Method for electroplating nickel onto titanium alloys

Abstract: This invention relates to an improved method for electroplating a layer of nickel onto titanium base alloy substrates. It is particularly useful in electroplating a layer of nickel onto titanium alloys which contain refractory metal elements, such as the alloy Ti-8A1-1V-1Mo. The method includes the steps of etching the surface of the substrate with a solution containing hydrofluoric acid and hydrochloric acid, followed by electroplating the etched surface in a nickel sulfamate solution.

Description: The present invention concerns a method for applying a layer of nickel onto a titanium base alloy component according to the precharacterizing portion of claim 1.(US-A-4 902 388) US-A- 4,127,209 to Ruben, 4,416,739 to Turner and 4,787,962 to Temprano all describe methods for electroplating a layer of nickel onto titanium and titanium base alloys. Turner indicates that it is difficult to achieve good adhesion between the titanium substrate and the layer of electroplated nickel because an oxide film is typically present on the surface of the substrate. Turner's invention is to use an aqueous solution of hydrofluoric acid and formamide to remove the oxide. Ruben removes the oxide by connecting the substrate as the cathode in an acid solution such as sulfuric acid, and then forming a layer of titanium hydride on the substrate. Temprano pickles the substrate surface with a 95% sulfuric acid solution. Other prior art methods for removing the oxide scale include a solution containing hydrofluoric acid and nitric acid. While the aforementioned methods may be successful with some titanium alloys, they are not useful on some specialty titanium alloys of the type used in the gas turbine industry. Accordingly, workers in this field seek improved techniques for applying electroplated layers of nickel onto state-of-the-art titanium alloys. This invention satisfies such a need. The method of applying a layer of nickel onto a titanium base alloy component is defined in the characterizing portion of claim 1. Other features and aspects of this invention will be apparent in light of the following description of the best mode for carrying out the invention. This invention relates to a process for electroplating a layer of nickel onto titanium and titanium base alloys. It should be understood that the term "titanium base" means those alloys in which titanium is the predominant element in the alloy composition. This invention has shown to be particularly applicable to the electrodeposition of nickel onto titanium base alloys which include refractory metal elements. Such alloys include, but are not limited to, the following compositions: Ti-3Al-2.5V; Ti-6Al-4V; Ti-8Al-1V-1Mo; Ti-6Al-2Sn-4Zr-6Mo; and Ti-6Al-2Sn-4Zr-2Mo. The key aspect of the invention is the use of a particular chemical solution for cleaning the surface of the titanium substrate prior to the electrodeposition step. The chemical solution etches the substrate surface, and, as a result, the step is referred to as an etching step. The preferred manner for carrying out this invention is described below. Portions of the component ( an alloy whose composition was Ti-8A1-1V-1Mo) which are not to be etched or electroplated are masked with an appropriate masking material; wax and polymer based resins are preferred. Dirt, oil and other residue which are present on the surface are removed by a dry pumice swab followed by a wet pumice swab. The surface is vapor blasted with aluminum oxide grit and then rinsed, preferably in water. The substrate is then immersed in a solution whose composition corresponds to that obtained by mixing, by volume, 4-6% of 70% hydrofluoric acid and 94-96% of 35-38% hydrochloric acid. The substrate is immersed in such solution for a period of time sufficient to clean and etch the surface but not so long as to excessively etch or pit the surface. Periods of time between 8 and 45 seconds are useful; 10 to 20 seconds are preferred, and 15 seconds is the most preferred immersion time. The substrate is removed from the HF-HCl bath and rinsed in water. To insure that no residual acid and/or smut is present on the substrate, it is ultrasonically cleaned for about 10 seconds in deionized water. The substrate may be etched in a solution containing hydrofluoric acid, glacial acetic acid and water. The solution contains, by volume, 11-15% of 70% hydrofluoric acid, 81-85% glacial acetic acid, and 2-6% water. The etching is done anodically, at a current of 140 amperes per square meter (ASM) for 6 minutes. The substrate is then rinsed and then cathodically plated in a conventional nickel sulfamate solution. The plating process takes place for 30 minutes at 270 ASM. The substrate is then preferably heat treated at about 400°C for 4 hours in air. Use of the aforementioned plating process produces an electrodeposited layer of nickel having a thickness of about 12-18 micrometres. The bond strength of the layer after heat treatment is in excess of 475 kilograms per square centimeter, as determined by lap shear specimens. The use of conventional prior art cleaning techniques produced nickel layers which were generally nonadherent to a Ti-8A1-1V-1Mo substrate. In particular, etching solutions containing, by volume, about 12% of 70% hydrofluoric acid and 1% of 70% nitric acid were not useful, as they produced a tenacious smut which could not readily be removed from the substrate surface. The HF-HNO3 solution is commonly used with much success on titanium alloys which are substantially free of refractory elements. While this invention has been shown and described with respect to a preferred embodiment, it should be understood by those skilled in the art that various changes in the form and detail may be made without departing from the scope of the invention as defined by the appended claims. Regardless of the particular concentrations used, the solution should have a composition corresponding to that obtained by mixing 4-6%of the 70% HF and 94-96% of the 35-38% HCl. The fluoride ion content of such solutions are readily measured using a conventional fluoride ion specific electrode.

Claims: A method for applying a layer of nickel onto a titanium base alloy component containing at least 1 weight % of a refractory metal element, comprising the steps of: (a) etching the surface of the component in a room temperature solution consisting essentially of, by volume, 94-96 % of 35-38 % HCl and 4-6 % of 70 % HF for at least 10 seconds; (b) anodically etching the component surface for 6 minutes in a room temperature solution consisting essentially of, by volume, 11-15 % of 70 % HF, 81-85 % glacial acetic acid and 2-6 % H 2 O; and (c) cathodically plating a layer of nickel onto the etched component surface for 30 minutes in a nickel sulfamate solution, characterized in that the anodically etching of the component surface is carried out at 140 amperes per square meter and the cathodically plating is carried out at 270 amperes per square meter. The method of claim 1, characterized in that said etching step is conducted in a solution containing 95 % of 35-38 % HCl and 5 % of 70 % HF.

Inventors: FORNWALT DONALD EDWARD, LOMASNEY GARY M, ROUTSIS KOSTAS, FORNWALT, DONALD EDWARD, LOMASNEY, GARY M., ROUTSIS, KOSTAS
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1991
Main classification: C25D5
IPC codes: C25F3, C23F1, C25D5, C25D3, C25F1