Microbiologically Influenced Corrosion of Stainless Steel by Iron-Oxidising Bacteria

Abstract

Microbiologically influenced corrosion (MIC) of stainless steel (SS) can be caused by the action of Metal-Oxidising Bacteria (MOB) in natural, relatively low-chloride waters. This type of corrosion is frequently associated with ennoblement and involves Iron Oxidising Bacteria (IOB) and Manganese Oxidising Bacteria (MnOB). This work focuses on the role of MOB and associated inorganic water chemistry processes that may cause corrosion of stainless steel. The surface analysis of the industrial 316L stainless steel field sample shows massive pitting corrosion at weld sites under the surface deposit. This damage is a consequence of ennoblement associated with the activity of manganese-oxidising bacteria and chlorination through the water treatment. The increase of the open circuit potential (OCP) value occurs through the deposition of two MnO2 phases: pyrolysite and ramsdellite. Laboratory and field studies in Hawkes Bay (New Zealand), have been conducted to investigate the effect of iron-biofouling and the influence of deposited ferric oxide (or oxyhydroxide) by (IOB) on the corrosion of 304L SS. Samples exposed to a natural spring water reached open-circuit potentials of +250 mV (SCE) and were covered by a biofilm containing a mixture of α-FeOOH, γ-FeOOH, Fe3O4 and γ- Fe2O3, together with low levels of manganese-based deposits. Analysis of the Raman spectra of the ennobled coupons is based on Raman spectra of the reference compounds, and literature data. Additionally, using atomistic simulation, the phonon energies and their eigenvectors have been calculated for Raman spectra of Fe3O4 and γ-Fe2O3, and Raman spectrum of α-Fe2O3. In the laboratory, an iron-based film was formed on some samples by oxidation of FeSO4 with Ca(ClO)2, giving a surface film with a composition similar to that produced by microbial activity. These samples did not show ennoblement, but they did show lower pitting potentials than were measured for control samples. The results of this work are consistent with the idea that MIC of SS in potable water involves manganese-based deposits causing ennoblement (W.H. Dickinson, F. Caccavo Jr, Z. Lewandowski, Corrosion Science, 38 (1996): p.1407; B. Olesen, R. Avci, Z. Lewandowski, Corrosion Science, 42 (2000): p. 211.), combined with iron-based deposits simultaneously causing a decrease in the pitting potential (M. Suleiman, I. Ragault, R. Newman, Corrosion Science,36 (1994): p.479). We have also found that very low levels of silicate (i.e. 5-10ppm) in the water can have a substantial effect on the pitting potential of stainless steel. The obtained reduction in pitting potential was around 120 mV, despite the extremely low concentration of chloride ions (0.001M). We propose a mechanism whereby at low silicate concentrations polymerisation occurs inside a pitting area through the acid-catalyzed polymerisation of monomeric silicic acid. This polymeric film possesses a positive charge and maintains a low pH, thereby accelerating pitting dissolution.