Sulfate Reducing Bacteria Influenced Corrosion of Carbon Steel—Nutrient Effects

Microbiolobically influenced corrosion (MIC) of carbon steel exposed to the sulfate reducing bacterium (SRB) Desulfovibrio africanus was investigated with electrochemical analytical tools. The corrosion of carbon steel exposed to mixed solution (synthetic seawater and growth media) in the presence and absence of SRB was characterized by Electrochemical Impedance Spectroscopy (EIS), Polarization Resistance (R_p) and Open Circuit Potential (OCP) measurements. EIS spectra show two time constants for the sterilized system of synthetic seawater and growth media. At high frequency, this constant is related to an outer layer due to the precipitation of chlorides, organic compound present in the growth media and corrosion products. The constant at low frequency can be attributed to the metal dissolution reaction. In the systems containing SRB, the formation of biofilm by D. desulfuricans accelerates the corrosion rate, and a diffusion-controlled process appears as a result of sulfide-rich iron corrosion products accumulation on the metal surface. Comparison of bacterial growth rate and effect on corrosion rate were conducted in different solutions, one of which was periodically provided with nutrients. The effect of adding fresh nutrients to the growth of D. desulfuricans in synthetic seawater was investigated by counting the planktonic cells in the solution.  The metal surface was examined by the scanning electron microscopy (SEM) and the elemental composition of the corrosion products was evaluated with energy dispersive spectroscopy (EDS).   Nutrient addition increased the amount of biogenic hydrogen sulfide present on the steel surface, resulting in an increase in metal dissolution rate.  Experimental result reveals a substantial decrease in the R_p values for the coupon in the solution provided with fresh growth media in the presence of SRB.