Variations of Microenvironments with and without ... - ACS Publications

Aug 14, 2013 - Tangqing Wu , Cheng Sun , Jin Xu , Maocheng Yan , Fucheng Yin , Wei ... Maocheng Yan , Cheng Sun , Jin Xu , Tangqing Wu , Shuang Yang ...
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Variations of Microenvironments with and without SRB for Steel Q 235 under a Simulated Disbonded Coating Jin Xu,* Cheng Sun, Maocheng Yan, and Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metals Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, China ABSTRACT: Distributions of potential and current and variations of crevice environments are studied in a crevice under a simulated disbonded coating in soil extract solutions (SES) with and without sulfate-reducing bacteria (SRB). Results show that forming of the crevice blocks the penetration of cathodic currents in the crevice and weakens the cathodic polarization of the steel Q 235. The cathodic currents in the presence of SRB are nearly three times more than those without SRB at the same cathodic protective potential, which indicates that SRB increases the energy consumption of the cathodic protection systems. SRB leads to basification of the solution in the crevice. The forming of inner electric field enhances the anionic migration into the crevice. The forming of the reversed potential increases the cathodic current.

1. INTRODUCTION Crevice corrosion of buried pipeline steel beneath a disbonded coating has been widely concerned since the 1970s.1−5 Many studies have shown that an effective cathodic protection can be achieved with proper applied potentials and enough polarizing times.6−8 The results by Fessler el al. have shown that there is only a 30−40 mV difference between the holiday and the bottom of the crevice after about 3 mouths at an applied potential of −850 mV (SCE).9 Peterson and Lennox have concluded that the cathodic protection can be achieved for the crevices with a distance to crevice opening ratio of 12000:1.10 The studies by Gan et al. have shown that the local potentials of the steel X65 in the crevice are more negative than the potential of −850 mVvsCSE.11 Li et al. have found that the NACE standard of the cathodic protection (−0.776 VvsSCE) can be met under the disbonded coating when the control potential is applied to −1.076 V. However, other studies have found that the steel cannot be efficiently protected at any applied potential.12 The results from Yan et al. have shown that the potential of the steel in the crevice does not reach the standard of the cathodic protection.13,14 Eslami et al. have found that the steel at the bottom of the crevice is not polarized, and the potential is the same as the free potential.15 The crevice environmental variations have also been investigated under the disbonded coatings. Some studies have shown that the pH of the solutions in the crevice is influenced by the applied potentials and the polarizing times. Jack et al. have found that the pH increases from 7.20 to 7.49 in the crevice after polarizing for 1 month.16 The results of Chen et al. have shown that the pH increases with time in the absence of the applied potential and reaches above 8.0 at the distance of 25 cm away from the holiday.17 Lara and Klechka have found that the pH has a relation with the applied potential in the crevice, and the pH value reaches 11 at the applied potential of −950 mV vsCSE.18 The studies by Perdomo and Song have shown that the concentrations of the dissolved oxygen decrease with time.19 Diakow et al. have found that the redox potential decreases with increasing of the distance.20 © 2013 American Chemical Society

The above studies have mainly investigated the corrosion under the disbonded coating in the absence of sulfate-reducing bacteria (SRB). As we all know, microbiologically influenced corrosion (MIC) of metals has been widely found in many environments, e.g., soils around buried pipelines, mine, seaport, lake, power plants, sewage treatment systems, and oil wells, and SRB are the most important microbes for anaerobic corrosion of the steel in soils. However, few studies are reported on the variations of the microenvironments in the presence of SRB. In this paper, the distributions of the cathodic potentials and currents and variations of crevice environment are investigated in the crevice under the disbonded coating in a soil extract solution (SES), and the effects of SRB on the distributions of the local potential and current and crevice environmental variations are also discussed.

