Patent Publication Number: US-2020291797-A1

Title: Electrodeposited nickel-chromium alloy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/914,313 filed on 10 Dec. 2013 and titled Electrodeposited Nickel-Chromium Alloy, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF USE 
     The present disclosure relates to an electrodeposited nickel-chromium (Ni—Cr) alloy that can be coated on turbine engine components intended to operate in hostile environments to provide improved resistance to oxidation, hot corrosion, and/or erosion. Specifically, the present disclosure relates to processes and chemistry used to repair engine components that have been damaged in service by adding wall thickness to restore the dimension of those components for extended useful life. The added materials include primarily electrodeposited Ni—Cr alloy. 
     BACKGROUND 
     High and low pressure turbine engine components like vanes, stators, and rotor blades are made of nickel based superalloys. Typically, these components are protected from the high temperature environment by a thermal barrier coating (TBC). However, the coating can be damaged due to oxidation, corrosion, and/or erosion during service, requiring scheduled repairs or being scrapped if material loss has thinned down the wall of the structure below allowable limits. 
     Traditional repair methods entail removing the existing coatings and apply new coatings to the engine components. The repair process generally causes material loss of the base metal. As the wall thickness approach allowable limit as a result of repair, the engine parts can no longer be reused. Therefore, dimensional restoration in engine repair service can lead to economic gain and reduce the amount of scrap parts that still have substantial remaining material value. 
     One of the current practices of engine repair is to deposit nickel (Ni) onto the damaged parts followed by a high temperature diffusion process to convert the nickel deposit to a desired alloy composition. While diffusion of chromium (Cr) into the Ni deposit layer can enhance the high temperature oxidation resistance of the repaired part, the diffusion process can gradually consume the chromium (Cr) and other minor compositions from the parent parts, i.e., vanes. Since the major composition of the vanes is Ni and Cr, plating a Ni—Cr alloy to satisfy the composition requirement can greatly retard or even reverse the depletion of the Cr from the parent parts. Thus, Ni—Cr deposit is attractive to enable engine dimensional restoration. 
     Electrodeposition is a non-light-of-sight coating application technique suitable for the parts with complex geometry, such as engine vanes and airfoils. Electrodeposition of Ni—Cr alloy in traditional plating chemistry has not been successful in forming a deposit thick enough for the structural repair (&gt;125 μm) with dense structure. The challenge is suspected to be related to the inability to deposit thick Cr deposits greater than 10 μm from conventional aqueous trivalent chromium plating baths. 
     Although thick hard chromium has been produced in hexavalent chromium solution, i.e. chromic acid, the hard chromium deposit has cracks and hexavalent chromium is highly carcinogenic. Therefore, it is desirable to develop plating chemistry using only trivalent chromium as the Cr source to produce Ni—Cr alloys for the engine dimensional restoration applications. 
     SUMMARY 
     According to an aspect of the present disclosure, a coated article is disclosed. The coated article includes a turbine component and a Ni—Cr alloy coated on a surface of the turbine component, wherein the Ni—Cr alloy includes from 2 to 50 wt % chromium and a remaining weight percentage of nickel, and wherein the Ni—Cr alloy is heat-treated to homogenize the composition similar to that of the base metals to restore the wall thickness reduced during repair of the turbine component. The electrodeposited Ni—Cr alloy is thicker than 2 mils (0.05 mm). It is desirable to apply a thick Ni—Cr deposit with sufficiently high Cr content to increase repair cycles of the turbine engine components. 
     According to another aspect of the present disclosure, a method for electrodepositing a thick nickel-chromium (Ni—Cr) alloy suitable to be plated on a turbine component is disclosed. The method includes pre-treating the turbine component prior to electrodeposition. The method further includes providing a plating bath filled with a solution including a solvent, a surfactant, and an ionic liquid (deep eutectic solvent) including choline chloride, nickel chloride, and chromium chloride, wherein a molar ratio of the choline chloride, the combined chromium chloride, and nickel chloride ranges from 0.5 to 3.5, and the solvent amounts to 5 to 80 vol. % (pre-mixing volume) relative to a mixture of the choline chloride and metal chlorides. 
