Abstract:
A method of stripping gamma/gamma prime polycrystalline alloy bond coats from single crystal gamma/gamma prime nickel based superalloys, including the steps of grit blasting followed by the use of hydrochloric acid solutions, followed by rinsing. The cycle may be repeated several times, with visual inspection between cycles.

Description:
BACKGROUND 
       [0001]    The present invention relates to removing a coating from a superalloy substrate. In particular, the invention relates to systems and methods for improving the removal of a coating from a superalloy substrate. 
         [0002]    It is advantageous to provide gamma/gamma prime bond coats for nickel base superalloys of the gamma/gamma prime type because this type of coating has demonstrated greater spallation resistance over conventional gamma/beta systems. Gamma/gamma prime alloys are solid solutions of the alloy with intermetallic compounds as a second phase. However, gamma/gamma prime coating provides difficulties in stripping without damage to the substrate due to the close similarities of the coating and the base material. 
         [0003]    During operation, the coatings suffer environmental damage and need to be periodically replaced or repaired to extend the life of the gas turbine hardware. Because of the corrosion and oxidation products that form on the surfaces, it is necessary to completely remove and reapply the coatings in selected areas before placing the component back in service. Mechanical means of removing the coatings such as abrasive blasting or machining are not preferred because of cost and the chance of harming the underlying substrate. 
         [0004]    The method of choice for those experienced in the art is chemical removal (or chemical stripping) wherein the coating to be removed is exposed to a solution that weakens and eventually dissolves the coating. However, care must be taken to avoid damage to the base material if replacement is to be avoided. 
       SUMMARY 
       [0005]    The present invention provides a method of stripping bond coats from nickel based superalloys that have been used in gas turbine engines, such as on turbine blades. The bond coats are gamma/gamma prime polycrystalline alloys, and the substrates are gamma/gamma prime single crystal alloys. The coatings are subjected to turbine operating conditions and in time need to be removed and replaced. The method includes grit blasting followed by the use of hydrochloric acid solutions, followed by rinsing. The cycle may be repeated several times, with inspection between cycles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates the steps of removing a bond coat from a substrate used in gas turbine parts. 
           [0007]      FIG. 2  is a table showing the composition of coatings that were applied to a superalloy base bar. 
           [0008]      FIG. 3  is a table showing the composition of different coatings that were applied to a superalloy base bar. 
           [0009]      FIGS. 4 and 5  are microphotographs of a strip cycle over five hours total of the method of this invention on a first bond coat. 
           [0010]    FIGS. band  7  are microphotographs of a strip cycle over nine hours total of the method of this invention on a second bond coat. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Method  100  for removing a bond coat from a substrate of a superalloy part is used to prepare the part for subsequent repair. First, an appropriate part needing repair is selected (Step  101 ). The part may be any of the many parts in gas turbine engines as well as other apparatus that are formed of superalloy metals and have a bond coat on their surfaces. Method  100  has been found to be effective, for example, on turbine air foils and stator vanes. 
         [0012]    The selected part is grit blasted as a first step in removing the bond coat (Step  103 ). Grit blasting is intended to remove surface oxides and may be performed, for example, using 240 aluminum oxide. Good results have been obtained using 240 aluminum oxide particles at a pressure ranging from about 30 psi to about 60 psi for suction type machines.  240  aluminum oxide has a grit size of 240 on the ANSI Standards macro grade grit size. 
         [0013]    The part is then placed in an agitated soak bath containing a high concentration of hydrochloric acid (HCl) in water (Step  105 ). The HCl concentration does not significantly affect the strip rate. Acceptable concentrations may range for 55% to 100% HCl. The part is then placed in the bath of HCl for a period of time, such as for about one to three hours. The bath temperature can be elevated and Step  105  is effective if the bath is at 150° F. (65.6° C.), though lower and higher temperatures of plus or minus 10° F. (5.5° C.) are also effective. 
