In order to shorten the E_sp measurement period, the potential of four types of stainless steel pipes in an 80℃ hot water environment containing R-Cl was measured using the R-Cl concentration as a parameter. As a result, an approximate linear equation was obtained that shows the relationship between E_sp and R-Cl concentration. Because the obtained potential data and the slope of the linear approximation equation were close to published and estimated values, it is believed that E_sp can be calculated from the approximate linear equation obtained this time when the usage environment is close to the test conditions used in this study.

In sulfate- or chloride-ion environments, the current density in the active/passive transition region in Fe-11 mass％ Cr steels decreases with the addition of 2 mass％ Al or Si. This reduced current density lowers the passivation pH; consequently, the corrosion resistance of the Al/Si-added Fe-11 mass％ Cr steel is superior to that of the Fe-11 mass％ Cr steel. The Al （or Si） content in the passive film formed on Fe-11 mass％ Cr-2 mass％ Al （or -2 mass％ Si）, is higher than that in the substrate. The excellent corrosion resistance of the Al-added low-Cr stainless steel is attributed to the generation of Al（III） hydroxides, which dominates over that of Fe（II） and Cr（III）. By contrast, the excellent corrosion resistance of the Si-added low-Cr stainless steel is ascribed to the formation of Si oxides.