities have a negative sign, and anodic DC current densities have a positive sign. All potentials are referred to the saturated cop-per/copper sulfate electrode (CSE).

rEsults And dIscussIon

It is well known that cathodic protection results in an increase of the pH value in front of the metal surface due to oxygen reduction and hydrogen evo-lution. Depending on the current density, the spread resistance, the morphology of the soil, and the hydro-dynamic conditions, very high pH values can be obtained. It is often pointed out that at high pH values the formation of soluble iron compounds is thermody-namically possible.19 To investigate the electrochemi-cal behavior of steel in highly alkaline environments, polarization scans in deaerated electrolyte solutions were performed. The results are shown in Figure 2. In all investigated electrolytes, passive behavior is observed. However, an increasing pH value results in increased passive current densities. In the case of saturated NaOH solution of about pH 16, the cur-rent density is increased by more than a factor of 10. Hence, increased dissolution rates could be expected in the case of potentials anodic of the cathodic protec-tion potential if the pH is higher than 14. It is impor-tant to note that the high passive current densities in Figure 2 are attributed to the comparably high scan rate of 10 mV/s.

It is generally assumed that AC corrosion is caused by short-term anodic polarization of the metal surface during the anodic half wave.18 This anodic polarization results in metal dissolution. To investi-gate the electrochemical processes occurring during

(1) UNS numbers are listed in Metals and Alloys in the Unified Num-bering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

TaBlE 1Composition of the Artificial Soil Solution

NaHCO3 2.5 mmol/L NaSO4·10 H2O 5 mmol/L NaCl 5 mmol/L

FIGURE 1. Experimental setup for determining the pH value in front of the steel surface. The steel plate had a surface of 1 cm2.



alternating anodic polarization, cyclic voltammograms in deaerated 0.1 M NaOH of pH 13 were recorded. The results are shown in Figure 3. The following descrip-tion is the summary of a detailed analysis of the pro-cess obtained by means of in situ x-ray absorption near-edge spectroscopy (XANES) investigation and in situ light absorption measurements given else-where.20-21 The first cycle shows passivation com-parable to the results obtained in Figure 2. In the second cycle anodic oxidation peaks at –0.72 VCSE and cathodic reduction peaks at –1.1 VCSE are occurring. Their size increases with every consecutive cycle. This behavior can be explained as follows (Figure 4[a]):20-21 After the formation of a protecting passive film in the first anodic cycle, the cathodic polarization results in the reduction of the passive film. In the alkaline envi-ronment, the Fe(II) formed during reduction of the

passive film has only limited solubility. As a conse-quence, it is accumulated in front of the metal surface forming a porous rust layer. In the consecutive cycle a new passive film is formed underneath the rust layer. Additionally, the Fe(II) in the rust layer is oxidized at about –0.72 VCSE. This rust layer does not exhibit

FIGURE 2. Polarization scan of steel in deaerated alkaline solutions (dU/dt = 10 mV/s).

FIGURE 3. Cyclic voltammetry of iron in deaerated 0.1 M NaOH of pH 13 (dU/dt = 10 mV/s).

FIGURE 4. Schematic representation of the processes taking place on steel under AC interference at various DC current densities: (a) high DC current density, (b) very high cathodic DC current density, and (c) low cathodic DC current density.

(a)

(b)

(c)



any protective properties since the passive current density is identical in every cycle. In the consecutive reduction, both the rust layer and the passive film are reduced. The Fe(II) formed during the dissolution of the passive film is added to the rust layer. There-fore, the thickness of the rust layer is increased with every oxidation/reduction cycle. Although the metal surface is changing from immunity to passivity and never passes through an active dissolution range, a metal loss is taking place. This is due to passivation and dissolution of this passive film with every cycle. The amount of iron being oxidized for forming the pas-sive layer is extremely small in every cycle. Yet, con-siderable metal loss can be obtained. Assuming 50 Hz alternating current and the formation of one single oxide layer in every anodic half wave, a metal loss of about 100 mm/y is obtained.

