Electrochemical Study of Stainless Steel Corrosion by Marine Sulphate-Reducing Bacteria
(Kajian Elektrokimia Terhadap Kakisan Keluli Kalis Karat oleh Bakteria Penurun-Sulfat)
FATHUL KARIM SAHRANI, ZAHARAH IBRAHIM, MADZLAN AZIZ & ADIBAH YAHYA
ABSTRACT
Corrosion caused by sulphate-reducing bacteria (SRB) isolated from seawater nearby to Pasir Gudang has been studied.
The test coupon was a AISI 304 stainless steel. Potential and corrosion rate measurements were carried out in various types of culturing solutions, with SRB1, SRB2, combination of SRB1 & SRB2 and without SRBs inoculated (sterilized). From Tafel plots a higher corrosion rate has been found in medium inoculated with SRBs than that of the sterilized medium (control). When SRBs were present in the medium, the Tafel plot shifted towards more negative values (Ecorr was shifted to much less anodic values) and increase in current density compared to that of the sterilized medium (control). Localized corrosion was observed on the metal surface, and it was associated to the SRB activity. X-ray analysis (EDAX) showed that the corrosion product has higher content of sulphur for medium containing SRBs than that of the sterilized medium. X-ray diffraction analysis carried out on corrosion products which showed the presence of iron sulphide. This indicates the influence of the presence of SRB in corrosion process.
Keywords: Sulphate-reducing bacteria; stainless steel; electrochemical test; Tafel plots
ABSTRAK
Kajian mengenai kakisan oleh bakteria penurun-sulfat (SRB) dipencilkan dari air laut berhampiran Pasir Gudang telah dilakukan. Kupon ujian adalah keluli kalis karat AISI 304. Pengukuran keupayaan dan kadar kakisan dijalankan dalam pelbagai keadaan iaitu dalam medium yang mengandungi SRB1, SRB2, kombinasi SRB1 & SRB2 dan tanpa kultur SRB
(steril). Daripada plot Tafel, didapati kadar kakisan tinggi pada medium yang mengandungi kultur SRB berbanding medium steril. Pada medium yang mengandungi kultur SRB, keupayaan kakisan berubah ke arah nilai yang lebih negatif (Ecorr berubah kearah anodik dengan nilai yang semakin berkurangan) dan meningkatkan ketumpatan arus berbanding medium kawalan. Kakisan setempat diperhatikan berlaku pada permukaan keluli yang dikaitkan dengan aktiviti SRB. Analisis sinar-x (EDAX) menunjukkan produk kakisan mengandungi kandungan sulfur yang tinggi bagi medium yang mengandungi kultur SRB berbanding medium steril (kawalan). Selanjutnya analisis belauan sinar-x terhadap produk kakisan menunjukkan kehadiran sebatian besi sulfida. Ini menunjukkan pengaruh SRB dalam proses kakisan.
Kata kunci: Bakteria penurun-sulfat; keluli kalis karat; ujian elektrokimia; plot Tafel
of sulphide film, i.e., their characteristic form of respiration uses sulphate and results in sulphide formation (Posgate 1984).
The petroleum production environment is particularly suitable for the activities of SRB because it handles large volumes of deaerated water from underground reservoirs.
The water is rich in nutrients and can become very sour with H2S, if infected by SRB. Therefore, the economical importance of this type of corrosion is unquestionable in petroleum industry (Rainha & Fonseca 1997).
In this present work, the role of SRB in corrosion of stainless steel has been studied based on Tafel plots. Surface characterization and composition of the corrosion products were determined by Environmental Scanning Electron Microscope (ESEM), energy dispersive analysis of x-ray
(EDAX) and X-ray diffraction analysis (XRD).
INTRODUCTION
Microbiologically Influenced Corrosion (MIC) is the deterioration of a metal by corrosion processes that occurs directly or indirectly as a result of the metabolic activity of microorganisms. MIC can be observed anywhere: in chemical, power, pulp and paper industries, metal working prosesses and hydraulic systems, ship engines, refineries, marine industry and nuclear power plants (Crum & Little 1991; Sarioglu et al. 1997). Failures related to MIC have been reported at every stage of the lifetime of engineering systems; during fabrication, test, service and shutdown periods.
