ABSTRACT

Sulfate reducing bacterial (SRB) influenced corrosion of carbon steel has been studied to determine the role of corrosion product/biofilm interactions on the electrochemical behavior of the metal. The influence of sulfides, chloride and ferrous ions on the electrochemistry processes has been assessed by using various electrochemical techniques.

Corrosion products were analyzed by energy dispersion X-ray analysis (EDAX), X ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPM). Microscopic observations of the metal surface were made by using light and scanning electron microscopy (SEM). Results show that carbon steel dissolution is due to simultaneous changes caused by synergistic or inhibitory interactions between anions, cations, corrosion products and biofilms. SRB alter local chemical conditions and the corrosion products layers. Moreover, there are differences in the composition and structure of biogenic sulfide films, when compared with those formed in abiotic media. The electrochemical behavior of the steel can be changed from active to passive as a result of corrosion product/biofilm interactions. These changes can also help in the understanding of different hydrogen effects observed in steel in biotic and abiotic media.

INTRODUCTION

SRB influenced corrosion of carbon steel has been a focus of biocorrosion research due to its economic importance and frequent occurrence in a variety of industrial environments1-2.

The importance of microbial effects on corrosion caused by biofilms containing consortia of SRB and other bacteria has been stressed in recent publications3-5. The final result of these effects at the biofilm/metal/solution interface is the generation of a wide variety of sites that are markedly different from non or differently colonized surrounding areas on the metal surface. The non homogeneous coverage of the metal facilitates the initiation of localized corrosion processes6.

Passivity breakdown and localized corrosion of carbon steel strongly depends on the aggressive anions present in the medium and on the protective characteristics of the corrosion product films formed on the metal surface7,8.

Strongly adherent homogeneous films can be formed in biogenic media, inhibiting localized corrosion. This has been proposed to explain one mechanism of microbial corrosion inhibition in a recent publication9. This kind of deposits formed in biogenic media are also able to induce a reduction in crack grow propagation as reported for sour SRB environments containing biogenic sulfides10,11.

The aim of this work is to quantify characteristics of sulfide films formed in abiotic and biotic media and the influence of ferrous ion additions on the electrochemical behavior of carbon steel by using different electrochemical and surface analysis techniques complemented with optical and electron microscopy.

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