Platinum-rhodium-containing high temperature alloy coating method

A method for improving the heat and corrosion resistance of a heat resistant article formed of an alloy selected from the group consisting of nickel-base, cobalt-base and nickel-cobalt-base alloys which comprises, providing said articles with a clean surface for receiving a metal coating,

• thermally diffusing first and second coatings of rhodium and platinum into the surface of said article, said rhodium and platinum having been applied to said article prior to diffusion therein as first and second coatings, respectively, at a Rh/Pt thickness ratio of about 0.5/1 to 2.5/1 at thicknesses of each ranging from about 0.00005 inch to 0.002 inch,
• and then pack aluminizing said article at an elevated aluminizing temperature by embedding it in a cementation pack containing by weight about 5 to 40% chromium metal and about 1/8 to 20% aluminum sufficient to effect the diffusion coating thereof into the surface of said article, said pack also containing about 1/8 to 2% of a vaporizable halogen material as a carrier for said aluminum,
• thereby producing a multi-layered protective coating characterized metallographically by an outer zone containing platinum and an inner zone substantially adjacent the surface of the article containing rhodium with an intermediate zone enriched in aluminum in the form of at least one intermetallic compound selected from the group consisting of nickel aluminide in the case of nickel-base and nickel-cobalt-base alloys, and cobalt aluminide in the case of nickel-cobalt-base and cobalt-base alloys.