The sulfur-iodine thermo-chemical cycle (S-I cycle) is one of the promising nuclear hydrogen production methods combined with a high temperature gas-cooled reactor. However, extremely corrosive environments limit the selection of structural materials. Therefore, in this study, corrosion behaviors of several metallic materials were investigated to screen the candidate metallic materials. Coupon type specimens were exposed for 100 h in simulated SO3 and HI decomposer conditions at 850 °C. After 100 h exposure, the surface treated Alloy 617 showed the superior weight change in both environments. However, scanning electron microscope observation showed oxide spallation for EB-treated and NiAl coated Alloy 617. On the other hand, the Ni3Al coated Alloy 617 showed better corrosion resistance in SO3 decomposer condition, such that only formed external Al-rich oxide layer. Especially, in a HI decomposer condition, the damage on the Ni3Al coated Alloy 617 was considerably less significant probably due to the protection by very thin aluminum-rich oxide on the surface.