Abstract:
Components which are subject to operating loads can often be passed for refurbishment by means of an acid treatment. The time for which the components remain in the acid has hitherto been determined empirically, which means that individual loads are not taken into account. The process according to the invention for the surface treatment of a component proposes that at least repeatedly a measurement voltage be applied to the component, resulting in the flow of a current, the time profile of which represents the state of the surface treatment and is used to decide upon when to terminate or interrupt the acid treatment.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority of the European application No. 04015424.7 EP filed Jun. 30, 2004, which is incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to a process for the surface treatment of a component in accordance with the preamble of claim  1  and to an apparatus for carrying out a process for the surface treatment of a component.  
       BACKGROUND OF THE INVENTION  
       [0003]     Components which are subject to operating loads, such as for example turbine blades and vanes of gas turbines, are subjected to an electrolyte treatment, so that the component can then be refurbished. In the case of gas turbine blades and vanes, the MCrAlX layers on the component, which are subject to operating loads, are removed by being immersed in 20% strength hydrochloric acid at approx. 50°-80° C. After a period of time derived from values gained through experience, the blades or vanes are removed from the acid bath, rinsed with water and then abrasively blasted. The process sequence of electrolyte bath followed by blasting is repeated a number of times until the entire MCrAlX layer has been removed or dissolved. The repetition of the individual process steps is generally necessary, since the electrolyte only dissolves aluminum-containing phases of the MCrAlX layer close to the surface. Deeper-lying regions of the MCrAlX layer therefore cannot be dissolved in one step. A porous layer matrix remains on the surface and is subsequently removed by blasting, for example mechanically.  
         [0004]     The time for which the blades or varies remain in the electrolyte does not in this case reflect the time which is actually required for the individual blade or vane to conclude the dissolution process, but rather is set as standard to a specific time. The residence time in the electrolyte is in this case determined on the basis of general empirical values.  
         [0005]     However, each individual component is subject to different levels of load, which means that a fixed preset time leads to different or incomplete dissolution of the surface of the component which is subject to load. In many cases, the components remain in the acid bath until the predetermined period of time has elapsed without any further progress being made in the removal of the coating.  
         [0006]     EP 1 094 134 A1 and US 2003/0062271 A1 disclose processes for the electrochemical removal of layers.  
         [0007]     U.S. Pat. No. 4,539,087 discloses a method in which the current of an electrolytic process is measured, so that on the basis of the current profile it is possible to reach a decision as to when to terminate the process.  
       SUMMARY OF THE INVENTION  
       [0008]     Therefore, it is an object of the invention to provide a process which allows the minimum treatment time required for each individual component (type, coating thickness, state after operating load, etc.) to be determined individually.  
         [0009]     The object is achieved by a process for the surface treatment of a component as claimed in the claims.  
         [0010]     A further object of the invention is to provide an apparatus which allows the minimum treatment times required to be determined individually for each individual component.  
         [0011]     This object is achieved by an apparatus for the surface treatment of a component as claimed in claim  27 .  
         [0012]     Further advantageous measures, which can be advantageously combined with one another in any desired way, are listed in the subclaims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     In the drawing:  
         [0014]      FIG. 1  shows an apparatus for carrying out the process according to the invention,  
         [0015]      FIGS. 2, 3 ,  4  show a time/voltage profile,  
         [0016]      FIGS. 5, 6  show time profiles for voltages and current which result when carrying out the process according to the invention,  
         [0017]      FIG. 7  shows a turbine blade or vane,  
         [0018]      FIG. 8  shows a combustion chamber, and  
         [0019]      FIG. 9  shows a gas turbine.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 1  shows an example of an apparatus  1  according to the invention which can be used to carry out the process according to the invention.  
         [0021]     The apparatus  1  comprises a vessel  3 , for example metallic, ceramic or made from plastic (Teflon polymer, etc.), in which there is a treatment agent  6 , for example an acid  6  or an electrolyte  6  (comprising coating material), which is used for the surface treatment of, such as the removal of a coating from or application of a coating to, at least one component  9 .  
