Compositional Factors Affecting Protective Alumina Formation Under Type II Hot Corrosion Conditions

The Type II (700 °C) hot corrosion resistance of Ni–36Al (at.%) base alloys with various additions of Pt, Co, and/or Cr was investigated. It was found that the addition of either 5 at.% Pt or 5 at.% Cr was highly beneficial, i.e., extended the protective incubation stage. This was shown, via a thorough examination of the scales formed on these alloys during 700 °C oxidation, to be due to an enhanced ability to rapidly form a protective, pure Al₂O₃ scale. Addition of either 5 at.% Cr or 5 at.% Co to a Ni–36Al–5Pt alloy was found to be highly detrimental. These latter results are explained by examining subsurface phase transformations that occur during exposure; extensive transient oxidation was also found to play a significant role in the presence of Co.