It is reported for the first time that irradiation of seawater by sunlight eliminates the ennoblement‐causing capacity of marine biofilms. In laboratory tests under strict conditions of darkness, freshly sampled Tuticorin seawater produced the previously reported 300 mV ennoblement whereas in the same seawater with prior, natural photo‐irradiation there was little variation in the corrosion potential. The irradiation procedure did not alter the primary water quality characters, nor did it perturb the process of biofilm formation. The absence of ennobling in irradiated seawater was consistent for all of the four alloys examined. Supplementary tests indicated that adequate interpretation of the results was not possible with any of the mechanisms suggested in literature.
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