Patent Application
20060093849
This invention relates to a method for applying a chromium-containing coating to a metal substrate of an article, such as a turbine airfoil, to provide corrosion protection for the surface of the substrate. This invention further relates to a corrosion resistant article that has such a coating.
Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. Significant advances in high temperature capabilities have been achieved through formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as turbine rotors, blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflector plates, augmentors and the like. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase, including resistance to the corrosive environments that surround and permeate these turbine components.
Turbine engine components, such as airfoils used in turbine blades and vanes, are typically heated to temperatures in excess of 1500° F. (815° C.) during service and exposed to highly corrosive exhaust gases from the gas turbine. At such temperatures, oxygen and other corrosive components of the exhaust gas can cause undesired corrosion of the metal substrate of the turbine airfoil, even metal substrates that comprise nickel and cobalt-base superalloys. In addition, cooling of turbine airfoils is typically necessary to remove excessive heat. For example, the turbine airfoil can be provided with internal cooling passages with air being forced through these cooling passages and out openings at the external surface of the airfoil, thus removing heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil. See, for example, commonly assigned U.S. Pat. No. 6,183,811 B1 (Conner), issued Feb. 6, 2001; and U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999.
Many protective coatings have been developed for metal substrates to improve the life of turbine airfoils. These protective coatings are typically 2 to 5 mils (51 to 127 microns) in thickness and provide protection to the metal substrate from oxidation and corrosion at higher temperatures that the airfoil is subjected to during operation. These include oxidation-resistant aluminide diffusion coatings such as, for example, nickel aluminide and platinum aluminide coatings. These aluminide diffusion coatings can be applied to the metal substrate by pack cementation techniques, or more recently by chemical vapor phase deposition (CVD) techniques. See, for example, U.S. Pat. No. 4,148,275 (Benden et al), issued Apr. 10, 1979; commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994, U.S. Pat. No. 5,928,725 (Howard et al), issued Jul. 27, 1999; U.S. Pat. No. 6,039,810 (Mantkowski et al), issued Mar. 21, 2000, U.S. Pat. No. 6,183,811 B1 (Conner), issued Feb. 6, 2001; and U.S. Pat. No. 6,224,941 B1 (Chen et al), issued May 1, 2001, which disclose various apparatus and methods for applying aluminide diffusion coatings.
For additional protection against corrosion at lower temperatures, or in marine environments where corrosive salts can be present, it can be desirable to include chromium in the protective coating. Chromium can be applied to the metal substrate surface by spraying a chromium-containing powder onto the surface thereof. However, for turbine airfoils having internal air cooling passages, the heterogeneity and especially surface roughness of such spray coatings on the external surface of the airfoil can be undesirable. Chromium can also be applied by depositing the chromium on the metal substrate, and then interdiffusing the chromium with the metal alloy in the substrate. See commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al), issued Sep. 4, 2001. This is typically followed by applying an aluminide diffusion coating by pack cementation or CVD techniques to the deposited chromium-containing layer.
This aluminide diffusion coating applied to the deposited chromium-containing layer typically forms an inner diffusion layer adjacent to the chromium-containing layer, and an outer additive layer adjacent to the diffusion layer. It has been found that insufficient chromium is delivered to this outer additive layer during subsequent diffusion processes that occur to provide beneficial corrosion protection. In particular, the level of chromium delivered to this outer additive layer is about 6% by weight or less of this outer layer.
Accordingly, it would be desirable to be able to incorporate chromium as a component of a coating for a metal substrate that also includes an aluminide diffusion coating in a manner that provides beneficial corrosion protection to the metal substrate. It would also be desirable to be able to incorporate this chromium into the protective coating of a metal substrate that is used with a turbine airfoil or other component that has internal cooling air passages or similar passages. It would be further desirable to be able to incorporate this chromium using a process that is compatible with various metal substrates, as well as other materials, that the turbine airfoil is made of and that provides a relatively inexpensive protective coating.
An embodiment of this invention relates to a method for applying a corrosion resistant chromium-containing coating to an underlying metal substrate where the metal substrate has an overlaying platinum-containing layer. This method comprises the steps of:
Another embodiment of this invention relates to a corrosion resistant coated article. This article comprises:
The method of this invention, well as the resulting corrosion resistant coated article, provides several benefits. This method allows effective incorporation of chromium as a component of the corrosion resistant protective coating, in particular the aluminide diffusion layer of the coating, that provides effective corrosion resistance and protection for the underlying metal substrate. In particular, sufficient chromium (i.e., at least about 10%) can diffuse into the outer additive layer of the aluminide diffusion layer of the coating. This method provides a chromium-containing coating that is compatible with various metal substrates and other materials that turbine airfoils comprise. This method can also be used to incorporate desired, beneficial chromium into the protective coating for an underlying metal substrate that is used with a turbine airfoil (e.g., turbine blade) or other component that has internal cooling air passages or similar passages without causing other undesired effects such as closure of such internal cooling passages, or increasing surface roughness and damage due to excessive heat treatments. This method also allows for the repair of components, especially turbine airfoils, that previously have had no protective coating thereon.
