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EP0318886B1 - 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

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 Download PDF

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Publication number
EP0318886B1
EP0318886B1 EP88119777A EP88119777A EP0318886B1 EP 0318886 B1 EP0318886 B1 EP 0318886B1 EP 88119777 A EP88119777 A EP 88119777A EP 88119777 A EP88119777 A EP 88119777A EP 0318886 B1 EP0318886 B1 EP 0318886B1
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Prior art keywords
parts
protective layer
electrolyte
weight
following composition
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German (de)
French (fr)
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EP0318886A1 (en
Inventor
Vladimir Sova
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BBC Brown Boveri AG Switzerland
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BBC Brown Boveri AG Switzerland
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25FPROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F5/00Electrolytic stripping of metallic layers or coatings
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23FNON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F1/00Etching metallic material by chemical means
    • C23F1/44Compositions for etching metallic material from a metallic material substrate of different composition

Definitions

  • 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.
  • 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.
  • 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.
  • 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.
  • coatings of metals Ni, Cr, Zn, Sn, Cu, Cd, Ag
  • nitric acid or nitrate-containing solutions with iodine content as electrolytes, to which organic chlorine compounds are additionally added will.
  • 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).
  • 1 shows a schematic cross section through the active part of an electrolytic cell for carrying out the method.
  • 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.
  • 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.
  • the main ions present in the present example H+; Ni2+; Co2+, NO 2nd 3rd ⁇
  • the electrochemical attack is first carried out on the surface of the protective layer 7 by NO 2nd 3rd ⁇ Ions, which mainly release the nickel (through with NO 2nd 3rd ⁇ And Ni2+ marked arrows). This loosens the protective layer 7, which is indicated by the formation of the pores 10.
  • 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).
  • 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).
  • 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 other values quantitatively, but always shows the same picture of the Cr depletion and the Ni enrichment in the diffusion zone 9.
  • FIG. 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.
  • 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.
  • the protective layer 7 can be detached without simultaneously attacking the base body 4, since the ions are the same.
  • 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.
  • 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 ⁇ m 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.
  • the bucket was activated. For this purpose, it was again placed in 20% NaOH and then placed in concentrated HCl for 2 h.
  • 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.
  • the scoop was removed from the bath, rinsed, brushed and dried.
  • 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 ⁇ m average surface protection layer had the following composition: Cr 49% by weight Si 6% by weight Fe 2% by weight Ni rest
  • a sheet made of corrosion-resistant Cr-Ni steel served as the cathode.
  • the electrolytic detachment of the surface protective layer was carried out under a cell voltage of 1100 mV at an anodic current density of 0.2 A / dm 2 for 120 h.
  • the bath temperature was 20 ° C.
  • 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 Mon 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 ⁇ m and had the following composition: Cr 20% by weight Fe 2% by weight B 3% by weight Ni rest
  • Example 2 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.
  • 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 Mon 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 ⁇ m and had the following composition: Cr 10% by weight Si 6% by weight Fe 4% by weight Co 20% by weight Ni rest
  • 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.
  • a sheet of corrosion-resistant 18/8 steel served as the cathode.
  • 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.

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  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Mechanical Engineering (AREA)
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Description

Technisches GebietTechnical field

Gasturbinen für höchste Ansprüche. Kritisches Bauteil ist die Schaufel, wobei Schutzschichten gegen Erosion, Verschleiss Korrosion und Oxydation bei hohen Temperaturen an Bedeutung gewinnen. Die Schutzschicht hat meist eine geringere Lebensdauer als der Kernwerkstoff der Schaufel, weshalb die Erneuerbarkeit der ersteren mehr und mehr in den Vordergrund rückt.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.

Die Erfindung bezieht sich auf die Weiterentwicklung von Verfahren zur Reparatur, Instandstellung und Erneuerung von durch Erosion, Verschleiss, Korrosion, Oxydation oder mechanische Beschädigung unbrauchbar gewordenen, mit Schutzschichten versehenen Bauteilen thermischer Maschinen. Dabei muss zunächst die alte bestehende Schutzschicht entfernt werden, was grundsätzlich mechanisch oder chemisch erfolgen kann.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.