2. EXPERIMENTAL SECTION 2.1. Simulated Crevice Cell. All the experiments were performed in an electrochemical cell (Figure 1) designed to simulate a crevice under a disbonded coating. A rectangular crevice with 500 × 100 mm in dimensions was formed by bolting a polytetrafluoroethylene (PTFE) gasket between two polymethylmethacrylate (PMMA) plates. The thickness of the crevice was 1.0 mm in this research. A cuboid (150 mm in length, 100 mm in width, and 150 mm in height) was adhered to the top plate to serve as the bulk solution container. The dimensions of the holiday were 150 × 100 mm. Six small electrodes and one big electrode of carbon structural steel were sealed in the bottom plate along the crevice length (Figure 1(a)). The small electrodes were used for measuring of cathodic currents, and the big one was used for working electrodes. A BNC plug was connected into the parallel conducting wire of every small steel electrode so that a Received: Revised: Accepted: Published: 12838

December 3, 2012 July 3, 2013 August 14, 2013 August 14, 2013 dx.doi.org/10.1021/ie303335n | Ind. Eng. Chem. Res. 2013, 52, 12838−12845

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Figure 1. Schematic of the electrochemical cell simulating a crevice under a disbonded coating.

KEITHLEY 6485 picoammeter can easily measure the current (Figure 1(b)). Five holes for placement of salt bridges were drilled through the top plate at 100, 200, 300, 400, and 500 mm from the holiday of the crevice (Figure 1(c)), and SES instead of the NaCl solution was filled in salt bridges in order to decrease the influence of chloride ions. A piece of stainless steel 316 was used as a counter electrode of cathodic protection. The cathodic protection (CP) potential of −0.976 VvsSCE was applied at the holiday. 2.2. Preparation of the Combination Electrode. A composite electrode (as shown in Figure 1(c)) was constructed based on a W/WO3 pH electrode, a chloride ion-selective electrode, and a redox electrode. 2.2.1. pH Electrode. The pH electrode, made from a tungsten wire (99.99%), was fabricated by using the cyclic voltammetry method.21 The pH sensing characteristics were evaluated in a series of pH solutions by measuring their open circuit potentials against a saturated calomel electrode (SCE). Commercial buffer solutions used here involved potassium hydrogen phthalate (pH 4.00), a mixture of potassium dihydrogen phosphate and disodium hydrogen phosphate (pH 6.86), and sodium tetraborate (pH 9.18). To enlarge the range of pH, the buffer solutions mentioned above were used as a background solution. The pH of the pH 4.00 solution was adjusted to pH 2 by adding 1 M HCl solution, and the pH of

the pH 9.18 solution was adjusted to pH 12 by adding 1 M NaOH solution. The pH of the buffer solutions was monitored with a commercial combined glass electrode (the model number is pHB-4). Figure 2 shows the potential versus SCE at standard solutions with different pH values. It proved that the pH electrode offered a good linear response for pH over a wide concentration range with a slope of 41 mV/pH unit.

Figure 2. Potential response of the chloride ion electrode in the standard solutions. 12839

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the SES is shown in Table 1. The SES was autoclaved at 121 °C for 20 min and stored at 4 °C before use. SRB, Desulfovibrio desulfuricans, used in this study was isolated from soils in order to investigate MIC of the steel in soil environments.23 In order to eliminate the experimental media from the disturbances of SO42‑ and Fe2+ in traditional cultures of SRB, they were anaerobically incubated in the beefprotein medium (g/L) containing 0.5% beef extract, 1% protein, and 0.5% NaCl. The seed bacteria were isolated from the soil at Shenyang, China, and then inoculated in the beef-protein medium and incubated for 2 days. The inoculated culture (300 mL) and sterilized culture (300 mL) were subsequently transferred into the sterilized SES (1700 mL) in the anaerobic box before the experiment, respectively. SRB numbers were determined by the three tube multiple most probable number (MPN) method according to the American Society of Testing Materials (ASTM) Standard D 4412-84. 2.5. Sulfate Ion Concentration and Electrolytic Conductivity. Samples of the SES were extracted in the bulk solution after different durations and in the crevice after the finish of the experiment by using a sterile syringe for the purpose of the measurement of sulfate ion and electrolytic conductivity. The sulfate ion concentrations were measured using the UV spectrophotometric method, and the conductivity was measured using a conductometer (DDS-307A).

