In our previous papers, it was showed that sprayed and fused coatings of some Ni-Cr-Mo or Co-Cr-Mo alloys could effectively prevent crevice corrosion of austenitic stainless steel in sea water and that additions of B and Si which are essential to fused coatings might cause considerable depletion of Mo in the matrix of fused layer.
In this report, the effect of B and Si to crevice corrosion resistance of above alloys is studied precisely by means of cyclic polarization behavior in 3% NaCl solution. Then, sprayed and fused coatings with several selected alloys are performed by the newly developed “temperature control method, ” and the effects of conditions in coating procedure to crevice corrosion resistance of fused layers are studied.
Obtained results were as follows.
(1) For Ni-base and Co-base alloys, the satisfactory region of Cr and Mo contents to pervent crevice corrosion sifts to higher ones with the increase of B and Si addition. This shift is thought to be due to the decrease of Mo content in the matrix resulting from the appearance of Mo and B rich phase. No Ni-base alloy containing 4% B and 4% Si reveals resistance to crevice corrosion.
(2) Fused layer contains a certain extent of Fe from base metal. In the case of fused layer of Co-35% Cr-20% Mo-4% B-4% Si alloy, the resistance to crevice corrosion is unexpectedly lost because of relative decrease in Cr and Mo contents resulting from large quantities of Fe dilution.

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Electrochemical behavior of pits developed on aluminum in 30ppm chloride ion solution was measured by the recently developed scanning vibrating electrode technique (SVET) and compared with that of the total exposed surface. It was found that the pit showed markedly different polarization characteristics compared with the total surface owing to the very different environment and electrochemistry within the pit. Furthermore it was shown that the size of the pit strongly affected the catholic activity of the pit because of the diffusion effect. An attempt was made to relate the growth rate of the pit to the variation of the open circuit potential of the total surface.

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In order to investigate the effects of environmental factors on the atmospheric corrosion of carbon steel in Toyosaka city, Niigata prefecture, the rate of corrosion and the cocentrations of chemical pollutants in atmosphere were determined at 15 sites and the data of some meteorological elements were obtained for three years from May 1978 to April 1981. The summary of the results are as follows: 1) Corrosion rate was high in autumn-winter season and this fact agreed with the observed data of chloride concentration, wind direction and wind velocity. This matter showed chloride was carried from seaside by northwestern monsoon. 2) The ratio of the corrosion rates of the rural and coastal area was 1:1.1 in spring-summer season, but 1:1.4 in autumn-winter season. The facts agreed with the description above. 3) According to the single correlation coefficients of atmospheric pollutants (chloride, sulfate, sulphur oxides and nitrogen oxides) and meteorological elements (precipitation, relative humidity, air temperature, solar radiation and wind velocity) for corrosion rate, chloride, sulfate and wind velocity showed the positive correlation of 1% level of significance, and also sulphur oxides showed 5% level of significance, however, air temperature and solar radiation showed the negative correlation of 1% level of significance. The increasing of corrosion rate in autumn-winter season is understandable because a lot of sea salt particles are carried by wind in the season of low air temperature and poor solar radiation. Sulfate and sulphur oxides are also increased by discharged gases of combustion in the cold season. This tendency was very similar to the results in Niigata presented in the previous paper. 4) The multiple regression equations of (3) and (4) which were obtained fro monly two variables of chloride and sulfate in Niigata were available in the estimation of the corrosion rates in Toyosaka city and the results were satisfactorily agreed with the observed ones in the city.

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The critical hydrogen content of steels and the critical temperature for hydrogen attack were evaluated by using the Nelson chart and various data on hydrogen content. The critical contents of hydrogen were 0.6ppm for carbon steel and 2.4-4.3ppm for 2%Cr-1/2%Mo steel. The critical temperatures were 494K for carbon steel and 670K for 2%Cr-1/2%Mo steel. The relation between the critical hydrogen content of steel and the critical temperature for hydrogen attack was discussed tentatively.

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This review deals with selective dissolution and surface enrichment of binary alloys subjected to anodic polarization in aqueous solution. The theoretical and experimental aspects of selective dissolution were described on the basis of volume diffusion mechanism. Furthermore, the corrosion resistivity of iron base alloys was discussed from the point of view of selective dissolution of iron component followed by chromium-enrichment in the alloy layer near to the film/alloy interface.

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