Die elektrochemische Methode als umgekehrter Vorgang zur galvanischen Beschichung nimmt dabei eine Sonderstellung ein.The electrochemical method as a reverse process to the galvanic coating has a special position.

Insbesondere betrifft sie ein Verfahren zum elektrolytischen Ablösen einer einen hohen Cr- und Ni- und/oder Co-Gehalt aufweisenden Oberflächenschutzschicht vom Grundkörper eines aus einer Nickel- oder Kobaltbasis-Superlegierung bestehenden Bauteils.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.

Stand der TechnikState of the art

Die Entfernung von Schutzschichten auf Substraten aus Superlegierungen wird unter anderem auf chemischem Wege durchgeführt. Das elektrolytische Auflösen ist für derartige Legierungen bisher praktisch nicht angewandt worden. Es sind einige Verfahren bekannt, durch Umkehrung der Methode des galvanischen Auftragens Metalle von ihren Substraten abzulösen. Aus der US-A-2 907 700 ist bekannt, Ueberzüge von Metallen (Ag, Ni, Cd, Zn, In) von einem Plutonium-Substrat elektrolytisch zu entfernen. Als Elektrolyt wird Schwefelsäure oder Natriumphosphatlösung verwendet. Aus der DE-B-21 46 828 ist ein elektrolytisches Verfahren bekannt, um Metallüberzüge (Cr, Au, Cd, Cu, Ag, Zn, Sn, Ni) von rostfreiem Cr/Ni-Stahl abzulösen. Als Elektrolyte werden bromhaltige Lösungen von Nitraten, Azetaten, Chloriden etc. verwendet. Angeblich soll der Angriff auf das Substrat gering sein. Gemäss DE-C-25 27 152 sollen Ueberzüge aus Metallen (Ni, Cr, Zn, Sn, Cu, Cd, Ag) von Stahl elektrolytisch entfernt werden, indem Salpetersäure oder nitrathaltige Lösungen mit Jodgehalt als Elektrolyte benutzt werden, denen zusätzlich organische Chlorverbindungen zugesetzt werden.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 organic chlorine compounds are additionally added will.

Diese bekannten Verfahren, welche auf der genügenden Verschiedenheit des Auflösungspotentials des Metallüberzugs gegenüber demjenigen des Substrats beruht, sind in der vorliegenden Form nicht auf Schutzschichten auf Nickelbasis-Superlegierungen übertragbar. Die enge Verwandtschaft des chemischen Aufbaus zwischen Schutzschicht und Substrat ermöglicht normalerweise ein elektrolytisches Auflösen der ersteren nicht, ohne dass das Substrat gleichzeitig in unzulässiger Weise angegriffen wird. Auch ein Ausweichen auf komplexbildende Zusätze zum Elektrolyten schafft keine Abhilfe.
Es kommt dazu, dass die Bedingungen für das Nichtangreifen des Substrats im Falle von Bauteilen aus einer Superlegierung (Gasturbinenschaufel) viel strenger sind als für irgendwelche andere, z.B. oben genannte Gegenstände. Eine auch nur leicht im Kernwerkstoff veränderte Gasturbinenschaufel wäre in den wenigsten Fällen wieder verwendbar.
Es besteht daher ein starkes Bedürfnis, die obigen Mängel weitgehend zu beseitigen und Wege zur erfolgreichen Anwendung eines elektrolytischen Ablöseverfahrens für auf Nickelbasis- oder Kobaltbasis-Superlegierungen aufgetragene Oberflächenschutzschichten aufzuzeigen.
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-based 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 stripping method for surface protective layers applied on nickel-based or cobalt-based superalloys.

