Practical Applications

For aircraft gas turbine blades fabricated from moderately-corrosion resistant nickel alloys (those containing 12 to 15% Cr), simple aluminide coatings of the inward and outward diffusion types provide adequate protection for many contemporary engines. When alloys are more corrosion prone (those containing 7 to 10% Cr), current practice is to modify aluminide coatings by chromizing prior to aluminizing. Aluminizing should be by inward diffusion of aluminum to locate the higher chromium concentration in the outer layer of the coating (Ref 14). For more severe Type I hot corrosion resulting from exposure of engines to salt spray from marine environments, modification of aluminide coatings with silicon (or better, by platinum) can prove to be the more cost effective solution.

If more expensive structure modifications of nickel superalloys, such as single crystals, are used to full strength-temperature capabilities to enhance fuel efficiency, contemporary diffusion coatings may not meet design objectives for oxidation and/or thermal fatigue service lives of the turbine. For these conditions use of the more expensive MCrAlY overlay and/or ceramic (zirconia-based) thermal barrier coatings may be in order.

Ground-Based and Marine Gas Turbines. Achieving cost effective hot corrosion protection for ground- or marine-based gas turbines is a more complex problem because of wide variations in local environments with respect to air contaminants (salts, sulfur oxides, and other industrial pollutants) and fuel constituents (salts, sulfur, vanadium, etc.). Burgel (Ref 11) identified chromizing as one of the best solutions in ground-based engines subjected to severe Type II hot corrosion in the metal temperature range of 740 °C (1365 °F)--this in comparison to silicide, platinum-aluminide, and some MCrAlY type coatings. Chromide coatings are, however, not satisfactory above 800 °C (1470 °F). Platinum aluminides provide moderate protection at somewhat higher temperatures but may not be sufficiently resistant to lower temperature Type II hot corrosion. Similar considerations appear to apply for marine environments (Ref 52).

The trend for most ground-based gas turbines is toward the use of various modifications of MCrAlY type coatings, particularly those with higher chromium and perhaps silicon additions (Ref 53, 54). For the very latest machines operating at the higher temperatures for improved fuel efficiency, these coatings may be overaluminized (Ref 55, 56) and internal cooling passages simultaneously aluminized to resist increasingly severe oxidation conditions.

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