Corrosion Behavior of Type 304 Stainless Steel in a Simulated

3008

Ind. Eng. Chem. Res. 2008, 47, 3008-3020

Corrosion Behavior of Type 304 Stainless Steel in a Simulated Seawater-Based Medium in the Presence and Absence of Aerobic Pseudomonas NCIMB 2021 Bacteria S. J. Yuan,† S. O. Pehkonen,‡ Y. P. Ting,† E. T. Kang,*,† and K. G. Neoh† Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, Singapore, and DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, 9 Engineering DriVe 1, Singapore 117576, Singapore

A comparative study of the corrosion behavior of type 304 stainless steel (304 SS) in a nutrient-rich simulated seawater-based medium in the presence and absence of a marine aerobic Pseudomonas NCIMB 2021 bacterium was carried out. Electrochemical studies (including Tafel plots and electrochemical impedance spectroscopy) revealed that 304 SS underwent different corrosion processes under the attack of Pseudomonas bacteria. In the absence of the Pseudomonas bacterium, the passivating film on 304 SS remained relatively stable, and the anodic reaction was under diffusion control. Colonization of the Pseudomonas biofilm on the coupon surface led to the acceleration of corrosion rates and a dramatic decrease in resistance of passivating film due to localized breakdown of the film. Scanning electron microscopy (SEM) results revealed an increase in heterogeneity and coverage of the biofilms of Pseudomonas bacterium with exposure time. Extensive micropitting corrosion underneath the biofilms was also observed, in comparison to minor pitting corrosion observed on the control coupon surface. Energy dispersive X-ray spectroscopy (EDX) analysis showed enrichment of carbon, oxygen, and chlorine and depletion of the metallic elements, iron, chromium, and nickel in the pitted regions, indicating that pitting corrosion was caused by the synergistic effect of aggressive chloride anions, colonized bacterial cells, and extracellular polymeric substances (EPS). 1. Introduction Type 304 stainless steel, or 304 SS, exhibits excellent corrosion resistance in aquatic environments due to the formation of a thin, compact, and chromium-enriched oxide film.1,2 It has therefore been widely used in industrial cooling water systems and for marine applications, particularly in power plants and in offshore industries. Nevertheless, 304 SS, as a low-grade austenitic stainless steel, is particularly susceptible to microbiologically influenced corrosion (MIC).1 The presence and activities of microorganisms can alter the structure of inorganic passive layers and increase their dissolution and removal from the metal surface. Generally, microorganisms attach themselves to the surface of materials, colonize, proliferate, and finally form biofilms.3 Chemical changes at the metal/biofilm interface, such as localized high concentration of the electrolyte constituents, lowering of pH due to the secretion of acidic metabolites, localized depletion of oxygen as a result of microbial respiration within the biofilm, and selective dissolution of alloying elements can occur.4-6 These changes may have effects ranging from facilitating or impeding anodic and cathodic reactions of the corrosion process to the induction of localized corrosion.7 The MIC of stainless steel has been manifested in many forms of localized corrosion, including pitting, crevice corrosion, under deposit corrosion, and stress corrosion cracking.8 Excellent works on the involvement and contribution of microorganisms, in particular the anaerobic sulfate-reducing bacteria (SRB), in the corrosion processes of stainless steel have been extensively reported.9-13 Relatively fewer studies have been devoted to the MIC of stainless steel by aerobic bacteria. * To whom correspondence should be addressed. Tel.: +6565162189. Fax: +65-67791936. E-mail address: [email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Division of Environmental Science and Engineering.

Figure 1. Tafel plots of 304 SS coupons in the sterile nutrient-rich medium after (a) short-term exposure of 7, 14, 21, and 35 days, and (b) long-term exposure of 49, 63, and 77 days.

In recent years, there is an increasing awareness of the complexity of the MIC process.1,7 MIC is rarely linked to a unique mechanism or to a single species of microorganisms. A

10.1021/ie071536x CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3009 Table 1. Analysis Parameters of Tafel Plots of 304 SS Coupons in the Sterile Medium after Different Exposure Periods exposure time βca βab (days) (mV‚decade-1) (mV‚decade-1) 7 14 21 35 49 63 77

-197 -150 -168 -143 -150 -187 -168

377 322 314 305 318 331 389

Ecorrc (V) -0.241 -0.203 -0.207 -0.202 -0.206 -0.197 -0.196

corrosion icorr rate (µA‚cm-2) (mm‚y-1) 2.08 1.38 1.36 1.38 1.29 1.63 1.77

0.0215 0.0142 0.0141 0.0143 0.0140 0.0168 0.0183

a β is the Tafel slope of cathodic polarization curve. b β is the Tafel c a slope of anodic polarization curve. c Ecorr refers to the potential, where the current reaches zero under polarization.

Table 2. Analysis Parameters of Tafel Plots of 304 SS Coupons in the Pseudomonas-Inoculated Medium after Different Exposure Periods exposure time βc βa (days) (mV‚decade-1) (mV‚decade-1) 7 14 21 35 49 63 77

Figure 2. Tafel plots of 304 SS coupons in the Pseudomonas-inoculated medium after (a) short-term exposure of 7, 14, 21, and 35 days, and (b) long-term exposure of 49, 63, and 77 days.

wide variety of microorganisms, including aerobic bacteria, anaerobic bacteria and fungi, has been implicated in MIC of various metals.14 Pseudomonas bacterial species, most prevalent in industrial water and seawater environments, have been found to be involved in the corrosion of mild steel, stainless steel, and aluminum alloys in numerous marine habitats.15-25 Initially, aerobic Pseudomonas strains were recognized to be the pioneer colonizer in the process of bioiflm formation, and their primary role was to create an oxygen-free environment to harbor the SRB. However, it is subsequently found that these strains are aerobic slime-formers. They often grow in patchy distribution over the metal surface via excreting copious amounts of slimes (extracellular polymeric substances). The slime impedes oxygen diffusion, creating an oxygen concentration cell.1 Various Pseudomonas strains isolates have also been shown to reduce Fe(III) to Fe(II), rendering steel to further oxidation because Fe(II) is more soluble and the protective Fe(III) layer has been solubilized in the initial process.15 Several fine studies have been devoted to elucidating the contribution of Pseudomonas to the corrosion process.17-27 Nevertheless, the long-term effect of the aerobic Pseudomonas bacteria and their biofilms on MIC of stainless steel has yet to be studied in detail. In our previous study,28 the influence of biofilm of a marine aerobic Pseudomonas NCIMB 2021 bacterium on the passive layer of 304 SS was studied by atomic force microscopy and X-ray photoelectron spectroscopy. Extensive pitting corrosion occurs underneath the Pseudomonas biofilm, and the depth of pits increases linearly with exposure time. Moreover, the outermost surface undergoes a subtle change in chemical composition, with the enrichment in chromium and the depletion in iron concentration. In the present study, the role of Pseudomonas bacterium in the corrosion behavior of 304 SS as a function of exposure time (including short-term and long-term exposures)

