WO2011110323A1

WO2011110323A1 - Method for electrochemically stripping coatings from gas turbine components

Info

Publication number: WO2011110323A1
Application number: PCT/EP2011/001122
Authority: WO
Inventors: Thiemo Ullrich; Marcel SCHLÖSSER; Michael Sies
Original assignee: Lufthansa Technik Ag
Priority date: 2010-03-09
Filing date: 2011-03-08
Publication date: 2011-09-15
Also published as: DE102010010771A9; DE102010010771A1

Abstract

The invention relates to a method for electrochemically stripping coatings from gas turbine components (1) made of titanium alloys or stainless steels in an electrolyte solution (2), wherein the gas turbine component (1) comprises a multi-layer system (13) on the surface and has a polarity as an anode, and wherein the electrolyte solution (2) comprises sodium hydroxide.

Description

Process for the electrochemical stripping of gas turbine components

The present invention relates to a method for the electrochemical stripping of gas turbine components having the features of the preamble of claim 1.

Components in gas turbines, in particular in aircraft turbines, are subject to different requirements. In the front region of the gas turbine and in the region of the compressor, the temperatures occurring in comparison to the remaining part of the turbine are relatively low and therefore is less used in the components used in this area, the temperature resistance, but rather the resistance of the components used, in particular the guide and Blades, against oxidation, corrosion and erosion in the foreground. This wear is caused inter alia by solid particles, such as sand and dust, which are sucked by the gas turbine and moved with the gas flow through the turbine. The components can be damaged by the erosive effect of the particles, resulting in performance losses of the turbine and requires a regular replacement of expensive components. In order to increase the service life of the guide vanes and rotor blades used in the compressor, they are coated with wear protection layers. Over time, however, this wear protection layer wears off as well. Before the anti-wear layer can be re-applied to the turbine blades as part of a routine overhaul of the gas turbine, the remnants of the old layer must be removed from the turbine blades. Such removal of the wear protection layer can often

confirmation copy not done without damaging the base material of the turbine blades. Above all, physical processes, such as sandblasting, are always associated with a removal of the base material. The already relatively thin wall thickness of the turbine blades is thereby further reduced, which aggravates added that by such removal of the wear protection layer, the thickness ratios of the turbine blades and thus the flow conditions can be changed. As an alternative to physical methods, such as sandblasting, chemical or electrochemical processes have become established. These methods are usually developed specifically for the particular combination of base material and layer material, since depending on the nature of the materials, different parameters, such as. Compositions of the liquid medium, temperatures, voltages, currents, pH or additives must be selected. Some layers can be deducted from the base material satisfactorily even with very special cathode geometries, pulsed current or other aids.

Thus, for example, DE 10 2004 009 757 A1 describes such a cathode adapted to the geometry of the component which is intended to improve the quality of the stripping of turbine blades. In addition to the cathode adapted to the turbine blade, a pulsed current is also used here.

For stripping another coating, US Pat. No. 6,454,870 B1 describes an electrochemical stripping process, also called "stripping", which is particularly gentle on components and removes the wear protection layer, in this case a chromium oxide layer, with the aid of a hydrochloric acid-based medium , Novel complex wear protection layers make it possible to realize ever higher wear protection for turbine components. An example of such an innovative wear protection layer variant is a so-called multilayer coating system, wherein the multilayer coating system comprises one or more layer systems arranged one above the other, wherein a single layer system always comprises a metal and a ceramic which is element-related to the metal (for example Cr and CrM). Such multilayer coating systems are preferably applied to gas turbine components in the vacuum coating technique by EB-PVD (Electron Beam Physical Vacuum Deposition) processes. Gas turbine components often consist of nickel-based alloys (eg Inconel 718), since they have good corrosion and / or high-temperature resistance. A material is a nickel-based alloy if the alloying element with the largest alloying content is nickel. For example, in the case of the nickel base material Inconel 718, the alloying content of nickel is 50-55%.

