Abstract:
A method for chromizing an article includes applying a slurry to an article. The slurry has active chromium and a residue-removal agent. The method also includes heating the article and slurry to diffuse chromium from the slurry into the article. The heating leaves a residue on the article with the residue-removal agent. The heating also includes removing the residue-removal agent to thus remove the residue from the article, using a cleaning solution. A method for chromizing parts and a method of cleaning a chromized part are also disclosed.

Description:
BACKGROUND 
       [0001]    Articles that are subject to corrosion, such as gas turbine engine components, may include a coating to protect an underlying material from corrosion. Some articles have internal passages which are subject to corrosion and can be protected by such a coating. 
         [0002]    Various techniques can be used to deposit a coating, such as “chromizing,” which results in a chromium-rich coating. Chromizing can be accomplished by, for instance, a vapor deposition process, or by the application of a slurry containing active chromium to the article and processing it. 
       SUMMARY 
       [0003]    A method for chromizing an article according to an example of the present disclosure includes applying a slurry to an article. The slurry has active chromium and a residue-removal agent. The method also includes heating the article and slurry to diffuse chromium from the slurry into the article. The heating leaves a residue on the article with the residue-removal agent. The heating also includes removing the residue-removal agent to thus remove the residue from the article, using a cleaning solution. 
         [0004]    In a further embodiment of any of the foregoing embodiments, the article includes internal passages, and the residue is in the internal passages. 
         [0005]    In a further embodiment of any of the foregoing embodiments, the slurry flows into the internal passages. 
         [0006]    In a further embodiment of any of the foregoing embodiments, the amount of solids in the slurry is greater than about 25 percent by weight of the slurry. 
         [0007]    The method of claim  2 , wherein the amount of solids in the slurry is between about 50 and about 75 percent by weight of the slurry. 
         [0008]    In a further embodiment of any of the foregoing embodiments, the residue-removal agent is inert with respect to the article and the slurry in the heating. 
         [0009]    In a further embodiment of any of the foregoing embodiments, the residue-removal agent includes silica. 
         [0010]    In a further embodiment of any of the foregoing embodiments, the residue-removal agent forms a matrix during the heating. The matrix traps the residue. 
         [0011]    In a further embodiment of any of the foregoing embodiments, the slurry contains an amount of residue-removal agent sufficient to form a continuous matrix of residue-removal agent during the heating step. 
         [0012]    In a further embodiment of any of the foregoing embodiments, the slurry contains an amount of solids, and greater than about 50% by weight of the solids of the residue removal agent. 
         [0013]    In a further embodiment of any of the foregoing embodiments, the article is an airfoil. 
         [0014]    A method for chromizing parts according to an example of the present disclosure includes applying a slurry to an article. The slurry has active metal and a residue-removal agent. The method also includes forming a matrix of the residue-removal agent and which residue in the matrix, and heating the article and the slurry. The heating diffuses the metal into the article to form a coating. The method also includes dissolving the matrix to remove the matrix and release the residue. 
         [0015]    A further embodiment of any of the foregoing embodiments includes conducting the dissolving in a pressure chamber. 
         [0016]    In a further embodiment of any of the foregoing embodiments, the dissolving includes cleaning the article with a hydroxide cleaning solution. 
         [0017]    A method of cleaning a chromized part according to an example of the present disclosure includes cleaning a residue from a chromized article. The residue is trapped in a matrix of residue-removal agent on the chromized article. The cleaning is by dissolving the matrix to release the residue. 
         [0018]    In a further embodiment of any of the foregoing embodiments, the article is cleaned with a hot pressurized cleaning solution in a pressure chamber. 
         [0019]    In a further embodiment of any of the foregoing embodiments, the residue removal agent is soluble in the cleaning solution. 
         [0020]    In a further embodiment of any of the foregoing embodiments, the cleaning solution is a hydroxide, and the residue-removal agent includes silica. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    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. 
           [0022]      FIG. 1A  illustrates an example article. 
           [0023]      FIG. 1B  illustrates schematically illustrates a section view of the example article with internal passages. 
           [0024]      FIG. 2  illustrates a method of chromizing the example article. 
           [0025]      FIG. 3  illustrates the example article with chromizing residue and a chromium-enriched coating. 
           [0026]      FIG. 4A  illustrates the cleaned example article with a chromium-enriched coating. 
           [0027]      FIG. 4B  illustrates a section view of the cleaned example article with a chromium-enriched coating. 
           [0028]      FIG. 5  illustrates a schematic detail view of a matrix formed by residue removal agent on the example article. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1A  illustrates an example article  10 . In this example, the article  10  is an airfoil for a gas turbine engine. The article  10  may potentially be exposed to hot corrosion during the operation of the gas turbine engine, for example, up to temperatures of about 1900° F. (1038° C.). The article  10  may be formed of steel or a superalloy, such as a cobalt- or nickel-based superalloy. It should be understood, however, that this disclosure will benefit other articles or gas turbine engine components with internal passages.  FIG. 1B  illustrates a representative section view of the article  10  with internal passages  12 . 