Based on these results, it can be assumed that the iron dissolved during anodic polarization due to AC interference in alkaline solution is not soluble and is therefore accumulated in front of the coupon.20 In soil, this process is even more pronounced, since the hydrodynamic is more limited. Moreover, the cathodic protection current assures that at least parts of the accumulated iron ions are reduced to Fe(II). Therefore, the amount of accumulated corrosion products in front of the coupon can be determined with the so-called coulometric oxidation. The results for coupons exposed to artificial soil solution in quartz sand for seven days under cathodic protection current density of –3 A/m2 and different AC currents are shown in Figure 5. A galvanostatic anodic current load of 1 A/m2 was applied and the potential development was recorded over time. The sample exposed to 100 A/m2 AC current density showed first some increase in potential, which is followed by a plateau. Based on the results in Figure 3, this plateau can be attributed to the oxidation of Fe(II) to Fe(III).20-21 At about 250 C/m2 a steep increase in potential occurs, which stops sharply when the oxygen evolution is reached. With increasing charge the potential stays almost constant. For the coupon exposed to 0 A/m2 AC current density, a similar behavior is observed. The increase of the potential, however, takes place at a significantly smaller charge. No corrosion was observed on the coupon exposed to 0 A/m2 AC current density while significant corrosion was observed on the coupon with 100 A/m2. Therefore, it can be concluded that the charge required for complete oxidation of the rust layer, for formation of the passive film, and for reach-ing of the oxygen evolution in Figure 5 indicates the amount of Fe(II) accumulated in front of the coupon, which is an indicator for the amount of AC corro-sion.3,8-11 Additionally, the experiment shown in Figure 5 clearly shows the presence of a pH value in front of the coupon that is sufficiently high for passivation of steel. This is demonstrated by the fact that the polar-ization of the steel surface up to the oxygen evolution

is possible. Additionally, the increase of the pH value due to the cathodic protection current can be deter-mined by the potential level at which the oxygen evo-lution takes place.8,16 Based on this evaluation, a pH value of 12.9 is reached for both coupons due to cathodic protection during one week.

The systematic application of the coulometric oxi-dation in the control intervals of the cathodic protec-tion of high-pressure pipelines in Switzerland allowed the collection of the amount of oxidation charge data and the comparision to the actual corrosion based on an excavation of the coupons.3,11,15 The results are shown in Figure 6. A certain correlation between the charge density determined in coulometric oxidation and the mass loss determined on the coupons can be found. Based on the obtained results, an oxidation charge of more than 10 C indicates corrosion attack in the range of 1 mm. In some cases at lower charges, however, the amount of corrosion is underestimated based on the coulometric oxidation. This effect is attributed to an incomplete reduction of the corrosion products due to a low cathodic protection current or the loss of corrosion products during operation. The loss of corrosion products is possible, for example, as a result of soil settlement. An increased cathodic polarization of the coupon previous to the coulometric oxidation may be useful to ensure the complete reduc-tion of all corrosion products in front of the coupon and exclude the possible insufficient reduction of cor-rosion products.

During the coulometric oxidation, the Fe(II) in the corrosion products is oxidized to Fe(III). It was found that a subsequent cathodic reduction of the corrosion products allows for reversible reduction of the Fe(III). In field investigations, it was found that the charge determined in coulometric oxidation allows for moni-toring of the corrosion situation over time. In some

FIGURE 5. Coulometric oxidation of coupons polarized for 7 days in artificial soil solutions and quartz sand with –3 A/m2 cathodic DC current density and different AC current densities.



cases, it was possible to demonstrate the progress of corrosion over time or the effectiveness of mitigation measures.3,15

Various authors have observed that the operation level of the cathodic protection has an effect on the AC corrosion rate.12-14,16 To evaluate this effect, labo-ratory investigations were performed in quartz sand flooded with artificial soil solutions. This artificial soil had a resistivity of 17 Ω·m, which corresponds to an extremely conductive soil. From the point of view of conductivity, the artificial soil may be considered to be a worst-case situation. The tests were run at vari-

ous on-potentials of the cathodic protection system and at 14 V and 30 V AC voltage. Furthermore, two different sets of coupons of 1 cm2 were used. Type 1 had a remaining coating of 7 mm thickness simulat-ing a punctuation through the coating and the pro-tective cement layer, while Type 2 was planar with a surface of the surrounding polymer coating simulat-ing an open defect excluding the pore resistance in the coating. Both situations represent extreme cases of coating defects.

In Figure 7, the corrosion rate of all investi-gated coupons is plotted vs. the absolute value of the cathodic DC current density. An increasing abso-lute DC current density results in increased corro-sion rates. It is interesting to note that the difference between the two levels of AC voltage disappears at sufficiently high absolute DC current density. At small cathodic current densities the corrosion rate increases presumably due to an insufficient cathodic protection. However, the increase in corrosion rate at high cathodic current densities is even stronger. This observation is in good agreement with the high cor-rosion rates observed in field applications at AC volt-ages below 10 V and at cathodic current densities more negative than –10 A/m2, which showed high cor-rosion rates.3 All off-potentials of the DC current den-sity more negative than 0.1 A/m2 were in the range from –0.98 VCSE to –1.12 VCSE.

