Microbiologically influenced corrosion of stainless steel by sulfate reducing bacteria – A tale of caution M.A. Javed*,**** W.C. Neil**, G. McAdam**, J.W. Moreau*** and S.A. Wade* *Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne, Australia **Defence Science and Technology Group, Melbourne, Australia ***School of Earth Sciences, The University of Melbourne, Australia ARTICLE INFO Article history: Received Day Month Year Accepted Day Month Year Available Day Month Year Keywords: A. microbiologically influenced corrosion B. pitting C. stainless steel D. sulphate reducing bacteria ****Corresponding author: Email: mjaved@swin.edu.au; telephone no. +61392144970; fax no.+61392145050; Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne, Australia ABSTRACT The influence of different experimental media composition and air purging on the potential for microbiologically influenced corrosion (MIC) of 304 stainless steel with sulfate reducing bacteria (SRB) was investigated. Modified Baar’s (MB) medium, MB medium without iron ions and supplemented with sodium chloride (MBN) and air purged MBN medium (MBO) were used. Pitting corrosion attack was found on the surface of the coupons for all of the conditions tested including the abiotic tests, and detailed statistical analysis showed no significant difference between the pitting results. General corrosion and maximum pit penetration rates also showed no difference between the coupons exposed to different test conditions. Interestingly, the pits found on the surface of the coupons in all the tested conditions were comparable in size/shape and depth to that of the inclusions present on the surface of the stainless steel coupons. These findings suggest that (i) the test conditions studied do not lead to increased corrosion rates of stainless steel with SRBs and (ii) care needs to be taken to avoid the pitfall of misinterpreting the corrosion of inclusions present on the surface of stainless steels, which can occur as a result of cleaning of the coupons, as MIC pits. INTRODUCTION Stainless steels, in general, exhibit excellent corrosion resistance. This has led to their widespread use in various applications such as in the chemical/petrochemical industries, power generation and for use in water supply systems [1, 2]. The corrosion resistance of stainless steels is primarily due to the presence of a stable passive film that forms on their surface [1]. However, localized pitting corrosion attack in the presence of microorganisms, chloride ions and/or reduced sulfur compounds is a potential issue with stainless steels [3-6]. Sulfate reducing bacteria (SRB) were one of the first, and remain one of the most frequently studied groups of microorganisms in relation to MIC [7-10]. SRB are anaerobic microorganisms that use sulfate as a terminal electron acceptor for their respiratory metabolism and produce sulfide as a metabolic by-product [11, 12]. They have been shown to cause localized corrosion attack on the surface of metallic materials, especially ferrous alloys, through the production of corrosive chemical agents (e.g. hydrogen sulfides, H2 S) and/or via withdrawal of electrons directly by metabolic coupling [13-15]. SRB have been reported as a contributing factor in many of the field reports of MIC failures in stainless steels [16-25]. Laboratory studies have been performed to try to replicate the rapid corrosion (i.e. on the order of several mm per year) of stainless steels that occur in the field due to the presence of SRB, or to investigate MIC with various types of stainless steels and microbes. Most of these studies, however, have either produced insignificant rates of corrosion [26-28] or used electrochemical measurements as evidence to show that MIC has taken place [29-31]. It is worth mentioning that some caution needs to be taken when interpreting corrosion rates measured by electrochemical techniques in the presence of microbes, as in some cases these methods have been considered to (i) affect the bioactivity of the biofilm due to externally applied electric field [32, 33] and/or (ii) are subject to interference due to test media constituents e.g. yeast extract [34]. In addition, most of the reports of laboratory tests of MIC of stainless steel often provide only very limited (indeed if at all) information on any pitting attack observed during the testing (Table 1). TABLE 1. Examples of pitting corrosion data reported in various laboratory-based corrosion studies of stainless steel and SRB. The specific composition of test media has been shown to play an important role in the outcome of MIC tests [44-46]. In relation to microbial corrosion of stainless steels, the effect of different test media has not received a great deal of attention. Work has suggested however that the chlorides to anions ratios, soluble iron ions and/or the presence of oxygen in the media can play a role in the MIC of stainless steels. Despite this, there is generally no consistency in the testing media and/or conditions used for studies of MIC of stainless steels, with virtually every individual research group using different testing arrangements. The potential effects of test media on MIC are discussed in more detail below: Role of chlorides and other anions: Chlorides are well known to be aggressive to stainless steels, where the pitting potential tends to drop as the concentration of chloride ions increases [47]. Conversely, there are several anions present in common test media used for laboratory- based SRB corrosion studies that can act as inhibitors to the corrosion of stainless steel, such as sulfates, hydroxides, phosphates, carbonates, nitrates and acetates [48-50]. Literature has indicated that the relative concentration of the aggressive anions to inhibiting anions (i.e. chlorides to ∑ anions ratio) is very important for the severity of MIC of stainless steel [51, 52]. Role of iron ions: The presence of iron ions in test solutions has been shown to play an important role in the growth and metabolism of microorganisms including SRB [53, 54]. The absence of iron ions in test media containing SRBs could possibly increase the production of H 2 S,