Composition dependence of chemical activation energy was considered by separating apparent activation energy（ΔQa）of pure Fe, Cr and Fe-Cr alloys 1N-H2SO4 solution into two factors, chemical and electric one. Corrosion reaction of Fe-Cr alloys was found to be determined by anodic dissolution of metals and cathodic reaction of H＋ reduction had less effect on the reaction. Thus, relationship between chemical activation energy and Cr content was investigated in terms of metal dissolution. We assume that the standard electrode potential of Fe-Cr alloy decreased linearly with increasing Cr content based on the fact that the standard electrode potential of pure Fe is more noble than that of pure Cr and also on the results indicating that corrosion potential of Fe-Cr alloys decreased linearly with Cr content. From the assumption, chemical activation energy（Q0）was estimated by subtracting electric factor from ΔQa. It was found that Q0 decreased lineally by the increase of Cr content in the range of no more than 40％ Cr, but it increased with Cr content in the range higher that 40％ Cr. We estimated chemical activation energy of Fe and Cr（partially mole activation energy）in Fe-Cr alloys for obtained Q0 by applying the concept of the additive of thermodynamic partial molar quantity. And it was revealed that Q0 of Cr was remarkably decreased in Fe-Cr alloys compared with one in pure Cr. It was considered that increase of corrosion rate with Cr content of Fe-Cr alloys was attributable to decrease of Q0 of Cr in pure Cr comparing to one of Cr in Fe-Cr alloys.
In order to clarify the influence of sulfur content on the pitting corrosion behavior of surface, L-section （longitudinal section）and T-section（transverse section）of stainless steel plate, cyclic corrosion test（CCT）and anodic polarization measurement in 0.85 M NaCl aqueous solution have conducted. Thick plate specimens （t 25 mm）of type 304 stainless steels containing 7, 29 and 57 ppm S were subjected to these tests. In CCT, rusting was more significant over T-section than surface and L-section. However, the number of initial rust spots over the T-section decreased with a reduction of S content. In pH 7 solution, the pitting potential（Epit）for 57 ppm S specimen was less noble at T-section than surface and L-section. But Epit at the T-section was shifted toward noble side with a reduction of S content. On the other hand, in pH 1 solution, Epit was low value regardless of S content. Furthermore, the cross-sectional shape of pits originated from MnS inclusion at the T-section was investigated. The MnS inclusion stretched into the steel matrix along the rolling direction and the growth of pitting corrosion along the MnS inclusion were observed.