-301 -317 -293 -312 -295 -301 -310

311 287 301 282 306 286 272

Ecorr (V) -0.224 -0.236 -0.239 -0.248 -0.244 -0.254 -0.262

corrosion icorr rate -2 (µA‚cm ) (mm‚y-1) 2.58 3.24 3.27 3.62 5.45 6.92 7.38

0.0267 0.0335 0.0339 0.0375 0.0564 0.0716 0.0765

is determined quantitatively by electrochemical studies, including Tafel plots, cyclic polarization curves, and electrochemical impedance spectroscopy (EIS). To understand the initiation of pitting corrosion underneath the Pseudomonas biofilm, scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDX), is also used to monitor the development of biofilms and to assess the degree of corrosion damage after removal of the biofilms at a predetermined exposure time. For comparison purposes, electrochemical analyses and SEMEDX measurements are also carried out on the control specimens in the absence of bacteria. 2. Experimental Section 2.1. Preparation of Metal Samples. The stainless steel specimens were purchased from Metal Samples Company (Alabama). The nominal elemental composition (wt %) of the 304 SS sample was: Fe 71.376%, Ni 8.18%, C 0.053%, Cr 18.08%, Cu 0.06%, Mn 1.68%, Mo 0.05%, N 0.047%, P 0.037%, S 0.007%, and Si 0.43%. Disk-shaped specimens with a diameter of 15 mm and thickness of 3 mm were used for electrochemical measurements. Square specimens with the dimension of 10 × 10 × 3 mm3 were used for SEM-EDX analysis. Both specimens were cut from the original plate samples with the dimension of 100 × 50 × 3 mm3. Prior to the experiments, each specimen was sequentially ground with a series of silicon carbide grit papers (180, 500, 800, and 1200 Mesh) to a smooth surface, rinsed with sterile deionized water thrice, degreased in acetone, followed by sterilizing in 70% ethanol for 8 h, and then dried aseptically in a laminar flow cabinet. The newly prepared specimens were immediately immersed in the test medium for all of the corrosion experiments. 2.2. Medium and Inoculum Cultivation. All of the tests were conducted using a nutrient-rich simulated seawater-based medium. According to Burkhoder’s formulation B,29 the medium consists of 23.476 g/L NaCl, 3.917 g/L Na2SO4, 0.192 g/L NaHCO3, 0.664 g/L KCl, 0.096 g/L KBr, 10.61 g/L MgCl2‚

3010

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008

Figure 3. Impedance spectra of 304 SS coupons recorded at the OCP in the sterile nutrient-rich medium after short-term exposure (Ia, Ib, and Ic) of 7 days (0), 14 days (O), 21 days (4), and 35 days (3), and long-term exposure (IIa, IIb, and IIc) of 49 days (]), 63 days ( ) and 77 days ( ). Solid lines represent the fitted results based on the equivalent circuit a. (a) Nyquist plots, (b) Bode magnitude plots, and (c) Bode phase angle plots.

6H2O, 1.469 g/ L CaCl2‚2H2O, 0.026 g/L H3BO3, 0.04 g/L SrCl2‚6H2O, 3 g/ L bacteriological peptone, and 1.5 g/L yeast extract. The pH of the medium was adjusted to 7.2 ( 0.1 using a 5 M NaOH solution and sterilized by autoclaving for 20 min at 121 °C and at a pressure of 1 bar. The marine aerobic Pseudomonas NCIMB 2021 bacteria were obtained from the National Collection of Marine Bacteria (Sussex, U.K.). A new culture was resuscitated from a freezedried ampule and was subcultured twice in 5 mL of NCIMB 2021 medium before use. After the resuscitation, the bacteria

were cultured for 3 days in a 125 mL Erlenmeyer flask containing 20 mL of the fresh culture medium on a rotary shaker (20 °C, 150 rpm). A total of 20 mL of the cultured bacteria, stored in a -20 °C refrigerator, was used as the inoculum in all of the experiments to ensure a monoculture of the Pseudomonas bacteria. 2.3. Corrosion Experiments with Pseudomonas Inoculation. A 1 mL aliquot of the 3-day-old Pseudomonas culture was introduced into 500 mL of the culture medium in individual 2 L conical flasks. The conical flasks were subsequently

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3011

Figure 4. Impedance spectra of 304 SS recorded at the OCP in the Pseudomonas-inoculated medium after short-term exposure ((Ia), (Ib), and (Ic)) of 7 days (0), 14 days (O), 21 days (4), and 35 days (3), and long-term exposure ((IIa), (IIb), and (IIc)) of 49 days (]), 63 days ( ), and 77 days ( ). Solid lines represent the fitted results based on the equivalent circuit b. (a) Nyquist plots, (b) Bode magnitude plots, and (c) Bode phase angle plots.

placed on a rotary incubator shaker at 150 rpm and 25 °C. The bacterial cell concentration was estimated from the optical density (OD) of the suspension at 600 nm based on the standard calibration that an optical density of 1.0 is equivalent to about 109 cells/mL.30 When the OD value was close to 1.0, the prepared specimens hung on Nylon strings were aseptically introduced into the inoculated medium. All of the flasks were capped with Bug-stoppers (Whatman, USA) to prevent contamination by airborne bacteria. To maintain the bacterial density at the steady-state growth phase throughout the study period, a semicontinuous mode of Pseudomonas

culture growth was employed, that is, 75% of the medium was drained and replaced with an equal amount of a fresh sterile medium every 7 days. The specimens were retrieved from the individual medium after 7, 14, 21, 35, 49, 63, and 77 days for electrochemical measurements and surface analyses. For comparison purposes, similar stainless steel specimens were exposed to a sterile nutrient-rich medium of the same composition as the original composition of the inoculation medium to act as the control samples. All of the experiments were carried out in a batch process at 25 °C in an incubator.

3012


Figure 5. Physical models and the corresponding equivalent circuits used for fitting the impedance spectra of 304 SS coupons in the sterile and the Pseudomonas-inoculated nutrient-rich media.

