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

• Published in: Advanced engineering materials Vol. 22; no. 6
• Main Authors: Wang, Yaping, Pillai, Rishi, Yazhenskikh, Elena, Frommherz, Martin, Müller, Michael, Naumenko, Dmitry
• Format: Journal Article
• Language: English
• Published: 01.06.2020
• Subjects: aluminide coatings; Na2SO4–K2SO4 deposits; temperature dependence; type II hot corrosion
• ISSN: 1438-1656; 1527-2648
• DOI: 10.1002/adem.201901244

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 pSO3 on phase equilibrium of the salt and oxide mixture is calculated with an in‐house developed thermodynamic database. The predicted minimum pSO3 for liquid formation calculated with the database is consistent with the experimental results. Corrosion morphologies change significantly with the change in exposure temperature. The influence of temperature and SO3 partial pressure (pSO3) on phase equilibrium of the salt and oxide mixture is calculated. The predicted minimum pSO3 for NiSO4–Na2SO4 liquid formation is consistent with the experimental results. The possible mechanisms of hot corrosion at different temperatures are extensively discussed.