Darstellung der ErfindungPresentation of the invention

Der Erfindung liegt die Aufgabe zugrunde, ein Verfahren zum Ablösen einer auf einer Ni- und/oder Co-Legierung mit hohem Cr-Gehalt basierenden Oberflächenschutzschicht vom Grundkörper eines Bauteils anzugeben, der aus einer chromhaltigen Ni- und/oder Co-Basislegierung besteht. Dabei soll die Oberflächenschicht vollständig entfernt werden, ohne dass der Werkstoff des Grundkörpers angegriffen, abgetragen oder beschädigt oder in seinen chemisch-physikalischen Eigenschaften und in seinem Verhalten bezüglich Verträglichkeit insbesondere beim nachträglichen Wiederaufbringen (Erneuern) einer Oberflächenschutzschicht beeinträchtigt oder verändert wird.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).

Diese Aufgabe wird erfindungsgemäss mit den kennzeichnenden Merkmalen des Anspruchs 1 gelöst.This object is achieved according to the invention with the characterizing features of claim 1.

Weg zur Ausführung der ErfindungWay of carrying out the invention

Die Erfindung wird anhand der nachfolgenden, durch Figuren näher erläuterten Ausführungsbeispiele beschrieben. Dabei zeigt:

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

In Fig. 1 ist ein schematischer Querschnitt durch den aktiven Teil einer Elektrolysezelle zur Durchführung des Verfahrens dargestellt. Die unwesentlichen, am prinzipiellen Verfahrensablauf nicht aktiv beteiligten Teile wie Gefäss, Stromzuführungen, Klemmen, Rühreinrichtungen, Steuergeräte etc. sind der Uebersichtlichkeit halber weggelassen worden. 1 ist die Kathode (in der Regel ein Blech aus korrosionsbeständigem Cr/Ni-Stahl), 2 der Elektrolyt (durch horizontale Striche angedeutet), 3 die aus Grundkörper und Oberflächenschutzschicht bestehende Anode. Der Grundkörper (Substrat) 4 besteht aus einer Nickel- oder Kobaltbasis-Superlegierung, der normalerweise überweigend als unveränderter Teil 5 (Kernwerkstoff) vorliegt. An der Grenze zur Oberflächenschutzschicht 7 befindet sich eine Diffusionszone 6 im Grundkörper 4.1 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.

Die Oberflächenschutzschicht 7 ihrerseits setzt sich aus einem ursprünglich unveränderten Teil 8 und einer Diffusionszone 9 zusammen. Letztere bildet sich in der Regel nach dem Aufbringen der Schutzschicht 7 durch eine Diffusionsglühung in der Fabrikation, spätestens aber beim Erreichen der hohen Temperaturen im Betrieb aus. Sie zeichnet sich in der Regel durch eine Verarmung an Chrom und eine Anreicherung an Nickel aus. Im Elektrolyten 2 sind die im vorliegenden Beispiel in der Hauptsache vorhandenen Ionen (H⁺; Ni²⁺; Co²⁺, NO 2 3

Figure imgb0001
⁻) angedeutet. Der elektrochemische Angriff erfolgt zunächst an der Oberfläche der Schutzschicht 7 durch NO 2 3
Figure imgb0002
⁻-Ionen, welche vor allem das Nickel herauslösen (durch mit NO 2 3
Figure imgb0003
⁻ 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 2nd 3rd
Figure imgb0001
⁻) Indicated. The electrochemical attack is first carried out on the surface of the protective layer 7 by NO 2nd 3rd
Figure imgb0002
⁻ Ions, which mainly release the nickel (through with NO 2nd 3rd
Figure imgb0003
⁻ 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).

Fig. 2 stellt den stark schematisierten Verlauf des Cr- und Ni-Gehalts in der Oberflächenschutzschicht und der darunterliegenden Zone des Grundkörpers dar. Auf der Abszisse x ist die Tiefe, gemessen von der Oberfläche in m aufgetragen, die Abszisse gibt den Cr- bzw. Ni-Gehalt in Gew.-% wieder. 4 ist der sich am stärksten elektropositiv (angedeutet durch ++) verhaltende Grundkörper. 7 ist die Oberflächenschutzschicht, deren ursprünglich unveränderter Teil 8 sich unter den Bedingungen der Elektrolyse elektropositiv, jedoch weniger hoch als der Grundkörper 4 einstellt (angedeutet durch +).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 +).