2.4. Electrochemical Characterization. A conventional three-electrode glass corrosion cell with a capacity of 500 mL was used to obtain the Tafel plots and the electrochemical impedance spectra. An Ag/AgCl electrode was used as the reference electrode, and a platinum rod was used as the counter electrode. After the prescribed exposure time, the steel specimens were mounted in a special holder, leaving a circular area of 0.785 cm2 and were fixed at the bottom of the corrosion cell containing the transferred culture medium from the exposure flask to serve as the working electrode. All of the electrochemical measurements were performed using an Autolab PGSTAT 30 (Ecochemie Co., The Netherlands), controlled by GPES software for the Tafel plot measurements and FRA software for the EIS measurements. The Tafel plots were recorded at a scan rate of 2 mV‚s-1 within the range of -250 to 250 mV versus the open circuit potential (OCP), and analyzed to determine the Tafel slopes (βa and βc), the corrosion potentials (Ecorr), the corrosion current densities (icorr), and the corrosion rates. The cyclic polarization curves were determined potentiodynamically at a scan rate of 10 mV‚s-1 in the potential range of -0.5 and 1.0 V to confirm the presence of pitting corrosion on the coupon surfaces. EIS measurements were carried out under OCP using a 10 mV amplitude sinusoidal signal over the frequency range of 0.005 to 100 000 Hz. 2.5. Surface Analysis by SEM-EDX. The type 304 SS specimens were examined for their surface morphology and corrosion features using SEM and EDX (JEOL Co., Tokyo, Japan, JSM-5600 model). The coupons with biofilms were prepared for SEM imaging using the following procedure: the coupons were gently washed twice with a sterile PBS solution to remove the dead and loosely attached cells and fixed in a 3 vol % PBS solution of glutaradehyde for 8 h at 4 °C. Thereafter, the coupons were removed from the glutaradehyde solution and

washed twice with deionized water, followed by stepwise dehydration with 25, 50, 75, 90, and 100 vol % ethanol for 10 min each. The specimens were then dried in an airtight desiccator prior to SEM imaging. To observe corrosion damages at the coupon surface, the biofilm and the corrosion products were removed from the coupon surface with sterile cotton swabs immediately after the coupons were taken out, followed by sonication in a 0.05 M EDTA solution for 3 min, rinsing with deionized water thrice, drying with purified nitrogen, and finally storing in an airtight vacuum desiccator prior to analysis. Control coupons from the sterile medium were also prepared using similar washing and drying procedures as mentioned above. 3. Results and Discussion 3.1. Corrosion Behavior of 304 SS in Sterile and Pseudomonas-Inoculated Media. 3.1.1. Tafel Polarization Measurements. Polarization measurements, widely used for monitoring instantaneous corrosion rates of a metal or an alloy exposed to a corrosive environment, are suitable for the detection of changes in corrosion rate due to the presence of bacteria, inhibitors, and biocides.31 Parts a and b of Figure 1 show the respective Tafel plots of 304 SS after short-term and long-term exposures in the sterile nutrient-rich medium. The Tafel analytical data are shown in Table 1. The corrosion current densities, icorr , of 304 SS remain relatively constant throughout the exposure periods in the sterile medium, albeit there is a slight increase after the initial exposure period of 7 days and after the long-term exposures (i.e., 63 and 77 days), indicative of no significant changes in the corrosion rates of 304 SS in the sterile nutrientrich medium. This phenomenon is consistent with the presence of a stable, passivating chromium-enriched oxide film on the

Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3013 Table 3. Fitting Parameters of Impedance Spectra of 304 SS Coupons in the Sterile Medium after Various Exposure Periods time (days) parameter

7

14

21

35

49

63

77

Rsol (Ω) Rct (kΩ) Cdl (µF) Rf (kΩ) Qf Y0 × e-5 n1 Of Y0 × e-3 B Zf (kΩ) ∑χ2 × e-3

13.29 1.22 59.58 24.59 1.54 0.86 3.10 5.68 1.83 2.60

14.33 1.32 68.90 26.19 3.12 0.88 1.67 4.13 2.47 9.26

12.63 1.46 55.69 30.60 1.90 0.87 1.99 5.34 2.68 3.41

13.64 2.13 70.61 30.92 1.11 0.82 1.28 5.31 4.25 1.98

14.56 2.30 73.12 27.43 1.37 0.84 1.78 5.19 2.92 5.73

13.76 2.03 71.17 25.19 1.82 0.87 2.21 5.66 2.56 4.55

14.14 2.01 69.23 24.54 1.76 0.85 1.40 4.73 3.38 7.90

Table 4. Fitting Parameters of Impedance Spectra of 304 SS Coupons in the Pseudomonas-Inoculated Medium after Different Exposure Periods

Figure 6. Cyclic polarization curves of 304 SS coupons in the (a) sterile and (b) Pseudomonas-inoculated media after 35 days of exposure.

alloy surface. The passivating films on stainless steel are reported to be very thin (about 1 to 10 nm). Their formation depends on the solution pH, chloride ion concentration in the electrolyte, anodic aging time, and the composition of the substrate on which they are formed.32 The corrosion potential, Ecorr , undergoes a slightly positive shift of 50 mV with exposure time. This is probably attributable to the inhibitive effect of the passivating film on the anodic oxidation reaction. The cathodic Tafel slopes, βc, show no discernible trends. They fluctuate in the range of about -140 to -200 mV‚decade-1, indicating that the cathodic oxygen reduction reaction is independent of the exposure time. The anodic Tafel slopes, βa, remain more than 300 mV‚decade-1 throughout the exposure period, indicating that the anodic oxidation reactions are under diffusion control. The Tafel plots of 304 SS after short-term and long-term exposure periods in the Pseudomonas-inoculated nutrient-rich medium are shown, respectively, in parts a and b of Figure 2. The analytical results are summarized in Table 2. During the short-term exposure, the corrosion current density, icorr , increases slowly by about 50% to 3.62 µA‚cm-2 after 35 days of exposure. With exposure time extending beyond 49 days, icorr increases dramatically and attains the high value of 7.38 µA‚cm-2 after 77 days of exposure. This phenomenon is attributed to the breakdown of the passivating film after the prolonged synergistic attack of Pseudomonas bacteria and aggressive chloride anions. The corrosion potential, Ecorr , undergoes a negative shift in the Pseudomonas-inoculated medium with time. The phenomenon is probably caused by the colonization of Pseudomonas bacteria and the subsequent modification of the chemical composition of the passive film on the surface of 304 SS. However, this phenomenon cannot be interpreted by the mixed potential theory alone because the effect of Pseudomonas on the corrosion process is complicated and unpredictable. The anodic Tafel slope, βa, decreases with exposure time, indicative of the enhancement of the kinetics of anodic oxidation reactions under

time (days) parameter

7

14

21

35

49

63

77

Rsol (Ω) Rct (kΩ) QEDL Y0 × e-6 n1 Rf (kΩ) Qf Y0 × e-5 n2 Rb (kΩ) Cb (mF) ∑χ2 × e-3