The described structure of the innovative multi-layer system described is to be explained briefly with reference to FIGS. 2 and 3. FIG. 2 shows a multi-layer system 13 which is formed from four individual layer systems 11 arranged one above the other, which are applied to a substrate 10, for example by EB -PVD were applied. In this case, a layer system 11 always comprises a metal 14 and a ceramic 15 which is element-related to the metal 14. Related to an element means that the metallic binding partner of the ceramic 15 (in the case of chromium nitride, this would be chromium) comprises the same element as the metal 14. Thus, the ceramic 15 is formed of a non-metallic binding partner and a metallic binding partner, wherein the metallic binding partner - The same metal is therefore present on the one hand as a metal 14 in elemental form and in addition it forms with at least one non-metallic element, the ceramic 15. In the case of a layer system 11 of chromium and chromium nitrite Thus, chromium is present on the one hand as metal 14 and additionally as bound form together with nitrogen as ceramic 15 (chromium nitrite). The ceramic 15 is then elementally related to the metal 14. Besides Cr and CrN, Cr and CrAlN, Ti and TiN, and Ti and TiAlN are further examples of such elementally related metals 14 and ceramics 15.

However, the layer system 11 can also comprise further layers, this state being illustrated in FIG. Between the layer of metal 14 and the layer of ceramic 15, for example, an intermediate layer of metal alloy material 17 and an intermediate layer of graded metal-ceramic material 16 may be provided.

Layers applied with EB-PVD have a significantly higher adhesion to the base material and less porosity than is the case with other methods, such as, for example, the plasma spraying method. However, this also means that the layers are more difficult to remove than those that were applied by means of plasma spraying.

In general, ceramic layers are much worse than metallic layers by electrochemical methods of components solve (due to the usually low e- lektrischen conductivity of ceramics), the peeling is particularly problematic when several alien foreign layers to be removed simultaneously. In addition, not all electrochemical processes are generally component- nend, so that the sometimes very aggressive media can attack the base material of the component. The electrochemical stripping methods known from the prior art can lead to a direct planar attack of the base material, but also to the intercrystalline dissolution of the microstructure in gas turbine components with this multilayer system. The electrolyte solutions used in this process, known as galvanic baths, are based, for example, on molten sodium hydroxide (Molten Salt Method) or on hydrochloric acid and can damage materials such as titanium alloys or nickel-base alloys. In addition to the direct surface attack of the base material, such electrolyte solutions can also trigger intercrystalline corrosion in the base material. In such an attack, the grain boundaries of the structure are stronger than the grains themselves decomposed, which leads to whole grains are dissolved out of the structure and the base material is gradually dissolved. Even if this damage effect is not visually as visible as the erosion during sandblasting, the microstructure of the base material is significantly weakened by the stripping process and the component strength is reduced. This effect reoccurs every time a peel operation is performed, so that the damage effect of the intergranular attack accumulates over the entire life of the turbine blade. The basic material structure is increasingly worn away by the stripping of the old wear protection layer which is necessary for the routinely performed repainting of the wear protection layer, which is in contradiction to the actual aim of the maintenance, specifically the avoidance of wear of the turbine parts. Since no gentle removal process is known for the new multi-layer system described, it is usually removed by abrasive blasting, which, as described above, also a material removal of the base Stoffs and thus a component damage brings with it. An example of abrasive blasting is the blasting with aluminum oxide (corundum), which in itself can cause a high erosion of material and, moreover, usually follows the Molten Salt Method.

The invention is therefore based on the object to provide a gentle method for electrochemical stripping of gas turbine components and a corresponding apparatus for performing the method.

The invention achieves the object by a method having the features of claim 1 and a device having the features of claim 9. Further preferred embodiments of the invention can be taken from the subclaims and the associated descriptions and the drawing.

To solve the problem, the present invention proposes a method in which the gas turbine component to be stripped of a nickel-base alloy having a multilayer film system on the surface is poled as an anode in an electrolytic solution and the electrolytic solution comprises sodium hydroxide. The method described here offers important advantages compared to the already established methods used for stripping gas turbine components. As has been found in experiments, the base material of the gas turbine component is not attacked by the method according to the invention, wherein in particular the material removal or the harmful change in the microstructure observed in other chemical and physical processes does not take place. The reason for this is that a solution is created by the sodium hydroxide, which is the electrochemically detached metal ions (eg chromium ions) of the multilayer Layer complexed very efficient and thus keeps in the galvanic bath in solution and the base material is passivated during stripping and thus additionally counteracts damage to the base material. The ceramic components of the multilayer coating system can accumulate as sludge during the stripping process. Another advantage lies in the fact that sodium hydroxide is readily available in large quantities and also takes account of the environmental aspects of the galvanic process, since it can be easily neutralized before disposal.

Preferably, the electrolyte solution has a pH of 13 to 14, preferably 14, as it has been found in experiments that in this pH range, the best results can be achieved. Due to the high hydroxide ion concentration in the pH range of 13 or 14, the detached metal ions can be kept in solution particularly well and without additional complexing agents in the galvanic bath.