Die Diffusionszone 9 der Oberflächenschutzschicht 7 stellt sich gegenüber den ihr benachbarten Bereichen elektronegativ ein (angedeutet durch -). Kurve "a" zeigt den Verlauf des Chromgehalts, Kurve "b" denjenigen des Nickelgehalts in Funktion der Tiefe x. Die Werte sind stark schematisierte Mittelwerte von zahlreichen Proben. Der Verlauf kann quantitativ andere Werte annehmen, zeigt aber grundsätzlich stets das gleiche Bild der Cr-Verarmung und der Ni-Anreicherung in der Diffusionszone 9.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 other values quantitatively, but always shows the same picture of the Cr depletion and the Ni enrichment in the diffusion zone 9.

In Fig. 3 ist ein Fliessdiagramm in Form eines Blockschemas einer möglichen Ausführungsart des Verfahrens dargestellt. Das Diagramm erklärt sich von selbst und bedarf keiner weiteren Erläuterungen.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.

Elektrolytische Trennverfahren beruhen auf der Verschiedenheit des Abscheide- bzw. Auflösungspotentials der beteiligten Komponenten und/oder Phasen. Im vorliegenden Fall liegen die Potentiale des Grundkörpers (Substrat) 4 und der Oberflächenschutzschicht 7 normalerweise nahe beisammmen, da es sich je um Nickellegierungen mit nicht wesentlich voneinander abweichenden Chromgehalten handelt. Auf den ersten Blick scheint es deshalb beinahe ausgeschlossen, dass eine Ablösung der Schutzschicht 7 ohne gleichzeitigen Angriff des Grundkörpers 4 möglich sei, da es sich um gleiche Ionen handelt. Es konnte jedoch gezeigt werden, dass durch die thermische Behandlng von beschichteten Bauteilen auch bei sehr verwandten Legierungen für Schutzschicht und Grundkörper durch Diffusion signifikante Konzentrations- und Potentialunterschiede auftreten. Durch Interdiffusion bildet sich eine Zwischenschicht (Diffusionszone 9), welche (in einem oxydierenden Elektrolysebad) gegenüber ihrer Umgebung ein negatives elektrochemisches Potential annimmt und demzufolge leichter angegriffen und abgelöst wird.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.

Ausführungsbeispiel 1:Example 1:

Es lag eine mit einer Oberflächenschutzschicht versehene, an ihrem Kopfende durch Erosion teilweise beschädigte Gasturbinenschauel folgender Abmessungen des Schaufelblattes vor: Länge 175 mm Grösste Breite 90 mm Grösste Dicke 23 mm Profilhöhe 28 mm 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 Greatest width 90 mm Greatest thickness 23 mm Profile height 28 mm

Der Kernwerkstoff der Gasturbinenschaufel bestand aus einer Nickelbasis-Knet-Superlegierung mit dem Handelsnamen Nimonic 80A von folgender Zusammensetzung: Cr 19,5 Gew.-% Al 1,4 Gew.-% Ti 2,4 Gew.-% Zr 0,06 Gew.-% Mn 0,30 Gew.-% Si 0,30 Gew.-% B 0,003 Gew.-% C 0,06 Gew.-% Ni Rest 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

Die Oberflächenschutzschicht von 100 bis 150 µm Dicke war durch Plasmaspritzen auf dem Kernwerkstoff aufgetragen worden und hatte folgende Zusammensetzung: Cr 17 Gew.-% Si 4,5 Gew.-% Fe 4,5 Gew.-% B 3,5 Gew.-% 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

Die Gasturbinenschaufel wurde gereinigt, indem sie bei einer Temperatur von 100 °C während 2 h in eine Lösung von 20% NaOH eingelegt wurde, gespült und nochmals in konzentrierter HCl nachbehandelt wurde. Dann wurde die Schaufel mit einer Stahlbürste gebürstet.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.

Nach der Reinigung wurde die Schaufel aktiviert. Zu diesem Zweck wurde sie nochmals in 20% NaOH gebracht und anschliessend während 2 h in konzentrierte HCl eingelegt.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.

