Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment

Definitions

• the invention relates to a coating on a metallic substrate, the coating comprising an alloy of an aluminum-comprising component and at least a second component.
• the invention further relates to a method of producing this coating and finally the invention relates to the use of the coating.
• Such coatings may comprise pure aluminum or predeposited aluminum which was transformed into an aluminide coating by interdiffusion of substrate components during a special heat treatment.
• One of the techniques for forming a coating comprising an alloy of aluminum and a second component uses electroplating methods in a non-aqueous electrolyte.
• the non-aqueous solvent may be e.g. toluene and the salts of the metals to be electroplated are added to the s'olvent in the form of bromides and/or chlorides.
• transition metal-rich precipitates at the coating surface lead to catastrophic local oxidations due to the rapid diffusion rate of oxygen through the transition metal oxide scale (see "Coatings for High Temperature Applications, Ed. by E. Lang, Appl. Pub. p. 68).
• Another object of the invention is the provision of a coating which is applicable to complicated as well as simple structures.
• a still further object of the invention is to provide a method for electroplating the above mentioned metals or alloys in a maximum wide range of compositions of the deposit, whereby the desired composition of the deposit as well as the desired structure thereof may be obtained by suitable choice of additives and operating conditions such as plating current density, bath composition and method of bath preparation.
• Coatings of the above mentioned type which have special properties in particular for corrosion protection are characterized in the second component being at least one transition metal of the groups IVB, VB or VIB of the periodic table, and the coating comprising a homogeneous distribution of the second component within the aluminum comprising component in form of a stable solid solution or a metastable, pseudo solid-solution of the transition metal or the compound thereof in the aluminum-comprising component.
• Coatings according to the invention may advantageously be produced in a range of compositions from 2-95 w% of the transition metal or metals and 98-5 w% of aluminum. Of particular advantage are composition between 5-30 w% of titanium and 95 -70 w% of aluminum.
• Coatings of the described structure and composition may be applied to any metallic substrate, but are specially advantageous in connection with so called super-alloys, comprising nickel, cobalt and iron, but also on other metals comprising valve metals which are difficult to electroplate because of their oxide surface layer, or other substrates comprising an intermediary layer comprising nickel.
• the coatings as described above may be submitted to a preliminary or in situ heat treatment (during the beginning of its use) which may be carried out in order to form an interdiffusion layer which extends at least over a part of the depth of the coating.
• a preliminary or in situ heat treatment (during the beginning of its use) which may be carried out in order to form an interdiffusion layer which extends at least over a part of the depth of the coating.
• Such heat treated coatings may comprise a matrix of NiAl, CoAl, FeAl (or e.g. TiAl if the substrate contains Ti) wherein the transition metal is contained as a dispersed phase.
• the final composition of such heat treated coatings may comprise 2-50w% of the transition metal(s) and 98-50w% of the aluminide phase.
• the content thereof may be within 2-20w% and the aluminide phase 98-80w%.
• One of the preferred methods to obtain the above comprises therefore electroplating at least one or an alloy of several transition metals of the groups IVB, VB and VIB of the periodic table of elements or an alloy of one or more of said transition metals with Al at near ambient temperature onto an electrically conductive substrate in a non-aqueous electrolyte, characterized in that the electroplating is carried out in an electrolyte comprising an aromatic hydrocarbon and an Al halide, wherein said transition metal(s) is (are) dissolved in the form of halides of a high oxidation state and in that said transition metal(s) is (are) pre-reduced to a lower oxidation state.
• the structure which may be obtained by one of the methods according to the invention is metastable, since the two components of the coating alone without the influence of the substrate material would segregate when heated up to a certain temperature.
• the definition of "pseudo" solid-solution relates to the fact, that the two alloyed metals would not form a solid solution by simple alloying techniques.
• the coating may be heat treated prior to or during its use, whereby the metastable pseudo-solution is transformed into a stable solid solution of compounds of the coating components and components of the substrate which penetrate the coating by outwards diffusion during the heat treatment.
• the coatings as described above may be used for corrosion protection in aqueous solutions of gaseous environments at temperatures up to 1400°C.