13.45 2.18 2.77 0.66 20.89 1.04 0.91 1.22 17.96 4.89

13.38 1.83 2.56 0.64 16.25 3.23 0.84 1.37 17.22 1.37

14.07 1.85 2.59 0.67 14.29 1.79 0.79 1.34 29.55 0.59

13.57 1.35 3.02 0.66 13.25 2.04 0.86 1.73 15.10 0.76

11.88 0.63 3.16 0.64 9.59 2.26 0.80 1.37 21.18 0.37

14.01 0.48 2.86 0.68 7.65 1.62 0.75 1.93 12.68 2.69

12.52 0.59 3.14 0.58 8.49 1.59 0.77 2.00 16.21 1.12

the attack of Pseudomonas bacteria. It has been reported that a smaller value of the Tafel slope is indicative of the relative ease in electron transfer.33 As compared to that of the corresponding control specimen, the smaller value of βa after long-term exposure to the Pseudomonas inoculated medium also reveals the detrimental effect of Pseudomonas bacteria on the integrity of the passive oxide film, such as pitting corrosion. On the other hand, the relatively larger values of the cathodic Tafel slopes, βc, as compared to those of the corresponding control coupons, indicate that cathodic oxygen reduction reactions might be correlated with the development of biofilm of Pseudomonas bacteria on the coupons. The cathodic reaction has been reported to be influenced by the presence of Pseudomonas bacteria: (i) the kinetics of oxygen reduction may be affected by the evolution of passive film composition with the exposure time34 and (ii) oxygen reduction can be catalyzed in the presence of Pseudomonas bacteria by catalase.25,35 The amplification of the cathodic reaction rate can thus accelerate the propagation of initiated oits. The aforementioned results reveal that the stainless steel specimens undergo different corrosion processes in the presence and absence of the Pseudomonas bacterium in the nutrient-rich medium. 3.1.2. Electrochemical Impedance Measurements. EIS is a powerful, nondestructive electrochemical technique for the characterization of the electrochemical reactions at the metal/ biofilm interface, as well as for the study of the formation of corrosion products and biofilms in MIC.31 The impedance spectra of a 304 SS coupon after short-term and long-term exposure to the sterile nutrient-rich medium are shown in parts I and II of Figure 3, respectively. The Nyquist plots (part I a of Figure 3) reveal that the real component of the impedance spectrum increases slightly with time, and then reaches a maximum value after 35 days of exposure. During the longterm exposure, the diameter of the impedance loop undergoes a slight decrease with time (part IIa of Figure 3). The Bode magnitude plots (parts Ib and IIb of Figure 3) also show that the impedance spectra oscillate in a narrow range (from the

3014


logarithmic values of about 4.45-4.55) at the lowest frequency. These results indicate that the passivating film on the 304 SS coupon surface remains in a relatively stable state over time. The Bode phase angle plots of the short-term coupons have an apparent maximum angle with a small shoulder. The maximum angles appearing in the frequency range of 1 to 100 Hz are ascribed to the formation of a protective oxide film; the small shoulder located in the frequency range between 0.1 and 1 Hz is probably associated with the electrical double layer (EDL) (part Ic of Figure 3). For the long-term coupons, the maximum angles due to the passivating films appear in the same frequency range as those of the short-term coupons. However, the smaller shoulder has almost vanished and only exhibits a weak signal (part IIc of Figure 3). The phase maxima in the Bode phase angle plot and a slope close to -1 in the Bode magnitude plot usually provide information on the relaxation time constants.36 The Bode phase angle plots for 304 SS coupons in the sterile medium thus reveal the existence of two time constants. Parts I and II of Figure 4 show the respective impedance spectra of 304 SS after short-term and long-term exposure in the Pseudomonas-inoculated nutrient-rich medium. As can be seen in the Nyquist plots (parts Ia and IIa of Figure 4), the diameter of impedance loop decreases gradually with time. The respective Bode magnitude plot (parts Ib and IIb of Figure 4) reveals that the total impedance magnitude decreases gradually from a logarithmic value of about 4.4 to about 3.9. The diameter of impedance loops and the magnitude of impedance of the long-term specimens have decreased dramatically as compared to those of the short-term specimens, indicative of the breakdown of the passivating film on the stainless steel surface under the synergistic attack of Pseudomonas bacterium and aggressive anions. The Bode phase angle plots have undergone marked changes after different exposure periods due to the effect of Pseudomonas on the passivating film. As for the shortterm samples, the maximum peak angles are significantly broadened, as compared to their counterparts in the sterile nutrient-rich medium, and span a frequency range of 0.01 to 1000 Hz. On the other hand, two maximum phase angles appear for the long-term coupons in the Pseudomonas-inoculated nutrient-rich medium: the one in the high-frequency range of 100 to 1000 Hz is ascribed to the protective oxide film, whereas the one in the frequency range of 0.1 to 10 Hz is probably associated with the biofilm or the electrical double layer (part IIc of Figure 4). The impedance spectra obtained in the sterile and Pseudomonas-inoculated media after different exposure periods are analyzed and fitted to circuit parameters using the nonlinear least-square method in the program EQUIVCRT by Boukamp.37 Figure 5 shows the physical models and the corresponding equivalent circuits, which can be satisfactorily used to fit the EIS data, as the Chi-square (χ2) values are around 10-3. The equivalent circuit a in Figure 6 models two relaxation time constants and is frequently used to represent a thin and compact protective passivating film formed on the metal surface. Of is introduced to represent the path of a finite diffusion process in equivalent circuit a. The cotangent hyperbolic term Of is used to describe the diffusion of mobile species through a thin layer, such as an oxide layer or coating, followed by a reaction at the electrode/oxide layer interface.38 It is defined by the following equation:37-40

Y ) tanh(B xjω)/Y0 xjω

(1)

