Influence of Ferrous Iron on the Rate and Nature of Microbiologically Influenced Corrosion of High-Strength Steel Under Sulfate-Reducing Conditions

The influence of ferrous iron, sulfate reduction, and stress were examined in the microbiologically influenced corrosion (MIC) of mild steel by Desulfovibrio desulfuricans subsp. aestuarii (American Type Culture Collection [ATCC] 29578). Corrosion was measured by monitoring the electrical resistance of a thin steel wire used as an electrical resistance probe (ERP) over time. Measurable corrosion required active sulfate reduction and ferrous iron concentrations of 1.2 mM or greater. Corrosion occurring at the highest Fe(II) levels tested (12 mM and 18 mM Fe[II]) was characterized by a marked increase in resistance that was readily explained by the electrochemical dissolution of the wire. Corrosion at lower iron levels resulted in a more rapid failure of the ERP (severance of electrical continuity) but less electrochemical dissolution (as determined by resistance increase). Increasing ferrous iron levels, therefore, appeared to result in a transition from a highly localized rapid attack to a more uniform attack with a lower maximum rate of penetration. Average corrosion rates, based upon resistance, were found to increase during incubation in all cases where a resistance increase was detectable. Similar patterns were observed with rates derived from potentiodynamic polarization. Corrosion at the lowest Fe(II) level (1.2 mM), which consistently produced failure of the wires, appeared to occur through a mechanism involving both sulfide and tensile stress. Continued sulfate reduction was required at the low iron level to produce rapid failure of the wire samples, indicating that sulfate reduction not only initiated the observed corrosion but also was important in sustaining it. Elevated sulfide levels at low Fe(II) levels may have resulted in biologically induced sulfide stress corrosion cracking (SSCC). The results of this study emphasize the importance of sulfate reduction in initiating and sustaining aggressive MIC. In addition, an unexpected relationship was observed between the rate and uniformity of corrosion as a function of the ferrous iron concentration.