2.2.2. Chloride Ion Electrode. To prepare the chloride ionselective electrode, a silver wire (99.95%) was anodized for 30 min in a 0.1 M HCl solution at a current density of 0.4 mA/cm2 by using a PARSTAT2273 potentiostat. The electromotive force of the chloride ion electrode was measured as a function of chloride ion concentration, [Cl−], in NaCl solution. Figure 3 shows the potential versus SCE at

Figure 3. Potential response of the chloride ion electrode in the standard solutions.

different [Cl−] varying from 0.14 to 84.5 mM. It proved that the chloride ion electrode offered a good linear response for log[Cl−] over a wide concentration range with slope of 54 mV/ log[Cl−] unit. 2.2.3. Redox Electrode. The redox potential was measured by using a redox potential between a Pt electrode and a reference electrode (SCE). The redox electrode was made of a platinum wire (99.99%). In order to make a comparison at different conditions, the redox potential was transformed by the following relationship Eredox = Em + ESCE + 60 × (pH‐7)

3. RESULTS AND DISCUSSION 3.1. SRB Numbers and Sulfate Concentration. Figure 4 shows SRB numbers and sulfate ion concentrations with time in

(1)

where Eredox = redox potential at pH = 7 (mV, standard hydrogen electrode), and Em = the potential measured from the platinum electrodes (mV).20,22 2.3. Materials and Sample Preparation. Six small (15 × 15 × 3 mm) electrodes and one big (30 × 550 × 3 mm) working electrode (Figure 1(c)) were cut from a carbon structural steel plate with the nominal composition (wt %) of 0.30C, 0.019P, 0.029S, 0.01Si, 0.42Mn, and balance Fe. Six counter electrodes of 30 × 30 × 2 mm dimensions were cut from a plate of stainless steel 316L with the nominal composition (wt %) of 17.82Cr, 12.93Ni, 2.19Mo, 0.06C, 0.037P, 0.029%S, 0.35Si, 1.69Mn, and balance Fe. The coupons were abraded with a series of grit papers (200, 400, 600, 800) followed by cleaning in acetone and alcohol and dried. All electrodes were sterilized under ultraviolet rays for 30 min prior to the experiment. 2.4. Soil Extract Solution Preparation and SRB Culture. The soil used in this work was collected at Shenyang, China. The SES was prepared by extracting the soil solution with the water-soil ratio of 1:1. The chemical composition of

Figure 4. Variations of SRB numbers and sulfate ion concentrations with time at the potential of −0.976 mVvsSCE in the bulk solution.

bulk solution with SRB. SRB numbers increase from 7 Mcfu/ mL on the first day to 250 Mcfu/mL after 16 days and then decrease to 40 Mcfu/mL after 30 days. It shows that SRB grows well in the whole period. The sulfate ion concentrations decrease from 159.6 mg/L on the first day to 21.9 mg/L after 16 days. Figure 5 shows SRB numbers and sulfate ion concentrations with distance in the crevice after 32 days. SRB numbers sharply increase with distance, reaching a peak at the distance of 100 mm, and then dramatically decrease. The

Table 1. Compositions of the Soil Extract Solution (mg/L) main anion composition soil type

pH

Cl−

SO42‑

organic content

whole nitrogen content

total salt content

meadow soil

6.8 ± 0.1

127 ± 1

296 ± 2

23300 ± 100

1170 ± 10

680 ± 5

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increase with distance in the crevice in both conditions and then slowly decrease. The electric conductivities are higher in the crevice than those at the holiday. The conductivity is also higher in the SES with SRB than that without SRB. SRB accelerate cathodic depolarization of the steel, which leads to increasing of ion concentrations of the SES in the crevice. They indicate that the increasing of the polarizing times and SRB both increase the electric conductivities of the SES. 3.3. Local Potential and Current Distributions. Figure 8 and Figure 9 show the local potential and cathodic current Figure 5. Variations of SRB numbers and sulfate ion concentrations with distance at the potential of −0.976 mVvsSCE in the crevice after 32 days.

sulfate ion concentrations increase with distance and then decrease. The results mentioned above show that the cathodic depolarization induced by SRB leads to the decreasing of the sulfate ion concentrations. 3.2. Electric Conductivity. Figure 6 shows variations of electric conductivities with time in the bulk solution. The

Figure 8. Local potential distributions with time and distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice.

