The effect of nitrate and nitrite supplementation on the biocorrosion of mild steel coupons in a loam soil system.
Microbially influenced corrosion is the participation of microorganisms in the corrosion process. This study determined the effect of nutrients, viz., nitrate and nitrite supplementation on the microbial corrosion of mild steel in loam soil. The optimal concentration of nitrate and nitrite needed to sufficiently inhibit biocorrosion was determined by incubating mild steel coupons in loam soil supplemented with increasing concentrations of nitrate and nitrite (5, 10, 20 and 40 mM respectively). Coupons were removed every 4 weeks and used to determine the extent of corrosion based on the weight loss method. The surface of the coupon was analysed using Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray (EDX) analysis. The biofilm formed on the coupon surface was studied by determining the protein and carbohydrate content as well as the species diversity within the biofilm using Denaturing Gradient Gel Electrophoresis (DGGE). Optimal nitrate and nitrite concentration were used in an in situ study to determine the efficacy in the external environment. Individual aerobic species were isolated from the coupons supplemented with nitrite and used to determine their potential to inhibit corrosion in a corrosive saltwater environment. Nitrate supplementation was found to increase the extent of corrosion and biofilm formation significantly, with greater concentrations leading to higher corrosion rates, when compared to the non-supplemented coupons. SEM observations confirmed the presence of extensive corrosion product and biofilm formation. EDX analysis determined the main components of the corrosion products to be iron and oxygen. Maximum corrosion rate was determined at 40 mM at week 20 (123.85 mg/cm²). The in situ study revealed similar results in which 20 mM nitrate supplementation increased corrosion rate significantly. Nitrite supplementation led to a decrease in corrosion rates as well as biofilm formation, with no corrosion or biofilm formation detected at 20 mM nitrite supplementation. SEM observations determined no corrosion or biofilm formation at 20 mM. The in situ results using 20 mM nitrite showed a decrease in corrosion rate. However this was not significant when compared to the unsupplemented controls. Sequence data in the laboratory experiments revealed phylotypes belonging to 2 major distinct phylogenetic groups, the Firmicutes and α-Proteobacteria. In situ experiments showed bacterial diversity exhibited phylotypes belonging to the Firmicutes, α-Proteobacteria, and γ-Proteobacteria. The community was found to differ between the non-autoclaved and nitrate-treated systems, with a higher bacterial diversity observed in the nitrate treated systems, however, the dominant microorganisms were found to be Bacillus species. This group of microorganisms are iron-oxidizing bacteria that could also promote the corrosion process. Nitrate addition, in this study, was found to increase corrosion rate of mild steel in loam soil, however, nitrite addition was found to significantly reduce corrosion rates as well as decrease biofilm formation. Furthermore, aerobic microorganisms were observed to play a role in the corrosion process in mild steel. Further studies would require a multidiscipline approach into the various soil factors involved and their interplay in the corrosion reaction to determine the viability of nitrite in the long term control of mild steel corrosion in loam soil.