Method for removing aluminide coating from metal substrate and turbine engine part so treated

#This invention relates to a method for removing an aluminide coating from a metal-based substrate. More particularly, the invention is directed to a method for selectively removing an aluminide coating by using a stripping composition to degrade the coating and then removing it without damaging the substrate. The invention also relates to a turbine engine part having an aluminide coating, at least a portion of which has been selectively removed by the above method.

#A variety of coatings are often used to protect metal parts exposed to high temperatures, such as parts made from superalloys. For example, gas turbine engine components (and other industrial parts) are often formed of superalloys that can withstand a variety of extreme operating conditions. Such parts are usually covered with coatings to protect them from environmental degradation, including the adverse effects of corrosion and oxidation. Coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, turbine shrouds and transition pieces, generally belong to one of two classes: diffusion coatings or overlay coatings.

#Diffusion coatings are typically formed of aluminide-type alloys, such as nickel-aluminide; a noble metal-aluminide such as platinum-aluminide; or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAl(X), where M is an element selected from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element selected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof. Diffusion coatings are formed by depositing constituent components of the coating, and reacting those components with elements from the underlying substrate, to form the coating by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate.

#The above methods comprise the step of contacting the surface of the substrate with at least one stripping composition comprising from about 20% to about 40% nitric acid, by weight of the composition, at a temperature less than about 20° C. for at least about 2 hours to degrade the coating without damaging the substrate.

#Within the above ranges, various stripping compositions and processing conditions can be used in the process of the invention. The choice of a particular composition or condition will depend on various factors, such as the type of substrate, the type of aluminide coating being removed from the substrate, the intended end use for the substrate, and the presence or absence of additional treatment steps (e.g., pretreatment, desmutting, neutralization and/or rinsing steps). Those skilled in the art will be able to choose appropriate stripping compositions and processing conditions for a given situation, based on the teachings herein.

#The substrate of the present invention can be any metallic material or alloy typically protected by an aluminide coating. As used herein, “metallic” refers to substrates that are primarily formed of metal or metal alloys, but which may also include some non-metallic components. Non-limiting examples of metallic materials comprise at least one element selected from the group consisting of iron, cobalt, nickel, aluminum, chromium, titanium, and mixtures thereof (e.g., stainless steel).