3 No marked difference in the off-potential with respect to the corrosion rate was observed. The effect of corrosion rate compared to the on-potential for the coupon type under the given experimental conditions can be summarized as fol-lows.3 Based on the available data a minimal corro-sion rate is obtained between on-potentials of about –1.2 VCSE and –0.9 VCSE. Apparently, the difference in geometry strongly affects the corrosion rate. The dif-ference between the two types of coupons at nega-tive potentials may be explained with the increase in cathodic protection current due to the difference in spread resistance.

In Figure 8, the corrosion rate is plotted vs. the AC current density. Clearly, an increase in corrosion rate is observed with increased AC current densities. However, even at AC current densities as high as 300 A/m2, corrosion rates below 10 µm/y are observed.

The obtained data clearly demonstrate the influence of both the AC voltage and especially the cathodic operation conditions. The influence of the AC current density is well known and its contribution to the corrosion rate is generally explained with higher anodic polarization during the anodic part of the half wave. In the past, one attempt was to decrease the AC corrosion by increasing the DC current density.22 The higher DC current density decreases the amount of charge that passes the steel/medium interface dur-ing the anodic half wave. If the DC current density is sufficiently high to prevent the passage of any anodic current during the AC cycle, the corrosion process

FIGURE 6. Comparison of the charge determined in coulometric oxidation with the mass loss determined after excavation of the coupons. The thickness loss is calculated based on the charge density assuming a homogeneous corrosion attack on a 1-cm2 coupon surface.

FIGURE 7. Corrosion rate of coupons of type 1 and type 2 at 14 V and 30 V AC plotted vs. the absolute value of the cathodic current density. The squares represent data obtained in 1 M NaOH.



described in Figure 4(a) can be completely stopped according to Figure 4(b). However, to eliminate any anodic current, on-potentials in the range of several volts are required, resulting in significant hydrogen evolution and an increased risk of cathodic disbond-ing of the coating. Based on the obtained results in Figures 7 and 8, an increase in DC current den-sity that does not completely eliminate anodic charge transfer results in a strong acceleration of the corro-sion rate.

Hence, the question arises regarding the negative effect of the cathodic current on corrosion rate. Gen-erally, the decrease of the spread resistance result-ing in a higher AC current density is assumed to be responsible for the increased corrosion rate. Moreover, the increase of the pH value above 14 in front of the metal surface and the formation of iron hydrates is often discussed as possible reasons for the increased corrosion rate.12

In discussing the effect of the pH value it is important to determine the pH value that is obtained on the steel surface under cathodic protection. The results of the pH measurement at various cathodic DC current densities are shown in Figure 9.4,23 The results clearly demonstrate an increase of the pH value over time. At all current densities, the pH value reaches a constant level within a week. While a good agreement between measuring the spread resistance and the potential for oxygen evolution was observed at lower current densities, some problems were encoun-tered in determining the oxygen evolution in the case of the tests at cathodic current densities of –50 A/m2. This was attributed to the high concentration of hydrogen interfering with the measurement. Neverthe-less, it can be concluded that the pH value does not exceed 14, even for cathodic DC current densities that were found to result in very high corrosion rates (Figure 7). The limited increase of the pH in front of the metal surface is explained with the increased migration and diffusion of the hydroxide ions into the bulk electrolyte. As soon as a steady state between formation and migration and diffusion is established, no further increase of the pH will be observed. An increase in current will cause a higher formation rate but will also increase removal of hydroxide ions due to migration. After exposure to cathodic current for one week, the steel samples were passivated by coulomet-ric oxidation. The amount of charge required for pas-sivation was comparable for all current densities. This observation confirms that the pH value is not exceed-ing 14 as observed in Figure 9. Moreover, no corrosion was found on the steel surface, demonstrating that in the absence of AC current the steel is efficiently protected against corrosion even at high DC current densities.

Based on the obtained results, it may be con-cluded that the formation of soluble iron compounds and thus the increase in pH is not primarily respon-

sible for the occurrence of AC corrosion. As a conse-quence, tests were run in quartz sand soaked with 1 M NaOH at pH 14. An AC current density of 500 A/m2 and 1,200 A/m2 was imposed under no or limited cathodic protection current.4,23 Over a test period of 3 months, no significant corrosion was observed. In Figures 7 and 8, the results are rep-resented with squares. In the presence of cathodic current densities more negative than –1 A/m2, high corrosion rates of up to 1 mm/y were found in both 1 M and 0.1 M NaOH. This is in line with results reported by Leutner, et al.,12 and Nielsen, et al.13

This result confirms that the alkaline environ-ment is not primarily responsible for the increased corrosion rate.

