Upgrading aluminide coating on used turbine engine component

Abstract

A method of upgrading an aluminide coating on a used turbine engine component to a platinum aluminide coating. The method involves cleaning at least one surface of the component to remove hot corrosion products from the surface without damaging the aluminide coating. In one embodiment, the cleaning step involves immersing the component in a heated solution comprising acetic acid while agitating the solution using ultrasonic energy. A layer of platinum is then deposited onto the cleaned surface of the component. A second aluminide coating is then formed on the surface of the component to upgrade the component. The invention also relates to a turbine engine component, e.g., a turbine blade, having a metal-based substrate and a platinum aluminide coating on at least one surface thereof, which coating has been upgraded from an aluminide coating originally on the component using the above method.

Description

BACKGROUND OF THE INVENTION [0001] This invention relates to a method for upgrading an aluminide coating on a used turbine engine component to a platinum aluminide coating. More particularly, this invention is directed to such a method that comprises cleaning at least one surface of the component to remove hot corrosion products from the surface without damaging the aluminide coating, depositing a layer of platinum onto the cleaned surface, and then forming a second aluminide coating on the surface of the component. The invention also relates to such an upgraded used turbine engine component. [0002] The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the tu...

AI Technical Summary

Benefits of technology

[0023] The method of this invention comprises the step of cleaning the surface of the component to remove hot corrosion products from the surface of the component without damaging the aluminide coating. The cleaning step may include conditioning or activating the surface to be cleaned by processing through caustic autoclave or grit blasting operations, immersing the component in a heated liquid solution comprising a weak acid, and / or agitating the surfaces of the component while it remains immersed in the solution. In this manner, the hot corrosion products on the surfaces of the component can be removed without damaging or removing the diffusion aluminide coating.

[0026] Baths comprising the cleaning composition are often stirred or otherwise agitated while the process is carried out to permit maximum contact between the cleaning composition and the corrosion products being removed. A variety of known techniques can be used for this purpose, such as using impellers, ultrasonic agitation, magnetic agitation, gas bubbling, or circulation-pumps. Immersion time in the bath will vary based on many of the factors discussed above. On a commercial scale, the immersion time will usually range from about 1 hour to about 10 hours, which may be split among two or more steps. In some embodiments, the total immersion time will be from about 1.5 to about 5 hours, typically from about 2 to about 4 hours. Longer times within the above ranges promote more complete removal of the corrosion products but can cause damage to the coating and / or substrate. Thus, the time, the concentration of acid in the composition, and the temperature of the composition are selected to provide the desired balance between maximizing removal of the corrosion products and minimizing damage to a particular coating and metal substrate.

[0027] In one embodiment, a weak acid solution, such as an acetic acid solution, e.g., white vinegar, which typically comprises from about 2% to about 10% acetic acid, more typically from about 4% to about 8% acetic acid, by weight, is used to remove hot corrosion products at certain temperatures, supplemented with sufficient agitation following a surface conditioning or activation step. Advantageously, such weak acetic acid solutions do not attack aluminide coatings, permitting rejuvenation of an aluminide coating instead of complete removal of the coating and application of a new coating. Another advantage of this invention is that acetic acid does not foul wastewater treatment facilities, and can be disposed of without concern for exceeding allowable levels for metal ion concentrations in wastewater. Accordingly, the treatment of this invention is environmentally friendly. While vinegar is generally preferred as the treatment solution of this invention due to availability and cost, it is foreseeable that stronger and weaker acetic acid solutions derived by other methods could be used.

[0029] In one embodiment, the cleaning composition further comprises a wetting agent. The wetting agent reduces the surface tension of the composition, permitting better contact with the substrate and the aluminide coating, particularly on internal surfaces of metal parts, to improve cleaning of the aluminide coating. Suitable wetting agents include polyalkylene glycols, glycerol, fatty acids, soaps, emulsifiers, and surfactants. The wetting agent is usually present at a level in the range of from about 0.1% by weight to about 5% by weight, based on the total weight of the composition.

[0030] Removal of the hot corrosion products without damaging the aluminide coating may be accomplished by various other methods known in the art. For example, the corrosion products may be removed by abrading the surface, such as by using a gentle abrasion step that minimizes damage to the coating. As an example, light grit blasting can be carried out by directing a pressurized air stream comprising aluminum oxide particles across the surface at a pressure of less than about 40 psi (about 2.8 kgf / cm2), typically less than about 20 psi (about 1.4 kgf / cm2). Various abrasive particles may be used for the grit blasting, e.g., metal oxide particles such as alumina, as well as silicon carbide, glass beads, crushed glass, sodium carbonate, and crushed corn cob. The average particle size usually is less than about 500 microns, and typically less than about 100 microns.

[0053] The turbine blades can then be upgraded to have a platinum aluminide coating, as described above. Since the present method does not remove or damage the original aluminide coating, there is little or no removal of substrate metal. Turbine engine blades and other components can thus go through multiple repair cycles without loss of wall thickness. In addition, the oxidation-resistance properties of platinum aluminide coatings formed using the present method generally are equivalent to those of platinum aluminide coatings formed on components that are stripped of their original aluminide coating or that are originally uncoated.

Problems solved by technology

The operating environment within a gas turbine engine is both thermally and chemically hostile.

Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor.

In the latter situation, hot corrosion typically occurs on hot section turbine blades and vanes under conditions where salt deposits on the surface as a solid or liquid.

The salt deposits can break down the protective alumina scale on the aluminide coating, resulting in rapid attack of the coating.

A disadvantage of completely removing an aluminide coating from a turbine engine component is that a portion of the substrate metal is removed with the coating, which can significantly shorten the useful life of the component.

However, coating rejuvenation technologies for turbine blade and vane repair cannot be used in the presence of hot corrosion products, which attack the rejuvenated coating upon exposure to engine temperatures.

However, current processes for recoating typically involve removing the aluminide coating by chemical stripping, grit blasting or other mechanical means.

As noted above, these processes often result in the removal of a portion of the substrate metal, which can reduce the strength and useful life of the component.