Resistance of a Mo–Si–B-Based Coating to Environmental Salt-Based Hot Corrosion

During operation of gas turbine engines, different salts develop that lead to a hot-corrosion attack. While the hot corrosion associated with Na2SO4 and Na3VO4 has received considerable attention, MgSO4 is also a main salt component, but it has received limited examination. For operation at temperatures beyond the capabilities of Ni-base superalloys, refractory-metal alloys with a Mo–Si–B-based coating have demonstrated a robust environmental resistance including hot corrosion attack by Na2SO4 and Na3VO4. In order to evaluate the performance of Mo with a Mo–Si–B-based amorphous coating, samples were exposed to MgSO4 under thermal cycling at temperatures from 800 to 1300 °C. Mass change measurements revealed minimal attack from the MgSO4 exposure except in local regions where cristobalite and MgMoO4 were detected. Even with the local region reaction, the mass change after a cumulative 100 h of exposure was about 3.5 mg/cm2. Exposure to a composite salt mixture containing Na2SO4, NaVO3, MgSO4, CaCl, and KCl revealed signs of interactivity compared to the corrosion due to the separate effects of the individual salts, but again the mass change stabilized to a minimal level demonstrating that the Mo–Si–B-based coating is resilient to hot corrosion.