FIGURE 8. Corrosion rate of coupons of type 1 and type 2 at 14 V and 30 V AC plotted vs. the AC current density. The squares represent data obtained in 1 M NaOH.

FIGURE 9. Development of pH over time at different cathodic DC current densities. The open symbols were determined based on the measurement of the spread resistance of the sensor.



Another possible reason for the increase in AC corrosion with increasing DC current density could be related to the decrease in spread resistance due to the increased pH at the metal surface. Based on Fig-ures 7 and 8, a high AC current density is observed in combination with an increased DC current density at given AC voltages. Nevertheless, the tests in 1 M NaOH (squares in Figures 7 and 8) did not reveal any AC corrosion even at very high AC current densities. It can be concluded that low or no DC current does not result in AC corrosion at elevated pH even at extremely high AC current densities. Similar results were obtained in cementitious material at elevated pH.24 As a consequence, neither the highly alkaline environment created by the cathodic protection nor the high AC current densities caused by the low spread resistance can primarily be responsible for the increased AC corrosion rate observed at high cathodic DC current densities.

Based on these results, the cathodic protection current must have an inherent effect on the corro-sion process that is not associated with the changes in the conductivity and pH at the metal surface. It is possible, in principle, to eliminate the corrosion pro-cess shown in Figure 4(a) by preventing the dissolu-tion of the once-formed passive film. If no cathodic dissolution (i.e., reduction) of the passive film takes place, no or only limited additional oxidation will take place during the next anodic half wave. This cathodic reduction can be prevented if the charge in the cathodic half wave is consumed without dissolving the passive film. Visually and with coulometric oxidation, a black rust layer is always observed on the steel sur-

face under AC interference in alkaline environments. This is even the case if the corrosion rate is close to zero. According to Figure 3, the Fe(II) in this rust layer can be oxidized at –0.72 VCSE and reduced at –1.1 VCSE. As a consequence, a significant amount of anodic and cathodic charge can be consumed in the rust layer (Figure 6). As long as the charge passing through the metal surface is consumed for changing the oxida-tion state of the rust layer, the passive film remains untouched. In contrast, if the cathodic charge is too large to be consumed for the reduction of the rust layer, the passive film will be electrochemically reduced. The iron released by this (partly) reduction of the passive film will be added to the rust layer. Hence, the amount of iron available for consuming anodic and cathodic charge will increase. As a consequence, the buffer required for consuming the AC current is formed by the system itself. This process explains the lack of AC corrosion in an alkaline environment with-out or at low DC current density, where the entire AC current is consumed by the rust layer without reduc-tive dissolution of the passive film, according to Figure 4(c).4 As soon as an increased DC current density is applied, the capacity of the rust layer to consume the cathodic charge of the AC current without reducing the passive film is lowered or even completely elimi-nated. A high cathodic DC current density, therefore, favors the partial reductive dissolution of the passive film according to Figure 4(a). A higher cathodic DC current results in a more efficient cathodic dissolution of the passive film, in a higher reformation during the anodic half wave, and in a higher corrosion rate.

If this mechanism is correct, the presence of a passive film on the metal surface is required at low cathodic current densities. Intuitively, this is in con-trast to the common understanding of the processes taking place during cathodic protection. The forma-tion of a passive film appears to be difficult under cathodic current flow. However, from the thermody-namic point of view, the presence of a passive film is possible, as can be concluded from the Pourbaix dia-gram shown in Figure 10.19 When a cathodic protec-tion current is imposed on a steel surface in neutral soil, the oxygen is reduced at the metal surface, resulting in a decrease of the potential. The decrease of the potential is limited by the hydrogen evolution. Hydrogen evolution and oxygen reduction cause an increase of pH on the metal surface over time. This increase of the pH value at the metal’s surface allows for the conditions where passivity is thermodynami-cally possible (Figure 9). Additionally, it is impor-tant to point out that the critical cathodic protection potential of –0.85 VCSE is in the passive range of the Pourbaix diagram. Comparing typical off-potentials obtained on pipelines that are generally in a range between –0.9 VCSE and –1.1 VCSE with the polarization scans in Figures 2 and 3 additionally confirms that the presence of a passive film is thermodynamically

FIGURE 10. Pourbaix diagram of iron. Effect of cathodic protection on pH and potential of the steel surface.



favored at low cathodic protection currents. Thus, at sufficiently high pH and low cathodic current density it has to be expected that the steel surface is covered with a passive film. The formation of a passive film is additionally favored by the rectification of the AC current caused by the asymmetry of the anodic and cathodic current-potential dependence (Figure 2). The AC current results in a shift of the potential to more positive potentials (Figure 11).4 An AC current, there-fore, will cause a shift of the potential in the anodic direction, resulting in an enhancement of the oxide film formation. Based on this consideration, the pres-ence of a passive film is likely at low DC current den-sities and high AC current densities.