Figure 6. Variation of electric conductivity with time in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the bulk solution.

electric conductivities of the solution increase with time in the SES with and without SRB. The conductivity is higher in the presence of SRB than that without SRB. Figure 7 shows variations of electric conductivities with distance in the crevice after 32 days. The electric conductivities of the solution Figure 9. Cathodic current distributions with time and distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice.

distributions in the crevice under the disbonded coating in the SES with and without SRB. The potentials shift in the negative direction with time and remain stable. The potentials shift in the positive direction with the increasing of the distance away from the holiday in the crevice. The potentials are more negative in the crevice with SRB than those without SRB. The potential differences between the holiday and the bottom of the crevice are 222.9 mV in the SES with SRB and 263.2 mV without SRB after one day but only 50.4 mV in the presence of SRB and 90.8 mV in the absence of SRB after 32 days, respectively. The results of the potential distributions show that the polarizing level of the steel is lower in the crevice in the initiate period, but the steel can be efficiently protected in the later period.

Figure 7. Variation of electric conductivity with distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice after 32 days. 12841

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Table 2. Cathodic Currents and Reversed Currents between Electrodes in the Crevice before and after Removing the Applied Potential after 32 Days SRB with without

applied potential turn turn turn turn

on off on off

distance (mm)

0

100

200

300

400

500

current (μA)

−106 113 −43.2 38.2

−3.17 3.24 −2.02 1.96

−1.42 1.32 −1.00 0.93

−1.30 1.26 −0.65 0.36

−0.94 0.93 −0.41 0.23

−0.67 0.69 −0.26 0.11

The cathodic currents increase with time and then remain stable (as shown in Figure 9). With distance away from the holiday increasing, the cathodic currents sharply decrease between 0 mm and 100 mm and slowly decrease when the distance is more than 100 mm. The ratios of cathodic currents at the holiday at the distance of 100 mm are 33.58 in the SES with SRB and 21.33 without SRB after 32 days, which indicates that the crevice blocks the penetration of cathodic current, and SRB enhance the hindering effects. The cathodic currents are much larger in the SES with SRB than those without SRB. The cathodic current is 106.47 μA at the holiday in the presence of SRB after 32 days and 43.16 μA without SRB. The cathodic currents with SRB are nearly three times more than those without SRB at the same cathodic protective potential. Because the energy consumption of the cathodic protection systems has a positive relationship with the cathodic currents, it indicates that SRB can increase the energy consumption of the cathodic protection systems. This result is accorded with the results by Jack et al.16 The distributions of the local potential and the cathodic current in the crevice are controlled by two factors, solution resistances and surface states of electrodes. Studies have shown that IR drops can weaken the polarization of the steels in the crevice.6,11,25 However, the effects of the interface variations of the electrodes on the potential and current distributions are seldom reported. Distributions of the local potential and the cathodic current will be explained by the variations of electrical double layer (EDL) of the electrodes. The positive and negative charges are equal on either side of the EDL when the electrode system reaches an equilibrium state. There are excess electrons on the metal side of the EDL when a cathodic potential is applied to the electrode. In order to reach a new equilibrium state, the corresponding cations are needed on the side of the solution. Because the supplying rate of the electrons is much faster on the side of the metal than that of the cations on the side of the solution, the charge balance of the EDL is controlled by the cation transfer rate. The electrode quickly reaches the equilibrium state in the bulk solution because there is no hindering for the cationic transfer. However, the electrode in the crevice needs to spend much more time on reaching the equilibrium state because the cationic transfer is limited by the crevice shape. With the increasing of the distance away from the holiday, the equilibrium time increases. It leads to the positive shift of the local potential in the crevice with the crevice deepness increasing. However, with the cations gradually closing to saturation in the crevice, it is more difficult for new cations to further transfer into the crevice due to the repulsion among cations. Just then, the cations numbers are still not enough for the cationic demanding of the EDL of electrodes, which causes the steels not to meet the potential as negative as the CP potential in the crevice. The above reasons lead to the results that the potentials in the crevice are much higher than those at the holiday in the initiate period and shift in the negative direction with time, but

