Abstract:
A method of applying a protective coating to an article comprises the steps of a) depositing aluminum in a surface region of an article, and b) depositing chromium is the surface region of the article subsequent to step a), whereby at least a portion of the chromium replaces at least a portion of the aluminum. Another method and an article are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present disclosure claims priority to U.S. Provisional Patent Application No. 62/026,143, filed Jul. 18, 2014. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to chromium-enriched metallic coatings. 
         [0003]    Coatings are used to enhance corrosion resistance of gas turbine engine components, or other components in severe environments. Coatings can be, for example, aluminum coatings or chromium coatings. 
       SUMMARY 
       [0004]    In a featured embodiment, a method of applying a protective coating to an article comprises the steps of a) depositing aluminum in a surface region of an article, and b) depositing chromium is the surface region of the article subsequent to step a), whereby at least a portion of the chromium replaces at least a portion of the aluminum. 
         [0005]    In another embodiment according to the previous embodiment, prior to step b) there is a first amount of aluminum in the surface region of the article, and subsequent to step b) there is a second amount of aluminum in the surface region of the article, the second amount less than the first amount. 
         [0006]    In another embodiment according to any of the previous embodiments, at least one of the aluminum and the chromium are deposited by chemical vapor deposition. 
         [0007]    In another embodiment according to any of the previous embodiments, the chemical vapor deposition is performed at a temperature of between approximately 1900° F. and 2100° F. (1037.78° C. and 1148.89° C.) for a time of between approximately 1 and 6 hours. 
         [0008]    In another embodiment according to any of the previous embodiments, step b) is performed with an activator. 
         [0009]    In another embodiment according to any of the previous embodiments, the activator is a halide activator. 
         [0010]    In another embodiment according to any of the previous embodiments, step a) results in an aluminum layer between approximately 0.5 and 3.0 mils (0.01 to 0.08 mm) thick. 
         [0011]    In another embodiment according to any of the previous embodiments, step b) results in a chromium layer having approximately 20-40% chromium by weight in an outer 30 to 60% of the coating thickness. 
         [0012]    In another embodiment according to any of the previous embodiments, step b) results in a chromium layer substantially free of alpha chromium. 
         [0013]    In another featured embodiment, a method of applying a protective coating to an article comprises the steps of a) vapor depositing aluminum in a surface region of an article formed of a low-chromium nickel-based alloy, b) vapor depositing chromium in the surface region of the article while removing at least a portion of the aluminum using an activator, and c) heating the article such that the chromium replaces at least a portion of the aluminum through action of the activator. 
         [0014]    In another embodiment according to the previous embodiment, step c) is performed at a temperature of between approximately 1900° F. and 2100° F. (1037.78° C. and 1148.89° C.) for a time of between approximately 4 and 20 hours. 
         [0015]    In another embodiment according to any of the previous embodiments, the activator is a halide activator. 
         [0016]    In another embodiment according to any of the previous embodiments, the activator interacts with the aluminum such that chromium replaces the aluminum. 
         [0017]    In another featured embodiment, an article comprises a base alloy, a corrosion-resistant coating on the base alloy, the corrosion-resistant coating containing a region of approximately 20%-40% chromium, and an aluminum diffusion zone disposed between the base alloy and the corrosion-resistant coating. 
         [0018]    In another embodiment according to any of the previous embodiments, the region extends between a depth of approximately 5 to 60 microns (0.2 to 2.3 mils) from a surface of the coating. 
         [0019]    In another embodiment according to any of the previous embodiments, the region is a first region, and further comprising a second region extending from the first region to the aluminum diffusion zone, the second region having a thickness of approximately 0.6 mils (15 microns) and comprising less than approximately 10% chromium by weight. 
         [0020]    In another embodiment according to any of the previous embodiments, the aluminum diffusion zone comprises less than approximately 20% chromium by weight. 
         [0021]    In another embodiment according to any of the previous embodiments, the article is a turbine blade. 
         [0022]    In another embodiment according to any of the previous embodiments, the article comprises external and internal surfaces and wherein the coating is applied to at least one of the external and internal surfaces. 
         [0023]    In another embodiment according to any of the previous embodiments, the base alloy is at least one of a nickel-based superalloy and a low-chromium superalloy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
           [0025]      FIG. 1  illustrates an example article. 
           [0026]      FIG. 2  illustrates an example method of applying a protective coating to an article. 
           [0027]      FIG. 3  illustrates a representative micrograph of a chromium-enriched coating. 
           [0028]      FIG. 4  illustrates another representative micrograph of a chromium-enriched coating. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  schematically illustrates an example article  100 . As will be described, a method  200 , shown in  FIG. 2 , can be employed to apply a protective coating to the article  100 . 
         [0030]    In  FIG. 1 , the example article  100  is an airfoil having one or more external surface regions  102  and one or more internal surface  104  regions. The external surface region  102 , the internal surface region  104 , or both are coated with a protective coating  106 . In another example, only a portion of the external and internal surface regions  102 ,  104  are coated with the protective coating  106 . The airfoil can be an airfoil for a gas turbine engine, such as a blade or vane. However, it is also to be understood that the features descried herein are applicable to other types of articles as well. 