         [0022]     In the case of the removal of a coating, it is preferable for an acid or an acid mixture to be present in the vessel  3 .  
         [0023]     By contrast, in the case of the application of a coating, the electrolyte  6  includes the corresponding chemical elements for the coating. In this case, by way of example, a single component  9 , the surface region of which is to be dissolved, is arranged in the treatment agent  6 . This dissolution is effected, for example, by the acid attack on, for example, the surface of the component  9  which is subject to operating loads.  
         [0024]     If the coating is to be removed from two or more components  9 , by way of example the two components  9  in each case form an electrode (i.e. anode and cathode), and in this case the treatment agent  6  used should be a nitrogen-containing treatment agent  6 .  
         [0025]     According to the invention, there is at least one voltage/current source  18 , which is electrically connected to the component  9  and a further electrode  12  via electrical connection means  15 . A first circuit can be closed by the connection means  15  being connected to a further electrical pole, i.e. the electrode  12 , which is arranged in the treatment agent  6  or connected to the vessel  3 , so that a current I can flow between component  9  and the pole  3 ,  12  and can also be measured. The current flows across the component  9  via the surface of the component  9  which is subjected to load and then flows through the treatment agent  6  to the electrode  12  (or to the vessel  3 ).  
         [0026]     It is also possible for a plurality of components  9  to be arranged in a vessel  3  in order for their coating to be removed, in which case a current curve I(t) can be determined individually for each component  9 , so that the components  9  if appropriate remain in the treatment agent  6  for different lengths of time.  
         [0027]     A further second circuit comprising lines  15 ′ and current/voltage source  18 ′, for example for a measurement voltage  33  ( FIG. 2 ), may also be present in accordance with the invention, so that a current likewise flows through this circuit and can also be measured.  
         [0028]     The lines  15 ′ are then likewise connected to the component  9  and the electrode  12 .  
         [0029]      FIG. 2  shows an example of a voltage profile according to the invention.  
         [0030]     To remove the coating from a large component  9 , a pulsed treatment voltage  30  with a pulse duration t 30  is applied, generating currents of up to 100 A, for example, for correspondingly large components  9  (length 38 cm), such as gas turbine blades or vanes  120 ,  130  ( FIGS. 7, 9 ).  
         [0031]     The pulse duration t 30  may always be the same or may change with time t. The magnitude of the treatment voltage may also change with time t.  
         [0032]     However, these currents are too high for it to be possible to obtain more accurate information about the progress of the surface treatment from the transient properties of the current profile (cooling times are too long, for example).  
         [0033]     Therefore, according to the invention, a lower, for example pulsed, measurement voltage  33  (1 mV to 50 mV) is superimposed on the higher treatment voltage  30  (for removal of the coating) in the circuit ( 18 ,  15 ,  9 ,  6 ,  12 ), or the treatment voltage  30  is briefly (i.e. at least at times) increased by the magnitude of the measurement voltage  33 .  
         [0034]     The pulse duration t 33  of the measurement voltage  33  may be shorter than, equal to or longer than the pulse duration t 30  of the treatment voltage  30 .  
         [0035]     If the pulse duration t 33  of the measurement voltage  33  is shorter than the pulse duration t 30  of the treatment voltage  30 , the measurement voltage  33  may be applied at the start, in the middle or at the end of the pulsed treatment voltage  33 .  
         [0036]     The lower measurement voltage  33  generates very much lower currents, which can be measured more successfully.  
         [0037]     The signals relating to the treatment voltage  30  and the measurement voltage  33  are separated, for example, by analysis of the current curve by means of mathematical signal separation methods, such as for example Fourier analysis.  
         [0038]     By way of example, it is possible to use three electrodes corresponding to the treatment voltage  30  for the removal of the coating and to the measurement voltage  33  (a further electrode  12 ′ for a second circuit ( FIG. 1 ) with lines  15 ′ and current/voltage source  18 ′ for a measurement voltage  33  may also be present in accordance with the invention; in this case, the lines  15 ′ are likewise connected to the component  9  and, for example, to the electrode  12 ′ (indicated by dashed lines) and not to the electrode  12 ), in which case the voltages are superimposed on the large surface. The separation of the current signals by measurement means is effected, for example, by the use of two partially decoupled circuits ( 15 + 18 + 9 + 6 + 12 ;  15 ′+ 18 ′+ 9 + 6 + 12  or + 12 ′).  
