EP0318886A1 - Process for the electrolytic stripping of a protective coating, having a high content of chromium and nickel and/or cobalt, from the substrate of an object made of a superalloy - Google Patents

Abstract

Process for the electrolytic detachment of a surface protection layer (7) having a high Cr and Ni and / or Co content from a base body (4) made of a Ni or Co-based superalloy by activation in NaOH and HCl and subsequent immersion as anode into an electrolyte (2) containing oxygen-releasing, oxidizing components and submission to an electrolysis process. In the electrolysis process, the diffusion zone (9) of the surface protective layer (7) which adjusts itself electronically to the surrounding areas, is depleted in Cr and enriched in Ni, is preferably attacked and detached.

Description

Technical field

Gas turbines for the highest demands. The blade is a critical component, with protective layers against erosion, wear, corrosion and oxidation becoming more important at high temperatures. The protective layer usually has a shorter lifespan than the core material of the blade, which is why the renewability of the former is becoming increasingly important.

The invention relates to the further development of methods for repairing, repairing and renewing components of thermal machines which have been rendered unusable by erosion, wear, corrosion, oxidation or mechanical damage and are provided with protective layers. The old existing protective layer must first be removed, which can basically be done mechanically or chemically.

The electrochemical method as a reverse process to the galvanic coating has a special position.

In particular, it relates to a method for the electrolytic detachment of a surface protective layer having a high Cr and Ni and / or Co content from the base body of a component consisting of a nickel or cobalt-based superalloy.

State of the art

The removal of protective layers on substrates made of superalloys is carried out, among other things, by chemical means. Electrolytic dissolution has hitherto not been practically used for such alloys. Some methods are known for removing metals from their substrates by reversing the method of electroplating. From US-A-2 907 700 it is known to electrolytically remove coatings of metals (Ag, Ni, Cd, Zn, In) from a plutonium substrate. Sulfuric acid or sodium phosphate solution is used as the electrolyte. An electrolytic process is known from DE-B-21 46 828 for detaching metal coatings (Cr, Au, Cd, Cu, Ag, Zn, Sn, Ni) from stainless Cr / Ni steel. Bromine-containing solutions of nitrates, acetates, chlorides etc. are used as electrolytes. The attack on the substrate is said to be low. According to DE-C-25 27 152, coatings of metals (Ni, Cr, Zn, Sn, Cu, Cd, Ag) are to be removed electrolytically from steel by using nitric acid or nitrate-containing solutions with iodine content as electrolytes, to which additional organic chlorine compounds are added will.

These known methods, which are based on the sufficient difference in the dissolution potential of the metal coating compared to that of the substrate, are not in the present form on protective layers based on nickel. Superalloys transferable. The close relationship of the chemical structure between the protective layer and the substrate normally does not permit electrolytic dissolution of the former without the substrate being attacked in an unacceptable manner at the same time. Switching to complex-forming additives to the electrolyte also does not help.
In addition, the conditions for the non-attack of the substrate in the case of components made of a superalloy (gas turbine blade) are much stricter than for any other objects, for example those mentioned above. A gas turbine blade that has only been slightly modified in the core material would rarely be reusable. There is therefore a strong need to largely eliminate the above shortcomings and to show ways of successfully using an electrolytic detachment process for surface protection layers applied on nickel-based or cobalt-based superalloys.

Presentation of the invention

The invention is based on the object of specifying a method for detaching a surface protection layer based on a Ni and / or Co alloy with a high Cr content from the base body of a component, which consists of a chromium-containing Ni and / or Co base alloy. The surface layer should be completely removed without the material of the base body being attacked, removed or damaged or its chemical-physical properties and its behavior with regard to compatibility being impaired or changed, particularly when a surface protective layer is subsequently reapplied (renewed).

