Role of corrosion products in biofilms in microbiologically induced corrosion of carbon steel

Abstract

The effects on microbiologically induced corrosion (MIC) of carbon steel resulting from the presence of corrosion products in the biofilm of sulphate reducing bacteria (SRB) have been investigated. The SRB were isolated from the Changqing oilfield in China. Electrochemical impedance spectrometry and a hydrogen monitoring device were used to study the accumulation of the biofilms. The corrosion products were analysed by both electron probe microanalysis and X-ray diffraction, and the metabolites of the SRB in the biofilm were detected by gas chromatrography and mass spectrometry techniques. The results have shown that the first corrosion product to form in the biofilm is mackinawite, followed by the appearance of troilite and greigite. Only the mackinawite layer has protective properties; the others accelerate the corrosion of carbon steel. Both the activity of the SRB and the structure of the SRB biofilm influence the initiation and development of corrosion. The process of MIC involves first the development of SRB activity, which promotes the entry of hydrogen into the carbon steel. This is then prevented by the formation of a layer of mackinawite corrosion product. Finally, metabolites of the SRB, including S^2- and organic acids, destroy the protective layer and promote hydrogen permeation.