Platinum enriched, silicon-modified corrosion resistant aluminide coating

A platinum-enriched silicon-modified aluminide coating on a nickel-base superalloy substrate, which substrate contains refractory metals, the coating comprising at least three distinguishable layers in a continuum of nickel aluminide which extends throughout the coating, including a first surface layer comprising dispersed therein platinum aluminide phases and refractory metal silicide phases throughout the nickel aluminide continuum thereof, a second layer below said surface layer having dispersed therein refractory metal silicide phases throughout the nickel aluminide continuum thereof and being relatively free of platinum aluminide phases as compared to the surface layer, and a third layer below said second layer in which the nickel aluminide continuum thereof is relatively free of platinum aluminide and refractory metal silicide phases as compared to the surface and second layers, the coating having resistance to hot corrosion conditions,

• wherein the refractory metal silicide phases of the first and second layer are formed by molten silicon powder from a slurry of aluminum and silicon powder or from an aluminum-silicon eutectic alloy powder, which molten silicon diffuses into the substrate scavenging and reacting with the refractory elements, thereby forming stable silicide phases with said refractory elements, and
• wherein the platinum aluminide phase in the first layer is formed by molten aluminum powder from said slurry of aluminum and silicon powder, which molten aluminum dissolves the silicon and diffuses simultaneously inwardly into the substrate with the silicon, thereby reacting with the nickel of the nickel substrate to form intermetallic nickel aluminide phases and said molten aluminum reacting simultaneously with the platinum to form platinum aluminide phases, the platinum having been diffused into and enriching the substrate before the diffusion of the molten aluminum and silicon into the substrate, thus, the aluminum and silicon diffusing through the platinum enriched first layer,
• wherein, in the refractory metal silicide phases, the refractory metal elements are selected from the group consisting of chromium, molybdenum, vanadium, titanium, tungsten, niobium, tantalum, hafnium and rhenium,
• wherein the slurry contains at least one other elemental metal powder component and the maximum aluminum content of the metallic powder of the slurry is about 98% and the minimum is about 34%,
• wherein the slurry is in an aqueous liquid which cures and/or volatilizes at the diffusion temperature of the metals into the substrate,
• wherein the temperature of the diffusion of the aluminum is in the range of 660.degree. to 1050.degree. C., and
• wherein the nickel base superalloy substrate has a high chromium content of over 12% or a low chromium content of less than 12%.