Here, B ) δ/xD, Y0 ) 1/σ x2 and δ is the Nernst diffusion layer thickness, D is the average value of the diffusion

coefficients of the species, and σ is the Warburg coefficient. The quotient of B divided by Y0 is accepted as a description for the diffusion resistance of a protective film of finite length. In the current study, Of reflects the diffusion process within the passive oxide film on the stainless steel surface. The equivalent circuit b contains three relaxation time constants and illustrates the formation of a duplex surface film on the metal surface: the outer layer is a heterogeneous biofilm, whereas the inner part is a porous oxide film. In part a of Figure 3 and part a of Figure 4, the depression in the semicircle with their center below the real axis is well-known as the dispersing effect.41 The frequency dispersion behavior is probably related to the inherent heterogeneous nature of the solid surface.42 Because of the fact that the bulk solution/EDL and the EDL/electrode interface do not behave as an ideal capacitor in the presence of the dispersing effect, a constant phase element (CPE) is often used as a substitute for the capacitor in the equivalent circuit to fit the EIS data more accurately. The CPE defined in the usual impedance and admittance formats are as follows:

ZCPE ) 1/[Y0(jω)n]

(2)

YCPE ) Y0(jω)n

(3)

Here, j is the imaginary root and ω () 2πf) is the angular frequency. The factor n, defined as the CPE power, is an adjustable parameter that lies between 0 and 1, and can be used as a measure of the surface inhomogeneity. Its decrease is connected to the increase in metal surface roughness. The factor Y0 denotes a parameter related to the capacitance. The capacitance can be calculated from the experimentally determined CPE parameters using the following equation:39-41

C ) ωn-1/Y0 sin(nπ/2)

(4)

The EIS data of 304 SS coupons after the short-term and long-term exposures in the sterile nutrient-rich medium are both fitted using the equivalent circuit a. The parameters used for fitting are given in Table 3. The resistance of the surface passivating film, Rf, increases with exposure time during the short-term exposure and reaches a maximum value of about 31 kΩ after 35 days. Thereafter, Rf decreases slightly with time, probably due to the formation of conditioning layers by the adsorption of organic compounds and the accompanied changes in composition of the passivating films with time. The porous and nonprotective conditioning layers adhere strongly to the underlying oxide film and probably have contributed to the complexity of surface conditions.28 The charge-transfer resistance, Rct, undergoes changes, similar to those in Rf, with exposure time. The variation in Rf is, nevertheless, very small (below 20%) for all of the exposure periods, indicative of the stability of the passivating film on the stainless steel surface in the sterile nutrient-rich medium. Another noticeable circuit element in Table 3 is Zf () B/Y0), which represents the diffusion resistance of active species through the thin oxide layer. The diffusion resistance, Zf, is dependent on the resistance of the passivating film, Rf. The diffusion resistance is probably associated with one of the following diffusion processes: (i) the diffusion of metal ions (such as iron or chromium) through the oxide film, (ii) the diffusion of metal ions from the surface of the oxide film to the bulk solution, (iii) the diffusion of oxygen in the oxide film, and (iv) the diffusion of dissolved oxygen from the bulk solution to the film surface.


Figure 7. Representative SEM images and EDX spectra of 304 SS coupons in the sterile nutrient-rich medium after different exposure periods: (a) 14, (b) 35, and (c) 63 days. The EDX spectra correspond to the rectangular area of the corresponding SEM images.

The temporal impedance spectra of 304 SS coupons in the Pseudomonas-inoculated nutrient-rich medium are fitted with the equivalent circuit b and the fitted parameters are given in Table 4. Both the charge-transfer resistance, Rct, and the passivating film resistance, Rf, decrease gradually with time. During the initial 7 days of exposure, the values of Rct and Rf are close to those of the corresponding specimen in the sterile nutrient-rich medium. This result suggests that the passivating film retains its integrity initially, although some Pseudomonas cells have colonized on the specimen surface. Upon prolonging exposure time, both the Rct and Rf values decrease dramatically, attributable to the deleterious effect of Pseudomonas bacteria and their biofilm, such as pitting corrosion on the integrity of the passivating film. The resistance of the biofilm, Rb, varies with time, consistent with the fact that the development of biofilm is a dynamic process and is accompanied by the continuous attachment, growth, and detachment from the solid

surface.21 This result is consistent with the onset of pitting corrosion under the heterogeneous biofilms because the biofilm of the Pseudomonas NCIMB 2021 bacteria has been reported to induce extensive micropitting corrosion on the surface of type 316 stainless steel.18 3.1.3. Cyclic Polarization Curves. Figure 6 shows the cyclic polarization curves of the stainless steel coupons after exposure to the sterile and the Pseudomonas-inoculated media for 35 days. It is widely recognized that the hysteresis of cyclic polarization curves can provide information on the initiation of pitting corrosion.36 Positive hysteresis occurs when a damaged passivating film is not repaired and/or pits are initiated.43 Negative hysteresis occurs when a damaged passivating film repairs itself and pits are not initiated.44 As shown in part a of Figure 6, the onset of anodic current occurs at a potential of about -0.34 V for a 304 SS coupon exposed to the sterile medium for 35 days. A negative hysteresis is observed, as the reverse scan current

3016


Figure 8. Representative SEM images of (a) 14 day old, (c) 35 day old, (e) 63 day old biofilms formed on the 304 SS coupon surface by Pseudomonas NCIMB 2021 bacteria. Representative SEM images of the corroded coupon surfaces upon removal of the biofilms after exposure periods of (b) 14, (d) 35, and (f) 63 days.

density is less than that of the forward scan, indicative of the absence of pitting corrosion on the coupon surface. However, for the cyclic polarization curves of 304 SS coupon in the Pseudomonas-inoculated medium (part b of Figure 6), the onset of the anodic current occurs at about -0.42 V, which is more negative than the corresponding sterile coupons. Pitting corrosion has occurred on the coupon surface because the current hysteresis is positive. These results confirm that the accumulation of biofilms on the coupon surface has induced and initiated pitting corrosion. 3.2. SEM and EDX Studies. Representative SEM micrographs and the corresponding EDX spectra of stainless steel specimens in the sterile nutrient-rich medium are shown in Figure 7. No localized corrosion was observed on the metal surface, except for the formation of conditioning layers by the spontaneous adsorption of organic macromolecules from the bulk solution after 14 and 35 days (parts a and b of Figure

