Role of Temperature in Na₂SO₄–K₂SO₄ Deposit Induced Type II Hot Corrosion of NiAl Coating on a Commercial Ni-Based Superalloy

The life span of gas turbine coatings may be restricted by type II hot corrosion when exposed at elevated temperatures in aggressive environments during service. Herein, the temperature dependence of corrosion morphologies and kinetics of NiAl coating on a second-generation single crystalline Ni-based superalloy is studied to provide an insight into the possible corrosion mechanisms. A series of tests are performed at 600–800 °C in air-300 ppm SO₂ atmosphere with Na₂SO₄–20% K₂SO₄ salt mixture as deposit. Severe attack is observed at both 700 and 750 °C after 24 h exposure, whereas at 600 and 800 °C, only a minor attack is found. The results indicate that the corrosion rate is strongly governed by Na₂SO₄–NiSO₄ liquid formation, and temperature affects the attack rate of the outer coating (mainly β-NiAl phase) primarily by changing the required minimum SO₃ partial pressure to stabilize this liquid. The influence of temperature and (Formula presented.) on phase equilibrium of the salt and oxide mixture is calculated with an in-house developed thermodynamic database. The predicted minimum (Formula presented.) for liquid formation calculated with the database is consistent with the experimental results.