Abstract
When stainless steels are immersed in natural sea water, their electrode potential (Esp) ennobles up to about 400mV vs. SCE in summer, while the Esp does not ennoble significantly in winter. In this paper, the relationship between ennoblement of Esp and attached diatoms in biofilm formed on the sample was investigated.
Firstly, as Stage I, Type 316 stainless steel was immersed in natural sea water for a few days. The Esp value, Esp, I, at the end of Stage I was controlled to be less than 100mV vs. SCE so that biofilm formed on the sample was mainly consisted of bacteria. As the next stage, Stage II, the steel was transfered to a diatom containing solution in laboratory, which consisted of sterilized sea water, non-oxidizing culture medium and raised diatom suspending in the solution, for 10 days and final Esp value, Esp, II, was determined in relation with amount and kind of attached diatoms on the sample.
Without Stage I, Stage II alone could not make significant ennoblement of Esp, II, because diatoms could not attach and develop on the sample. Stage I immersion, even in winter, followed by Stage II immersion in the diatom solution with sufficient density of Chaetoceros gracilis was found to make Esp, II value ennobled as high as in the immersion in natural sea water in summer season. The extent of ennoblement in Esp, II increased with increasing Esp, I value and diatom density in the diatom solution in Stage II.
Similar experiments were conducted with other three kinds of diatoms, isolated from natural sea water at Orido Bay, Shizuoka prefecture. Nitzschia sp. A, the most dominant species in summer, showed the highest level of ennoblement in Esp, while Melosira moniliformis, the most dominant species in winter, showed the lowest. This fact could explain the seasonal change of Esp ennoblement in natural sea water from a viewpoint of diatoms.