7). The formation of conditioning layers on the solid surface is a ubiquitous phenomenon in aquatic environments and is recognized to be the initial step of biofilm formation.21 Previous studies reported that the amount of organic species adsorbed increased with time, and discrete or patchy films were formed after a short exposure time.45,46 In the present study, the conditioning layers are found to be thin, continuous, and nonhomogeneous after 35 days of exposure (part b of Figure 7). However, after 63 days of exposure, minor corrosion occurs on the specimen surface (part c of Figure 7). EDX was used to evaluate the chemical composition of corrosion products on the coupon surface. In the case of the short-term samples, the metallic elements of iron, chromium, and nickel are predominant on the surface (parts a and b of Figure 7). The EDX spectrum of the long-term specimens shows that the amount of carbon has increased to about 21%. About 8% oxygen and a trace amount of chlorine (about 0.5%) are also


Figure 9. SEM images and EDX spectra of representative pits upon removal of the biofilms on the 304 SS coupon surface after short-term exposures of (a) 14 days and (b) 35 days to the Pseudomonas-inoculated medium.

observed (part c of Figure 7). The presence of chlorine can probably be used to account for the cause of minor corrosion upon the long-term exposure. It has been reported that stainless steel is susceptible to localized corrosion by chloride and sulfide ions.47 Furthermore, the occurrence of minor localized corrosion on the long-term specimen also explains the slight decrease in resistance of the passivating film, Rf. SEM results reveal that the development of biofilms on the stainless steel surface is a dynamic process (parts a, c, and e of Figure 8). The SEM image of the 14-day specimen reveals dense Pseudomonas cells at localized sites. They aggregate to form bacterial colonies or tubercles (part a of Figure 8). The patchy biofilms or tubercles provides physicochemical conditions in localized environments to facilitate the initiation of pitting corrosion.48 Some shallow pits are discernible on the 14-day coupon surface upon removal of the biofilms (part b of Figure 8). Upon prolonging the exposure time to 35 days, bacterial cells associated with amorphous extracellular polymeric substances (EPS) are observed on the coupon surface (part c of Figure 8). The aggregates of bacterial cells are interspersed throughout the EPS matrix, providing pathways for the diffusion of active and nutrient species and giving rise to local gradients in metabolic products, aggressive ions (such as Cl-), and/or dissolved oxygen (i.e., differential aeration cells). All of the above can generate active electrochemical corrosion cells to facilitate the occurrence of localized corrosion. Upon removal of the biofilm, the SEM image of a 35-day specimen reveals severe localized corrosion, visible as intense micropits and some deep macropits (part d of Figure 8). After the long-term (63 days) exposure, the entire surface of the metal coupon is

covered by a highly heterogeneous, porous, and continuous biofilm (part e of Figure 8). Individual cells are no longer discernible on the biofilm because the bacterial cells are probably embedded within the EPS matrix. After removal of the biofilm, extensive micropits and macropits are present on the 63-day specimen surface (part f of Figure 8). Thus, the dramatic decrease in resistance of the passivating film, Rf, can be ascribed to extensive pitting corrosion in the presence of Pseudomonas bacteria. EDX analysis was carried out on representative pits of the specimens to quantify the chemical composition of the pitted areas on the metal surface. The macropits and the corresponding EDX spectra of the 14-day and 35-day specimens are shown in parts a and b of Figure 9, respectively. The EDX spectra are very similar to each other, except for the variation in elemental peak intensities. The enrichment of carbon, oxygen, and chlorine in the pitted areas with exposure time are indicative of the presence of bacterial cells and their EPS, as well as aggressive chloride ions (Cl-) in the pitted region. The depletion of metallic elements of iron, chromium, nickel, and manganese in the pitted areas is consistent with the occurrence of localized corrosion in the presence of aggressive chloride ions (Cl-) and the colonizing bacteria. To further demonstrate the involvement of chloride ions in initiating and propagating pitting corrosion, EDX analysis was further performed on several locations with different surface characteristics on a representative 63-day specimen. Figure 10 shows the SEM image and three EDX spectra corresponding to the indicated locations. Among the three EDX spectra, the most noticeable feature is the change in the amount of chlorine, which is directly correlated to pitting corrosion. The relative abundance of chlorine is as high as about

3018


Figure 10. EDX spectra from various locations on a representative SEM image of a 63 day old specimen upon removal of the biofilms.

9% in the deep and large pit (point A), whereas it is about 2.2% in the shallow and small pit (point B) and only about 1.3% in a newly initiated pit (point C). In addition, the enrichment of carbon and oxygen, and the depletion of metallic elements of iron, chromium, and nickel all appear to be associated with the pitted regions as well. It can therefore be concluded that both Pseudomonas bacterial colonization and aggressive chloride (Cl-) ions play significant roles in the initiation and propagation of pitting corrosion. This conclusion is consistent with that of Morales et al.17 in studying the role of Pseudomonas aeruginosa in localized corrosion of stainless steel. They concluded that the presence of growing bacterial cells enhanced the breakdown of passivity by the aggressive Cl- ions. Comparison of the EDX spectra of bacteria-colonized specimens with those of sterile control specimens suggests that the formation of heterogeneous biofilms, aided by the aggressive chloride ions, is responsible for the initiation and propagation of pitting corrosion on the bacteria-colonized specimen surface. Hakkarainen48 reported that three conditions must be fulfilled for the occurrence of pitting corrosion: (i) the presence of aggressive anions (such as Cl-) in the electrolyte, (ii) the existence of potential difference between the interior of the pit and the open surface, and (iii) the reaction temperature must be beyond a critical temperature. These conclusions help to explain the occurrence of extensive micropitting corrosion in the present study. The colonization of a bacterial biofilm gives rise to a potential difference between the area under the biofilm, which serves as the anodic site, and the surrounding regions, which act as the cathodic sites. The synergistic effect of aggressive chloride ions (Cl-) and colonized bacterial cells can therefore trigger the partial loss of passivity of the stainless steel and the initiation of pitting corrosion. The initial damage to the