         [0014]    After the soaking has been completed, the part is removed and rinsed (Step  107 ). Care should be taken to avoid contact with the HCl when rinsing, as well as all other times. After the part is rinsed, it is inspected to see whether or not the coating has been removed. An effective inspection is a “smut check” (Step  109 ). A smut check involves placing the part in the acid solution of Step  105  for a short time, such as five minutes or so, followed by a visual inspection. The part without a coating will have a gun metal finish. If some or all of the part has a black or gray color, this is evidence that the coating or some of it remains. If it appears that some coating remains, as is normally the case after Step  105  has been performed only once, Step  105  is repeated for a period of time, such as an additional hour, including the smut check time of Step  109 , followed by Step  107  and, once again, Step  109 . In most instances the total number of hours of soak time may range from about three to about ten hours. Total soak times of six to nine hours have been found to be effective. No damage to the substrate was observed during the prescribed time for this process. 
         [0015]    Alternatively, the repeat cycle may also include repeating Step  103  prior to Steps  105  and  107 . The cycle of grit blast, soak in HCl and rinse is to be repeated until the smut check of Step  109  fails to show any dark color. 
         [0016]    When Step  109  does not reveal smut, the part is then subjected to an additional inspection, which is a heat tint test (Step  111 ). The part is carefully removed, using white gloves or other non-contaminating devices and put in an air circulating oven. One effective air circulating oven is known as a Blue-M oven, which are available from Thermal Product Solutions in New Columbia, Pa.. Hot air circulates over the part in Step  111 . The temperature will be determined by the reaction of the part to the hot air as it will form oxides. The oxide color of the coating is different from the oxide color of the base alloy. Typically, the temperature may be 1050° F.,±25° F. (579.4° C.±13.9° C., although higher temperatures are also contemplated, such as 1300° F.±25° F. (704.4 ° C.±13.9° C.). After about an hour of Step  111 , the part is again visually inspected. If the color is like flat gold or dusty brass, some coating remains, and Steps  105 - 111  are repeated. If the coating is removed, the base metal will have a violet or blue color. If the color of the stripped area is the same as the uncoated area, the part is stripped. A good comparison to determine if the coating is fully stripped is to compare the heat tinted surface of a previously coated area to an uncoated heat tinted surface on the same part. If those surfaces look similar then the coatings is stripped. The coated area will heat tint differently than the base metal. After heat tint confirms that the coating has been removed, the part is ready for further processing, which may include further repair and then recoating of the base coat, or, may only involve recoating. (Step  113 ). The part is then ready for reuse. 
         [0017]    A number of experiments were performed using method  100  on different superalloy coated bars. Specifically, the coatings shown in  FIGS. 2 and 3  were placed on a second generation nickel based superalloy. Bond coats such as those in  FIGS. 2 and 3 , as well as all others used in the present invention, are polycrystalline alloys. Polycrystalline alloys have grain boundaries at the point where each of the multitude of crystals forming the polycrystalline material so the process solution can dissolve the coating. In contrast, a single crystal alloy has no grain boundaries to be attached by the acid. The surface of the single crystal substrate material is the only grain boundary of that material. There are no short-circuit paths or surface area to attack with the process solution. It has been discovered that the method of this invention is capable of removing the coatings, using abrasion, heat, acid and rinses, of polycrystalline coatings on single crystal substrates without significantly affecting the substrate. 
         [0018]    The specific test procedure included the following steps. All grit blasting was performed with 240 aluminum oxide at 45 psi in a suction cabinet. The soak solution was approximately 75% HCl in water at a temperature of 150° F. All heat tints were performed in an air circulating Blue-M oven at 1050° F.±25° F. for one hour. 
         [0019]    Grit blast 
         [0020]    Soak for one or more hours 
         [0021]    Rinse to remove residue 
         [0022]    Smut check 
         [0023]    Repeat the grit blast, soak, rinse and smut check until the test indicates the coating has been removed. 
         [0024]    Heat tint for one hour 
         [0025]    Remove a section for metallographic review 
         [0026]      FIG. 4  shows a microphotograph of the surface of a coating having the composition shown in  FIG. 3  on a second generation nickel-based alloy as identified above.  FIG. 5  illustrates the result of treating after five hours of soaking using the strip cycles as set forth above. The surface is sufficiently cleared of bond coat that an additional grit blasting at most would be necessary to have the surface ready to use. 
         [0027]      FIG. 6  shows a microphotograph of the surface of a coating having the composition shown in  FIG. 2  on a second generation nickel-based alloy as identified above.  FIG. 7  illustrates the result of treating after nine hours of soaking using the strip cycles as set forth above. The surface is sufficiently cleared of bond coat and no further treatment would be necessary to have the surface ready to use. 
         [0028]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.