Influence of denitrification on the corrosion of iron and stainless steel powder

The purpose of this research was to address the anoxic oxidation of metallic iron and stainless steel powder by nitrate, nitrite, and anaerobic mixed cultures. In sterile batch reactors, both nitrate and nitrite(10 mg N/L) could chemically oxidize metallic iron, with a concomitant reduction to ammonium. Nitrate or nitrite reduction coupled to metal corrosion was not observed in the case of stainless steel powder. Combination of an anaerobic mixed culture and metallic iron led to (cathodically produced) Hg consumption and a complete nitrate or nitrite reduction (mainly to NH4+). This caused a slightly enhanced metal oxidation. In the case of stainless steel, corrosion caused by denitrifying microorganisms was evidenced by data on nitrate/nitrite ions and solubilized iron. Experiments with increasing nitrite concentrations indicated that nitrite in the range of 50 mg of NO2- -N/L inhibited the corrosion processes. Moreover, at concentrations above 140 mg NO2- -N/L, a significant production of nitric oxide (NO) was detected. Differences between iron and stainless steel powder at low concentrations of nitrate or nitrite are most probably due to differences in kinetics: metallic iron exhibited faster chemical than biological reactions as opposed to stainless steel. It is postulated that the inhibitory effect of higher nitrite concentrations could partly be due to the chemical formation of NO and its toxic effect on the microorganisms acting at the steel surface