Die gereinigte und aktivierte Schaufel wurde als Anode in ein Elektrolysebad eingehängt. Der Elektrolyt hatte die nachfolgende Zusammensetzung:

30 Teile konzentrierte
HNO₃
 2 Teile
Ni(NO₃)₂
 1 Teil
Co(No₃)₂
70 Teile
H₂O.
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.

Als Kathode diente ein Blech aus korrosionsbeständigem 18 Cr/8 Ni-Stahl.A sheet made of corrosion-resistant 18 Cr / 8 Ni steel served as the cathode.

Nun wurde unter einer Zellenspannung von 1000 mV bei einer anodischen Stromdichte von 0,2 A/dm² während einer Zeit von 144 h elektrolysiert. Die Badtemperatur betrug dabei 25 °C. Nach dieser Behandlung wurde die Schaufel aus dem Bad herausgenommen, gespült, gebürstet und getrocknet.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.

Ausführungsbeispiel 2:Example 2:

Eine auf einem grossen Teil ihrer Oberflächenschutzschicht des Schaufelblattes abgenutzte Gasturbinenschaufel mit den Abmessungen: Länge 180 mm Grösste Breite 93 mm Grösste Dicke 22 mm Profilhöhe 29 mm wurde einer elektrolytischen Behandlung zur Entfernung der verbliebenen Schutzschicht unterworfen. Der Kernwerkstoff hatte den Handelsnamen IN 939 von INCO, stellte eine Nickelbasis-Guss-Superlegierung dar und hatte folgende Zusammensetzung: Cr 22,4 Gew.-% Co 19,0 Gew.-% Ta 1,4 Gew.-% Nb 1,0 Gew.-% Al 1,9 Gew.-% Ti 3,7 Gew.-% Zr 0,1 Gew.-% C 0,15 Gew.-% Ni Rest 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 Greatest 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

Die ca. 120 µm im Durchschnitt messende Oberflächenschutzschicht hatte folgende Zusammensetzung: Cr 49 Gew.-% Si 6 Gew.-% Fe 2 Gew.-% 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

Zunächst wurde die Gasturbinenschaufel gemäss Beispiel 1 gereinigt, gebürstet und aktiviert. Dann wurde die Schaufel als Anode in ein Elektrolysebad eingehängt. Der Elektrolyt hatte folgende Zusammensetzung:

10 Teile konzentrierte
HNO₃
 5 Teile
AgNO₃
90 Teile
H₂O.
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.

Als Kathode diente ein Blech aus korrosionsbeständigem Cr-Ni-Stahl. Die elektrolytische Ablösung der Oberflächenschutzschicht wurde unter einer Zellenspannung von 1100 mV bei einer anodischen Stromdichte von 0,2 A/dm² während 120 h vorgenommen. Die Badtemperatur betrug 20 °C.A sheet made of corrosion-resistant Cr-Ni steel served as the cathode. The electrolytic detachment of the surface protective layer was carried out under a cell voltage of 1100 mV at an anodic current density of 0.2 A / dm 2 for 120 h. The bath temperature was 20 ° C.

Ausführungsbeispiel 3:Example 3:

Eine mit einer Oberflächenschutzschicht versehene, an ihrem Kopfende stark beschädigte Gasturbinenschaufel musste vor ihrer Reparatur zuerst von ihrer Schutzschicht befreit werden. Die Abmessungen des Schaufelblattes waren die gleichen wie bei Beispiel 1. Der Kernwerkstoff der Schaufel bestand aus einer Nickelbasis-Guss-Superlegierung mit dem Handelsnamen IN 738 von INCO mit der folgenden Zusammensetzung: Cr 16,0 Gew.-% Co 8,5 Gew.-% Mo 1,75 Gew.-% W 2,6 Gew.-% Ta 1,75 Gew.-% Nb 0,9 Gew.-% Al 3,4 Gew.-% Ti 3,4 Gew.-% Zr 0,1 Gew.-% B 0,01 Gew.-% C 0,11 Gew.-% Ni Rest 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 Mon 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

Die Schutzschicht wies eine mittlere Dicke von 100 µm auf und hatte folgende Zusammensetzung: Cr 20 Gew.-% Fe 2 Gew.-% B 3 Gew.-% 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

