Microbiologically Influenced Corrosion of Submerged Steel Bridge Piles in Natural Waters in Presence of Marine-Fouling

Recent findings at a Florida bridge showed that submerged steel piles had severe localized corrosion, up to 3 in diameter. Testing indicated strong presence of microbes that can be associated with microbiologically-influenced corrosion (MIC), including sulfate-reducing bacteria (SRB). In addition, the site had heavy marine growth. It was thought that the localized crevice environments created by the macrofoulers support MIC. The objective of the research was to identify (1) if marine fouling can enhance proliferation of bacteria that can support MIC, (2) if macrofouling can affect the efficacy of cathodic protection (CP) to mitigate MIC, and (3) if application of coatings can be used to mitigate the degradation. Field site visits and review of environmental databases identified common environmental conditions, water chemistry, and micro- and macro-fouling activity. Steel specimens were installed at three estuarial, brackish, and freshwater sites. Microbiological analysis verified that bacteria associated with MIC had developed under the fouling organisms. Testing identified that crevice environments can affect SRB interactions in natural waters. Different modes of nutrient availability allow for SRB activity in supportive environments (including anerobic environments) under foulers. Electrochemical tests showed that MIC can develop under fouling organisms. Testing was conducted to differentiate the CP currents that develop in the presence of microorganisms under the marine fouling. Results showed that proliferation of the bacteria was not inhibited in the presence of cathodic polarization at -1000 mVCSE and CP alone may not be adequate to mitigate MIC under heavy fouling. It was shown that the cathodic charge was related to the sulfate reduction by SRB, and SRB can be a significant contributor to the electrochemical process for steel corrosion with cathodic polarization. Testing was conducted to identify the mitigation properties of antifouling and polyurea coating in presence of microorganism and marine fouling. Application of electrochemical impedance spectroscopy (EIS) and development of a computational assessment approach identified microbial activity and degradation of an antifouling coating. The results from EIS indicated degradation of the coating due to its self-polishing characteristics and that formation of surface layers associated with SRB can form as biocide components of the coating become depleted