HEOPTAHHHECKHE MATEPHAMbI, 2007, moM 43, № 6, c. 698-705
UDC 620.141
Pitting Initiation of 316L Stainless Steel in the Media
of Sulfate-Reducing and Iron-Oxidizing Bacteria © 2007 r. Y. H. Zhang*, C. M. Xu*, G. X. Cheng*, W. S. Zhu**
*Department of Chemistry Engineering, Xi'an Jiaotong University, China **Research and Technology Center of Lanzhou Oil Refinery Factory, Petro China Company limited, China e-mail: gxcheng@mail.xjtu.edu.cn Received 22.05.2006
Pitting corrosion behavior of stainless steel 316L in the presence of aerobic and anaerobic bacteria isolated from cooling water system in oil refinery was investigated using open circuit potential measurement, electrochemi-cally impedance spectroscopy, scanning electron microscopy examinations and energy dispersive spectrum analysis. The results shown the corrosion potential (£cor) and polarization resistance (Rp) decrease in the presence of sulfate-reducing bacteria (SRB), iron-oxidizing bacteria (IOB) and the combination of SRB and IOB, in comparison with those observed in the sterile medium for the same exposure time. The presence of SRB demonstrated higher corrosion rates than IOB. The combination of SRB and IOB created the highest corrosion rate. The metabolic activity of bacteria and the integrality and compactness of biofilm influenced the pitting corrosion process, increased the corrosion damage degree of the passive film, accelerated the pitting corrosion. It is suggested that SRB and IOB in influencing the pitting corrosion of 316L SS is highlighted.
INTRODUCTION
Type 316L stainless steel has good corrosion resistance and has been used increasingly for cooling water service in the chemical, petrochemical and power utility industries. The excellent corrosion resistance of stainless steel is due to the formation of a stable passive layer on the surface. But stainless steel is susceptible to localized corrosion by chloride ions and reduced sulfur compounds [1, 2]. The presence of microorganisms on a metal surface often leads to highly localized changes in the concentration of the electrolyte constituents, pH and oxygen levels [3, 4]. These microorganisms and their metabolic activity have influenced severely the corrosion process; they often stimulated forms of localized corrosion [5, 6].
In recent years it has become more and more common phenomenon that microbiologically activity creates pitting corrosion of stainless steel in many industrial equipments [7]. The advances in biofilm theory and techniques have allowed a better understanding of the interactions between microorganisms, metal surfaces and corrosion processes [8]. Microorganisms scale deposit and corrosion have become three big harmness factors of cooling circuit water system, the damage of microorganisms is the principal. Many products of oil refinery, on the one hand, posses on strong inhibitory characteristics and on the other hand they are the rich carbon source for microorganisms growth. Oil refinery plants have high risk of water contamination with these products [9]. Various cases of corrosion damage caused by microorganisms were observed during the last several years in cooling water system of oil refineries, sul-
fate-reducing bacteria (SRB) and iron-oxidizing bacteria (IOB) were the most troublesome group of bacteria on tubercular corrosion and induced microbiologically influenced corrosion in cooling circuit, caused poor water quality and equipment clogging, which included flow blockages of pipelines due to tubercle formation, choking of valves and strainers, pipe punctures and high corrosion rates, made a great deal of biofouling, resulted in the serious pitting corrosion of stainless steel equipment [10-12]. SRB increased rapidly in the locally anaerobic conditions beneath the iron-rich tubercles produced by IOB [13], the interaction between SRB and IOB accelerated the corrosion process of stainless steel.
The purpose of the research is to investigate the influence factor of aerobic IOB and anaerobic SRB on the pitting behavior of 316L stainless steel in cooling water system of oil refinery and micro-mechanism of pitting formation on the metal surface using open circuit potential measure, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and energy dispersive spectrum (EDS) techniques.
EXPERIMENTAL
The specimens used for corrosion were cut from Type 316L stainless steel sheet, the nominal elemental composition (wt%) of 316L SS specimens was: C 0.029, Cr 16.97, Ni 10.11, Mo 2.04, Mn 1.38, Si 0.39, P 0.031, S 0.005. Disc shape specimens with a diameter of 18 mm and thickness of 2 mm were used for electrochemical measurements, rectangle specimens with di-
Table 1. Analytical results for cooling water sampled from oil refinery
Cl- C O32 HC O3 Ca2+ Mg2+ S O42- pH Total hardness TDS SAL
242.66 19.246 435.504 112.30 200.15 240.24 8.16 1105.35 932 0.9
mensions 30 x 25 x 2 mm were used for biofilm observation. To create working electrodes, an electrical contact to each sample was provided by a length of copper wire connected to the back of each specimen mounted in an epoxy resin, then the specimens were abraded through 240, 400 and 600-grit silicon carbide metallurgical paper, degreased in acetone, washed with sterile distilled water and dried in a desiccator until use.
