Abstract:
Components comprising corrosion products are often reused, in which case the corrosion product has to be removed. According to the prior art, this takes a very long time since the reaction times with the corrosion product are often very long. According to the invention, the corrosion product is pretreated in order to produce a larger attackable surface area, so that the corrosion product can be removed more quickly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is the US National Stage of International Application No. PCT/EP2005/000405, filed Jan. 17, 2005 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 04002158.6 filed Jan. 30, 2004. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a process for removing a layer as described in the claims.  
       BACKGROUND OF THE INVENTION  
       [0003]     After they have been used, components, such as for example turbine blades or vanes, have corrosion products, such as for example oxides, sulfides, nitrides, carbides, phosphates, etc. which form a layer. Components of this type, after they have been used, can be reused if, inter alia, the corrosion products have been removed. The complete removal of the corrosion products is effected, for example, by sand-blasting, although this can lead to damage to the substrate.  
         [0004]     It is also possible for the component to be completely treated by means of acid stripping or fluoride ion cleaning (FIC). However, this is very time-consuming since the material-removal rates of the corrosion products over the course of time are in some cases too low with respect to the acid or the fluorine and/or fluoride.  
         [0005]     U.S. Pat. No. 5,575,858 describes a process for removing a removal region, in particular a corrosion product of a component, in which the removal region is pretreated prior to final cleaning, so as to damage the removal region, so that then the material-removal rate during the final cleaning of the removal region is greater than without the damage to the removal region.  
       SUMMARY OF THE INVENTION  
       [0006]     Similar processes are disclosed in U.S. Pat. No. 4,439,241, U.S. Pat. No. 5,464,479 and EP 1 013 797. Therefore, the object of the invention is that of providing a process in which the removal of layers on a component is facilitated and therefore takes less time.  
         [0007]     The object is achieved by the process as claimed in the claims.  
         [0008]     The subclaims list further advantageous measures of the process according to the invention.  
         [0009]     The measures listed in the subclaims can be combined with one another in advantageous ways. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention is explained schematically with reference to the figures, in which  
         [0011]      FIG. 1  shows a component with a corrosion product,  
         [0012]      FIG. 2  diagrammatically depicts the execution of the process according to the invention,  
         [0013]      FIG. 3, 4 ,  5  show the component after the process according to the invention has been carried out,  
         [0014]      FIG. 6  shows a gas turbine,  
         [0015]      FIG. 7  shows a combustion chamber,  
         [0016]      FIG. 8  shows a turbine blade or vane, and  
         [0017]      FIG. 9  shows a steam turbine. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]      FIG. 1  shows a component  1  which can be treated by the process according to the invention.  
         [0019]     The component  1  comprises a ceramic or metallic substrate  4  (base body) which is for example, in particular for turbines, a cobalt-base, iron-base or nickel-base superalloy.  
         [0020]     The component  1  is, for example, a guide vane  130  or rotor blade  120  ( FIGS. 6, 8 ) of a gas turbine  100  ( FIG. 6 ), of a steam turbine  300 ,  303  ( FIG. 9 ) or of an aircraft turbine, a combustion chamber lining  155  ( FIG. 7 ) or another component of a turbine which is exposed to hot gases.  
         [0021]     The component  1  may be either newly produced or refurbished. Refurbishment means that components  1 , after they have been used, if appropriate have layers (thermal barrier coating) detached and corrosion and oxidation products removed. If appropriate, cracks may also have to be repaired.  
         [0022]     A component  1  of this type can then be coated again; this is particularly advantageous because the base body is very expensive.  
         [0023]     For use, the component  1  may have at least one ceramic or metallic layer on the surface  13 , such as for example an MCrAlX layer and/or a thermal barrier coating resting thereon, which can be roughly removed in a first process step.  
         [0024]     The MCrAlX layer may also represent the removal region  10  which is treated by the process according to the invention.  
         [0025]     In the text which follows, the removal region  10  is considered to be a corrosion product  10  (corrosion layer  10 ). However, the removal region  10  may equally be a functional layer without corrosion products.  