         [0039]     The contribution of the lower measurement voltage  33  to the electrolytic removal of the coating is low or negligible.  
         [0040]     When using a pulsed treatment voltage  30 , it is likewise possible to use a DC measurement voltage  33 ″ (indicated by dashed lines).  
         [0041]      FIG. 3  shows a further example of a voltage profile according to the invention for the method according to the invention.  
         [0042]     Here, once again a high pulsed treatment voltage  30 , which generates very high currents, is used to remove the coating.  
         [0043]     The measurement voltage  33  is in this case, for example, likewise pulsed and is applied during the interpulse periods  36  (t 36 ) of the treatment voltage pulses  30  (t 36 &gt;t 33 ). This is done by synchronizing the voltage pulses  30 ,  33 .  
         [0044]      FIG. 4  shows examples of further voltage profiles.  
         [0045]     In this case, a treatment voltage  30  of a constant level (DC voltage) is applied to the component  9  for electrolytic coating removal, while the measurement voltage  33  is once again pulsed and superimposed on the treatment voltage  30 .  
         [0046]     In this case, the treatment voltage  30  can be briefly increased (corresponding to a pulsed increase) by the magnitude of the measurement voltage  33 , in which case only one circuit is required, or alternatively the measurement voltage  33 ′ (indicated by dashed lines) is superimposed on the treatment voltage, for example by a second circuit.  
         [0047]     It is likewise possible to use a lower DC measurement voltage  33 ″, in particular in a second circuit  18 ′,  15 ′,  9 ,  6 ,  12  or  12 ′.  
         [0048]     The pulse durations t 33 , t 30  may be identical or different (t 30 =t 33 , t 33 &lt;t 30 , t 33 &gt;t 30 , t 30 =t 33  and t 36 &gt;t 30 , etc.).  
         [0049]     A time profile of the current I(t) caused by the measurement voltage during electrolysis for coating removal is illustrated in  FIG. 5 .  
         [0050]     The current I(t) initially rises with time t and after a certain point in time is initially substantially constant. The coating removal is not yet complete, i.e. the coating removal rate is still high.  
         [0051]     After a certain time t, the current I drops. The drop (range or point  27  in curve I(t)) in the current I indicates that only a small amount of coating material is being dissolved. Consequently, the dissolution process can be stopped when, for example, a predetermined comparison value for the current intensity has been reached or the current intensity drops by a certain amount (cf. difference between measurement points  27 ,  22 ) or when a trend line indicates a falling profile for the current intensity.  
         [0052]     This applies analogously to the coating processes when the electrolyte  6  has been consumed or the coating thickness is determined from the surface area below the curve I(t).  
         [0053]     The process can also be carried out in substeps. In this case, in a process intermediate step an abrasive coating removal is in each case carried out, removing residues of acid products and/or accelerating the coating removal, since after a certain residence time of the component  9  in the treatment agent  6 , by way of example, a brittle layer forms, which can be removed more successfully by abrasive means.  
         [0054]     It is also possible for the component  9  to be washed (rinsed) in a process intermediate step.  
         [0055]     Then, the component  9  is once again positioned in the treatment agent  6 .  
         [0056]     The process steps of treatment of the component  9  in the treatment agent  6  and abrasive blasting can be repeated as desired.  
         [0057]     The removal of the coating from the component(s)  9  proceeds even without the presence of a treatment voltage, i.e. the coating removal process is not at that time electrolytic.  
         [0058]      FIG. 6  shows an experimentally determined profile for the currents and voltages measured or used.  