         [0031]    The article  100  may be a metallic article formed of a base alloy. For example, the base alloy is a superalloy. In a further example, the superalloy is a nickel-base alloy. In a still further example, the base alloy is a low-chromium superalloy, such as a superalloy having less than approximately 12% by weight chromium. 
         [0032]    The steps or actions described with respect to the method  200  to form the coating  106 , shown in  FIG. 2 , can be employed with additional steps or other processes as desired. In Step  202 , aluminum is deposited in one or more surface regions  102 ,  104  of the article  100  (aluminizing) The aluminizing is accomplished by chemical vapor deposition (CVD) or diffusion coating, for instance. 
         [0033]    In a further example, the article  100  is exposed to an aluminum source material such as a chromium aluminum alloy in the presence of an activator, such as a halide activator, and a cover gas such as argon or hydrogen, at a temperature of between approximately 1900° F. and 2100° F. (1037.78° C. and 1148.89° C.) for a time of between approximately 1 and 6 hours. The halide activator can be, for instance, a fluoride or a chloride in a solid or gaseous form. 
         [0034]    In another example process, the aluminum or aluminum source and activator can be combined with another material such as aluminum oxide powder prior to being deposited on the surfaces  102 ,  104  of the article  100 . The other material can enhance the deposition of aluminum or the properties of the deposited aluminum. Then, the article  100  is exposed to the aluminum or aluminum mixture at a temperature of less than approximately 1500° F. (815.56° C.) for a time of between approximately 2 and 4 hours. 
         [0035]    In another example process, the aluminum or aluminum source and activator can be combined with another material such as aluminum oxide powder prior to being deposited on the surfaces  102 ,  104  of the article  100 . The other material can enhance the deposition of aluminum or the properties of the deposited aluminum. Then, the article  100  is exposed to the aluminum or aluminum mixture at a temperature of between 1500° F. and 1900° F. (815.56° C. and 1037.78° C.) for a time of between approximately 2 and 8 hours. In another example process, the aluminum or aluminum source and activator can be combined with another material such as aluminum oxide powder and a binder to form a slurry prior to being deposited on the surfaces  102 ,  104  of the article  100 . The other material can enhance the deposition of aluminum or the properties of the deposited aluminum. Then, the article  100  is exposed to the aluminum or aluminum mixture at a temperature of between 1500° F. and 1900° F. (815.56° C. and 1037.78° C.) for a time of between approximately 2 and 8 hours. 
         [0036]    Any of the processes described above can be repeated to incorporate additional aluminum multiple times. Additionally, any of the processes described above can form an aluminum coating on the surface  102 ,  104  of the article  100  that is between approximately 0.5 and 3.0 mils (0.01 to 0.08 mm) thick, for example. 
         [0037]    In Steps  204 - 208 , chromium is deposited onto article  100  (chromizing). Chromizing is accomplished by CVD or diffusion coating, similar to the processes discussed above with respect to aluminizing, in one example. The chromium is applied to the article  100  from a pure chromium source material in the form of powder nuggets with an activator such as a halide activator in Step  204 . The activator may be, for example, ammonium chloride, chromium chloride, or another chloride or fluoride. Prior to step  204 , the chromium can be combined with aluminum oxide, which can enhance the deposition of chromium or the properties of the deposited chromium. In Step  206 , the article  100  is heated to temperature of between approximately 1900° F. and 2100° F. (1037.78° C. and 1148.89° C.) for a time of between approximately 4 and 20 hours. The heat up to temperature can coincide with the deposition, or be conducted separately. During the heating process, the activator interacts with the aluminum to allow the chromium to replace at least some of the aluminum in the coating to produce a chromium-enriched coating  106 . That is, subsequent to the chromizing process of Steps  204 - 208 , there is less aluminum in the surface regions  102 ,  104  of the article  100 . In one embodiment, the chromium-enriched coating  106  is substantially free of alpha-chromium. 
         [0038]      FIGS. 3-4  show representative micrographs of a chromium-enriched coating  106 . Areas  1 - 17  represent example areas of the coating  106  which were analyzed. The coating  106  includes a first region  108  at a surface  107  of the coating  106 . The first region  108  extends from approximately 5 microns (0.2 mils) below the surface  107  of the coating  106  to a depth of approximately 2.3 mils (60 microns) and comprises approximately 20-40% chromium by weight. In one example, the first region  108  comprises approximately an outer 30 to 60% of the coating thickness. 
         [0039]    The coating  106  further includes a second region  110  under the first region  108 . The second region  110  is approximately 0.6 mils (15 microns) thick and comprises less than approximately 10% chromium by weight. 
         [0040]    The coating  106  further includes an aluminum diffusion zone  112  under the second region  110  which comprises less than about 20% chromium by weight. The aluminum diffusion zone  112  is disposed between the second region  110  and the base alloy of the article  100 . The aluminum diffusion zone  112  is a region of aluminum deposited during the aluminizing process of Step  202  but not replaced by chromium during the chromizing process of Steps  204 - 208 . That is, the aluminum concentration in the portion of the aluminum diffusion zone  112  adjacent the second region  110  can have a higher aluminum concentration than the first and second regions  110 ,  112 . The aluminum concentration in the aluminum diffusion zone  112  decreases with distance to the base alloy of the article  100 . 
         [0041]    Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
         [0042]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.