         [0026]     The removal region  10  may be a metallic and/or ceramic layer, in which case the layer may be metallic and includes corrosion products. The corrosion product  10 , for example an oxide, a sulfide, a nitride, a phosphide or a carbide, etc., may be present on a surface  13  of the component  1  or in a crack  7  in the component  1 .  
         [0027]     The corrosion products  10  have to be removed from the crack  7  or from the surface  13  so that the crack  7  can be filled with a solder or welding material and the surface  13  can be coated again. Corrosion products  10  would otherwise prevent or at least reduce good bonding of the solder or renewed coating.  
         [0028]     The corrosion product  10  according to the prior art has a defined material-removal rate (mass per unit time) when it is cleaned for example using the FIC process. However, this material-removal rate is too low and after a certain time may even be zero.  
         [0029]      FIG. 2  diagrammatically depicts the execution of the process according to the invention.  
         [0030]     By way of example, a material  16 , for example a salt  16 , which can react chemically with the corrosion product  10  in order to damage the removal region  10 , is applied to the corrosion product  10  in order to damage the latter.  
         [0031]     The salt used is preferably Na 2 SO 4  (sodium sulfate) and/or CoSO 4  (cobalt sulfate). Further salts or combinations are conceivable.  
         [0032]     The corrosion products aluminum oxide and/or cobalt oxide and/or titanium oxide of the metals titanium, aluminum and/or cobalt which are contained in the alloy (for example super-alloy) of the substrate  4  can be removed very successfully in particular using these salts.  
         [0033]     It is also possible for a fused salt to be applied directly in the crack  7  or to the corrosion product  10  or for the component  1  to be immersed in a fused salt.  
         [0034]     It is also possible for the salt to be applied into the crack  7  and to the surface  13  in the form of a slurry. In the case of large-area applications, it is appropriate to lay down a sheet which contains the material  16  or salt  16 .  
         [0035]     The salt  16  can, for example, be heated, in particular locally, by means of a laser  19  and its laser beams  22 , resulting in a chemical reaction of the salt  16  with the corrosion product  10  or a thermal shock.  
         [0036]     The heating can also be effected by electromagnetic induction, in particular if the substrate  4  is metallic. The heating of the component  1  can be effected, for example locally, by means of induction or by means of a light source, for example by means of a laser, by the laser  19  radiating the laser beam  22  only into the crack  7 .  
         [0037]     The local heating can also be effected by means of tunable microwaves. Tunable means that, inter alia, the wavelength and intensity can be varied.  
         [0038]      FIG. 3  shows a component  1  with a corrosion product  10  following the damaging of the corrosion product  10  by a pretreatment according to the invention.  
         [0039]     The pretreatment produces cracks  25  which run from the surface  14  of the layer  10  in the direction of the substrate  4 , resulting in a larger attackable surface area of the corrosion product  10  with respect to the acid and/or the fluoride ions, etc.  
         [0040]     Cracks  25  of this type can also be produced by means of laser beams, high-pressure water jets, sand-blasting, in particular with coarse grains. The intensity and duration of the sand-blasting treatment, however, has to be set in such a way that the substrate  4  is not reached and the corrosion product  10  is only partially removed.  
         [0041]     In a final process step, the component  1  is subjected to a final cleaning by means of an acid or fluoride ion treatment, which leads to complete removal of the corrosion product  10 , since the damage to the corrosion product  10  means that the material-removal rate during FIC or another process is considerably increased and there is no significant reduction in the material-removal rate over the course of time.  
         [0042]      FIG. 4  shows another way of damaging the corrosion product  10 .  
         [0043]     The corrosion product  10 , which rests on a surface  13  of the substrate  4 , is subjected to a thermal shock. The thermal shock can be effected by immersion in a hot metal or salt bath or by rapid heating by means of electron beams or a laser  28 .  
         [0044]     The corrosion product  10  may also be partially melted during the thermal shock.  