passivation film has an important consequence to the long-term performance of stainless steel, as deep pits can cause wall penetration, leakage, and failure. It has been reported that a pit, once initiated, provides a recess area that hinders mass transfer between the pit interior and the exterior bulk solution.49 The growth of pits has been widely recognized to be a selfpropagating or autocatalytic process because hydrolysis of the corrosion products and aggressive chloride ions can result in acidic solutions that destroy passivity locally and create an active (corroding) anode within the pit.49,50 4. Conclusions The corrosion behavior of type 304 SS in the absence and presence of a marine aerobic strain Pseudomonas in a nutrientrich simulated seawater-based medium was investigated. Electrochemical results indicated that despite a slight decrease in resistance of the passivating film under long-term exposure, the passivating film formed on 304 SS in the sterile nutrient-rich medium was relatively stable, and the anodic reaction was under diffusion control. The colonization of Pseudomonas bacteria on the coupon surface resulted in the acceleration of the anodic reaction due to the deterioration of the passivity of the oxide film and the slowing down of the cathodic reaction accompanying the depletion of oxygen. Furthermore, the charge-transfer resistance, Rct, and resistance of the passivating film, Rf, both decreased with exposure time, indicative of the enhancement of the corrosion rate of 304 SS coupons. SEM images of the surface and interface revealed that the heterogeneity and the coverage of biofilms formed on the coupon surface increased with time, and that extensive micropitting corrosion occurred under the biofilms. EDX spectra further verified that the


occurrence of pitting corrosion was caused by the synergistic effect of aggressive chloride ions and the colonization of bacterial cells and their EPS. In comparison, no apparent signs of pitting corrosion were observed on the coupon surface upon short-term exposure in the sterile nutrient-rich medium, except for the formation of thin conditioning layers. Even after a longterm exposure, only sparse and minor pitting corrosion had occurred on the control samples. Acknowledgment The authors would like to thank National University of Singapore for the financial support for this study (FRC Research Grant R-279-000-236-112). Literature Cited (1) Borenstein, S. Microbiologically Influenced Corrosion Handbook; Woodhead Publishing Ltd.: Cambridge, 1994; p 113. (2) Fe´ron, D.; Dupont, I. DeVelopments in Marine Corrosion; Campbell, S.A., Campbell, N., Walsh, F. C., Eds.; Royal Society of Chemistry: Cambridge, 1998; p 89. (3) Costerton, J. W. Biofouling and Biocorrosion in Industrial Water Systems; Fleming, H. C., Geesey, G. G., Eds.; Springer-Verlag: New York, 1991; p 1. (4) George, R. P.; Muraleedharan, P.; Sreekumari, K. R. Influence of Surface Characteristics and Microstructure on Adhesion of Bacterial Cells onto a Type 304 Stainless Steel. Biofouling 2003, 19, 1. (5) George, R. P.; Muraleedharan, P.; Parvathavarthini, N.; Khatak, H. S.; Rao, T. S. Microbiologically Influenced Corrosion of AISI type 304 Stainless Steels under Fresh Water Biofilms. Mater. Corros. 2000, 51, 213. (6) Gubner, R.; Beech, I. B. The Effect of Extracellular Polymeric Substances on the Attachment of Pseudomonas NCIMB 2021 to AISI 304 and 316 Stainless Steel. Biofouling 2000, 15, 25. (7) Videla, H. A.; Herrera, L. K. Microbiologically Influenced Corrosion: Looking to the Future. Int. Microbiol. 2005, 8, 169. (8) Little, B.; Ray, R.; Wagner, P.; Jones-Meehan, J.; Lee, C.; Mansfeld, F. Spatial Relationships between Marine Bacteria and Localized Corrosion on Polymer Coated Steel. Biofouling 1999, 13, 301. (9) Angell, P.; Luo, J. S.; White, D. C. Microbially Sustained Pitting Corrosion of 304 Stainless Steel in Anaerobic Seawater. Corros. Sci. 1995, 37, 1085. (10) Rodrı´guez, J. J. S.; Hernańdez, F. J. S.; Gonza´lez, J. E. G. Comparative Study of the Behavior of AISI 304 SS in a Natural Seawater Hopper, in Sterile Media and with SRB Using Electrochemical Techniques and SEM. Corros. Sci. 2006, 48, 1265. (11) Duan, J. Z.; Hou, B. R.; Yu, Z. G. Characteristics of Sulfide Corrosion Products on 316L Stainless Steel Surfaces in the Presence of Sulfate-reducing Bacteria. Mater. Sci. Eng., C 2006, 26, 624. (12) Gonza´lez, J. E. G.; Hernańdez, F. J. S.; Mirza-rosca, J. C. Effect of Bacterial Biofilm on 316 SS Corrosion in Natural Seawater by EIS. Corros. Sci. 1998, 40, 2141. (13) Xu, C. M.; Zhang, Y. H.; Cheng, G. X.; Zhu, W. S. Localized Corrosion Behavior of 316L Stainless Steel in the Presence of Sulfatereducing and Iron-oxidizing Bacteria. Mater. Sci. Eng., A 2007, 443, 235. (14) Beech, I. B.; Gaylarde, C. C. Recent Advances in the Study of Biocorrosion: an Overview. ReVista de Microbiologia 1999, 30, 177. (15) Coetser, S. E.; Cloete, T. E. Biofouling and Biocorrosion in Industrial Water Systems. Crit. ReV. Microbiol. 2005, 31, 213. (16) Valcarce, M. B.; de Sańchez, S. R.; Va´zquez, M. Localized Attack of Copper and Brass in Tap Water: the Effect of Pseudomonas. Corros. Sci. 2005, 47, 795. (17) Morales, J.; Esparza, P.; Gonza´lez, S.; Salvarezza, R.; Are´valo, M. P. The Role of Pseudomonas aeruginosa on the Localized Corrosion of 304 Stainless Steel. Corros. Sci. 1993, 34, 1531. (18) Beech, I. B.; Zinkevich, V.; Hanjangsit, L.; Avci, R. The Effect of Pseudomonas NCIMB 2021 Biofilm on AISI 316 Stainless Steel. Biofouling 2000, 15, 3. (19) Pedersen, A.; Kjelleberg, S.; Hermansson, M. A Screening Method for Bacterial Corrosion of Metals. J. Microbiol. Methods 1988, 8, 191. (20) Moreno, D. A.; Ibars, J. R.; Beech, I. B.; Gaylarde, C. C. Biofilm Formation on Mild Steel Coupons by Pseudomonas and DesulfoVibrio. Bioufouling 1993, 7, 129. (21) Videla, H. A. Manual of Biocorrosion; Lewis Publishers, CRC Press Incorporated: Boca Raton, FL, 1996; p 129.