As used herein, the term “comprising” means various compositions, compounds, components, layers, steps and the like can be conjointly employed in the present invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.”
All amounts, parts, ratios and percentages used herein are by weight unless otherwise specified.
The embodiments of the method of this invention are useful in applying chromium-containing corrosion resistant protective coatings to metal substrates comprising a variety of metals and metal alloys, including superalloys, used in a wide variety of turbine engine (e.g., gas turbine engine) parts and components operated at, or exposed to, high temperatures, especially higher temperatures that occur during normal engine operation. These turbine engine parts and components can include turbine airfoils such as blades and vanes, turbine shrouds, turbine nozzles, combustor components such as liners, deflectors and their respective dome assemblies, augmentor hardware of gas turbine engines and the like. The embodiments of the method of this invention are particularly useful in applying chromium-containing corrosion resistant protective coatings to turbine blades and vanes, and especially the shank and airfoil portions of such blades and vanes. However, while the following discussion of embodiments of the method of this invention will be with reference to turbine blades and vanes, and especially the airfoil portions thereof, that comprise these blades and vanes, it should also be understood that the method of this invention can be useful with other articles comprising metal substrates that require corrosion resistant protective coatings.
The various embodiments of the method of this invention are further illustrated by reference to the drawings as described hereafter. Referring to the drawings,
Airfoil 22 extends upwardly from a platform 28, which extends laterally outwardly from the airfoil 22. Platform 28 has a top side 30 adjacent to the airfoil 22 and a bottom side 32 remote from the airfoil 22. As shown in
In some embodiments of turbine blade 20, a number of internal passages extend through the interior of airfoil 22, ending in openings indicated as 38 in the surface of airfoil 22. During operation, a flow of cooling air is directed through the internal passages to cool or reduce the temperature of airfoil 22.
Substrate 21 can comprise any of a variety of metals or metal alloys that are typically protected by aluminide diffusion coatings. For example, substrate 21 can comprise a high temperature, heat-resistant alloy, e.g., a superalloy. Such high temperature alloys are disclosed in various references, such as U.S. Pat. No. 5,399,313 (Ross et al), issued Mar. 21, 1995 and U.S. Pat. No. 4,116,723 (Gell et al), issued Sep. 26, 1978, both of which are incorporated by reference. High temperature alloys are also generally described in Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd Ed., Vol. 12, pp. 417-479 (1980), and Vol. 15, pp. 787-800 (1981). Illustrative high temperature nickel-base alloys are designated by the trade names Inconel®, Nimonic®, René® (e.g., René® 80 and René® N5 alloys), and Udimet®.
Protective coatings of this invention are particularly useful with nickel-base superalloys. As used herein, “nickel-base” means that the composition has more nickel present than any other element. The nickel-base superalloys are typically of a composition that is strengthened by the precipitation of gamma-prime phase. More typically, the nickel-base alloy has a composition of from about 4 to about 20% cobalt, from about 1 to about 10% chromium, from about 5 to about 7% aluminum, from 0 to about 2% molybdenum, from about 3 to about 8% tungsten, from about 4 to about 12% tantalum, from 0 to about 2% titanium, from 0 to about 8% rhenium, from 0 to about 6% ruthenium, from 0 to about 1% niobium, from 0 to about 0.1% carbon, from 0 to about 0.01% boron, from 0 to about 0.1% yttrium, from 0 to about 1.5% hafnium, the balance being nickel and incidental impurities.