MIC can be considered in two categories: anaerobic and aerobic. The sulphate (SO42-) reducing bacteria (SRB)
are the most destructive microorganisms in anaerobic MIC. They reduce sulphate to sulphide and promote formation
EXPERIMENTAL DETAILS CULTURAL CONDITIONS
The SRBs used in this work was isolated from seawater collected nearby Pasir Gudang, Malaysia. The collected samples were inoculated in a selective medium, in accordance to recommendations for SRB sampling. The microorganisms were maintained in the laboratory using the VMNI medium (Table 1) as proposed by Zinkevich et al. (1996) which was modified from Posgate’s Marine medium C. The medium was degassed under N2 for 30 minutes to create anaerobic condition. pH was adjusted to 7.2 using 1.0M NaOH before autoclaving at 121 oC. It was left to cool to room temperature before being inoculated with the SRB.
The bacterial cells were spun in 30 ml centrifuge tubes for 10 minutes at 1200 rpm. The supernatant was removed and the samples were ready to be used or stored in a freezer until needed.
ELECTROCHEMICAL EXPERIMENTS
These experiments were carried out in a ASTM standard cell (ASTM Designation: G3-89, 1999), with three electrode system: AISI 304 stainless steel as working electrode, graphite rod as counter electrode and saturated calomel electrode (SCE) as reference electrode (Figure 1). The electrolyte was 300 ml VMNI medium, deaerated with nitrogen gas to maintain anaerobic condition. The stainless steel sample was immersed in the electrolyte solution with test area of about 0.708 cm2. The potential of the stainless steel electrodes immersed in various conditions: (a) VMNI
(control); (b) VMNI + SRB1; (c) VMNI + SRB2 and (d) VMNI
+ SRB1 + SRB2.
A potentiodynamic method was used to obtain the potential-current curve by applying potential ± 1000mV over potential, with respect to the free corrosion potential Ecorr. The General Purpose Electrochemical System (GPES)
software was used for data analysis and management.
(a) (b)
FIGURE 1. (a) Schematic diagram of sample holder and (b) the electrochemical cell
TABLE 1. Composition of the VMNI medium
Chemical Reagents Composition (g/L)
KH2PO4 0.5
NH4Cl 1.0
NaSO4 4.5
Sodium citrate 0.3
CaCl2.6H2O 0.04
MgSO4.7H2O 0.06
Casamino acids 2.0
Tryptone 2.0
Lactate 6.0
Ascorbic acids 0.1
Thioglycollic acid 0.1
FeSO4.7H2O 0.5
Trace elements (stock solution) 1.0ml
Vitamins (stock solution) 2.0ml
From this technique, resistance values are obtained (Rp), which are used to calculate the corrosion current density, Icorr according to equation (1),
I B
corr R
P
= (1)
where B is the Tafel slope. The corrosion rate was determined by equation 2,
Corrosion rate (CR)
=K I× ×EW D
corr (mmpy) (2)
where K is a constant = 3272, EW is equivalent weight and D is density of the sample.
ESEM, EDAX AND XRD STUDIES
ESEM images were obtained using ElectroScan model Leo 1455, (Germany). The identification of corrosion product was carried out using XRD model D8 Advance, Bruker.
The identification of corrosion product compound is based on the measurement of atomic structures obtained from their x-ray diffraction pattern of diffractogram.
RESULTS AND DISCUSSION
Figure 2 shows the corrosion rate of stainless steel in the filter-sterilized seawater (SSSW) and VMNI medium (SSV). The corrosion rate of stainless steel in VMNI was always lower than that of the filter-sterilized seawater. The corrosion rate in SSSW increased from 0.032 mmpy to average value of 0.080 mmpy. The corrosion rate of SSV
has no significant increase and reached to a maximum value of 0.037 mmpy. In the cases of stainless steel, higher
corrosion rate was recorded in the (SSSW) compared to VMNI
medium (SSV). The presence of the yeast extract in VMNI
media may explain the decrease in the corrosion potential observed over 15 days of exposure. According to Dupont et al. (1998), yeast extract may be adsorbed on the electrode surface inhibiting the corrosion of steel. Further more,
VMNI medium was also enriched with more substance such as organic nutrients, sulphate, ion ferrous and corrosion products which increased the complexity of the electrolyte system.
