The method to determine a corrosion resistance by the electrochemical impedance with inductive behavior was discussed. The electrochemical impedance of iron electrode in sulfuric acid containing NaI shows an apparent inductive loop. In this case, two parameters concerning the resistance are obtained, namely, a charge transfer resistance in the middle frequency and a polarization resistance in the low frequency regions. Comparing the reciprocals of these resistances with Fe (II) dissolution rate, it was found that the reciprocal of the charge transfer resistance corresponded to the Fe (II) dissolution rate, and that the charge transfer resistance should be the corrosion resistance. Furthermore, the above-mentioned experimental results were examined by the discussion concerning the inductive behavior by adsorbed species.
The influence of firing Victorian brown coal on high temperature corrosion of boiler tubes was investigated in comparison with bituminous coal and the mix of the brown coal and the bituminous coal by combustion tests and laboratory immersion tests. The combustion tests were carried out using experimental furnace installed sample tubes with internal air cooling to simulate superheater tubes. Both the degree of corrosion of the tubes and the content of sodium sulfate and chloride in ash deposit on the tubes were highest in firing the brown coal. The immersion test revealed that sodium sulfate and chloride in synthetic ash both promoted the high temperature corrosion, indicating that sodium sulfate and chloride in deposit were the main factor of the corrosivity of brown coal. In firing the 1:1 mix of bituminous and brown coal, corrosion of the tubes was almost the same level with the bituminous coal and constituent of the ash also was nearly the same as the bituminous coal due to low ash of the brown coal. It was concluded that mixing brown coal with bituminous coal was effective in mitigating high temperature corrosion due to brown coal.
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.
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