the potentials are still higher in the crevice than the applied potential. SRB and extracellular polymeric substance (EPS) in biofilms are negative,26−28 which increases the carrying capacity of the cations of the solution in the crevice, and indirectly increase the cationic numbers on the side of the solution of the EDL of the electrodes in the crevice. It leads to negatively shifting of the potentials of the steels in the crevice compared with the conditions without SRB. The presence of SRB also increases the conductivity of the crevice solution because of the increasing of the ionic concentrations (Figure 6 and Figure 7), which shortens the polarizing time and lowers the potential of the steels. With accumulating of the cations in the crevice, these ions cannot easily migrate outward because of the shape limiting of the crevice, and then the concentration gradient is formed in the crevice. The gradient causes a current flowing of the cations from the deeper parts of the crevice toward the holiday, which is equivalent to a reversed potential. In order to verify the existence of the reversed currents, it is monitored by shutting off the applied power after the finish of the experiment. The currents before and after removing the applied potential are given in Table 2, and a negative sign represents the cathodic current. There are reversed currents between two electrodes after turning off the applied power supply. The reversed current is bigger in the SES with SRB than that without SRB at the same distance in the crevice. SRB increases the carrying capacity of the cations in the crevice because SRB are negative charged, which leads to the increasing of the cationic concentrations, the inner currents, and the applied cathodic current in the presence of SRB. The results mentioned above indicate that the reversed currents and SRB both increase the applied cathodic current of the steels in the crevice. 3.4. Redox Potential. Variations of redox potentials in the crevice under the disbonded coatings are given in Figure 10. The redox potentials decrease with time in the crevice. The

Figure 10. Variation of redox potential with time and distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice. 12842

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redox potentials are more negative in the SES with SRB than those without SRB. The potentials decrease to about −150 mVvsSHE in the presence of SRB after 32 days and about 100 mVvsSHE in the absence of SRB. The decreasing of the potential with SRB is due to the formation of reducing metabolites, e.g., FeS and H2S,20,24 and the decreasing of oxidizing substances is due to oxygen consuming during the process of the cathodic reaction. The decreasing of the redox potential in the absence of SRB is mainly due to the consuming of oxygen. Diakow et al.20 have found that the solution can be considered as an oxygen-free condition when the redox potential is lower than 100 mVvsSHE. 3.5. pH. Figure 11 shows variations of pH values with time in the crevice under the disbonded coating. The pH values keep

From the equation of cathodic reaction, hydroxyl ion concentrations will increase with time, which leads to increasing of the pH value. OH− migrates toward the inner direction of the crevice under the effect of the inner electric field. It results in increasing of the OH− concentrations in the crevice. The migration rates of the anions become slower and slower with the deepness of the crevice increasing because of the limitation of the crevice shape. The above reasons lead that the pH value increases with time in the crevice, and with the distance away from the holiday increasing, especially in the later period of the experiment. There are some substances with buffer abilities, e.g., soil colloid particles and amphoteric organic compounds, in the neutral and alkaline soils. The pH values remain stable as long as OH− concentrations are in the buffering range (as shown in Figure 11 without SRB). However, SRB can enhance the cathodic depolarization of the steel in the crevice by the following reaction. 4Fe + SO24 ‐ + 4H 2O → 3Fe(OH)2 + FeS + 2OH‐

(4)

The reaction mentioned above further increases the OH− concentrations, which leads to a dramatic increasing of pH values (Figure 10 with SRB after 32 days) once OH− concentrations exceed the buffering range. 3.6. Chloride Ions. Variations of chloride ion concentrations with time in the crevice under the disbonded coating are given in Figure 12. The chloride ion concentrations slowly Figure 11. Variation of pH values with time and distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice.