         [0030]      FIG. 2  illustrates a method  100  of chromizing the article  10  including the internal passages  12 . In Step  102 , a slurry is applied at least to the internal passages  12 . The slurry can be applied by, for example, dipping the article  10  into the slurry, spraying the slurry onto the article  10 , painting the slurry onto the article  10 , flowing the slurry across the article  10  and into internal passages  12 , or by another method of application. Although some of the slurry may drip off, the slurry at least forms a slurry coating on surfaces of the internal passages  12 . 
         [0031]    The slurry contains an active coating metal of chromium powder in liquid carrier material. The slurry may also include alumina powder and/or a diffusion activator, such as chromium(III) chloride or a halide activator. The amount of liquid carrier material controls the viscosity of the slurry. The slurry contains enough liquid carrier material such that the slurry can readily flow through internal passages  12  of article  10 . In one example, the amount of solids in the slurry is between about 50 and 75 percent by weight of the slurry. The slurry also includes residue-removal agent, such as silica, which is discussed in more detail below. 
         [0032]    In Step  104 , the article  10  with slurry coating is heated to diffuse chromium from the slurry coating into the article  10  as represented at D ( FIG. 3 , discussed in more detail below). In one example, the heating is conducted in a furnace having a continual flow of argon to produce an argon environment, in which argon is the most abundant gas, at a temperature greater than 1900° F. (103° C.), such as 1950° F. (1066° C.) to 2000° F. (1094° C.). The article  10  is heated for a selected amount of time, depending upon a desired thickness of the resulting chromium diffusion coating. In some examples, the selected amount of time is between  6  and 16 hours and the final chromium diffusion coating includes at least 20% by weight of chromium. The heating and diffusion creates a chromium-enriched coating  16 , but leaves a residue or crust  14  on a surface  15  of the article  10  or internal passages  12 , as shown in  FIG. 3 . For example, the coating  16  is the surface region of the article  10  that has diffused chromium. 
         [0033]    In Step  106 , the article  10  is processed to remove the residue  14  from the article  10 , yielding an article  10  with the chromium-enriched coating  16  having a clean surface  16 a, as is shown in  FIGS. 4A-4B . For example, as will be discussed in more detail below, the processing includes cleaning the article with a cleaning solution. In other examples, the article may be processed in a different way, such as by grit blasting. The coating  16  can be on an outer surface of the article  16  and/or on the surfaces  15  of the internal passages  12 , as shown in  FIGS. 4A-B , respectively. In this example, the residue  14  is removed from the article  10  with a hot, pressurized cleaning solution. In one example, the cleaning solution is a hydroxide, and the processing step is performed inside a pressure chamber, such as an autoclave. 
         [0034]    The residue removal agent permits removal of the residue  14 . The residue removal agent is thermodynamically inert with respect to the slurry and the article  10 . That is, the residue removal agent does not react or substantially react with the slurry or the article  10 . In one example, the residue removal agent is or includes silica (silicon dioxide). 
         [0035]    Turning to  FIG. 5 , the residue-removal agent  18  forms a matrix  20  on the surface  15  of the internal passage  12  during the heating step  104 . Though the surface of the internal passage  12  is shown in  FIG. 5 , it should be understood that the residue removal agent  18  can form a matrix  20  on the exterior surface of the article  10  as well. For instance, the heating diffuses the chromium into the article  10  and evaporates the liquid carrier, leaving the silica to consolidate on the surface  15 . The consolidated silica forms the matrix  20  and traps the residue  14 . For example, the residue  14  may be residual constituents of the powders in the slurry, byproducts of the powders, or foreign substances. 
         [0036]    The residue removal agent  18  is soluble in the cleaning solution. In one example, the residue  14  is insoluble or substantially insoluble in the cleaning solution. When the residue-removal agent  18  is dissolved by the cleaning solution, it releases the residue  14  from the article  10 . This cleaning solution can flow through the internal passages  12  to contact the residue  14  in the internal passages  12  and thereby remove the residue  14 . Other residue-removal methods, such as grit blasting or a similar mechanical process, can be difficult to perform in internal passages  12  or other non-line-of-sight surfaces of an article  10  because such methods require a clear physical line-of-sight to a surface for effective removal. 
         [0037]    In one example, the slurry contains an amount of residue removal agent  18  sufficient to form the matrix  20 . In a further example, the slurry contains an amount of residue removal agent  18  sufficient to form a continuous matrix  20 . That is, the continuous matrix  20  entraps all the residue  14  from the heating and diffusion, and when the matrix  20  is removed from the article  10  by the processing step  106 , substantially all of the residue  14  is removed with it. Although less residue removal agent  18  could be used, lower amounts increase the potential for residue  14  to bond to the surface  15  of the internal passage  12  or to the article  10 , thereby making removal more difficult. In a yet another example, the slurry contains greater than about 25% by weight of solids of residue removal agent  18 . More particularly, the slurry contains greater than about 50% by weight of solids of residue removal agent  18 . 
         [0038]    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. 
         [0039]    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 this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.