The present invention relates to diffusion coatings for components exposed to oxidizing environments (i.e., the hostile thermal environment of a gas turbine engine). In some of the more specific embodiments, this invention teaches a process for selectively stripping the additive sublayer of diffusion aluminide coatings from metal substrates.
In order to increase the efficiency of gas turbine engines, higher operating temperatures are continuously sought. For this reason, the high temperature durability of the engine components must correspondingly increase. With the formulation of superalloys, such as nickel-base and cobalt-base, significant advances in high-temperature capabilities are being achieved. Consequently, in the absence of a protective coating, sensitive superalloy components (e.g., the turbine and combustor) typically will not endure long service exposures, without wear or damage. One such coating is referred to as an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. This coating is typically formed by a diffusion process, e.g., using a pack cementation-type procedure, and usually contains aluminum.
The diffusion process generally entails reacting a surface component with an aluminum-containing gas composition. This forms two distinct sublayers, the outermost of which is referred to as the additive sublayer, and the innermost of which is a diffusion sublayer. The additive sublayer contains the environmentally-resistant intermetallic, represented by MAI; where M is iron, nickel or cobalt, depending on the substrate material. The MAI intermetallic is often the result of the diffusion of deposited aluminum into the substrate, and a general, outward diffusion of iron, nickel or cobalt from the substrate. During high temperature exposure in air, the MAI intermetallic forms a protective aluminum oxide (alumina) scale that inhibits oxidation of the coating and the underlying substrate. The chemistry of the additive sublayer can be modified by the presence of additional elements, such as chromium, silicon, platinum, rhodium, hafnium, yttrium and zirconium. As a result of changes in elemental solubility (in the local regions of the substrate and gradient), the diffusion sublayer is thus formed. Due to reactivity, the diffusion sublayer contains various intermetallic and metastable phasesxe2x80x94products of all alloying elements from the substrate and coating.
Though significant advances have been made with environmental coating materials (and processes for forming such), they still require repair necessitated by erosion and thermal degradation. Moreover, the component on which the coating is formed must often be refurbished. The current state-of-the-art repair method is to completely remove a diffusion aluminide coating, I.e., a xe2x80x9cfull stripxe2x80x9d of both the diffusion sublayer and the additive sublayer.
An example of a full stripping process is shown in FIGS. 1 and 2. FIG. 1 is a photomicrograph of a platinum aluminide diffusion coating applied over a superalloy substrate. In this figure, region A is a portion of a substrate, while region B generally represents the diffusion sublayer of a platinum aluminide diffusion coating. Region C is the additive sublayer of the diffusion coating. This additive sublayer serves as a protective layer over the substrate, e.g., a substrate in the form of a turbine wall. As shown in FIG. 1, diffusion sublayer B is incorporated into the upper region of the substrate. (Dotted line E represents the original surface of the substrate).
FIG. 2 is a photomicrograph of the coated substrate of FIG. 1, after a current, state-of-the art, full stripping technique. Both the additive sublayer and the diffusion sublayer have been removed. In this figure, region D is the remaining portion of the substrate. The original surface of the substrate is again indicated by dotted line E.
Although functional, this full stripping process relies on lengthy exposure to stripping chemicals at elevated temperatures, which causes complete removal of the additive and diffusion sublayers. The process may also significantly attack the underlying metallic substrate, resulting in alloy depletion and intergranular/interdendritic attack. This attack is most prevalent when a component being stripped has regions with different coating thicknesses, or has uncoated surface regions (e.g., the dovetail of a turbine blade). One unfortunate result may be the need for continuous, uneconomical coating replacements.
Accordingly, it is sometimes undesirable to remove a significant portion of the coating as shown in FIG. 2. For gas turbine blades and vane airfoils, removal of the diffusion sublayer can lead to excessively thinned walls, and drastically altered airflow characteristics.
It is apparent that a new process for removing coatings from substrates (i.e., metal substrates) should be welcome in the art. The process must be capable of removing only the additive sublayer of the coating, while not substantially affecting the diffusion sublayer.
The present invention generally provides a method of removing an additive sublayer of a diffusion aluminide coating from a metallic substrate. The process described herein is effective In selectively removing only the additive sublayer of a diffusion-aluminide coating. Such a procedure does not attack the underlying diffusion sublayer, including the substrate (see FIGS. 3 and 4, discussed below). In brief, FIG. 3 is a photomicrograph of a coated substrate similar to FIG. 1, prior to the stripping treatment according to the present invention. FIG. 4 depicts the same substrate, after treatment.
The present method comprises the step of contacting the diffusion aluminide coating with an aqueous composition comprising at least one acid having the formula HXZrF6, or precursors to said acid, wherein x is 1-6. The acid is typically present at a concentration in the range of about 0.05 M to about 5 M, where M represents molarity. (Molarity can be readily translated into weight or volume percentages, for ease in preparing the solutions). As described below, these compounds may sometimes be formed in situ.
An additional embodiment of the invention is directed to an aqueous composition used for the partial stripping of diffusion aluminide coatings. The substrate is a metallic material, and is often in the form of a superalloy component. As used herein, xe2x80x9cpartial strippingxe2x80x9d of the coating refers to the selective removal of substantially all of the additive sublayer, while removing little or none of the underlying diffusion sublayer. The composition comprises an acid having the formula Hx ZrF6, as mentioned above. The acid is usually present in the composition at levels described hereinafter.
Further details regarding the various features of this invention are found in the remainder of the specification.