Influence of orientation on corrosion behavior of directionally solidified cobalt‐base superalloy

Abstract

Oxidation behavior on intersecting and longitudinal surfaces of directionally solidified cobalt‐base superalloy DZ40M was studied respectively at 1050 °C for 0.1 h, 1 h and 10 h and electrochemical property before and after oxidation on two orientations in 3.5% sodium chloride solution at room temperature was investigated. Grain boundaries of DZ40M are conducive to ion diffusion at elevated temperature and the higher area fraction of grain boundaries on intersecting surface results in higher oxidation rate and more CoO in the oxide layers. Carbides with Cr depletion zone around vulnerable to corrosion are located in grain boundaries, and unoxidized intersecting surface with higher area fraction of grain boundaries is prone to more serious uneven general corrosion. After oxidation, the corrosion resistance of the alloy is enhanced because oxide layer preferentially covers the grain boundaries and relieves the unevenness of corrosion. The protective effect of oxide layer is mainly provided by the innermost compact and continuous chromium oxide (Cr2O3). The intersecting surface has lower Ecorr and higher icorr, for the protective Cr2O3 layer on this orientation is largely consumed by excessive cobalt oxide to form CoCr2O4.