Die Gasturbinenschaufel wurde gemäss Beispiel 1 gereinigt und aktiviert. Dann wurde sie in eine elektrochemische Zelle gegeben und einem Elektrolyseprozess unterworfen. Der Elektrolyt hatte folgende Zusammensetzung:

20 Teile
CrO₃
80 Teile
H₂O
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

Als Kathode diente, wie bei Beispiel 1 ein Blech aus korrosionsbeständigem 18/8-Stahl. Die Zellenspannung betrug 1050 mV, die Stromdichte an der Anode 0,2 A/dm². Bei einer Badtemperatur von 22 °C wurde während 140 h elektrolysiert.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.

Ausführungsbeispiel 4:Example 4:

Der Kernwerkstoff einer Gasturbinenschaufel mit den Schaufelblattabmessungen gemäss Beispiel 2 bestand aus einer Nickelbasis-Knet-Superlegierung mit der Handelsbezeichnung IN 105 von INCO mit der nachfolgenden Zusammensetzung: Cr 13,5 Gew.-% Co 18 Gew.-% Al 4,2 Gew.-% Mo 4,5 Gew.-% Ti 0,9 Gew.-% Mn 1 Gew.-% Si 1 Gew.-% C 0,2 Gew.-% Ni Rest 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 Mon 4.5% by weight Ti 0.9% by weight Mn 1% by weight Si 1% by weight C. 0.2% by weight Ni rest

Die Schutzschicht hatte eine Dicke von durchschnittlich 140 µm und wies folgende Zusammensetzung auf: Cr 10 Gew.-% Si 6 Gew.-% Fe 4 Gew.-% Co 20 Gew.-% 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

Nach der Reinigung und Aktivierung des Bauteils gemäss Beispiel 1 wurde das letztere als Anode in ein Elektrolysebad eingehängt. Der Elektrolyt hatte folgende Zusammensetzung:

10 Teile
H₂SO₄
10 Teile
Na₂S₂O₈
80 Teile
H₂O
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

Als Kathode diente ein Blech aus korrosionsbeständigem 18 Cr/8 Ni-Stahl.
Es wurde unter einer Zellenspannung von 1100 mV mit einer anodischen Stromdichte von 0,18 A/dm² während 150 h elektrolysiert. Badtemperatur 24 °C. Nach der Behandlung wurde das Bauteil in üblicher Weise gespült, gebürstet und getrocknet.
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.

Ausführungsbeispiel 5:Example 5:

Eine mit einer Oberflächenschutzschicht versehene, durch kombinierte Erosion und Korrosion teilweise beschädigte Gasturbinenschaufel wurde gemäss Beispiel 1 zunächst gereinigt und aktiviert. Die Schaufel hatte die gleichen Abmessungen wie in Beispiel 1. Der Kernwerkstoff bestand aus einer Nickelbasis-Guss-Superlegierung mit der Handelsbezeichnung IN 738. Zusammensetzung siehe oben! Die Schutzschicht hatte eine Dicke von 150 µm und entsprach in der Zusammensetzung derjenigen in Beispiel 1.
Nachdem das Bauteil gemäss Beispiel 1 gereinigt und aktiviert worden war, wurde es in ein Elektrolysebad als Anode eingehängt. Der Elektrolyt hatte folgende Zusammensetzung:

30 Teile
HNO₃
70 Teile
H₂O
10 g/l
AgNO₃
20 g/l
NH₄HF₂
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

Als Kathode diente ein Blech aus korrosionsbeständigem 18/8-Stahl. Nun wurde unter einer Zellenspannung von 1100 mV bei einer anodischen Stromdichte von 0,2 A/dm² elektrolysiert. Alle 20 min wurde die Zellenspannung während der Zeitdauer von 15 sec. auf den Wert von 2800 mV erhöht (zusätzliche Ueberspannung von 1700 mV bezogen auf den stationären Wert der Zelle). Dies führte zu einer rascheren Abtragung der schwer löslichen Oxyde von der jeweiligen akitven Oberfläche der noch verbliebenen Schutzschicht. Auf diese Weise wurde periodisch neuer Elektrolyt an die Oberfläche herangeführt. Nach einer totalen Betriebszeit von 60 h war die Oberflächenschutzschicht vollständig abgetragen, ohne dass der Grundkörper angegriffen worden war. Mit diesem Verfahren des gepulsten Verlaufs der Zellenspannung kann somit die Zeit für das Ablösen der Schutzschicht um 40 bis 70% verringert werden.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%.