Ideally, the temperature of the electrolyte solution is in the range of 35-80 ° C, preferably 45-55 ° C. It is known that temperature has a significant influence on the rate of chemical reactions. In the range of 35-80 ° C, preferably 45-55 ° C, the best results can be achieved, the galvanic process is controlled, and very short process times (up to two hours) can be achieved. In addition to the bath temperature and the foreign metal concentration is monitored in the solution and when exceeding a defined limit, the solution is disposed of according to the usual regulations.

Preferably, the voltage applied to the gas turbine component is in the range of 2-8 V, preferably 3-4 V. The to be applied Voltage depends on the component to be stripped, with the specified range being ideal for the majority of nickel base alloy gas turbine components. Too high voltages can lead to unwanted material removal, so the applied voltage must be kept constant by a power supply. Preferably, the voltage is slowly up-regulated to the desired level in order to ensure a gentle removal process.

Preferably, steel is used as the cathode material. Since the cathode material can have a significant influence on the quality of the peeling process, it is necessary to choose a cathode material which has the greatest possible positive effect on the quality of the peeling process. In the process according to the invention, steel as the cathode material gives the best results.

The process described is very suitable for stripping multilayer systems containing the ceramic components CrN, CrAlN, TiN, or TiAlN.

The invention will be explained in more detail with reference to a concrete embodiment with the aid of the figure. The figures show in detail:

Fig. 1: Schematic representation of a device for electrochemical stripping a gas turbine component.

Fig. 2: representation of a multi-layer system

Fig. 3: representation of a single layer system

FIG. 1 shows a container 4, which has an electric lytlösung 2 contains. Further, a lowering device 6 is provided, with which a gas turbine component 1 is lowered into the electrolyte solution 2. The gas turbine component 1 was previously cleaned of oil, grease, dirt or other contaminants. Such contaminants may adversely affect the stripping process and should therefore be carefully removed by known means.

The gas turbine component 1 is in this embodiment of a nickel-based alloy and has on the surface of a multi-layer system 13. The lowering device 6 simultaneously forms the anode and is connected to a power supply 5, which in turn is connected to a cathode 8.

Preferably, the lowering device 6 is designed so that it can simultaneously receive and lower several gas turbine components l. Advantageously, it is also possible to set individual voltages for different gas turbine components 1, if this is advantageous or necessary, for example, due to size and / or geometry differences. The power supply 5 is generally used to apply a positive voltage to gas turbine components 1, to switch them as an anode, and to apply a negative voltage to the cathode 8. The cathode 8 can be designed so that it is designed as a separate component and is lowered into the electrolyte solution 2 or permanently installed in the electrolyte solution 2 or, as in this embodiment, is realized by the container 4 itself and thus the electrolyte solution. 2 surrounds large area. The cathode 8 or the container 4 is preferably made of steel and insulated to the outside, for example by a plastic coating. Here is one of the great advantages of the method according to the invention, since the Kathodenge.ometrie can be relatively simple. A special, exactly to the geometry of the gas turbine component 1 fitted cathode geometry is not required. As a result, the method can be carried out with significantly less process complexity than many comparable methods according to the prior art. Another advantage of the method according to the invention without geometrically conforming cathode is that in a simple manner very high numbers of pieces can be stripped off at the same time. In order to further minimize the processing effort, the cathode 8 may also preferably be designed so that it can be easily replaced. Should the cathode 8 wear off in any way, it can easily be replaced by this replacement option and therefore the process need not be interrupted for a long time.

The composition of the electrolytic solution 2 depends on the base material and includes sodium hydroxide for gas turbine components 1 made of a nickel-based alloy. In this embodiment, the electrolyte solution comprises 40 to 90 g / L of sodium hydroxide and has a pH of 14. An electrolyte solution 2 based on sodium hydroxide offers important advantages compared to the already established methods. The base material used is not attacked; the material removal or the change in the microstructure observed in other chemical and physical processes does not take place. The temperature of the electrolyte solution 2 is adjusted by a temperature control 3 to 45- 55 ° C. The voltage applied to the gas turbine component 1 via the voltage supply 5 is slowly raised from 0 V to 3-4 V and held until the multilayer coating system 13 is drawn off from the gas turbine component 1. Depending on the age of the electrolyte solution 2, the ideal hold time is different. Based on empirical values and optical inspection, the exact point in time at which the multilayer system 13 is completely removed from the gas turbine component 1 is determined is detached, and the gas turbine component 1 is to be lifted out of the electrolyte solution 2 by the lowering device 6. Residues of the multi-layer system 13 can be removed if necessary with a soft brush. A bath monitoring 7, the composition of the electrolyte solution 2 can be controlled periodically. In this case, the concentration of metal impurities in the electrolyte solution 2 can be monitored. Possibly. is caused to prepare a fresh electrolyte solution 2. The used electrolyte solution 2 must then be disposed of according to current regulations. Since the composition of the electrolyte solution 2 is relatively environmentally friendly compared to other galvanic baths (sodium hydroxide can be neutralized very easily), the disposal is not only correspondingly simple and therefore inexpensive, but also advantageous from an ecological point of view, thus completing the inventive gentle electrochemical process for stripping of multilayer coating systems 13 of gas turbine components.