     The method further includes electrodepositing a Ni—Cr alloy on a metallic substrate cathode while using an anode that is either insoluble or soluble such as nickel under electrolytic conditions. Specifically, the insoluble anode is used to promote the oxidation of water to produce oxygen as the main by-product while other minor products can be produced concurrently as well. The soluble nickel anode is used to replenish the nickel deposited on the cathode. Alternating use of the combined insoluble and soluble (active) anodes is also included in this method to attain plating bath composition control. An external power supply is used for the electrodeposition and the current or potential can be regulated to achieve desired deposit properties such as adhesion, grain structure, hardness and residual stress. The electrodeposited Ni—Cr alloy is subsequently heat-treated to replenish the materials lost during repair of the turbine component and homogenize the composition. 
     The details of one or more embodiments of the present disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a plating bath filled with an electrolytic solution for electrodepositing a Ni—Cr alloy on turbine engine parts with a combined soluble and insoluble anode according to an aspect of the present disclosure. 
         FIG. 2A  illustrates a cross-sectional view of an article as coated with Ni—Cr alloy formed by electrodeposition. 
         FIG. 2B  illustrates a cross-sectional view of an article of  FIG. 2A  after high temperature heat treatment to homogenize the composition. 
         FIG. 3  is a flow chart of the process for electrodepositing a Ni—Cr alloy for dimensional restoration of an engine component. 
     
    
    
     The drawings depict various preferred embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION 
     Typically, electroplating is a process that uses electrical current to reduce dissolved metal ions, most likely metal ion complexes so that they form a coherent metal coating on an electrode that is, for example, a turbine engine component to be repaired. The process used in electroplating is called electrodeposition. The part to be plated with Ni—Cr alloy is a cathode, and an anode is made of such metal as Ni, Cr, Ni—Cr alloy, or any combination of these materials to be plated on the part, according to an embodiment. In another embodiment, an insoluble catalytic anode (e.g., iridium oxide, tantalum oxide, ruthenium oxide, or the like) can be used. Yet in another embodiment, an insoluble catalytic anode is used in conjunction with a soluble anode, and the soluble anode can be optionally used to adjust the bath composition as desired. 
       FIG. 1  illustrates an electroplating bath filled with an electrolytic solution for electrodepositing a Ni—Cr alloy suitable to be plated on a turbine engine part to be repaired according to an aspect of the present disclosure. The part to be plated is pre-treated prior to electrodeposition. The pre-treatment includes removing the existing coating, mechanically cleaning the surface, degreasing, acid or alkaline etching including electro-etching and final activation before the part is placed in the plating bath for deposit application. The electrodeposition inevitably decomposes water in the bath  102 , and thus the solution in the bath needs to be replenished to maintain consistent deposition quality. 
     Referring now to  FIG. 1 , there is provided a plating bath  102  containing an electrolytic solution that consists of a room temperature ionic liquid, namely deep eutectic solvent, including choline chloride, nickel chloride, chromium chloride, solvents, and surfactants including anionic, cationic, or Zwitterionic (amphoteric) surfactants. An example of the surfactant is a sodium dodecyl surfate, fluorosurfactants, cetyl trimethylammonium bromide (CTAB), or cetyl trimethyammonium chloride (CTAC). It is noted that the choline chloride based metal processing is low-cost and environmentally friendly. In one embodiment, a molar ratio of the choline chloride and chromium chloride ranges from 0.5 to 3.5. 
     In one embodiment, polar aprotic and polar protic solvents are used to adjust the viscosity and conductivity of the plating bath  102  to attain a high quality Ni—Cr alloy coating. Specifically, protic solvents are preferred due to their hydrogen bond donating ability. The solvents include formic acid, citric acid, Isopropanol (IPA), water, acetic acid, glycine (aminoacetic acide) and ethylene glycol. 
