IS (Iodine-Sulfur) thermochemical hydrogen production process has corrosive environments, such as a boiling sulfuric acid and hydrogen iodine. An Fe-Si alloy is known as a corrosion resistant material in H2SO4 environment because of SiO2 film formation. Although the corrosion resistance increases with Si concentration, it becomes brittle. Hence, a compositionally graded Fe-Si alloy (CG Fe-Si alloy), 14wt% Si at free surface and 3wt% at substrate, was tentatively made by using CVD (Chemical Vapor Deposition) technique. The corrosion resistance of the CG Fe-Si alloy in the boiling 17.7kmol/m3 sulfuric acid was examined through weight loss, SEM microscopic observation and an electron probe X-ray microanalyzer. The CG Fe-Si alloy shows equivalent corrosion resistance to the Fe-Si alloy including more than 12wt% Si homogeneously. However, after 300 hours immersion the SiO2 film formed on the surface became likely to break away around surfaces where microcracks caused by cooling-down process during the CVD treatment were present.