FIG. 2 shows an exemplary multilayer coating system 13 which can be gently removed from a substrate 10 using the stripping process according to the invention. Accordingly, the substrate 10 is a gas turbine component 1, for example. The multilayer system 13 was preferably applied to the substrate 10 or the gas turbine component 1 by means of a PVD (Physical Vapor Deposition) method. The multilayer system 13 in this example comprises four layer systems 11. The method according to the invention is preferably used for stripping multilayer system 13 comprising three to six layer systems 11, more preferably for multilayer systems 13 comprising four or five layer systems 11. The individual layer systems 11 comprise a layer of metal 14 and a layer of a ceramic 15 which is elementally related to this metal 14. A preferred layer system 11 consists of a layer of metal 14 of chromium and a layer of ceramic 15 of chromium nitrite or chromium aluminum nitrite. Another preferred layer system 11 comprises titanium and titanium nitrite or titanium aluminum nitrite.

In addition to metal 14 and ceramic 15, the layer system 11 may also comprise further layers. This is shown in FIG. The layer system 11 has here 2 more layers. The layer system 11 preferably additionally comprises a layer of metal alloy material 17 and a layer of graded metal-ceramic material 16. These intermediate layers can further increase the strength and adhesion of the multilayer system 13. Preferably, the intermediate layers are also made of element-related materials and homogenize the transition from metal 14 to ceramic 15th

Even for these even more complex multilayer systems 13, in which the layer systems 11 comprise more than the two layers of metal 14 and ceramic 15, the stripping process according to the invention is well suited.

List of reference numbers:

1 gas turbine component

2 electrolyte solution

3 temperature control

4 containers

5 power supply

6 lowering device

7 bath monitoring

8 cathode

10 substrate

11 shift system

13 multi-layer system

14 metal

15 ceramics

16 metal-ceramic material

17 metal alloy material

Claims

Claims :

Anspruch [en] A process for electrochemically stripping gas turbine components (1) from nickel-base alloys in an electrolyte solution (2), wherein the gas turbine component (1) has a multilayer coating system (13) on the surface and is poled as the anode, characterized

the electrolyte solution (2) comprises sodium hydroxide.

2. The method according to claim 1, characterized in that the electrolyte solution (2) has a pH of 13 to 14, preferably 14.

3. The method according to any one of the preceding claims, characterized in that the temperature of the electrolyte solution (2) in the range of 35-80 ° C, preferably 45-55 ° C.

4. The method according to any one of the preceding claims, characterized in that the voltage applied to the gas turbine component (1) voltage in the range of 2-8 V, preferably 3-4 V is located.

5. The method according to any one of the preceding claims, characterized in that the multi-layer system (13) comprises at least one layer system (11) made of Cr and CrN.

6. The method according to any one of the preceding claims, characterized in that the multi-layer system (13) comprises at least one layer system (11) of Ti and TiN.

7. The method according to any one of the preceding claims, characterized in that the multi-layer system (13) at least one layer system (11) comprises Cr and CrAlN.

8. The method according to any one of the preceding claims, characterized in that the multi-layer system (13) comprises at least one layer system (11) of Ti and TiAlN.

9. A device for carrying out a method according to one of the preceding claims, characterized in that a cathode (8) is provided, which is associated with the electrolyte solution (2).

10. The device according to claim 9, characterized in that the cathode (8) is exchangeable.

11. The device according to claim 9 or 10, characterized in that the cathode (8) is formed of steel.

12. Device according to one of claims 9 to 11, characterized in that a plurality of gas turbine components (1) can be clamped simultaneously.

13. The apparatus according to claim 12, characterized in that for each gas turbine component (1) a different high voltage is adjustable.