Comparison of sulphide corrosivity of sulphate- and non-sulphate-reducing prokaryotes isolated from oilfield injection water Z. Duque, J. R. Ibars, M. I. Sarro ´ and D. A. Moreno * The microbiologically influenced corrosion (MIC) of water injection systems by sulphate-reducing prokaryotes (SRP) has caused many problems in the oil industry. These prokaryotes produce H 2 S, which reacts aggressively with steel and is thus widely considered to be the main cause of bacterial corrosion of industrial oil equipment. However, current microbiological treatments and controls have not taken into account other groups of sulphidogenic prokaryotes, which also produce H 2 S or its derivatives and with the same adverse effects of MIC. In the present work, sulphidogenic prokaryotes were isolated from water injection systems and identified by DNA sequencing. The identified species included sulphate-reducing Desulfovibrio termitidis and non-sulphate-reducing Escherichia coli. Biocorrosion tests were carried out on API 5L grade X65 carbon steel. Electrochemical impedance spectroscopy, polarisation resistance, open circuit potential and weight loss were carried out. Steel corrosion resulting from the production of the metabolite H 2 S by SRP and non-SRP was observed, with sulphide generation by SRP much greater than that by non-SRP. These results confirm the need to investigate and consider the role of not only SRP but also non-SRP in order to improve the control over bacterial corrosion of oil-industry equipment. 1 Introduction In the 1980s, microbiologically influenced corrosion (MIC) was internationally recognised as the cause of serious problems in the oil industry [1], accounting for 50–90% of all localised corrosion and 30% of the total cost of corrosion. In MIC, the culprit microorganisms are mainly sulphate-reducing prokaryotes (SRP), but efforts aimed at their control have been largely unsuccessful and MIC continues to be a recurrent problem. Magot et al. [2] and Crolet and Magot [3] detected H 2 S- producing non-SRP strains in the corrosion products found in crude-production water pipelines in Africa, highlighting the capacity of these microorganisms to produce significant amounts of H 2 S and organic acids from thiosulphate and peptides, which for some bacterial species are the only source of carbon and energy. Additionally, H 2 S is responsible for souring (increased sulphide concentrations) in oil reservoirs. The importance of sulphide generation in MIC by non-SRP strains was also described by Lie et al. [4]. These results point to the relevance of identifying H 2 S- producing anaerobic groups (non-SRP) and assessing their influence, together with that of SRP, on corrosion processes. However, to date, there have been few published studies on non- SRP strains as agents of MIC and thus their importance to the control of this type of corrosion has been largely ignored. Here we present the results of a study of MIC by strains of SRP and non- SRP isolated from the injection water used in secondary crude recovery processes. Conventional microbiological, molecular biological, electrochemical and gravimetric techniques were applied to determine the relative influence of these strains on MIC. 2 Materials and methods The study was divided into three phases: 1) sampling of oil-related water and detection of H 2 S-producing prokaryotes; 2) isolation Materials and Corrosion 2011, 62, No. 9999 DOI: 10.1002/maco.201106298 1 Z. Duque, J. R. Ibars, M. I. Sarro ´, D. A. Moreno Universidad Polite ´cnica de Madrid, Departamento de Ingenierı ´a y Ciencia de los Materiales, Escuela Te ´cnica Superior de Ingenieros Industriales, Jose ´ Gutie ´rrez Abascal 2, E28006 Madrid (Spain) E-mail: diego.moreno@upm.es M. I. Sarro ´ Centro Nacional de Investigacio ´n Sobre la Evolucio ´n Humana (CEN- IEH), Paseo Sierra de Atapuerca s/n, E-09002 Burgos (Spain) www.matcorr.com wileyonlinelibrary.com ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim