Abstract
The biomachining mechanism of metals for mild steel and copper has been investigated both by electrochemical measurements in 9K medium and ferrous oxidizing bacteria-cultured solution and by observations of the surface film. The results are as follows: (1) In all tested pH, passivation was found at near -450mV vs. SCE on the anodic polarization curves of mild steel in 9K medium. Great passivation was found also in the region of -500 to 0mV vs. SCE on the anodic polarization curves of copper. (2) Cathodic polarization curves of mild steel depolarized greatly in the region of -750 to -900mV vs. SCE in the bacteria-cultured solution. This depolarization caused by the reduction reaction of Fe3++e→Fe2+accelerated the dissolution of metals. (3) Passivation was not found on the anodic polarization curves of mild steel in the bacteria-cultured solution. Natural electrode potential of copper in the bacteria-cultured solution shifted to the noble dirrection of -30 to +50mV vs. SCE. Passivation was not found also on the anodic polarization curves of copper. (4) Passivation at near -450mV vs. SCE in 9K medium seemed to be caused by the formation of FeSO4 film from the results of EPMA of Fe and S. This film formation was remarkable on copper. (5) In 9K medium the dissolution of mild steel and copper was prevented by FeSO4 film, however, in the bacteria-cultured solution FeSO4 film was not formed owing to the action of bacteria and the dissolution was accelerated by oxidizing effect of Fe3+. (6) The biomachining mechanism of metals for mild steel and copper seemed to be as follows. Fe2 (SO4)3 which was formed by the culture of bacteria accelerated the dissolution of both mild steel and copper as the oxidizing agent. The dissolution progressed as the two following reactions.
Fe2(SO4)3+Fe→3FeSO4 Fe2(SO4)3+Cu→CuSO4+2FeSO4