This object is achieved in that, in the method mentioned at the outset, the component coated with a protective layer is first activated in a solution of 20% NaOH and then at 40 ° C. for 2 hours in a solution of Concentrated HCl is immersed, that the component with its activated protective layer is brought as an anode into an electrolyte which contains oxygen and contains oxidizing components and is subjected to the electrolysis until the protective layer is completely dissolved and falls off.

Way of carrying out the invention

• Fig. 1 einen schematischen Querschnitt durch den aktiven Teil einer Elektrolysezelle zur Durchführung des Verfahrens,
• Fig. 2 den stark schematisierten Verlauf des Cr- und Ni-­Gehalts in der Oberflächenschutzschicht und der darunter liegenden Zone des Grundkörpers,
• Fig. 3 ein Fliessdiagramm (Blockschema) einer Ausführungs­art des Verfahrens.

The invention is described on the basis of the following exemplary embodiments which are explained in more detail by means of figures. It shows:

• 1 shows a schematic cross section through the active part of an electrolysis cell for carrying out the method,
• 2 shows the highly schematic course of the Cr and Ni content in the surface protective layer and the underlying zone of the base body,
• 3 shows a flow diagram (block diagram) of an embodiment of the method.

1 shows a schematic cross section through the active part of an electrolytic cell for carrying out the method. The insignificant parts that are not actively involved in the basic process flow, such as vessel, power supply, clamps, stirring devices, control devices etc., have been omitted for the sake of clarity. 1 is the cathode (usually a sheet made of corrosion-resistant Cr / Ni steel), 2 the electrolyte (indicated by horizontal lines), 3 the anode consisting of the base body and surface protection layer. The base body (substrate) 4 consists of a nickel or cobalt-based superalloy, which is usually present as an unchanged part 5 (core material). A diffusion zone 6 is located in the base body 4 at the boundary with the surface protective layer 7.

⁻) angedeutet. Der elektrochemische Angriff erfolgt zunächst an der Oberfläche der Schutzschicht 7 durch NO $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$

⁻-Ionen, welche vor allem das Nickel herauslösen (durch mit NO $\genfrac{}{}{0ex}{}{\text{2}}{\text{3}}$

⁻ und Ni²⁺ markierte Pfeile angedeutet). Dadurch wird die Schutzschicht 7 aufgelockert, was durch Bildung der Poren 10 angedeutet ist. Auf diese Weise kann der Angriff des Elektrolyten immer tiefer ins Innere der Schutzschicht 7 vorgetragen werden. Das Chrom wird durch den oxydierenden Angriff zum überwiegenden Teil oxydiert und wirkt passi­vierend. Die gebildeten Cr₂O₃-Partikel fallen sukzessive aus dem aufgelockerten Verband mechanisch heraus (durch Pfeil angedeutet). Schliesslich wird die an Chrom verarmte und an Nickel angereicherte, gegenüber den Nachbargebieten sich elektrochemisch negativ verhaltende Diffusionszone 9 der Oberflächenschutzschicht 7 bevorzugt angegriffen, indem das Chrom oxydiert wird und als Cr₂O₃ mechanisch heraus­fällt (durch Pfeil angedeutet).The surface protection layer 7 in turn is composed of an originally unchanged part 8 and a diffusion zone 9. The latter generally forms after the protective layer 7 has been applied by diffusion annealing in production, but at the latest when the high temperatures in operation are reached. It is usually characterized by a depletion of chromium and an enrichment in nickel. In the electrolyte 2, the main ions present in the present example (H⁺; Ni²⁺; Co²⁺, NO $\genfrac{}{}{0ex}{}{\text{2nd}}{\text{3rd}}$

⁻) Indicated. The electrochemical attack is first carried out on the surface of the protective layer 7 by NO $\genfrac{}{}{0ex}{}{\text{2nd}}{\text{3rd}}$

⁻ Ions, which mainly release the nickel (through with NO $\genfrac{}{}{0ex}{}{\text{2nd}}{\text{3rd}}$

⁻ And Ni²⁺ marked arrows). This loosens the protective layer 7, which is indicated by the formation of the pores 10. In this way, the attack of the electrolyte can be carried forward deeper inside the protective layer 7. The chromium is mainly oxidized by the oxidizing attack and has a passivating effect. The Cr₂O₃ particles formed fall out successively mechanically from the loosened dressing (indicated by an arrow). Finally, the chromium-depleted and nickel-enriched diffusion zone 9 of the surface protection layer 7, which is electrochemically negative compared to the neighboring regions, is preferably attacked by oxidizing the chromium and mechanically falling out as Cr₂O₃ (indicated by the arrow).

