Fine-Scale Bacterial Community and Chemical Changes within Steel Corrosion Tubercles in the Duluth-Superior Harbor

Abstract

Corrosion of steel structures in the Duluth-Superior Harbor (DSH) is a severe economic problem. This corrosion is characterized by deep pits covered by crusty, orange, blister-like structures called tubercles. Prior research demonstrated these tubercles contain both iron-oxidizing (IOB) and sulfate-reducing bacteria (SRB) but fine scale changes of bacterial community structure within these tubercles remain unknown. Experimental steel coupons were placed at four sites in the DSH in 2006 and 2012 and recovered in 2013 to measure chemical gradients and identify bacteria associated with young and old corrosion tubercles. Oxygen concentrations measured with microelectrodes decreased from saturation in the overlying water to nearly anoxic conditions at the surface of tubercles while ferrous iron (Fe2+) concentration increased with depth inside the tubercles. Bacterial DNA from different regions of the tubercles was sequenced using a next-generation sequencing technique to characterize the bacterial communities. Bacterial community composition changed within the tubercles. Families containing bacteria that oxidize iron and sulfur were consistently found on the outsides of tubercles and on non-tubercle steel. Families with bacteria that reduce iron and sulfur were found on the undersides of tubercles and in corrosion pits on the steel surface. Iron-oxidizing bacteria were more abundant after 0.8 years than 6.6 years and were distributed throughout tubercles while sulfate-reducing bacteria were most abundant in corrosion pits. Combined, these data are consistent with the cathodic depolarization (differential aeration cell model) and direct electron transfer theories of pitting corrosion, which provide explanations for the roles of bacteria in the accelerated corrosion of steel structures.