Investigations of Fine Biomachining of Metals by using Microbially Influenced Corrosion

摘要

The biomachining mechanism of metals for mild steel and copper has been investigated both by electrochemical measurements in 9 K 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 -450 mV vs. SCE on the anodic polarization curves of mild steel in 9 K medium. Great passivation was found also in the region of -500 to 0 mV vs. SCE on the anodic polarization curves of copper. (2) Cathodic polarization curves of mild steel depolarized greatly in the region of -750 to -900 mV vs. SCE in the bacteria-cultured solution. This depolarization caused by the reduction reaction of Fe~(3+) + e -> Fe~(2+) accelarated 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 +50 mV vs. SCE. Passivation was not found also on the anodic polarization curves of copper. (4) Passivation at near -450 mV vs. SCE in 9 K medium seemed to be caused by the formation of FeSO_4 film from the results of EPMA of Fe and S. This film formation was remarkable on copper. (5) In 9 K medium the dissolution of mild steel and copper was prevented by FeSO_4 film, however, in the bacteria-cultured solution FeSO_4 film was not formed owing to the action of bacteria and the dissolution was accelerated by oxidizing effect of Fe~(3+). (6) The biomachining mechanism of metals for mild steel and copper seemed to be as follows. Fe_2 (SO_4)_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.Fe_2 (SO_4)_3 + Fe -> 3FeSO_4 Fe_2 (SO_4)_3 + Cu -> CuSO_4 + 2FeSO_4