2 shows the highly schematic course of the Cr and Ni content in the surface protective layer and the underlying zone of the base body. The abscissa x plots the depth, measured from the surface, in m, the abscissa gives the Cr or Ni content in% by weight again. 4 is the most electropositive (indicated by ++) behavior. 7 is the surface protective layer, the originally unchanged part 8 of which is electropositive under the conditions of electrolysis, but less high than the base body 4 (indicated by +).

The diffusion zone 9 of the surface protective layer 7 sets itself electronegatively with respect to the regions adjacent to it (indicated by -). Curve "a" shows the course of the chromium content, curve "b" that of the nickel content as a function of depth x. The values are highly schematic mean values of numerous samples. The course can assume different values quantitatively, but always shows the same picture of the Cr depletion and the Ni enrichment in the diffusion zone 9.

3 shows a flow diagram in the form of a block diagram of a possible embodiment of the method. The diagram is self-explanatory and requires no further explanation.

Electrolytic separation processes are based on the difference in the separation or dissolution potential of the components and / or phases involved. In the present case, the potentials of the base body (substrate) 4 and the surface protective layer 7 are normally close together, since they are each nickel alloys with chromium contents that do not differ significantly from one another. At first glance, it therefore seems almost impossible that the protective layer 7 can be detached without simultaneously attacking the base body 4, since the ions are the same. However, it could be shown that the thermal treatment of coated components, even with very related alloys for the protective layer and base body, causes significant differences in concentration and potential due to diffusion. Interdiffusion forms an intermediate layer (diffusion zone 9) which (in an oxidizing electrolysis bath) assumes a negative electrochemical potential with respect to its surroundings and is therefore more easily attacked and detached.

Example 1:

There was a gas turbine blade provided with a surface protection layer and partially damaged at the head end by erosion of the following dimensions of the airfoil: Length = 175 mm Largest width = 90 mm Greatest thickness = 23 mm Profile height = 28 mm

The core material of the gas turbine blade consisted of a nickel-based wrought superalloy with the trade name Nimonic 80A with the following composition: Cr = 19.5% by weight Al = 1.4% by weight Ti = 2.4% by weight Zr = 0.06% by weight Mn = 0.30% by weight Si = 0.30% by weight B = 0.003% by weight C = 0.06% by weight Ni = rest

The surface protective layer with a thickness of 100 to 150 µm had been applied to the core material by plasma spraying and had the following composition: Cr = 17% by weight Si = 4.5% by weight Fe = 4.5% by weight B = 3.5% by weight Ni = rest

The gas turbine blade was cleaned by placing it in a solution of 20% NaOH at a temperature of 100 ° C. for 2 hours, rinsing it and treating it again in concentrated HCl. Then the shovel was brushed with a steel brush.

After cleaning, the bucket was activated. For this purpose, it was again placed in 20% NaOH and then placed in concentrated HCl for 2 h.

The cleaned and activated blade was hung as an anode in an electrolysis bath. The electrolyte had the following composition: 30 parts concentrated HNO₃ 2 parts Ni (NO₃) ₂ Part 1 Co (NO₃) ₂ 70 parts H₂O

A sheet made of corrosion-resistant 18 Cr / 8 Ni steel served as the cathode.

