Microbially influenced corrosion (MIC) of turbine materials in hydroelectric power plants
PAUL LINHARDT
From the corrosion point of view, hydro power should be a very active field, since large metallic structures are exposed to an aqueous environment. However, based on long term experience, designs are robust and significant corrosion of turbines is excluded to a high degree.

Nevertheless, sometimes, unexpected cases of corrosion of turbine components are experienced, which cannot be related to the chemical parameters of the water alone, but may be explained by considering microbial activity.

Based on the experiences gained from several such cases, so far two basic mechanisms of microbially influenced corrosion (MIC) could be identified in hydropower plants by the author: (1) MIC by manganese oxidizing microorganisms, and (2) MIC by peroxide producing organisms. Both mechanisms act via the phenomenon of Ennoblement, that is the effect of a strong biogenic oxidant acting on the metal more efficient than the usual oxygen.

Examples will be presented related to the worldwide standard material for hydropower turbines, the martensitic stainless steel alloy type CrNi 13/4 (rolled: Mat. No. 1.4313 equiv. X3CrNiMo13-4, cast: 1.4317 equiv. GX4CrNi13-4), which typically suffers from pitting corrosion in such cases. By a case of phase selective corrosion of nickel-aluminum-bronze, the complex interaction of materials in such MIC scenarios will be addressed and the usefulness of electrochemical investigations in the analysis of such failure cases will be demonstrated.

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