Patent Publication Number: US-2010108538-A1

Title: Method for stripping a component

Description:
This claims priority to German Patent Application DE 10 2007 022 832.7, filed May 15, 2007 through international application PCT/DE2008/000756, filed May 2, 2008, the entire disclosures of which are hereby incorporated by reference herein. 
     The present invention relates to a method for stripping a component, in particular a gas turbine component, to completely or partially remove a multilayer or sandwich antiwear coating from the surface of the component, the antiwear coating including at least one relatively hard ceramic layer and at least one relatively soft metallic layer. 
     BACKGROUND OF THE INVENTION 
     To provide gas turbine components, such as rotor blades, with oxidation-resistance, corrosion-resistance or also erosion-resistance, special antiwear coatings are applied to the surfaces thereof. During operation, gas turbine components are subject to wear or can become damaged in some other way. Repairing the damage typically requires localized, partial, or also complete removal or ablation of the antiwear coating from the component to be repaired. The process of removing or ablating coatings is also described as stripping. One distinguishes among the different stripping methods according to whether the coatings are removed mechanically, chemically or electrochemically. 
     Antiwear coatings typically take the form of what is known as multilayer coatings, which are composed of a plurality of layers alternately deposited on the gas turbine component. Thus, an antiwear coating in the form of a multilayer coating may include, for example, a relatively soft metallic layer and a relatively hard ceramic layer, a multiplicity of said layers being alternately deposited one above another on the gas turbine component. In practice, there are known antiwear coatings in which more than two different layers are alternately deposited one above another on the gas turbine component, such as, for example, multilayer coatings composed of four layers alternately deposited one above another on the gas turbine component, namely a first metallic layer which is adapted to the material composition of the gas turbine component and which is therefore relatively soft, a metallic layer which is composed of a metal alloy material and is also relatively soft, a third, relatively hard-grade metal-ceramic layer, and a fourth relatively hard ceramic layer. 
     The prior art has not yet disclosed a method that would allow multilayer antiwear coatings to be effectively removed without the risk of damage to the base material of the gas turbine component. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a novel method for stripping a gas turbine component. 
     The present invention provides a method for stripping a gas turbine component to completely or partially remove a multilayer or sandwich antiwear coating from the surface of the component, the antiwear coating including at least one relatively hard ceramic layer and at least one relatively soft metallic layer. 
     According to the present invention, in order to remove the multilayer or sandwich antiwear coating, the component is placed in a bath of an alkaline electrolyte, the component placed in the electrolyte being stripped at a current density of between 1 A/dm 2  and 20 A/dm 2 . 
     The method of the present invention for stripping a component has the advantage of allowing residue-free removal of multilayer coatings from the surface of the component to be stripped without the risk of the base material thereof being attacked and thereby damaged. In particular, titanium-based alloys and nickel-based alloys are not damaged during the stripping process according to the present invention. Another advantage of the method of the present invention is that it allows components to be stripped in a single bath of the alkaline electrolyte. Thus, the method according to the present invention for stripping a component eliminates the need to provide several different baths. 
     Preferably, the component placed in the electrolyte is anodically processed at room temperature. 
     In an advantageous embodiment of the present invention, the bath contains, in addition to the alkaline electrolyte, a surfactant to reduce surface or interfacial tension. In particular, a sodium hydroxide solution or a potassium hydroxide solution is used as the electrolyte. The surfactant used is especially a fluorinated surfactant, said surfactant being used in an amount to reduce the surface or interfacial tension to a value between 20 dynes and 50 dynes. 
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention is described in greater detail below. 
     The method of the present invention is used for stripping components, such as gas turbine components, which are coated with multilayer antiwear coatings composed of at least two different layers arranged alternately one above another, namely of relatively hard ceramic layers and relatively soft metallic layers arranged alternately one above another. 
     In accordance with the method of the present invention, in order to remove a multilayer antiwear coating from a component, and thus to strip the component, the component to be stripped is placed in a bath of an alkaline electrolyte. The component placed in the alkaline electrolyte is then stripped by applying an electric current having a current density of between 1 A/dm 2  and  20  A/dm 2 . 
     Accordingly, in accordance with the present invention, stripping is accomplished electrochemically. In this process, the component to be stripped is mainly processed anodically; i.e., it mainly functions as an anode during stripping. During the stripping process, the component to be stripped can also be used as a cathode at defined intervals for short periods of time in order to increase the stripping efficiency. 
     Preferably, the stripping of the component placed in the alkaline electrolyte is performed at a current density of between 1 A/dm 2  and 5 A/dm 2 , and preferably at room temperature. 
     Preferably, sodium hydroxide solution or potassium hydroxide solution is used as the alkaline electrolyte. 
     In order to reduce the surface or interfacial tension during stripping, it is preferred to use a bath which contains a surfactant in addition to the alkaline electrolyte. The surfactant is preferably a fluorinated surfactant. 
     The amount of the surfactant is selected to reduce the surface or interfacial tension to a value between 20 dynes und 50 dynes, in particular to a value between 25 dynes und 35 dynes. 
     If the component is to be stripped only in specific surface regions, the portions or regions of the component that are intended not to be stripped are covered with a wax-containing or wax-like material prior to stripping. This is useful, for example, when a gas turbine blade is to be stripped only in the region of its blade, but not in the region of its root. In such a case, the blade root is covered with a wax-containing or wax-like material prior to placing the gas turbine blade in the bath. 
     The method of the present invention enables effective, low-stress stripping of multilayer coatings from components. Stripping is accomplished electrochemically, it being sufficient to use a single bath of an alkaline electrolyte. There is no risk of the base material of the component to be stripped being eroded and thus damaged.