Electrolysis was then carried out under a cell voltage of 1000 mV at an anodic current density of 0.2 A / dm 2 for a period of 144 h. The bath temperature was 25 ° C. After this treatment, the scoop was removed from the bath, rinsed, brushed and dried.

Example 2:

A gas turbine blade that has worn out over a large part of its surface protective layer of the airfoil and has the dimensions: Length = 180 mm Largest width = 93 mm Greatest thickness = 22 mm Profile height = 29 mm was subjected to electrolytic treatment to remove the remaining protective layer. The core material had the trade name IN 939 from INCO, was a nickel-based casting superalloy and had the following composition: Cr = 22.4% by weight Co = 19.0% by weight Ta = 1.4% by weight Nb = 1.0% by weight Al = 1.9% by weight Ti = 3.7% by weight Zr = 0.1% by weight C = 0.15% by weight Ni = rest

The approx. 120 µm average surface protection layer had the following composition: Cr = 49% by weight Si = 6% by weight Fe = 2% by weight Ni = rest

First, the gas turbine blade was cleaned, brushed and activated in accordance with Example 1. Then the shovel was hung as an anode in an electrolysis bath. The electrolyte had the following composition: 10 parts concentrated HNO₃ 5 parts AgNO₃ 90 parts H₂O

A sheet made of corrosion-resistant Cr-Ni steel served as the cathode. The electrolytic detachment of the surface protection layer was carried out under a cell voltage of 1100 mV with an anodic current density of 0.2 A / dm 2 for 120 h. The bath temperature was 20 ° C.

Example 3:

A gas turbine blade provided with a surface protective layer and severely damaged at the head end had to be freed of its protective layer before it was repaired. The dimensions of the airfoil were the same as in example 1. The core material of the airfoil consisted of a nickel-based casting superalloy with the trade name IN 738 from INCO with the following composition: Cr = 16.0% by weight Co = 8.5% by weight Mo = 1.75% by weight W = 2.6% by weight Ta = 1.75% by weight Nb = 0.9% by weight Al = 3.4% by weight Ti = 3.4% by weight Zr = 0.1% by weight B = 0.01% by weight C = 0.11% by weight Ni = rest

The protective layer had an average thickness of 100 μm and had the following composition: Cr = 20% by weight Fe = 2% by weight B = 3% by weight Ni = rest

The gas turbine blade was cleaned and activated according to example 1. Then she was put in an electrochemical cell given and subjected to an electrolysis process. The electrolyte had the following composition: 20 parts CrO₃ 80 parts H₂O

As in Example 1, a sheet made of corrosion-resistant 18/8 steel was used as the cathode. The cell voltage was 1050 mV, the current density at the anode was 0.2 A / dm². Electrolysis was carried out at a bath temperature of 22 ° C. for 140 h.

Example 4:

The core material of a gas turbine blade with the blade dimensions according to Example 2 consisted of a nickel-based wrought superalloy with the trade name IN 105 from INCO with the following composition: Cr = 13.5% by weight Co = 18% by weight Al = 4.2% by weight Mo = 4.5% by weight Ti = 0.9% by weight Mn = 1% by weight Si = 1% by weight C = 0.2% by weight Ni = rest

The protective layer had an average thickness of 140 μm and had the following composition: Cr = 10% by weight Si = 6% by weight Fe = 4% by weight Co = 20% by weight Ni = rest

After cleaning and activation of the component according to Example 1, the latter was suspended as an anode in an electrolysis bath. The electrolyte had the following composition: 10 parts H₂SO₄ 10 parts Na₂S₂O₈ 80 parts H₂O

A sheet made of corrosion-resistant 18 Cr / 8 Ni steel served as the cathode.
It was electrolyzed under a cell voltage of 1100 mV with an anodic current density of 0.18 A / dm² for 150 h. Bath temperature 24 ° C. After the treatment, the component was rinsed, brushed and dried in the usual way.