stable, about 6.5, in the SES with SRB before 16 days, and increase with time. The pH value reaches 8.93 at the distance of 500 mm away from the holiday after 32 days. It is in accordance with the result by Brousseau and Qian.29 The pH values remain stable, about 6.5, in the SES without SRB in the whole experiment. The pH values in the presence of SRB are higher in the crevice than those at the holiday, which indicates that SRB enhance the basification of the crevice solution at the applied potential of −0.976 V vs SCE. The polarizing potential shifts in the positive direction with the distance away from the holiday increasing because of the electric shielding of the coatings against the cathodic currents, which results in formation of the inner electric field from the bottom to the holiday in the crevice. The anions migrate from the holiday toward the interior of the crevice under the action of the electric field. The concentration gradient of the anions is formed between in the bulk solution and at the holiday because of the decreasing of the anionic concentrations at the holiday, and the gradient lets the anions enter into the crevice. It leads that the anionic concentrations are much higher in the crevice than those at the holiday. The corrosion reaction of the carbon steel occurred in the neutral and alkaline mediums as follows: Anodic reaction: Fe → Fe2 + + 2e−

Figure 12. Variation of chloride ion concentration with time and distance in the presence and absence of SRB at the potential of −0.976 mVvsSCE in the crevice.

decrease with time at the holiday and are both lower than 4.8 mM in the presence and absence of SRB at the holiday after 32 days. However, the chloride ion concentrations increase with time in the crevice, and the maximum value reaches nearly 56 mM after 32 days. Two increasing trends of the chloride ions are observed. The chloride ion concentrations slowly increase with time in the presence of SRB and then dramatically increase, while the concentrations sharply increase with time in the absence of SRB and then slowly increase. The changing trends of the Cl− concentrations in the crevice are the same as those of the OH−. However, the increments of the Cl− concentrations in the crevice stem from the migration of chloride ions of the bulk solution, which are different from the hydroxide ions. The chloride ions at the holiday migrate toward the inner side of the crevice under the action of the inner electric field (Figure 12). At the same time, the chloride ions in the bulk solution migrate into the crevice under the

(2)

Cathodic reaction: O2 + H 2O + 4e− → 4OH−, 2H 2O + 2e− → H 2 + 2OH−

(3) 12843

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concentration gradient because of the decreasing of the Cl− concentrations at the holiday. The difference of the variations is mainly due to the fact that SRB and EPS are negative charged.26−28 In the earlier period of the experiment, there are numerous SRB in the crevice in the presence of SRB, which inhibit the migration of the chloride ions toward the crevice by the anionic repulsion force. In the later period, the cationic concentrations increase with time in the crevice due to electrostatic attractions, and the repulsion among the anions becomes weaker and weaker. However, in the absence of SRB, the Cl− ion concentrations quickly increase with time by the inner electric field and the concentration gradient in the initiate period. The repulsion force among the anions becomes stronger and stronger with the Cl− ion concentrations further increasing, which leads to the results that the increasing rates of the Cl− concentrations become slower.