Protective coatings of this invention are particularly useful with nickel-base alloy compositions such as René N5, which has a nominal composition of about 7.5% cobalt, about 7% chromium, about 6.2% aluminum, about 6.5% tantalum, about 5% tungsten, about 1.5% molybdenum, about 3% rhenium, about 0.05% carbon, about 0.004% boron, about 0.15% hafnium, up to about 0.01% yttrium, balance nickel and incidental impurities. Other operable nickel-base superalloys include, for example, René N6, which has a nominal composition of about 12.5% cobalt, about 4.2% chromium, about 1.4% molybdenum, about 5.75% tungsten, about 5.4% rhenium, about 7.2% tantalum, about 5.75% aluminum, about 0.15% hafnium, about 0.05% carbon, about 0.004% boron, about 0.01% yttrium, balance nickel and incidental impurities; René 142, which has a nominal composition of about 6.8% chromium, about 12.0% cobalt, about 1.5% molybdenum, about 2.8% rhenium, about 1.5% hafnium, about 6.15% aluminum, about 4.9% tungsten, about 6.35% tantalum, about 150 parts per million boron. about 0.12% carbon, balance nickel and incidental impurities; CMSX-4, which has a nominal composition of about 9.60% cobalt, about 6.6% chromium, about 0.60% molybdenum, about 6.4% tungsten, about 3.0% rhenium, about 6.5% tantalum, about 5.6% aluminum, about 1.0% titanium, about 0.10% hafnium, balance nickel and incidental impurities; CMSX-10, which has a nominal composition of about 7.00% cobalt, about 2.65% chromium, about 0.60% molybdenum, about 6.40% tungsten, about 5.50% rhenium, about 7.5% tantalum, about 5.80% aluminum, about 0.80% titanium, about 0.06% hafnium, about 0.4% niobium, balance nickel and incidental impurities; PWA1480, which has a nominal composition of about 5.00% cobalt, about 10.0% chromium, about 4.00% tungsten, about 12.0% tantalum, about 5.00% aluminum, about 1.5% titanium, balance nickel and incidental impurities; PWA1484, which has a nominal composition of about 10.00% cobalt, about 5.00% chromium, about 2.00% molybdenum, about 6.00% tungsten, about 3.00% rhenium, about 8.70% tantalum, about 5.60% aluminum, about 0.10% hafnium, balance nickel and incidental impurities; and MX-4, which has a nominal composition as set forth in U.S. Pat. No. 5,482,789 of from about 0.4 to about 6.5% ruthenium, from about 4.5 to about 5.75% rhenium, from about 5.8 to about 10.7% tantalum, from about 4.25 to about 17.0% cobalt, from 0 to about 0.05% hafnium, from 0 to about 0.06% carbon, from 0 to about 0.01% boron, from 0 to about 0.02% yttrium, from about 0.9 to about 2.0% molybdenum, from about 1.25 to about 6.0% chromium, from 0 to about 1.0% niobium, from about 5.0 to about 6.6% aluminum, from 0 to about 1.0% titanium, from about 3.0 to about 7.5% tungsten, and wherein the sum of molybdenum plus chromium plus niobium is from about 2.15 to about 9.0%, and wherein the sum of aluminum plus titanium plus tungsten is from about 8.0 to about 15.1%, balance nickel and incidental impurities. The use of the present invention is not limited to turbine components made of these preferred alloys, and has broader applicability.
As shown in
This protective coating 46 comprises a platinum-containing layer indicated generally as 50 that overlays and is directly adjacent to substrate 21. This platinum-containing layer 50 typically has a thickness of from about 0.1 to about 0.5 mils (from about 2.5 to about 13 microns), more typically from about 0.1 to about 0.2 mils (from about 2.5 to about 5 microns). The platinum-containing layer 50 typically comprises from about 99 to 100% platinum. During post-deposition heat treatment of platinum-containing layer 50 as described hereafter, elements from substrate 21 (e.g., aluminum and nickel) can diffuse into layer 50 and, to a more limited extent, platinum can diffuse from layer 50 into substrate 21.
As shown in
Corrosion resistant portion 54 of coating 46 includes a chromium-containing layer 58 that is directly adjacent to and overlays platinum-containing layer 50. This chromium-containing layer 58 typically has a thickness of from about 0.5 to about 2 mils (from about 13 to about 51 microns), more typically from about 0.5 to about 1 mils (from about 13 to about 25 microns). These thicknesses are usually with reference to the initial deposition of the chromium-containing layer 58. During deposition of this chromium-containing layer and especially subsequent heat treatment steps as described hereafter, the boundaries of layer 58 can become less distinct.
As shown in
As shown in
As also shown in
As shown in
As shown in
While the prior description of the embodiment of the method of this invention has been with reference to applying a new protective coating 46 to substrate 21 of a blade or vane 20, another embodiment of the method of this invention can also be used to repair or replace a prior existing partially or completely damaged coating 46, or at least the corrosion resistant portion 54 thereof, on substrate 21 of blade or vane 20. In the embodiment of this method, the existing partially or completely damaged coating is removed, if needed, from substrate 21, such as by grit blasting, so that a new protective coating 46, or at least the corrosion resistant portion 54 thereof, can be applied to substrate 21, as previously described and as shown in
While specific embodiments of this invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of this invention as defined in the appended claims.
