The oxidation resistance was investigated in air at 1173K for the TiAl alloy, which has been pre-sulfidized at 1173K in a H2-H2S gas mixture to form a TiAl3 layer on the alloy surface due to selective sulfidation of Ti, and changes of the TiAl3 into TiAl2 phase as well as their effects on oxidation behavior were considered. It was found that the pre-formed TiAl3 layer with 20μm thickness was changed into a TiAl2 layer after oxidation at 1173K for 86.4ks. Both the TiAl3 and TiAl2 layers indicated very good oxidation resistance for long time, up to 810ks. These two layers showed re-passivation capability of forming a protective Al2O3 scale.
Corrosion potential fluctuation of SUS 304 stainless steel during induction period of localized corrosion was measured in 4.0mol/l NaCl solution. The solution was exposed to air and its temperature was kept at 353K. It was found that new type of the potential fluctuation (Type II), differ from the potential fluctuaiton generally reported (Type I). From the result of SEM observation, the number of Type II and the number of micropits were agreed. In order to investigate the correlation between potential fluctuation and micropit, it was thought up that new method to estimate the local anode current quantitatively from the observed potential fluctuation. The observed corrosion potential fluctuation was applied to the specimen by a potentiostat and the change of the cell current (response current) was measured. Pit radius estimated from the integral of the response current was in good agreement with the radius of micropit evaluated from SEM micrograph.
When ferrous sulfates are mixed with cement slurries in setting processes of cement paste and aggregates, iron-rich layers are produced on the surface of the concrete. X-ray fluorescence spectrometry, X-ray analysis, and electron probe microanalysis have been applied to the cross section of the surfaces. The data indicates that iron and sulpher are dispersed to the surface layers ranging from the top of surface to 500μm and that amorphus iron oxide layers of 50μm in thickness are formed near surfaces and calcium sulfates are formed in next to the iron oxide layer.
Recently, corrosion of Power-Transmisson-Towers made of hot-dip-zinc galvanized steel are accelerated by acid rain. Corrosion is localized at a lower part of the tower placed under an aluminum rail. This paper is aimed to find the mechanism of this corrosion. Test pieces were exposed to rain water flowing on the aluminum rail, that flowing on the galvanized-steel tower and that falling from the sky. After 9 months, change in weight and appearance of test pieces were examined. Laboratory tests were also performed immersing test pieces in the three kinds of rain water described above. It was found that pH of rain water increases flowing on a galvanaized-steel tower as a result of the reaction between zinc and the rain water. On the other hand, increase in pH of rain water flowing on an aluminum rail is little sinse corrosion resistance of aluminum is higher than zinc. Consequently, corrosion rate of galvanized steel exposed to rain water flowing on an aluminum rail is higher than that exposed to that flowing on a galvanized-steel tower.
Chloride-pitting of iron group metals developed at noble potentials proceeds in the polishing state dissolution as far as hydrated metal chloride in the pit solution remains greater than a critical concentration. It ceases to progress by pit repassivation if the pit is smaller than a critical size, or transforms into the active state pitting if the pit becomes greater. The boundary potential between the polishing state and the active state pitting may be represented by the passivation-depassivation potential in the pit solution of the critical metal chloride concentration. Crevice corrosion is characterized by the crevice protection potential, at which the hydrogen ion concentration in the crevice solution is equivalent to pHpd, the passivation-depassivation pH of the crevice metal. It continues to corrode if the crevice solution is more acidic than pHpd, but is inhibited if the solution is less acidic, which is realized at potentials less noble than the protection potential. A potential-dimension diagram may be drown for the region of stalle pitting and crevice corrosion.