Abstract:
Apparatus and process for removing surface regions of a component. The prior art involves removing surface regions of a metallic component by means of electrochemical processes. The electrochemical process is accelerated by the use of a current pulse generator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the US National Stage of International Application No. PCT/DE03/00953, filed Mar. 21, 2003 and claims the benefit thereof. The International Application claims the benefits of German application No. 10215374.4 DE filed Apr. 8, 2002, and German application No. 10259365.5 DE filed Dec. 18, 2002, all of the applications are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to an apparatus and a process for removing surface regions of a component as described in the claims. 
     BACKGROUND OF THE INVENTION 
     Hitherto, components which have been coated with coatings of type MCrAlY or ZrO 2  have had the coating removed, for example, by acid stripping in combination with sand blasting or by high-pressure water blasting. 
     EP 1 122 323 A1 and U.S. Pat. No. 5,944,909 show examples of the chemical removal of surface regions. 
     EP 1 941 34 A1, EP 1 010 782 A1 and U.S. Pat. No. 6,165,345 disclose methods for the electrochemical removal of metallic coatings (stripping). 
     The processes listed above are time-consuming and therefore expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an apparatus and a process in which the removal of the coating takes place more quickly and economically. 
     The object is achieved by an apparatus and a process for the removal of surface regions from a component as described in the claims. 
     Further advantageous configurations and process steps are listed in the corresponding subclaims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawing: 
         FIG. 1  shows an apparatus according to the invention, 
         FIG. 2  shows a time curve of a current of a current pulse generator, and 
         FIG. 3  shows a further time curve of a current from a current pulse generator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an apparatus  1  according to the invention. The apparatus  1  comprises a vessel  4  in which an electrolyte  7  there is arranged. An electrode  10  and a component  13  are arranged in the electrolyte  7 . The electrode  10  and the component  13  are electrically connected to a current/voltage pulse generator  16 . The component  13  is, for example, a coated turbine blade or vane, the substrate of which is a nickel- or cobalt-base superalloy, to which a metallic layer has been applied to serve, for example, as a corrosion-resistant or anchoring layer. A layer of this type in particular has the composition MCrAlY, where M stands for an element iron, cobalt or nickel. 
     The coating has been corroded during use of the turbine blade or vane  13 . The surface region  25  which has been formed as a result (as indicated by dashed lines) is to be removed by the process according to the invention and the apparatus  1  according to the invention. It is also possible for layer regions  25  which have been formed by corrosion, oxidation or other forms of degradation to be removed from a component  13  which does not have a coating, these layer regions being in the vicinity of the surface. 
     The current pulse generator  16  generates a pulsed current/voltage signal ( FIG. 2 ). 
     An ultrasound probe  19 , which is operated by an ultrasound source  22 , may optionally be arranged in the electrolyte  7 . The ultrasound excitation improves the hydrodynamics of the process and thereby assists the electrochemical reaction. 
       FIG. 2  shows an example of a current/voltage curve of the current/voltage pulse generator  16 . 
     The current pulse signal or the voltage pulse is, for example, square-wave (pulse shape) and has a pulse duration t on . Between the individual pulses there is an interval of length t off . Furthermore, the current pulse signal is defined by its current level I max . 
     The current (I max ) which flows between the electrode  10  and the component  13 , the pulse duration (t on ) and the pulse interval (t off ) have a significant influence on the electrochemical reaction by accelerating the latter. 
       FIG. 3  shows an example of a series of current pulses  40  which are repeated. A sequence  34  comprises at least two blocks  77 . Each block  77  comprises at least one current pulse  40 . A current pulse  40  is characterized by its duration t on , the level I max  and its pulse shape (square-wave, delta, etc.). Other important process parameters are the intervals between the individual current pulses  40  (t off ) and the intervals between the blocks  77 . 
     The sequence  34  comprises, for example, a first block  77  of three current pulses  40  between each of which there is an interval. This is followed by a second block  77 , which has a higher current level and comprises six current pulses  40 . After a further interval, there then follow four current pulses  40  in the opposite direction, i.e. with a reversed polarity. 
     The sequence  34  is finished by a further block  77  of four current pulses. The sequence  34  can be repeated a number of times. 
     The individual pulse times t on  are preferably of the order of magnitude of approximately 1 to 10 milliseconds. The time duration of the block  77  is of the order of magnitude of up to 10 seconds, so that up to 500 pulses are emitted in one block  77 . 
     The application of a low potential (base current) both during the pulse sequences and during the intervals is optionally possible. 
     The parameters of a block  77  are matched to a constituent of an alloy which, by way of example, is to be removed in order to optimize the removal of this constituent. This can be determined in individual tests.