The discussion of the mechanism of AC corro-sion shows clearly that the DC current favors cathodic polarization and the reductive dissolution while the AC current favors the anodic polarization and the pas-sive film formation. Since AC and DC currents have opposing effects, it has to be assumed that their ratio should be significant for the corrosion process. In Fig-ure 12, the results of extended laboratory investiga-tions in various soils are shown.16 Clearly the ratio of the current densities affects the corrosion behavior. At low values a process is taking place, according to Fig-ure 4(b). At intermediate ratios, the corrosion follows the process in Figure 4(a). At high ratios, the low cor-rosion rates can be explained based on Figure 4(c).

Based on these considerations, the increase of AC corrosion with increasing DC current density could be explained by the increased dissolution of the passive film present on the steel surface. This proposed model can explain the experimental effects observed so far with AC corrosion.

conclusIons

v The electrochemical characterization of steel in an alkaline environment as it is observed under cathodic protection provides a fundamental understanding of the processes involved with the AC corrosion mecha-nism. The corrosion products formed under AC inter-ference can be quantified by means of the coulometric oxidation. v The laboratory investigation of AC corrosion in arti-ficial soil clearly demonstrates the strong effect of the on-potential on the corrosion rate. At high protection current densities obtained at negative on-potentials, the corrosion rate is higher than 0.1 mm/y and the effect of the AC voltage level is negligible. Even at low AC voltage, high corrosion rates may be observed. Hence, the cathodic protection parameters have to be optimized to minimize the AC corrosion rate signifi-cantly. The DC current density must be below 5 A/m2 to significantly decrease the corrosion rate. Presumably even lower current densities are desirable, especially in the case of certain defect geometries that lead to heterogeneous current distribution on the steel sur-

face. In this case, the local current density may be underestimated based on the average current density of the entire defect. v Based on the available data, neither the reduced spread resistance nor the high pH may entirely explain the negative effect of the high DC current den-sity on the corrosion behavior. A mechanism is pro-posed and discussed that is capable of describing the available data. It is known that the AC corrosion can be stopped if the formation of a passive film is entirely prevented by applying very negative on-potentials. Similarly, it can be concluded that AC corrosion may be decreased if the dissolution of the once-formed passive film is prevented. This is possible by decreas-ing the cathodic protection current. The significant AC current densities can be consumed by the rust layer formed on the metal surface, hence preventing

FIGURE 11. Effect of AC current density on the off-potential of steel in 0.01 M NaOH of pH 12.

FIGURE 12. Effect of the ratio between AC and absolute value of the cathodic DC current density on the corrosion rate.



dissolution and reformation of the passive film. The higher the cathodic current density, the more efficient is the cathodic dissolution of the passive film. As a consequence, more oxidation of iron will be required to reform this film in the following anodic half wave. With increasing cathodic current density, the dis-solution of the passive film and therefore the corro-sion rate will be increased since more passive film has to grow to compensate for the cathodically dissolved part.

AcknowlEdgmEnts

This work was possible thanks to the generous support of EON-Ruhrgas AG, Swissgas AG, SBB AG, BFE, BAV, ERI, and Transitgas AG.

rEfErEncEs

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19. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solu-tions (Houston, TX: NACE, 1974), p. 307.

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21. P. Schmucki, M. Büchler, S. Virtanen, H.S. Isaacs, M.P. Ryan, H. Böhni, J. Electrochem. Soc. 146 (1999): p. 2,097.

22. CEN/TS 15280, “Evaluation of AC Corrosion Likelihood of Buried Pipelines. Application to Cathodically Protected Pipelines,” 2006.

23. M. Büchler, C.-H. Voûte, H.-G. Schöneich, “Discussion of the Mechanism of AC Corrosion of Cathodically Protected Pipelines: The Effect of the Cathodic Protection Level,” in CEOCOR Int. Congress (Brussels, Belgium: CEOCOR-VIVAQUA, 2008).

24. H.-G. Schöneich, “Corrosion Protection Aspects of Some Cementi-tious Materials for the Backfilling of Casings,” in CEOCOR Int. Congress (Brussels, Belgium: CEOCOR-VIVAQUA, 2008).

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