Microbially deposited manganese and iron oxides on passive metals - their chemistry and consequences for material performance

Abstract

The open-circuit potential (OCP) values of Type 316L (UNS S31603) stainless steel and Ti-6Al-4V (UNS R56400) corrosion coupons, exposed to fresh river water, were ennobled to as high as 365 mV vs saturated calomel electrode (SCE) and 400 mVsce respectively. With microchemical imaging capabilities and high-detection sensitivity, a surface analysis technique based on time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was developed to identify the oxidation states and distribution of biominerals on the ennobled metal surfaces. ToF-SIMS spectra of the microbial deposits compared to spectra of different manganese and iron mineral standards indicated that the biominerals on the metal surfaces are a mixture of ferric oxide (Fe2O3), manganese oxide (Mn3O4), and manganese oxyhydroxide (MnOOH) on fully ennobled coupons, and a mixture of iron oxide (Fe3O4), Fe2O3, Mn3O4, and manganese(III) oxide (Mn2O3) on partially ennobled coupons. Biomineralized manganese and iron oxides on the Type 316L stainless steel surfaces, regardless of the oxidation states, endanger the material integrity in a similar manner, as evidenced by the elevated OCP and increased cathodic current density upon mild polarization.