(22) Franklin, M. J.; White, D. C.; Isaacs, H. S. Pitting Corrosion by Bacteria on Carbon Steel Determined by the Scanning Vibrating Electrode Technique. Corros. Sci. 1991, 32, 945. (23) Vaidya, R. U.; Butt, D. P.; Hersman, L. E.; Zurek, A. K. Effect of Microbiologically Influenced Corrosion on the Tensile Stress-strain Response of Aluminum and Alumina-particle Reinforced Aluminum Composite. Corrosion 1997, 53, 136. (24) Vaidya, R. U.; Brozik, S. M.; Deshpande, A.; Hersman, L. E. Protection of Beryllium Metal against Microbial Influenced Corrosion Using Silane Self-Assembled Monolayers. Metall. Mater. Trans. A 1999, 30A, 2129. (25) Busalmen, J. P.; Va´zquez, M.; de Sańchez, S. R. New Evidences on the Catalase Mechanism of Microbial Corrosion. Electrochim. Acta 2002, 47, 1857. (26) Busalmen, J. P.; Frontini, M. A.; de Sańchez, S. R. DeVelopments in Marine Corrosion; Campbell, S. A., Campbell, N., Walsh, F. C., Eds.; Royal Society of Chemistry: Cambridge, 1998; p 119. (27) Beech, I. B.; Sunner, J. Biocorrosion: Towards Understanding Interactions between Biofilms and Metals. Curr. Opin. Biotechnol. 2004, 15, 181. (28) Yuan, S. J.; Pehkonen, S. O. Microbiologically Influenced Corrosion of 304 Stainless Steel by Aerobic Pseudomonas NCIMB 2021 Bacteria: AFM and XPS Study. Colloids Surf., B 2007, 59, 87. (29) Bidwell, J. P.; Spotte, S. Simulated Seawaters: Formulas and Methods; Jones and Bartlett Publishers: Boston, 1985; p 14. (30) Hogt, A. H.; Dankert, J.; Feijen, J. Adhesion of Coagulase-Negative Staphylococci to Methacrylate Polymers and Copolymers. J. Biomed. Mater. Res. 1986, 20, 533. (31) Mansfeld, F.; Little, B. A Technical Review of Electrochemical Techniques Applied to Microbiologically Influenced Corrosion. Corros. Sci. 1991, 32, 247. (32) Alamr, A.; Bahr, D. F.; Jacroux, M. Effects of Alloy and Solution Chemistry on the Fracture of Passivating Films on Austenitic Stainless Steel. Corros. Sci. 2006, 48, 925. (33) Huang, G. T.; Chan, K. Y.; Fang, H. H. P. Microbiologically Influenced Corrosion of 70Cu-30Ni Alloy in Anaerobic Seawater. J. Electrochem. Soc. 2004, 151, B434. (34) Le Bozec, N.; Compe`re, C.; L’ Her, M.; Laouenan, A.; Costa, D.; Marcus, P. Influence of Stainless Steel Surface Treatment on the Oxygen Reduction Reaction in Seawater. Corros. Sci. 2001, 43, 765. (35) Lai, M. E.; Bergel, A. Electrochemical Reduction of Oxygen on Glassy Carbon: Catalysis by Catalase. J. Elecroanal. Chem. 2000, 494, 30. (36) Tait, W. S. An Introduction to Electrochemical Corrosion Testing for Practicing Engineers and Scientists; University of WisconsinMadison: Racine, WI, 1994; p 79. (37) Boukamp, B. A. A Nonlinear Least-squares Fit Procedure for Analysis of Immittance Data of Electrochemical Systems. Solid State Ionics 1986, 20, 31. (38) Zhang, X. H.; Pehkonen, S. O.; Kocherginsky, N.; Ellis, G. A. Copper Corrosion in Mildly Alkaline Water with the Disinfectant Monochloramine. Corros. Sci. 2002, 44, 2507. (39) Benedetti, A. V.; Sumodjo, P. T. A.; Nobe, K.; Cabot, P. L.; Proud, W. G. Electrochemical Studies of Copper, Copper-Aluminium and CopperAluminium-Silver Alloys: Impedance Results in 0.5M NaCl. Electochim. Acta 1995, 40, 2657. (40) Yu, Z.; Pehkonen, S. O. Copper Corrosion Kinetics and Mechanisms in the Presence of Chlorine and Orthophosphate. Water Sci. Technol. 2004, 49, 73. (41) Yuan, S. J.; Pehkonen, S. O. Surface Characterization and Corrosion Behavior of 70/30 Cu-Ni Alloy in Pristine and Sulfide-containing Simulated Seawater. Corros. Sci. 2007, 49, 1276. (42) Xu, X. J.; Ma, H. Y.; Chen, S. H.; Xu, Z. Y.; Su, A. F. General Equivalent Circuits for Faradaic Electrode Processes under Electrochemical Reaction Control. J. Electrochem. Soc. 1999, 146, 1847. (43) Tait, W. S. Comparison of Potentiodynamically Determined Pitting Rates with Actual Pitting Rates for Mild Steel and Admiralty Brass in Oxygen Bearing Waters. Corrosion 1978, 34, 214. (44) Tait, W. S. Effect of Physical Parameters on the Pitting Corrosion of Mild Steel in Low Total Dissolved Solids, Oxygen Bearing Waters: a Nonpassivating Metal-Environment System. Corrosion 1979, 35, 296. (45) Pradier, C. M.; Bertrand, P.; Bellon-Fontaine, N.; Compe`re, C.; Costa, D.; Marcus, P.; Poleunis, C.; Rondot, B.; Walls, M. G. Adsorption of Proteins on an AISI 316 Stainless-Steel Surface in Natural Seawater. Surf. Interface Anal. 2000, 30, 45. (46) Compe`re, C.; Bellon-Fontaine, M. N.; Bertrand, P.; Costa, D.; Marcus, P.; Poleunis, C.; Pradier, C. M.; Rondot, B.; Walls, M. G. Kinetics of Conditioning Layer Formation on Stainless Steel Immersed in Sea Water. Biofouling 2001, 17, 129.

3020


(47) Stott, J. F. D. The Initiation of Pitting Corrosion at MnS Inclusions. Corros. Sci. 1993, 35, 667. (48) Hakkarainen, T. J. Microbiologically Influenced Corrosion of Stainless Steels-What is Required for Pitting. Materials Corros. 2003, 54, 503. (49) Jones, D. A. Principles and PreVention of Corrosion, Prentice Hall: Upper Saddle River, NJ, 1996; p 209.

(50) Geiser, M.; Avci, R.; Lewandowski, Z. Microbially Initiated Pitting on 316 Stainless Steel. Int. Biodeter. Biodegr. 2002, 49, 235.

ReceiVed for reView November 12, 2007 ReVised manuscript receiVed February 10, 2008 Accepted February 17, 2008 IE071536X

Corrosion Behavior of Type 304 Stainless Steel in a Simulated

Recommend Documents