         [0059]     A constant treatment voltage  30  of 1.2 V is applied to a turbine blade or vane (length≈18 cm, surface area≈150 cm 2 ); the electrolyte used is, for example, 5% HCl (hydrochloric acid) containing 2% triethanolamine. The treatment voltage  30  is represented by the diamond shapes and generates a current I of 10 to 11 A (not shown).  
         [0060]     The pulsed measurement voltage  33  for determining the end point is in this case, for example, 50 mV and is applied by pulses with a pulse length of, for example, 0.5 s. The ratio of the measurement voltage  33  to the treatment voltage  30  is therefore 1:24; alternatively it may, for example, be 1:10 (or 1:20, 1:30 or greater than 1:50, 1:100).  
         [0061]     The measurement voltage  33  is represented by squares in  FIG. 6 . The current I, which is measured as a result of the measurement voltage  33 , is represented by the triangles in  FIG. 6 . A separating line (indicated in dashed lines) shows the intrapolated and expected time profile of the current. This curve corresponds to that shown in  FIG. 2 .  
         [0062]     The time profile  24  of the current I(t) can also be determined from individual measurement points  21  which are taken at regular or irregular intervals.  
         [0063]     The components from which the coating is removed in the following descriptions of figures can be coated again, as explained in the following descriptions of figures.  
         [0064]      FIG. 7  shows a perspective view of a blade or vane  120 ,  130  which extends along a longitudinal axis  121 .  
         [0065]     The blade or vane as an example of the component  9  may be a rotor blade  120  or a guide vane  130  of a turbomachine. The turbomachine may be a gas turbine of an aircraft or a power plant for generation of electricity, a steam turbine or a compressor.  
         [0066]     The blade or vane  120 ,  130  includes, in succession along the longitudinal axis  121 , a securing region  400 , an adjoining blade or vane platform  403  and a main blade or vane part  406 . When used as a guide vane  130 , the vane may have a further platform (not shown) at its vane tip  415 .  
         [0067]     In the securing region  400  there is a blade or vane root  183 , which is used to secure the rotor blades  120 ,  130  to a shaft or a disk (not shown).  
         [0068]     The blade or vane root  183  is designed, for example, in the shape of a hammerhead. Other configurations, such as a fir-tree root or a dovetail root, are also possible.  
         [0069]     The blade or vane  120 ,  130  has a leading edge  409  and a trailing edge  412  with respect to a medium which flows past the main blade or vane part  406 .  
         [0070]     With conventional blades or vanes  120 ,  130 , by way of example, solid metal materials are used in all regions  400 ,  403 ,  406  of the blade or vane  120 ,  130 .  
         [0071]     The blade or vane  120 ,  130  can in this case be produced by a casting process, or also by means of directional solidification, by means of a forging process, by means of a milling process or by combinations thereof.  
         [0072]     Workpieces with a single-crystal structure or structures are used as components for machines which are exposed to high mechanical, thermal and/or chemical loads in operation.  
         [0073]     Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metal alloy solidifies to form a single-crystal structure, i.e. a single-crystal workpiece, or solidifies directionally.  
         [0074]     In this case, dendritic crystals are oriented along the heat flow direction and form either a columnar grain structure (i.e. grains which extend over the entire length of the workpiece and are in this case referred to as directionally solidified, in accordance with the standard terminology employed in the field) or a single-crystal structure, i.e. the entire workpiece comprises a single crystal. In these processes, the transition to globulitic (polycrystalline) solidification has to be avoided, since non-directional growth inevitably results in the formation of transverse and longitudinal grain boundaries which negate the good properties of the directionally solidified or single-crystal component.  
         [0075]     Wherever the text speaks in general terms of directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angled grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. The latter crystalline structures are also known as directionally solidified structures.  
         [0076]     Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.  
         [0077]     Refurbishment means that protective layers may have to be removed (e.g. by sandblasting) from components  120 ,  130  after they have been used, by the process according to the invention. This is followed by removal of the corrosion and/or oxidation layers or products. If appropriate, cracks in the component  120 ,  130  are also repaired. This is followed by further coating of the component  120 ,  130 , for example by the process according to the invention, and renewed use of the component  120 ,  130 .  