Die Erfindung ist nicht auf die Ausführungsbeispiele beschränkt. Das Verfahren bezieht sich speziell auf das elektrolytische Ablösen von Oberflächenschutzschichten mit hohem Cr-Gehalt und mit hohem Ni- oder Co-Gehalt oder gleichzeitig hohem Ni- und Co-Gehalt. Es handelt sich also um hochchromhaltige Nickel- oder Kobaltbasis-Legierungen oder solche, die auf einer Nickel/Kobalt-Mischung basieren. Die Aktivierung erfolgt durch 20%ige NaOH und nachfolgendes Einlegen in konzentrierte HCl während 2 h bei 40 °C. Das Bauteil wird hierauf als Anode in einen Elektrolyten gebracht, der Sauerstoff abgebende, oxydierende Bestandteile enthält. Dort wird es bis zum völligen Auflösen und Abfallen der Oberflächenschutzschicht der Elektrolyse unterworfen. Die Oberflächenschutzschicht wird gegebenenfalls vor dem Elektrolysieren durch Schleifen und/oder Sand- oder Kugelstrahlen vorbehandelt. In hartnäckigen Fällen wird mit gepulster Zellenspannung gearbeitet. Der stationären Zellenspannung wird intermittierend in Intervallen von 10 bis 30 min. während einer jeweiligen Zeitdauer von 5 bis 10 sec eine zur Zellenspannung zusätzliche Ueberspannung von 1500 bis 2000 mV überlagert.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. Process for electrolytically stripping a surface protection layer (7) having a high Cr and Ni and/or Co content from the parent body (4) of a structural member consisting of a nickel-based or cobalt-based superalloy, characterised in that the structural member coated with a protective layer (7) is first immersed in a solution of 20%-strength NaOH and then at 40°C for 2h in a similar one of concentrated HCl for the purpose of activation, and in that the structural member with its activated protective layer (7) is introduced as anode into an aqueous electrolyte (2), which additionally contains an oxidising oxo acid and, optionally, salts of such an acid, and is subjected to electrolysis until the protective layer (7) completely dissolves and drops off.
  2. Process according to Claim 1, characterised in that the electrolyte (2) has the following composition:
    30 parts of concentrated   HNO₃
     2 parts   Ni(NO₃)₂
     1 part   Co(NO₃)₂
    70 parts   H₂O.
  3. Process according to Claim 1, characterised in that the protective layer (7) is pretreated by grinding and/or sand or shot blasting before immersion in the electrolyte (2), and in that the latter has the following composition:
    10 parts of concentrated   HNO₃
     5 parts   AgNO₃
    90 parts   H₂O.
  4. Process according to Claim 1, characterised in that the electrolyte (2) has the following composition:
    20 parts   CrO₃
    80 parts   H₂O.
  5. Process according to Claim 1, characterised in that the electrolyte (2) has the following composition:
    10 parts   H₂SO₄
    10 parts   Na₂S₂O₈
    80 parts   H₂O.
  6. Process according to Claim 1, characterised in that the electrolyte (2) has the following composition:
    30 parts   HNO₃
    70 parts   H₂O
    10 g/l   AgNO₃
    20 g/l   NH₄HF₂.
  7. Process according to Claim 1, characterised in that, during the electrolysis process, an additional overvoltage of 1500 to 2000 mV is intermittently superimposed on the steady cell voltage at intervals of 10 to 30 min during a respective time period of 5 to 10 sec.
EP88119777A 1987-12-01 1988-11-28 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 Expired - Lifetime EP0318886B1 (en)

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CH467387 1987-12-01
CH4673/87 1987-12-01

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