Effect of Bacteria Combined with Diatoms on Ennoblement of Electrode Potential of Stainless Steels in Natural Seawater

摘要

When stainless steel samples are immersed in natural seawater, their electrode potential (E_(sp)) ennobles up to approximately 400 mV vs. SCE due to the development of bioflim on the samples. The primary organisms in the biofilm are bacteria and diatom algae. We investigated the relationship between the ennoblement of E_(sp) and bacterial species in the bioflim, as well as the interaction between bacteria and diatoms in the ennoblement. To study the effect of various bacterial species in the biofilm on the ennoblement of E_(sp), we immersed Type 316 stainless steel samples in 0.4 #mu#m natural seawater that had been filtered to make sure that the only organisms in it were bacteria. Under this condition, E_(sp) did not ennoble up to 400 mV vs. SCE, showing that the presence of other organisms in the bioflim was necessary to promote ennoblement. When we analyzed the bioflim that formed in natural seawater on a sample with ennobled E_(sp), we detected the presence of sulfate-reducing bacteria and of acidic and neutral sulfur-oxidizing bacteria (A-SOB and N-SOB, respectively). We also observed that the density of N-SOB was high throughout the year. To study the effect of SOB on the ennoblement of E_(sp), we immersed Type 304 stainless steel in solutions containing Thiobacillus thioparus (as N-SOB) and T. thiooxidans (as A-SOB). Remarkable E_(sp) ennoblement was induced by N-SOB, but not by A-SOB. However, as the density of attached N-SOB on the ennobled sample was too much higher than the density that would be attained during immersion in unfiltered natural seawater, we concluded that the ennoblement of E_(sp) was caused by that single bacterial species. When samples were immersed in natural seawater for a few days to promote the attachment of bacteria, and then were transferred to a diatom-containing solution, the E_(sp) of the samples presented high values, similar to the values for samples immersed in natural seawater in the summer. After the samples were immersed in diatom-containing solution, we investigated the effect of the density of attached bacteria and diatoms on E_(sp). For E_(sp) ennoblement up to 200 mV vs. SCE, the density of the attached bacteria increased up to about 3.5 X l0~4 pg/cm~2, with increasing density of diatoms. For E_(sp) ennoblement higher than 200 mV vs. SCE, we concluded that the attachment of diatoms on the sample plays a more important role.