         [0045]      FIG. 5  shows further damage to the corrosion product  10  in accordance with the process of the invention.  
         [0046]     If the material of the corrosion product  10  has, for example, been melted, the material contracts again as it cools, resulting in mechanical stresses which can lead to crack formation.  
         [0047]     In addition to cracks  25  in the surface of the corrosion product  10 , it is also possible for cracks  31  to be produced within the corrosion product  10 .  
         [0048]     It is also possible for delaminations  34  to form between the corrosion product  10  and a surface  13  on which the corrosion product  10  rests.  
         [0049]     The particular feature of the process is that the component  1  having the corrosion products  10 , which has been damaged by these corrosion products  10  and needs to be repaired, is damaged still further in the region of the corrosion products  10 .  
         [0050]      FIG. 6  shows, by way of example, a partial longitudinal section through a gas turbine  100 .  
         [0051]     In the interior, the gas turbine  100  has a rotor  103  which is mounted such that it can rotate about an axis of rotation  102  and is also referred to as the turbine rotor.  
         [0052]     An intake housing  104 , a compressor  105 , a, for example, toroidal combustion chamber  110 , in particular an annular combustion chamber  106 , with a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas housing  109  follow one another along the rotor  103 .  
         [0053]     The annular combustion chamber  106  is in communication with a, for example, annular hot-gas passage  111 , where, by way of example, four successive turbine stages  112  form the turbine  108 .  
         [0054]     Each turbine stage  112  is formed, for example, from two blade or vane rings. As seen in the direction of flow of a working medium  113 , in the hot-gas passage  111  a row of guide vanes  115  is followed by a row  125  formed from rotor blades  120 .  
         [0055]     The guide vanes  130  are secured to an inner housing  138  of a stator  143 , whereas the rotor blades  120  of a row  125  are fitted to the rotor  103  for example by means of a turbine disk  133 . A generator (not shown) is coupled to the rotor  103 .  
         [0056]     While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake housing  104  and compresses it. The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107 , where it is mixed with a fuel. The mix is then burnt in the combustion chamber  110 , forming the working medium  113 .  
         [0057]     From there, the working medium  113  flows along the hot-gas passage  111  past the guide vanes  130  and the rotor blades  120 . The working medium  113  is expanded at the rotor blades  120 , transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the generator coupled to it.  
         [0058]     While the gas turbine  100  is operating, the components which are exposed to the hot working medium  113  are subject to thermal stresses. The guide vanes  130  and rotor blades  120  of the first turbine stage  112 , as seen in the direction of flow of the working medium  113 , together with the heat shield bricks which line the annular combustion chamber  106 , are subject to the highest thermal stresses.  
         [0059]     To be able to withstand the temperatures which prevail there, they have to be cooled by means of a coolant.  
         [0060]     The substrates may likewise have a directional structure, i.e. they are in single-crystal form (SX structure) or have only longitudinally oriented grains (DS structure). Iron-base, nickel-base or cobalt-base superalloys are used as material.  
         [0061]     It is also possible for the blades or vanes  120 ,  130  to have coatings which protect against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X stands for yttrium (Y) and/or at least one rare earth element) and heat by means of a thermal barrier coating. The thermal barrier coating consists, for example, of ZrO 2 , Y 2 O 4 —ZrO 2 , i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide.  
         [0062]     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).  
         [0063]     Despite the protective layers, corrosion products  10  can form on the component. For refurbishment, the corrosion products have to be removed by the process according to the invention if the component is to be coated again. If appropriate, cracks in the substrate of the component are then repaired.  
         [0064]     The guide vane  130  has a guide vane root (not shown here), which faces the inner housing  138  of the turbine  108 , and a guide vane head which is at the opposite end from the guide vane root. The guide vane head faces the rotor  103  and is fixed to a securing ring  140  of the stator  143 .  
