Effect of Low pH on Corrosion Behaviour of Duplex Stainless Steel in Seawater Containing Sulphate Reducing Bacteria

Sulphate reducing bacteria (SRB) are able to regulate the environmental pH because of their metabolism. This study focuses on effect of pH on corrosion of duplex stainless steel 2205 in a nutrient rich artificial seawater environment containing SRB species, Desulfovibrio vulgaris for 28 days at 370C in pH ranging from 4.0 to 7.4. The open circuit potential value, sulphide level, pH and number of bacteria in the medium were recorded. Electrochemical impedance spectroscopy (EIS) was used to study the properties of passive film and biofilm. Inductively coupled plasma mass was used to measure the concentration of cations Fe, Ni, Mo, Mn in the experimental solution after 28 days. Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) were used for surface analysis. The results showed the ability of D. vulgaris to regulate the environmental pH after 5 days. The highest iron concentration was at pH 4 and this was 3 times higher than at pH 7.4 indicating increased release of iron due to corrosion at lower pH. SEM revealed pitting on the stainless steel only at pH 4. EDX showed the presence of sulphide on all specimens but with more sulphide corrosion products at pH 4. EIS showed the film resistance of the specimen at pH 4 was much lower than at pH 7.4 which suggests the corrosion resistance of the stainless steel was better at pH 7.4 than at pH 4. The nature and mechanism of SRB attack on duplex stainless steel at different acidic environments are discussed.