Corrosion of stainless steel in VMNI medium inoculated with different SRBs isolates presented in Figure 3. The corrosion rate of stainless steel in the presence of
SRB1 was only slightly higher than that of the control. A longer exposure for over 15 days, the corrosion rates decreased from 0.065 mmpy to 0.032 mmpy. The corrosion rate of stainless steel in the presence of SRB2 has increased drastically after the first day exposure to the maximum value of 0.83 mmpy and then decreased slowly with the some fluctuated to the end point value of 0.19 mmpy. The corrosion rate of the mix-culture (SSVSRB1&2) was only slightly higher than those of else. After first day exposure, the corrosion rate increased rapidly to maximum point of 0.96 mmpy and then decreased slowly to a value of 0.66 mmpy in day 13 and dropped suddenly to the end point of 0.31 mmpy.
It has been reported that corrosion appears to be worse when a wide variety of microorganisms is present (Cheung et al. 1994; Starosvtesky et al. 2000). The mechanism of corrosion for di-culture was quite different from that presented in mono-cultures (Dowling et al. 1988). The mono cultures usually induced higher corrosion rates initially, but with time the corrosion rates decreased compared with that of the sterile control.
The Tafel plots of the quasi-steady polarisation curves of stainless steel in the VMNI medium (control) and
FIGURE 2. Corrosion rates as a function of time for stainless steel in VMNI medium and filter-sterilized seawater without bacterial inoculation
containing of SRB1, SRB2 and SRB1 & 2, obtained on the 8 days exposure are given in Figure 4. In the medium containing SRBs, the corrosion potential of stainless steel slightly shifted towards the negative values compared to that of control. The corrosion potentials, (Table 2) increased slightly from -0.74 V(SCE) (in control) to about of -0.89 V(SCE) for SRB1, -0.97 V(SCE) for SRB2 and -0.80
V(SCE) for SRB1&2 on the 8 days of immersion, respectively.
No changes in the cathodic Tafel slopes were observed but the anodic Tafel slopes increased significantly for
SRB1&2 from 1.43 V/dec (Control) to 11.99 V/dec as a results of the presence of the mix-culture (SRB1 and SRB2).
The corrosion rate were also high in presence of SRBs i.e.
0.068 mmpy (SRB1), 0.62 mmpy (SRB2), 0.67 mmpy (SRB1&2) compared to the control which was only 0.049 mmpy. The changes value of anodic Tafel slopes implies the higher corrosion rates of steel, that involved the changes in corrosion mechanism which controlled by anodic reactions. For the experiments with the SRBs inoculated, the formation of black film in corrosion product on the sample surface was observed. After 24 hours of immersion, the formation of the porous soluble film continued throughout the experiment and turned the test solution to black over period of the experiment.
FIGURE 3. Corrosion rates as a function of time for stainless steel in VMNI medium inoculated with different SRB isolates
(a) (b)
(c)
FIGURE 4. Tafel plots of the quasi-steady polarisation curves of stainless steel in sterile VMNI (control) and VMNI inoculated with (a) SRB1 (b) SRB2 and (c) SRB1&2 during the study on the 8 days of immersion
The changes of corrosion potential and current density is in agreement with previously data by Gaylarde and Johnston (1986); Dowling et al. (1992), and indicates the increase in the aggressivity of the medium due to the presence of bacteria. In most cases, the Tafel slope regions increased in the presence of bacteria. These changes may be induced by alterations in the solution pH, iron dissolution inside pits, specific adsorption species of anion, e.g. Cl-, SO42- or HPO4- (in VMNI medium), the electrode surface was highly covered by adsorbed species, etc. The changes in the Tafel slopes in both anodic and cathodic regions have allowed us to suggest that SRB induces a quite remarkable effect on both anodic and cathodic dissolution of steel.
The interaction between microbial species are complex. Gaylarde and Johnston (1986) showed that anaerobic corrosion of mild steel was enhanced in pure
cultures of Desulfovibrio vulgaris, but reduced to below control levels by pure Vibrio anguillarum, in the presence of both species, corrosion rates were the highest of all. On the other hand, a second facultatively anaerobic bacterium, probably of the genus Citrobacter, had little effect on corrosion rates, except in triple cultures, where it apparently modified the action of the other species microbial species.
