Preventing the accelerated low-temperature oxidation of MoSi2 (pesting) by the application of superficial alkali-salt layers

Previous work showed that MoSi2 diffusion coatings formed by a NaF-activated pack cementation process did not pest. A Na-Al-oxide by-product layer resulting from the NaF activator formed a Na-silicate layer to passivate MoSi2. Superficial NaF layers were then used to prevent the pesting of MoSi2 diffusion coatings that were otherwise susceptible to pest disintegration. In this study, the use of superficial alkali-salt layers to prevent the accelerated oxidation of bulk MoSi2 at 500 degrees C is investigated move broadly. The application of Na-halide, KF, LiF, Na2B4O7, or Na-silicate layers prior to oxidation prevented accelerated oxidation and pesting for at least 2000 hr at 500 degrees C in air. The formation of a fast-growing, Na-silicate layer passivates MoSi2. The MoO3 that forms during oxidation absorbs sodium by intercolation to form stable Na-molybdate precipitates. Na2B4O7, Na-silicate, LiF and KF prevented accelerated oxidation at 500 degrees C by a similar mechanism. The application of alkali-halide salts is a simple, effective solution to prevent the accelerated oxidation and pesting of MoSi2.