Microbiological and chemical aspects of corrosion of sheet steel in the Duluth-Superior Harbor

Abstract

Steel structures in the Duluth-Superior harbor show unusual patterns of corrosion characterized by raised blister-like nodules called tubercles and pitted steel. This corrosion phenomenon could possibly decrease the integrity and the lifespan of the structures. Microbiological and chemical factors that may be influencing corrosion processes were examined in this study from 2007-2009. A laboratory microcosm experiment was designed to examine several microbiology and water quality parameters. Terminal-restriction fragment length polymorphism (T-RFLP) analyses showed that bacterial communities on steel were different from one another in microcosm treatments with different types of water (e.g. autoclaved Duluth-Superior harbor water vs. unaltered Duluth-Superior harbor water and Lake Superior water vs. Duluth-Superior harbor water). Coupons with different bacterial communities also showed differences in sulfate-reducing bacterial abundance and iron-oxidizing bacterial abundance. Further, coupons with higher abundances of sulfate-reducing bacteria and iron-oxidizing bacteria had higher inverse polarization resistance values. Inverse polarization resistance is frequently used to approximate the instantaneous rate of corrosion. Mass lost and pit depth were measured on steel in the microcosm, and these two measurements showed a positive correlation. Microbiological and corrosion measurements were also performed on steel in the Duluth-Superior harbor, and these measurements were compared to steel in the microcosm experiment. These experiments showed correlations among water chemistry, bacterial community, and populations of specific bacteria that are associated with corrosion in other ecosystems.