The effect of pre-oxidation on the hot corrosion behavior of CoNiCrAlHf alloy in Na2SO4 environment at elevated temperature

The effect of pre-oxidation during hot corrosion test of cobalt-based alloy CoNiCrAlHf were corroded at 1000 ℃ with molten Na2SO4 salt and investigated the corrosion resistance. Pre-oxidized specimens showed good oxidation resistance and better adhesion to the oxide layer with a dense and unvarying layered structure. In the case of non-pre-oxidized specimens by increasing corrosion time, more aluminum was consumed due to the instability of aluminum content in salt, and the oxide scale decreased slightly which is not enough for corrosion resistance. Nevertheless, direct corroded specimens generated a non-protective and imperfect α-Al2O3 scale, which had thick but not uniform and compact. Therefore, non-pre-oxidized specimens permitted detrimental penetration of sulfur into the substrate. This sulfur diffused into the substrate by grain boundaries which increased the degree of thermal expansion and produced internal stress, cause spallation. The pre-oxidation treatment presented two advantages, one is the uniform and dense α-Al2O3 formation and the second one is impeded sulfur penetration.