THE MICROBIAL ECOLOGY OF METAL SURFACES (CORROSION, HYDROGEN EMBRITTLEMENT)

This thesis presents the results of several years of research on the microbial ecology of metal surfaces. Data are presented on the characteristics of metal biofilm communities and factors that control their development and activity in marine environments. Microbial populations on metals were found to differ quantitatively and qualitatively, depending upon the composition of the metal or alloy substratum. Metals that are relatively inert in seawater, such as titanium and 316 stainless steel, supported large, diverse populations of bacteria, algae, and protozoa. Protozoan communities on titanium were particularly diverse. Copper alloys were colonized less rapidly than other metals and their communities were much less diverse. Cell surface hydrophobicity, toxicity of corrosion products, and composition of the macromolecular conditioning film were examined as possible factors controlling the attachment and growth of bacteria on metals. Of these, metal ion toxicity appeared to be most important, with resistant communities being selected on the metals which show a toxic effect.; Also included in the thesis are studies of the involvement of bacteria in hydrogen embrittlement of metals and of the corrosion of hot water systems by thermophilic microorganisms--two aspects of microbiological corrosion that previously have received little or no attention. Pure cultures of a hydrogen-producing Clostridium were found to enhance uptake of hydrogen by steel. Experiments using a sealed Devanathan cell to measure hydrogen permeation showed that hydrogen and acid production by the bacteria acted synergistically to promote hydrogen absorption by the metal. In other experiments an obligately thermophilic bacterium was isolated from a nickel braze joint that failed during corrosion testing at 60(DEGREES)C. The bacterium was shown to enhance the corrosion rate of nickel. Evidence suggests that acid production and creation of differential aeration cells by the bacteria are involved in this process.