Leptothrix discophora SP-6: Effects of biofilms on passive film chemistry of 316L stainless steel and modeling of growth

This thesis examined effects of manganese oxidizing bacteria, Leptothrix discophora SP-6 biofilms, on passive film chemistry and pitting corrosion of 316L stainless steel.; Biofilms of manganese oxidizing bacteria attach to the passive films of metals and deposit manganese oxides on the surfaces. These oxides, being in equilibrium with manganese ions in water, shift the corrosion potential of the passive metals in the noble direction, causing a phenomenon known as “ennoblement”. Ennoblement is of interest because it increases corrosion potential, which may initiate localized forms of corrosion of passive metals, pitting corrosion. The mechanism of pit initiation on the ennobled coupons still is not completely known. For localized corrosion to occur, the passive film has to be broken down, and it has been speculated that the biofilms of manganese oxidizing bacteria have a direct effect on the chemistry of the passive films.; The effect of ennoblement of 316L stainless steel by biomineralized manganese deposits on chemistry of passive films was studied using surface-sensitive analytical techniques and cyclic polarization. Depth profiles of elements in the passive films on the ennobled coupons were analyzed using x-ray photoelectron spectroscopy (XPS), and distribution of metal elements were examined by time-of-flight secondary ion mass spectroscopy (ToFSIMS). The results showed that oxide layers on the ennobled coupons were thinner than those on the control coupons. Cyclic polarization curves showed that ennobled 316L stainless steel indicated significant loss of passivity. It was concluded that metabolic activity of manganese oxidizing bacteria, Leptothrix discophora SP-6 degraded the quality of the passive film on stainless steel coupons by locally reducing its thickness and lowering the pitting potential.; To relate the kinetics of ennoblement to the rate of deposition of manganese oxides on metal surfaces, growth kinetics of Leptothrix discophora SP-6 were quantified in biofilms and in planktonic form. In planktonic form, double-substrate growth kinetics, using Monod growth kinetics for pyruvate and Tessier growth kinetics for oxygen, showed the best agreement with the experimental data. Monod model of microbial growth kinetics adequately represents the growth of Leptothrix discophora SP-6 biofilms.