     In the embodiment, preferred solvent content is from 10 to 80 vol % relative to the mixture of choline chloride and metal chlorides including the nickel and chromium chlorides on a pre-mixing basis. Referring to  FIG. 1 , electroplating of the Ni—Cr alloy begins by providing an external supply of current to an anode and a cathode that is the part to be repaired. An external supply of the current can be a direct current or an alternating current including a pulse or pulse reverse current (not shown). The regime and magnitude of the current can be controlled during the deposition to achieve desired coating composition, density, and morphology. 
     The turbine part  104  to be plated is a cathode during electrodeposition. The anode  106  is, for example, a Ni—Cr alloy anode, a Ni and/or Cr anode, or any combination of these materials that can be chosen to satisfy different requirements. An insoluble catalytic anode (catalyzing oxygen evolution to suppress or eliminate other undesirable anodic reactions such as chlorine evolution, hexavalent chromium formation) is preferable, but the anode used is not specifically limited. A combination of soluble Ni anode and an insoluble catalytic anode can be used to control bath composition during the course of plating as well. 
       FIG. 2A  illustrates an article  200  as-coated by an electrodeposited Ni—Cr alloy  206 . Referring to  FIG. 2A , a part  202  includes a turbine component that has at least one surface  204 . A Ni—Cr alloy deposit  206  on the surface  204  of the turbine part  202  adds wall thickness and the chromium lost during repair of the part. The coated Ni—Cr alloy is compatible with the material forming the turbine part  202 . The coating  206  may be applied directly to the surface  204  of the turbine part  202  which is formed from a wide range of metallic materials including, but not limited to, a single crystal nickel-based superalloy. 
     The Ni—Cr alloy coating  206  is subsequently heat-treated at high temperature (over 1000° C.) to allow inter-diffusion of elements, resulting in homogenized composition in the restored wall.  FIG. 2B  illustrates a cross-sectional view of an article of  FIG. 2A  after high temperature heat treatment with a schematic inter-diffusion zone  208 . Referring to  FIG. 2B , an interdiffusion zone  208  is formed along the interface region between the turbine part  202  and the Ni—Cr alloy coating  206  as result of the high temperature heat-treatment. 
       FIG. 3  is a flow chart of an electrodeposited Ni—Cr coating process of the present disclosure. Forming a Ni—Cr deposit of substantial thickness, for example, over 1 mil (0.025 mm), by electrodepositing a Ni—Cr alloy on a turbine part begins at step  300  where the coating and damaged surface of the turbine part is first removed and cleaned down to the base alloy. Then, a mechanical and chemical cleaning of the part is carried out and the cleaned surface is then activated at step  301  prior to being placed into the plating bath for electrodeposition. At step  304 , the Ni—Cr alloy is electrodeposited on a metallic substrate of the turbine part by providing an external supply of current to an anode and the cathode. The electrodeposited Ni—Cr alloy is then heat-treated at step  306  to restore materials lost during repair of the turbine component and homogenize the composition. 
     In an embodiment, the electrodeposited Ni—Cr alloy formed by the method disclosed above comprises from 2 to 50 wt % chromium balanced by nickel, and is capable of rebuilding a vane wall by more than 2 mils (0.05 mm). In another embodiment, the electrodeposited Ni—Cr alloy formed by the method disclosed above comprises from 8 to 20 wt % chromium balanced by nickel, and is capable of rebuilding a turbine component wall by more than 5 mils (0.125 mm). The turbine component to be plated includes a vane, a rotor blade, or a stator. 
     The Ni—Cr alloy plated on the aero-engine parts including vanes minimizes the loss of key elements like chromium during repair services that are critical to high temperature oxidation resistance. Thus, the electrodeposited Ni—Cr alloy that is plated on the turbine parts extends the repair cycles of the parts. The electrodeposited Ni—Cr alloy is subject to the post heat treatment at high temperature (usually over 1000° C.) to homogenize the composition of the alloy and to restore materials lost during the repair of the turbine engine parts. 
     The disclosed choline chloride based electrodeposition is a metal forming process that is cost-effective to restore dimensions of high temperature turbine parts with complex geometries and tighter tolerance, and is environmentally friendly. 
     It is to be understood that the disclosure of the present invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts, and details of operation. The disclosure of the present invention rather is intended to encompass all such modifications which are within its spirit and scope of the invention as defined by the following claims.