Pack Cementation Coatings for Superalloys: A Review of History, Theory, and Practice

Nickel and cobalt-base superalloy blades and vanes in the hot sections of all gas turbines are coated to enhance resistance to hot corrosion. Pack cementation aluminizing, invented in 1911, is the most widely used coating process. Corrosion resistance of aluminide coatings can be increased by modification with chromium, platinum, or silicon. Chromium diffusion coatings can be used at lower temperatures. Formation and degradation mechanisms are reasonably well understood and large-scale manufacturing processes for these coatings are gradually being automated. Pack cementation and related diffusion coatings serve well for most aircraft engine applications. The trend for industrial and marine engines is more toward the use of overlay coatings because of the greater ease of designing these to meet a wide variety of corrosion conditions.