The electrochemical behaviors of coating films on the steel and Ni–Cr–Mo alloy (Hastelloy) were investigated to obtain fundamental data on their corrosion resistance characteristics. We compared several types of materials and coating layers. SUS316, Hastelloy, DLC (Diamond-like carbon) on SUS316 and DLC on Hastelloy samples showed stable potential-time characteristics. F₂ processing SUS316 and F₂ processing Hastelloy samples showed negative direction change of the potential, and an active dissolution peak appeared in the F₂ processing SUS316 samples. On the other hand, DLC samples on SUS316 and Hastelloy showed a small anode current. It appeared that DLC coating was a superior resistance layer for the corrosion.