It was suggested that V. Anguillarum produced a strongly- bound, protective film on the metal surface in pure cultures, but this film incorporated SRB cells when D. Vulgaris was present, turning it into a highly aggressive biofilm. The incorporation of the third organism into this biofilm would reduce the SRB population.
Figure 5 (a) and (b) shows the SEM and EDAX of deposits on the surface of stainless steel in sterile VMNI
(control) after 15 day exposure. The strong presence of
TABLE 2. Corrosion data for stainless steel in sterile VMNI (control) and VMNI inoculated with SRB1, SRB2 and combination of SRB1&2 during the study on the 8 days of immersion
Strain of Ecorr V( sce) Icorr (µAcm-2 ) Corr. rate (mmpy) !a (V/dec) !c (V/dec) inoculated
Control -0.74 3.56E-5 4.94E-2 1.43 0.55
SRB1 -0.89 4.64E-5 6.75E-2 0.54 0.21
SRB2 -0.97 2.31E-4 6.21E-1 0.83 0.34
SRB1&2 -0.80 4.82E-4 6.69E-1 11.99 0.48
FIGURE 5. (a) SEM images (Mag. 15,000X) and (b) EDS spectra of the deposits on the surface of stainless steel in sterile VMNI (control) after 15 day exposure; (c) SEM images (Mag. 15,000X) and (d) EDS spectra of the deposits on the surface
of stainless steel in VMNI medium containing SRBs culture after 15 day exposure
(a) (b)
(a) (b)
Fe, Cl, Na, Fe, O and Mg but low in S was observed for stainless steel control sample. The highest concentration of ferum and followed by oxygen indicates that the deposits are a form of iron oxide or hydroxide. The SEM and EDS of deposits on the surface of stainless steel in VMNI inoculated with SRBs after 15 day exposure presented in Figure 5 (c) and (d). The strong peaks of Fe, O and S, as seen in Figure 5 (d), indicating the presence of iron sulphide or iron oxide compounds recovered from the SRB growth on stainless steel. The appearance of S peak is due to the presence of metal sulphides formed as a result of SRB metabolic activities. It is well know that the higher concentration of sulphide in corrosion product indicated that the influenced of SRB in corrosion processes. The ferrous sulphide layer is formed on metal surface by Fe2+ reacting with hydrogen sulphide produced by SRB (Videla 1990). X-ray difraction
(XRD) was carried out on corrosion products deposited on metal surfaces. XRD analysis results showed that the corrosion product for sterile medium as iron oxide (Fe2O3) and medium inoculated with SRBs as mackinawite (tetragonal FeS1-x), respectively. The presence of mackinawite or greigite among corrosion products of iron is generally evidence that SRB participated in the corrosion reaction (Beech & Gaylarde 1999). The initial corrosion product formed as far as SRB is concerned is mackinawite, an iron rich sulphide, that forms a poorly protective layer on the metal surface. It is known that mackinawite is produced easily from iron and iron oxide by consortia of microorganisms including SRB (Hamilton 1994).
CONCLUSION
Electrochemical data show great changes in the kinetics and also in the mechanism of biocorrosion (changes in the anodic Tafel slopes) for stainless steel in presence of the SRB isolates compared to that of the blank solution. In the
VMNI containing SRBs, Ecorr was shifted to more negative anodic values, increase in current density which consequently raised the corrosion rates. A high amount of sulphide detected in corrosion products was due to the presence of SRBs and XRD result confirmed that the product was mackinawite (tetragonal-FeS).
ACKNOWLEDGEMENT
We wish to acknowledge the Malaysia Marine and Heavy Engineering Sdn. Bhd. (MMHE), Johor for providing the facilities and to UKM-JPA for generous financial support throughout the study.
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Fathul Karim Sahrani
School of Environment and Natural Resource Sciences Faculty of Science and Technology
Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor D. E Malaysia
Mazlan Abd. Aziz, Zaharah Ibrahim, Adibah Yahya Department of Biology/Chemistry
Faculty of Science
Universiti Teknologi Malaysia 81310 Skudai, Johor Darul Takzim Malaysia
Received : 25 May 2007 Accepted: 6 November 2007
