Oxidation of aero engine parts at high temperatures and its prevention

Structural materials for high temperature applications require sufficient oxidation resistance. For high temperature steels or nickel-based alloys this is usually achieved by the formation of a chromia layer which unfortunately has some disadvantages especially in water vapor containing environments or at very high temperatures. Titanium alloys cannot permanently be used at temperatures above approximately 600°C because TiO₂ loses its protective properties at such temperatures. Alumina is the preferred oxide because it offers superior behavior over Cr₂O₃ or TiO₂. Alumina formation can be achieved by Al-enrichment in the surface zone of the components. This enrichment was realized via a CVD process, i.e., powder pack cementation in the presented work. During this procedure Al is diffused into the surface zone and intermetallic aluminides are formed. These aluminides provide an Al reservoir for the formation of a protective Al₂O₃ layer. By using fluorine containing activators during the CVD process fluorine is additionally deposited onto the surface which promotes the alumina formation even more via the so-called fluorine effect. The fluorine effect leads to the formation of the thermodynamically stable a-Al₂O₃ even at temperatures below 1000°C where metastable Al-oxides usually are formed on untreated steels without the presence of fluorine. The fluorine dose must be kept below a certain limit otherwise negative corrosion will occur. Results of several technical alloys used for high temperature applications in aero engines with and without Al-enrichment plus fluorine deposition will be presented in this paper. The results will be discussed in view of a prolonged use of the treated components for more efficient flights, i.e., less inspection intervals or lower fuel consumption by using lighter materials.