In use in a gas turbine engine, airfoil components, such as gas turbine engine blades and vanes, are operated at extremely elevated temperatures and as a result are internally cooled by ducted airflow through one or more cooling passages located internally in the airfoil. Such components typically are externally coated with a protective coating that is resistant to high temperature degradation. The walls of the internal passages of such components also typically can be protectively coated to this same end. Over time, such airfoil components exhibit coating wear, cracking, corrosion, and other degradation to the coating and/or airfoil substrate that necessitates repair or replacement of the component. Repair, rather than replacement, is the preferred procedure for extending the service life of such components from an economic standpoint due to their high initial cost.
In typical repair procedures, engine-run airfoil components are subjected to external and possible internal recoating with protective coating materials using well known high temperature coating techniques, such as pack, out-of-pack, chemical vapor deposition, or plasma spray coating to form protective external and internal coatings. It is important prior to such coating operations that any deposits of foreign material accumulated or built-up on the external and/or internal surfaces of such airfoil components be removed to avoid harmful metallurgical contamination of the repaired airfoil component. Dirt build-up reduces airflow cooling which can create a "hot spot" in the airfoil, possibly resulting in blade failure. Removal of deposits from external surfaces of engine-run airfoil components is readily achieved as a result of the accessibility of the external surfaces to chemical cleaning agents, such as caustic solutions, or to grit blasting agents. However, removal of deposits from internal passages of engine-run airfoil components is rendered difficult by virtue of their small size and oftentimes convoluted nature that can hinder access of cleaning solution throughout the passage.
One technique developed to remove difficult-to-access internal passages of engine-run airfoil components employs a high pressure autoclave procedure wherein the engine-run components to be cleaned are disposed inside an autoclave containing a caustic cleaning solution, such as an aqueous 45% KOH solution. The autoclave is heated to elevated temperature greater than 400.degree. F. and pressurized to 200 psi for a prolonged time (e.g. 8-24 hours) to remove deposits from the internal airfoil passages. However, this autoclave procedure may not be effective to adequately remove heavy deposits from the internal airfoil passages such that the components are scrapped rather than subjected to remaining repair operations that would enable reuse of the repaired component in gas turbine engine service.