Classification of Materials

Pack compositions, process temperatures and process times depend on the type of base material to be aluminized. Materials fall into one of the following general classifications:

• Class I: carbon and low-alloy steels and copper

• Class II: ferritic and martensitic stainless steels and austenitic stainless steels with less than 20% Ni

• Class III: austenitic stainless steels with 21-40% Ni and iron-base superalloys

Class IV: nickel-and cobalt-base superalloys

As a general rule, overall aluminum diffusion is slower as the nickel, chromium, and cobalt contents increase. Therefore, higher temperatures and longer process times are required to produce greater aluminum diffusion thicknesses as the base material goes from Class I to Class IV. (The Class IV materials are covered in the Section "Diffusion Coatings for Gas Turbine Engine Hot Section Parts" of this article.)

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