Abstract
In order to use microbially influenced corrosion for the fine biomachining of metals, culture conditions of bacteria and differences between mild steel and copper in the metal biomachining through Tiobacillus Ferrooxidans of a kind of iron oxidizing bacteria have been investigated in three types of environment of 9K medium, the bacteria-cultured solution, and the cultured and sterilized solution. The results were summarized as follows. To culture bacteria in 9K medium of pH 2.5 gave the maximum amounts of the cell with the minimum culture time. However the maximum amount of bacteria cells became 20% less in pH 3.0 medium than in pH 2.5 one and so the former seemed to be suitable for biomachining test of metals from the point of view of reproduction of culture. Losses in mass of SS 400 by biomachining in 9K medium with cultured solution were larger than those of copper. Differences in loss in mass between SS 400 and copper were not clear in the solution cultured and sterilized. Losses in mass of SS 400 and copper by biomachining in the cultured solution were respectively about 30mm/y and 6mm/y of dissolution in thickness as the dissolved amount was converted into thickness. The surface of SS 400 by biomachining was in rough compared with copper's in 9K medium and the cultured solution, that is the surface roughness of SS 400 was about 30μm whilst one of copper was about 3μm. There is no difference in the roughness between SS 400 and copper in the cultured and sterilized solution. Both SS 400 and copper took a general attack in corrosion. But pitting was observed on the surface microstructures of SS 400 in 9K medium and the cultured solution. The crystallographic dissolution with step also was observed in grains through high magnification. In the cultured and sterilized solution, a few dispersed attacks were observed as if the surface was covered with films. In all solutions, copper took general corrosion. The biomachined surface of copper with the very fine structures was differed apparently from that of SS 400. On the copper surface, neither crystallographic attacks with step nor localized pitting were observed.
