Preventing the accelerated low-temperature oxidation of MoSi{sub 2} (pesting) by the application of superficial alkali-salt layers

Journal Article · Thu Feb 01 04:00:00 EST 1996 · Oxidation of Metals

DOI:https://doi.org/10.1007/BF01046821· OSTI ID:245134

Cockeram, B V; Wang, G; Rapp, R A ^[1]

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1. Ohio State Univ., Colombus, OH (United States)

Previous work showed that MoSi{sub 2} 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 MoSi{sub 2}. Superficial NaF layers were then used to prevent the pesting of MoSi{sub 2} 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 MoSi{sub 2} at 500{degrees}C is investigated more broadly. The application of Na-halide, KF, LiF, Na{sub 2}B{sub 4}O{sub 7}, 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 MoSi{sub 2}. The MoO{sub 3} that forms during oxidation absorbs sodium by intercalation to form stable Na-molybdate precipitates. Na{sub 2}B{sub 4}O{sub 7}, 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 MoSi{sub 2}.

• Sponsoring Organization: USDOE
• OSTI ID: 245134
• Journal Information: Oxidation of Metals, Journal Name: Oxidation of Metals Journal Issue: 1-2 Vol. 45; ISSN OXMEAF; ISSN 0030-770X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALKALI METAL COMPOUNDS
DIFFUSION COATINGS
MOLYBDENUM SILICIDES
OXIDATION
SALTS
TEMPERATURE RANGE 0400-1000 K