Example 5:

A gas turbine blade provided with a surface protection layer and partially damaged by combined erosion and corrosion was first cleaned and activated according to Example 1. The blade had the same dimensions as in Example 1. The core material consisted of a nickel-based cast superalloy with the trade name IN 738. Composition see above! The protective layer had a thickness of 150 μm and had the same composition as that in Example 1.
After the component had been cleaned and activated according to Example 1, it was suspended in an electrolysis bath as an anode. The electrolyte had the following composition: 30 parts HNO₃ 70 parts H₂O 10 g / l AgNO₃ 20 g / l NH₄HF₂

A sheet of corrosion-resistant 18/8 steel served as the cathode. Now was under a cell voltage of 1100 mV electrolyzed at an anodic current density of 0.2 A / dm². Every 20 min the cell voltage was increased to 2800 mV for a period of 15 seconds (additional overvoltage of 1700 mV based on the stationary value of the cell). This led to a more rapid removal of the poorly soluble oxides from the respective active surface of the remaining protective layer. In this way, new electrolyte was periodically brought to the surface. After a total operating time of 60 hours, the surface protective layer had been completely removed without the base body having been attacked. With this method of the pulsed course of the cell voltage, the time for the detachment of the protective layer can thus be reduced by 40 to 70%.

The invention is not restricted to the exemplary embodiments. The method relates specifically to the electrolytic detachment of surface protective layers with a high Cr content and with a high Ni or Co content or at the same time a high Ni and Co content. It is therefore a matter of high-chromium nickel or cobalt-based alloys or those based on a nickel / cobalt mixture. Activation is carried out by 20% NaOH and subsequent immersion in concentrated HCl for 2 h at 40 ° C. The component is then placed as an anode in an electrolyte that contains oxygen-releasing, oxidizing components. There it is subjected to electrolysis until the surface protective layer has completely dissolved and fallen off. The surface protective layer is optionally pretreated by grinding and / or sand or shot peening before electrolysis. In stubborn cases, pulsed cell voltage is used. The stationary cell voltage is intermittent at intervals of 10 to 30 min. over a period of 5 to 10 seconds, an additional overvoltage of 1500 to 2000 mV is superimposed on the cell voltage.

Claims (7)

1. A method for the electrolytic detachment of a surface protection layer (7) having a high Cr and Ni and / or Co content from the base body (4) of a component consisting of a nickel or cobalt-based superalloy, characterized in that it has a protective layer (7) the coated component is first immersed in a solution of 20% NaOH and then immersed in a solution of concentrated HCl at 40 ° C. for 2 h so that the component with its activated protective layer (7) acts as an anode in an oxygen-donating, brought electrolyte (2) containing oxidizing constituents and is subjected to the electrolysis until the protective layer (7) has completely dissolved and fallen off. 2. The method according to claim 1, characterized in that the electrolyte (2) has the following composition: 30 parts concentrated HNO₃ 2 parts Ni (NO₃) ₂ Part 1 Co (NO₃) ₂ 70 parts H₂O. 3. The method according to claim 1, characterized in that the protective layer (7) is pretreated by grinding and / or sand or shot peening before immersion in the electrolyte (2) and that the latter has the following composition: 10 parts concentrated HNO₃ 5 parts AgNO₃ 90 parts H₂O. 4. The method according to claim 1, characterized in that the electrolyte (2) has the following composition: 20 parts CrO₃ 80 parts H₂O. 5. The method according to claim 1, characterized in that the electrolyte (2) has the following composition: 10 parts H₂SO₄ 10 parts Na₂S₂O₈ 80 parts H₂O. 6. The method according to claim 1, characterized in that the electrolyte (2) has the following composition: 30 parts HNO₃ 70 parts H₂O 10 g / l AgNO₃ 20 g / l NH₄HF₂. 7. The method according to claim 1, characterized in that an additional overvoltage of 1500 to 2000 mV is superimposed during the electrolysis process of the stationary cell voltage intermittently at intervals of 10 to 30 minutes for a respective period of 5 to 10 seconds.

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