Patent Application
20070034524
The present invention relates to an improved technique for masking airfoils during electrochemical stripping operations.
It has been found that wall thinning of an airfoil portion of a turbine engine component will occur as a result of removing a coating applied to the airfoil portion and/or a diffusion layer formed on the airfoil portion using electrochemical stripping techniques. Wall thinning is highly undesirable because it leads to bending.
Thus, it is deemed desirable to mask the trailing edges of airfoils used in turbine engine components, such as vanes and blades, to prevent wall thinning and cooling hole closure caused by bending a thin wall. Many of the techniques employed today use non-conductive trailing edge maskants to prevent this from occurring. Typically, barrier types of maskants, such as plater's tape, lacquer, and UV-curable materials, have been used in the stripping processes.
Some of the maskants which have been used have caused trenching of the airfoil portion under the masked area. The trench is caused by crevice corrosion and is an unacceptable condition. It is speculated that the trench is formed as a result of a crevice being formed under the maskant as the coating and/or diffusion layer are removed. After the crevice is formed, crevice corrosion begins and propagates, causing the formation of the trench.
There is a need for improved maskant materials, particularly those which help avoid trenching.
Accordingly, it is an object of the present invention to provide an improved maskant for use in electrochemical stripping operations.
It is a further object of the present invention to provide a maskant as above which avoids trenching.
The foregoing objects are attained by the maskant of the present invention.
In accordance with the present invention, a maskant for use in an electrochemical stripping operation is provided. The maskant broadly comprises a bead of ultra violet (UV) curable maskant placed along a trailing edge of an airfoil portion of a turbine engine component covering any trailing edge windows (openings) and a clip formed from an electrically conductive material, such as a titanium based material, to prevent crevice corrosion under the masking line.
The present invention also relates to a method for removing a coating from a turbine engine component. The method broadly comprises the steps of placing a UV curable maskant along a trailing edge of an airfoil portion of the component, placing a clip formed from an electrically conductive material, such as a titanium based material, over said UV curable maskant, immersing the turbine engine component with the UV curable maskant and clip into a bath, and electrochemically stripping the coating from unmasked portions of the turbine engine component.
Other details of the masking techniques for electrochemical stripping, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.
As discussed above, the present invention relates to a masking technique for use in a method for electrochemically stripping coatings and/or diffusion layers from airfoil portions of turbine engine components, such as turbine blades, vanes, seals and shrouds. In one embodiment, the maskant for the trailing edge of the airfoil portion comprises a layer of a UV curable maskant material and a clip placed over the UV curable maskant formed from an electrically conductive material. Preferably, the electrically conductive material is formed from a titanium based material. Titanium is a preferred material because it will not corrode in many of the baths used in electrochemical stripping techniques. Root portions of the turbine engine component may be masked by dipping the root portions into a thin paint or by applying a lacquer in order to prevent any slight etching or pitting. In another embodiment, the maskant solely comprises a clip placed over the airfoil trailing edge.
The masking technique of the present invention may be used in conjunction with any suitable electrochemical stripping technique known in the art.
Referring now to
In accordance with the present invention, as shown in
Using the masking technique of the present invention, it is possible to obtain a coating remaining on the airfoil portion trailing edge of the turbine engine component which has a smooth transition between the fully stripped base alloy forming the turbine engine component and the fully protected coating. A smooth transition is desirable because it significantly reduces or eliminates any subsequent blending of the coating needed to remove any sharp corner.
As shown in
Prior to, or subsequent to, applying the electrically conductive member 10 or clip 30, root portions of the turbine engine component may be masked to prevent any slight etching or pitting. Masking of the root portions may be achieved by dipping them into a thin paint, such as DYKEM layout ink fluid, or by hand applying a maskant, such as a suitable lacquer. Preferably, two coats of the root portion masking material should be applied. After application of the root portion masking material, the turbine engine component may be subjected to a drying treatment which depends upon the nature of the root portion maskant.
As previously mentioned, the use of an electrically conductive member 10 or clip 30, particularly one formed from a titanium based material, helps prevent trenching. It also provides a smoother transition between the coating on the trailing edge portion of the turbine engine component and the underlying substrate.
Two high pressure turbine blades were stripped in a 4.7 vol. % hydrochloric acid solution at a temperature of approximately 20° C. The stripping potential set point was 0.08v with respect to an Ag/AgCl reference electrode. The turbine blades were stripped for 2 hours, water pressure sprayed, tripped for an additional two hours, burnt out, grit blasted, and heat tinted. All of the blades were masked at the root and the tip with Dymax X-391-17A. One of the blades had a U-channel mask in accordance with the present invention applied to the trailing edge. The other of the blades had a hinged clip in accordance with the present invention applied to the trailing edge. The test showed an absence of crevice corrosion at the trailing edge.
While the masking technique of the present invention preferably applies a UV curable material over the trailing edge portion, for removing certain coatings from some turbine engine components, the UV curable material may be omitted.
While the UV curable material may be applied to both sides of the trailing edge portion of the turbine engine component, it may also be applied to just one side such as the concave side.
After the trailing edge portions of the root portion have been masked, the turbine engine component is immersed in an acidic bath. The bath may be any suitable stripping bath known in the art. After immersion, the coating on the turbine engine component may be stripped using any suitable electromechanical stripping technique known in the art. The particular electrochemical stripping technique does not form part of the present invention.
While the present invention has been described with the maskant placed about a trailing edge of an airfoil portion of a turbine engine component, the maskant could also be applied to a leading edge or a tip of the airfoil portion and/or to a platform portion of the component.
It is apparent that there has been provided in accordance with the present invention masking techniques for electrochemical stripping which fully satisfy the objects, means, and advantages set forth hereinbefore. While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.
