Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D15/00—Electrolytic or electrophoretic production of coatings containing embedded materials, e.g. particles, whiskers, wires
  • C25D15/02—Combined electrolytic and electrophoretic processes with charged materials

Definitions

• the present invention relates to a composite electrodeposited coating comprising a metal matrix electrodeposited from a plating bath, the matrix incorporating particles deposited simultaneously from the bath in which the particles are substantially insoluble.
• US 3057048 (Hirakis) describes a method of protecting niobium in a high temperature oxidising environment by electro-depositing a coating comprising a matrix of iron chromium nickel or cobalt incorporating various solid particles for example alumina, thoria, silicon, aluminium, chromium and boron.
• the coating is found to have a particulate content of from 5 to 20% by volume.
• a composite electrodeposited coating is characterised in that the particles comprise a combination of particles of or containing chromium, particles of or containing aluminium and particles of an oxide of a rare earth metal and/or an oxide of zirconium, titanium or hafnium.
• the particles other than the oxide particles may comprise pure chromium powder and pure aluminium powder, or an alloy of chromium and aluminium in powder form, or an alloy of chromium and aluminium in powder form mixed with either both or one of pure aluminium or chromium, or one further possibility is to have a small quantity of a rare earth metal or a Group IV metal alloyed with either chromium or aluminium or both.
• the metal matrix comprises nickel or cobalt and the coating may be used to coat any suitable substrate for example steel, high temperature creep-resistant nickel alloys, nonferrous and light alloys and non-metallic substances with an outer conductive layer.
• the oxide present in the coating preferably comprises "lime-stabilized" zirconia.
• the oxide may constitute from 0.01 to 5% of the coating, preferably about 2% by weight, and may be present in the size range up to 10 pm.
• the chromium present in the coating may comprise up to 40% by weight of the coating, preferably comprising 10 to 20%, for example 15%.
• the aluminium present in the coating may comprise up to 25% by weight of the coating and preferably comprises 10 to 20%.
• the particles may be up to 10 pm in diameter and preferably fall largely within the range of 1 to 2 um.
• a method of forming a composite coating is characterised in that the particles comprise a combination of particles of or containing chromium, particles of or containing aluminium, and particles of an oxide of a rare earth metal and/or an oxide of zirconium, titanium or hafnium.
• the coating may be further heat treated, either in use or for example by being held at 1000°C for 4 hours.
• a metal atom diffuses outwards through the oxide it may leave behind a vacancy.
• the vacancies would normally coalesce at the interface between the metal and oxide to form voids such that the oxide would only be attached to the parent metal at a few places.
• the oxide formed might have a greater volume than the original metal from which it has been formed and so a compressive stress would develop in the growing oxide layer. This might give rise to a complicated stress situation at the oxide-metal interface which is believed to consist mainly of shear stresses and also transverse stress acting to pull the oxide away from the metal. The net result of these stresses coupled with the lack of coherency at the interface would be that the oxide would spall off and expose fresh metal to the hot gas. This process would repeat itself and so the metal would be progressively lost.
• a problem which may arise if oxide spallation does not occur is that the oxide simply gets thicker as time goes on and the component gets weaker as the cross section of the metal decreases.
• One method by which these problems may be overcome is to reduce simultaneously the rates at which the oxygen and metal diffuse through the oxide and to provide a means by which the vacancies may be annihilated and so prevent void formation. It is in this way that the inclusion of the oxide particles, which are both thermodynamically stable and chemically stable in an oxidising atmosphere, may greatly improve the adhesion of the aluminium oxide to the matrix metal at the surface of the coating. It is believed that the effect is achieved in three ways, either singly or in combination.
• the first of these is the “stress relief” mechanism in which the presence of oxide particles at the coating's surface provides “dead spots” over which the developing oxide film can grow laterally thereby relieving any stresses in the film.
• the second is the “vacancy sink” mechanism in which the vacancies left by aluminium atoms diffusing to the surface to react with oxygen are at least partially filled by oxide molecules so that the oxide particles undergo a rearrangement to occupy the voids which effectively "diffuse” through the matrix.
• the third is simply a reduction in the diffusion coefficients through the growing surface oxide layer of either the metal ions or the oxygen atoms or both.
• a method of forming a composite coating is characterised in that the particles comprise a combination of particles of or containing chromium and particles of an oxide of a rear earth metal and/or oxide of zirconium titanium or hafnium, and by subsequently aluminising the coating.
• the particles also include an aluminium-containing substance and, preferably, the aluminising step comprises a pack-aluminising process.
• the material used may be aluminium powder, or a mixture of aluminium and aluminium oxide.
• the aluminium is in the form of an alloy with chromium.
• the pack-aluminising is carried out at a high temperature.
• the aluminium is believed to diffuse into the surface of the electrodeposited coating and thus form a bond.
• the coating may be subsequently heat treated, for example by being held at 1000°C for about 4 hours, or may be heat treated in use.
• the coating process may involve an electrolytic or an electroless method and may be carried out using the apparatus and operating conditions described in the Applicants' British Patents Nos. 1 218 179, 1 224 166, 1 329 081 and 1 347 184.
• the panel to be coated was given a pretreatment comprising immersion in a cyanide cleaner for two minutes followed by a water rinse etching by immersion for 30 seconds in a 50% sulphuric acid followed by a water rinse, and a nickel strike by plating in a nickel bath for three minutes at a current density of 3.9 amps per square decimetre.
• the panel was secured in a plating barrel and connected to a cathode contact. 40 grams per litre of barrel capacity of lime-stabilised zirconia powder and 600 grams per litre of barrel capacity of chromium/aluminium alloy powder, both with a mean particle size of to 5 pm, were added to the barrel and the opening in the barrel through which the panel to be coated and the powder were admitted was closed.
• the barrel was then completely submerged in the solution in the tank and was rotated at three revolutions per minute while composite plating took place at a voltage of between 2.5 and 3 volts with a current density of approximately 2.7 amps per square decimetre.
• the solution temperature was maintained at 50°C and the solution had a pH of between 4.5 and 5.
• plating was stopped.
• the panel was then held at 1000°C for 4 hours, cooled and examined. It was found that the panel had been given a tenacious coating having an even distribution of particles.
• the coating had a particle content of approximately 2% by volume of zirconia and 30% by volume of chromium/ aluminium alloy.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Chemically Coating (AREA)

Applications Claiming Priority (2)

Publications (2)

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ID=10514464

Family Applications (1)

Country Status (7)

Cited By (1)

Families Citing this family (7)

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• 1980
  • 1980-07-02 GB GB8021614A patent/GB2083076B/en not_active Expired
• 1981
  • 1981-07-02 CA CA000381000A patent/CA1176596A/en not_active Expired
  • 1981-07-02 BE BE0/205298A patent/BE889491A/fr not_active IP Right Cessation
  • 1981-07-02 JP JP56502220A patent/JPS643960B2/ja not_active Expired
  • 1981-07-02 EP EP81901799A patent/EP0055272B1/en not_active Expired
  • 1981-07-02 WO PCT/GB1981/000124 patent/WO1982000162A1/en not_active Ceased
  • 1981-07-02 IT IT48809/81A patent/IT1171356B/it active
• 1988
  • 1988-06-01 JP JP63135397A patent/JPH01212800A/ja active Granted

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