         [0065]      FIG. 7  shows a combustion chamber  110  of a gas turbine. The combustion chamber  110  is configured, for example, as what is known as an annular combustion chamber, in which a multiplicity of burners  102  arranged circumferentially around the turbine shaft  103  open out into a common combustion chamber space. For this purpose, the combustion chamber  110  overall is of annular configuration positioned around the turbine shaft  103 .  
         [0066]     To achieve a relatively high efficiency, the combustion chamber  110  is designed for a relatively high temperature of the working medium M of approximately 1000° C. to 1600° C. To allow a relatively long service life even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side which faces the working medium M, with an inner lining formed from heat shield elements  155 . On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer or is made from material that is able to withstand high temperatures. A cooling system is also provided for the heat shield elements  155  and/or their holding elements, on account of the high temperatures in the interior of the combustion chamber  110 .  
         [0067]     The materials of the combustion chamber wall and their coatings may be similar to the turbine blades or vanes  120 ,  130 .  
         [0068]     The combustion chamber  110  is designed in particular to detect losses of the heat shield elements  155 . For this purpose, a number of temperature sensors  158  are positioned between the combustion chamber wall  153  and the heat shield elements  155 .  
         [0069]      FIG. 8  shows a perspective view of a blade or vane  120 ,  130 , which extends along a longitudinal axis  121 . The blade or vane  120 ,  130  has, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane region  406 . A blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to the shaft, is formed in the securing region  400 . The blade or vane root  183  is designed in hammerhead form. Other configurations, such as a fir-tree or dovetail root are possible. In the case of conventional blades or vanes  120 ,  130 , solid metallic materials are used in all the regions  400 ,  403 ,  406  of the rotor blade  120 ,  130 . The rotor blade  120 ,  130  may in this case be produced by a casting process, by a forging process, by a milling process or combinations thereof.  
         [0070]      FIG. 9  illustrates, by way of example, a steam turbine  300 ,  303  with a turbine shaft  309  extending along an axis of rotation  306 .  
         [0071]     The steam turbine has a high-pressure part-turbine  300  and an intermediate-pressure part-turbine  303 , each with an inner casing  312  and an outer casing  315  surrounding it. The high-pressure part-turbine  300  is, for example, of pot-type design. The intermediate-pressure part-turbine  303  is of two-flow design. It is also possible for the intermediate-pressure part-turbine  303  to be of single-flow design. Along the axis of rotation  306 , a bearing  318  is arranged between the high-pressure part-turbine  300  and the intermediate-pressure part-turbine  303 , the turbine shaft  309  having a bearing region  321  in the bearing  318 . The turbine shaft  309  is mounted on a further bearing  324  next to the high-pressure part-turbine  300 . In the region of this bearing  324 , the high-pressure part-turbine  300  has a shaft seal  345 . The turbine shaft  309  is sealed with respect to the outer casing  315  of the intermediate-pressure part-turbine  303  by two further shaft seals  345 . Between a high-pressure steam inflow region  348  and a steam outlet region  351 , the turbine shaft  309  in the high-pressure part-turbine  300  has the high-pressure rotor blading  354 ,  357 . This high-pressure rotor blading  354 ,  357 , together with the associated rotor blades (not shown in more detail), constitutes a first blading region  360 . The intermediate-pressure part-turbine  303  has a central steam inflow region  333 . Assigned to the steam inflow region  333 , the turbine shaft  309  has a radially symmetrical shaft shield  363 , a cover plate, on the one hand for dividing the flow of steam between the two flows of the intermediate-pressure part-turbine  303  and also for preventing direct contact between the hot steam and the turbine shaft  309 . In the intermediate-pressure part-turbine  303 , the turbine shaft  309  has a second blading region  366  comprising the intermediate-pressure rotor blades  354 ,  342 . The hot steam flowing through the second blading region  366  flows out of the intermediate-pressure part-turbine  303  from an outflow connection piece  369  to a low-pressure part-turbine (not shown) which is connected downstream in terms of flow.  
         [0072]     The components of the steam turbine  300 ,  303  likewise have protective layers and/or corrosion products  10  which are removed by the process according to the invention before the components can be refurbished.