Experimental SRB and IOB were isolated from cyclic cooling water system of an oil refinery plant, the chemical composition of cooling water is provided in Table 1. SRB and IOB were cultivated separately in appropriate media. SRB culture was grown in corrected Post-gate'C medium (g/l): 0.5KH2P04, 1.0NH4Cl, 4.5Na2SO4, 0.06CaCl2 ■ 2H2O, 0.06MgS04 ■ 7H2O, 6 sodium lactate, 1.0 Yeast extract, 0.004FeS04 ■ 7H2O, 0.3 sodium citrate (pH 7.2) under aerobic conditions. IOB mixed culture was grown in Winogradski nutrient medium (g/l): 0.5K2HP04, 0.5NaN03, 0.2CaCl2, 0.5MgS04 ■ 7H2O, 0.5NH4N03, 6.0 ammonium iron citrate (pH 6.8) under anaerobic chamber. These solutions were autoclaved at 121°C for 20 min. These cultures were incubated at 30°C. Enrichment cultures were used as the corrosion cell inoculum. Test cells were inoculated with 5% (V/V) of each of the selected cultures.
Anaerobic SRB were added in some experiments to already growing aerobic IOB to examine the combined effect of IOB with that for SRB. We observed that SRB can be active in an anaerobic bottom layer when the bulk solution contains oxygen. Formation and maintenance of such anaerobic conditions are due to the presence of aerobic bacteria. The respiration of aerobic IOB scavenges oxygen and favors growth conditions for SRB. In these experiments, the SRB were added from 3 to 5 days after the inoculation of the medium with the aerobic bacteria.
All electrochemical tests were carried out in a 2L cell, with three-electrode system, the measurements were done with M263A potentiostat and phase lock-in amplifier (EG & G, USA). Working electrode potentials were referred to a saturated calomel electrode. The counter electrode was a Pt-plate. EIS measurements were made at the open circuit potential using a 10 mV amplitude sinusoidal signal over frequencies ranging from 5 mHz to 100 kHz. All measurements were carried out at 30°C for optimum bacteria growth.
Corrosion pits and morphology on the coupon surfaces were analyzed using SEM and EDS. The coupons with biofilm were immersed for 15 min in a 4% glut-araldehyde solution in order to fix the biofilm to the stainless steel surface, and then become dehydrated us-
ing four ethanol solutions (15 min each): 25, 50, 75 and 100% successively. After that, the samples were taken to the SEM and EDS for their surface analysis.
RESULTS AND DISCUSSION
The variations of Ecor with exposure time for 316L stainless steel in sterile, SRB, IOB and SRB + IOB mediums at 30°C is shown in Fig. 1. In the sterile medium, no significant changes in Ecor of 316L SS occur, the electrode surfaces were observed and didn't find the clear corrosion after the termination of test, indicating that the steel specimens were in a passive state during the whole test session. In the presence of only SRB, the drastic decrease of Ecor occurred from -0.02 to -0.46 V during one day, then practically kept this value constant up to the termination of the experiment, indicating the activation of the initially passive steel electrodes, this drop was attributed to the presence of the SRB metabol-ically products sulfide. In the presence of only IOB, the Ecor dropped to approximately -0.32 V during 0-3 days indicating an activation of the initially passive sample. Then the activated sample repassivated with time increase and the Ecor shifted toward the positive direction during 3-7 days, subsequently it dropped to -0.4 V again during 7-10 days. After 10 days the Ecor kept this value constant up to the termination of the test. In the SRB + IOB solution, the Ecor dropped sharply to -0.48 V after one day, indicating the steel activation occurrence. Ecor practically did not change during the following two days of exposure. Upon additional exposure it gradually decreased and after 24 days it reached value
Ecor (vSCE)
Time, h
Fig. 1. Variation of corrosion potential with time for 316L SS in different solutions.
Fig. 3. SEM micrograph and EDS analysis showing corrosion products formed on 316L SS surface after 30 days exposure, with presence of SRB (a), IOB (b) and SRB + IOB (c).
close to -0.54 V. The combination of SRB and IOB shifted Ecor values at a rate faster than that observed in the presence of SRB or IOB alone. The presence of SRB leads to the open circuit potential further drop than that observed in IOB solution.
SEM was carried to validate the adhesion of the microorganisms to the 316L SS surface and also to analyze microbial diversity. Figure 2 shows a detail of biofilm developed on 316L SS surface exposed to: (a) SRB medium; (b) IOB medium; (c) SRB and IOB medium
Fig. 4. SEM micrographs showing corrosion pits on
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