The Corrosion Resistance of Some Diffusion Coating Layers on Ni-Base Superalloys

Abstract

The corrosion tests of the Ni-base superalloy with Al and Cr single coating, and Cr-Al, Cr-Si and Al-Si composite coating, the structure of which was reported in the previous paper, were carried out by cyclic oxidation at 1100°C and cyclic sulfidation at 900°C. The corrosion resistance of each coating specimen was evaluated by the measurement of weight change during cyclic corrosion test, the observation of the cross section by micro- and macrophotography and the analysis by EPMA. From the results of above measurements, the following corrosion resistance behavior was found. The Al rich coating layers of NiAl phase formed in Al and Cr-Al coating showed good resistance for oxidation test, although these layers showed poor resistance for the sulfidation test. On the other hand, the Cr rich coating layers of α-Cr phase formed in Cr and Cr-Si coating showed good resistance for sulfidation test. Among the coating specimens, the Si-Cr composite coating formed by combination of ion plating of Si and diffusion coating of Cr showed very good performance for severe sulfidation test. These Cr rich coating layers, however, showed not so good oxidation resistance at high temperature.