         [0078]     The blade or vane  120 ,  130  may be of hollow or solid design. If the blade or vane  120 ,  130  is to be cooled, it is hollow and may also include film-cooling holes (not shown). To protect against corrosion, the blade or vane  120 ,  130  by way of example has corresponding, generally metallic coatings, and, to protect against heat, generally also a ceramic coating.  
         [0079]      FIG. 8  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 large number of burners  102  arranged circumferentially around the turbine shaft  103  open out in a common combustion-chamber space. For this purpose, the combustion chamber  110  overall is configured as an annular structure positioned around the turbine shaft  103 .  
         [0080]     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 operating time even under these operating parameters, which are unfavorable for the materials, the combustion chamber wall  153  is provided, on its side facing the working medium M, with an inner lining formed from heat shield elements  155  (a further example of component  9 ). On the working medium side, each heat shield element  155  is equipped with a particularly heat-resistant protective layer or is made from material which is able to withstand high temperatures. Moreover, on account of the high temperatures in the interior of the combustion chamber  110 , a cooling system is provided for the heat shield elements  155  or for the holding elements thereof.  
         [0081]     The materials of the combustion chamber wall and their coatings may be similar to the turbine blades or vanes.  
         [0082]      FIG. 9  shows, by way of example, a gas turbine  100  in the form of a longitudinal part-section.  
         [0083]     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.  
         [0084]     An intake casing  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 casing  109  follow one another along the rotor  103 .  
         [0085]     The annular combustion chamber  106  is in communication with a, for example, annular hot-gas duct  111 , where, for example, four turbine stages  112  in succession form the turbine  108 .  
         [0086]     Each turbine stage  112  is formed, for example, from two blade/vane rings. As seen in the direction of flow of a working medium  113  in the hot-gas duct  111 , a row of guide vanes  115  is followed by a row  125  of rotor blades  120 .  
         [0087]     The guide vanes  130  are secured to an inner casing  138  of a stator  143 , whereas the rotor blades  120  belonging to a row  125  are, for example, fitted to the rotor  103  by means of a turbine disk  133 .  
         [0088]     A generator or machine (not shown) is coupled to the rotor  103 .  
         [0089]     While the gas turbine  100  is operating, the compressor  105  sucks in air  135  through the intake casing  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 mixture is then burnt so as to form the working medium  113  in the combustion chamber  110 . From there, the working medium  113  flows along the hot-gas duct  111  past the guide vanes  130  and the rotor blades  120 . At the rotor blades  120 , the working medium  113  expands, transferring its momentum, so that the rotor blades  120  drive the rotor  103  and the latter in turn drives the machine coupled to it.  
         [0090]     The components exposed to the hot working medium  113  are subject to thermal loads when the gas turbine  100  is operating. 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 lining the annular combustion chamber  106 , are subject to the highest thermal loads.  
         [0091]     To be able to withstand the prevailing temperatures, these components can be cooled by means of a coolant.  
         [0092]     It is likewise possible for substrates of the components to have a directional structure, i.e. for them to be in single-crystal form (SX structure) or to have only longitudinally directed grains (DS structure).  
         [0093]     By way of example, iron-base, nickel-base or cobalt-base superalloys are used as material for the components, in particular for the turbine blade or vane  120 ,  130  and components of the combustion chamber  110 .  
         [0094]     Superalloys of this type are known, for example, from EP 1204776, EP 1306454, EP 1319729, WO 99/67435 or WO 00/44949; these documents likewise form part of the present disclosure.  
         [0095]     It is also possible for the blades or vanes  120 ,  130  to have coatings to protect against corrosion (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth) and against heat (thermal barrier coating).  
         [0096]     The thermal barrier coating consists, for example, of ZrO 2 , Y 2 O 4 —ZrO 2 , i.e. it is not stabilized, or is partially or completely stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide.  
         [0097]     Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).  
         [0098]     The guide vane  130  has a guide vane root (not shown here) facing the inner casing  138  of the turbine  108 , and a guide vane head 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 .