It is often encountered in the exhaust gas treatment facilities that fly ash contains high amounts of chlorides or flue gas contains several hundred ppms of hydrochloric acid. In these cases, sulfuric acid dew corrosion behavior of steels may be affected with chlorides and hydrochloric acid atmosphere can develop at the steel/ rust interface5). shows corrosion rates of steels in hydrochloric acid solutions (10.5mass% and 1.0mass%) at 80 deg.C. In 10.5mass% HCl solution, corrosion rate of S-TEN1 is substantially lower than even that of type 316L. In more dilute 1%HCl solution, S-TEN1 still showed comparable low corrosion rate to type 316L steel, otherwise CORTEN showed higher corrosion rate in both concentrations. This results suggest that S-TEN1 would show lower corrosion rate in applications in which hydrochloric acid atmosphere could develop. Fig.4 showed a result of field test in which several mass percent of chloride containing ash involved in sulfuric acid dew corrosion. Steels were actually applied as duct panels without external heat insulator. As predicted from Fig.3, S-TEN1 showed substantially lower corrosion rate compared to mild steel. This study clarified and confirmed that a low alloyed steel containing Cu-Sb without Cr shows better corrosion resistance than Cu-Cr-Ni bearing steel in (1) relatively lower metal temperature/ lower acid concentration environments, and (2) sulfuric acid dew corrosion environments involving chlorides. Effect of micro alloying elements such as Cu, Sb, Cr were also investigated electrochemically and discussed in terms of anodic dissolution and hydrogen evolution reactions.