(6) Song, Y. Q.; Du, C. W.; Li, X. G. Electrochemical corrosion behavior of carbon steel with bulk coating holidays. J. Univ. Sci. Technol. Beijing 2006, 13, 37. (7) Chen, X.; Li, X. G.; Du, C. W.; Cheng, Y. F. Effect of cathodic protection on corrosion of pipeline steel under disbonded coating. Corros. Sci. 2009, 51, 2242. (8) Chen, X.; Du, C. W.; Li, X. G.; He, C.; Liang, P.; Lu, L. Effects of solution environments under disbonded coatings on the corrosion behaviors of X70 pipeline steel in acidic soils. Int. J. Miner., Metall. Mater. 2009, 16, 525. (9) Fessler, R. R.; Markworth, A. J.; Parkins, R. N. Cathodic protection levels under disbonded coating. Corrosion 1983, 39, 20. (10) Peterson, M. H.; Lennox, T. J. A study of cathodic polarization and pH changes in metal crevices. Corrosion 1973, 29, 406. (11) Gan, F.; Sun, Z. W.; Sabde, G.; Chin, D. T. Cathodic protection to mitigate external corrosion of underground steel pipe beneath disbonded coating. Corrosion 1994, 50, 804. (12) Li, Z. F.; Gan, F. X.; Mao, X. H. A study on cathodic protection against crevice corrosion in dilute NaCl solutions. Corros. Sci. 2002, 44, 689. (13) Yan, M. C.; Wang, J. Q.; Han, E. H.; Ke, W. Local environment under simulated disbonded coating on steel pipelines in soil solution. Corros. Sci. 2008, 50, 1331. (14) Yan, M. C.; Wang, J. Q.; Han, E. H.; Ke, W. Electrochemical measurements using combination microelectrode in crevice simulating disboned of pipeline coatings under cathodic protection. Corros. Eng. Sci. Technol. 2007, 42, 42. (15) Eslami, A.; Fang, B.; Kania, R.; Worthingham, B.; Been, J.; Eadie, R.; Chen, W. Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment. Corros. Sci. 2010, 52, 3750. (16) Jack, T. R.; Van Boven, G.; Wilmott, M.; Sutherby, R. L.; Worthinghan, R. G. Cathodic protection potential penetration under disbonded pipeline coating. Mater. Perform. 1994, 33, 17. (17) Chen, X.; Du, C. W.; Li, X. G.; Huang, Y. Z. Effects of cathodic potential on the local electrochemical environment under a disbonded coating. J. Appl. Electrochem. 2009, 39, 697. (18) Lara, P. F.; Klechka, E. Corrosion mitigation under disbonded coating. Mater. Perform. 1999, 38, 30. (19) Perdomo, J. J.; Song, I. H. Cathodically protecting underground asphalt enamel coated pipes. Corros. Rev. 2000, 18, 221. (20) Diakow, D. A.; Van Boven, G. J.; Wilmott, M. J. Polarization under disbonded coatings: conventional and pulsed cathodic protection compared. Mater. Perform. 1998, 37, 17. (21) Yamamoto, K.; Shi, G. Y.; Zhou, T. S.; Xu, F.; Zhu, M.; Liu, M.; Kato, T.; Jin, J. Y.; Jin, L. T. Solid-state pH ultramicrosensor based on a tungstic oxide film fabricated on a tungsten nanoelectrode and its application to the study of endothelial cells. Anal. Chim. Acta 2003, 480, 109. (22) Booth, G. H.; Cooper, A. W.; Cooper, P. M.; Wakerley, D. S. Criteria of soil aggressiveness towards buried metals. I. Experimental methods. Br. Corros. J. 1967, 2, 104. (23) Sun, C.; Xu, J.; Wang, F. H. Interaction of sulfate-reducing bacteria and carbon steel Q 235 in biofilm. Ind. Eng. Chem. Res. 2011, 50, 12797. (24) Li, F. S.; An, M. Z.; Liu, G. Z.; Duan, D. X. Roles of sulfurcontaining metabolites by SRB in accelerating corrosion of carbon steel. Chin. J. Inorg. Chem. 2009, 25, 13. (25) Perdomo, J. J.; Song, I. Chemical and electrochemical conditions on steel under disbonded coatings: the effect of applied potential, solution resistivity, crevice thickness and holiday size. Corros. Sci. 2000, 42, 1389. (26) Zuo, R. J.; Kus, E.; Mansfeld, F.; Wood, T. K. The importance of live biofilms in corrosion protection. Corros. Sci. 2005, 47, 279. (27) Sheng, X. X.; Ting, Y. P.; Pehkonen, S. O. The influence of sulphate-reducing bacteria biofilm on the corrosion of stainless steel AISI 316. Corros. Sci. 2007, 49, 2159.

5. CONCLUSION The cathodic polarization of the steel is seriously influenced by the migrating rates of the cations onto the interface of the electrode in the crevice under the disbonded coating. The inner electric field lets the anionic ions migrate toward the interior of the crevice, which leads to the increasing of the pH values and Cl− concentrations in the crevice. The negative charging of the SRB and EPS increases the carrying capacity of the cations in the crevice. The cationic increasing results in the electric double layer of the steel quickly reaching the charge balance. SRB increases the cathodic currents of the steel in the crevice under the disbonded coating.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+86) 24 2391 5867. Fax (+86) 24 2391 5867. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The project was financially supported by the National Natural Science Foundation of China (Grant No. 51131001), the National R&D Infrastructure and Facility Development Program of China (Grant No. 2060503), and the Doctor startup fund of Liaoning Province of China (Grant No. 20131122).

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Industrial & Engineering Chemistry Research

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dx.doi.org/10.1021/ie303335n | Ind. Eng. Chem. Res. 2013, 52, 12838−12845