• a preliminary or in situ heat treatment may be carried out in order to form an interdiffusion layer which extends at least over a part of the depth of the coating.
• metastable means that such fine, normally not existing distributions of Ti in Al represent some kind of "frozen” status, which would segregate if e.g.
• the Ti-Al system is present in the form of a coating on a substrate which contains components which interdiffuse with the coating during a suitable heat treatment, whereby a stable solid-solution of the titanium in a matrix of an intermetallic compound of aluminum and the respective substrate component is formed.
• the super fine distribution of the titanium with the aluminum may be obtained by electroplating in a non-aqueous electrolyte.
• Aluminide coatings according to the invention were formed by predeposition of an aluminum-transition metal alloy deposit which in certain cases was deposited on substrates comprising Ni, Co or Fe based alloys. In the latter cases, the predeposition step was followed by an interdiffusion reaction at elevated temperatures. This interdiffusion step may be carried out prior to the use of the coated substrate or during its use in situ.
• the diffusion reaction between the principal component of the substrate and aluminum of the deposit occurs under the solid form.
• the creation of local concentrations of transition metal was kinetically disfavoured by the low diffusion rate in solid phases, by the homogeneous composition of the initial deposits and by the higher solubility of Ni, Co or Fe in the aluminum matrix.
• the transition metal was surrounded by stable intermetallic compounds of NiAl, CoAl or FeAl, and the subsequent segregation of the transition metal dispersions in the coating matrix was thermodynamically unfavourable.
• the final coatings were composed of a matrix of NiAl, CoAl or FeAl intermetallic compounds, containing a uniform homogeneous and stable dispersion of transition metals, with a low concentration varying between 3 and 15% following the composition of the initial deposits. It was found that the presence of a uniform dispersion of transition metals in the aluminide coating in this concentration range improved greatly not only the adherence of the A1 2 0 3 scale and the corrosion resistance towards sulphidisation but also the stability of the coating towards interdiffusion.
• a duplex coating and a modification of the chemical composition of the coating may be envisaged. It is known that the resistance of NiAl is better than that of CoAl or FeAl, and the performance of aluminide coatings produced by conventional/thermal diffusion processes is considerably improved by the presence of a pre-deposited layer of Ni.
• the optimum thickness of the Ni under layer was determined as a function of the composition and the thickness of the top layer of the aluminum-transition metal alloy deposit. For the useful composition range of the latter, the best results were obtained with a thickness of the Ni under layer according to the following equation:
• the Ni under layer could be obtained by usual deposition techniques such as ion plating, plasma spraying, electroless deposition, electrodeposition etc...
• the Ti (IV) species were reduced to Ti (II) by reaction with an excess of about 5g of Al particles, at 60-80°C during 24 hours.
• the electrolyte was placed afterwards in a cylindrical glass electrolysis cell. Two plane Al anodes of dimensions 5.0 x 2.5 x 0.2 cms were used. The agitation was insured by a magnetic stirrer.
• the coated Inconel 738 sample was introduced into a furnace heated at 1000°C, in air. The diffusion treatment lasted 24 hours.
• the composition and the X-ray images of different coating components showed that the coating layer was principally composed of a matrix of NiAl with some CoAl containing a homogenous dispersed phase of TiAl or NiTiAl compounds and some Cr rich precipitates forming a solid solution of the minor constituents in the Ni/Al matrix.
• a TiAl coated sample of Inconel 738 was prepared as in example 1.
• the TiAl deposit composition and thickness were in the range of 20% Ti - 80% Al and 35-40 um.
• the diffusion formation step of the aluminide coating from the TiAl deposit was performed directly under the test conditions.
• the oxidation resistance of the coating was tested under thermal cycling conditions in static air.
• the thermal cycle was defined as follows: 23.5 hours at 1000°C followed by 0.5 hours at room temperature.
• a TiAl coated sample of Nimonic 90 (dimensions 2.5x6.0x0.15 cms) was prepared as in example 1. The deposit thickness and composition were in the range of 35-40 um and 20% Ti-80% Al.
• the coated sample was submitted directly to the hot corrosion conditions simulated by spraying on the sample surface a solution of 0.9 mole/l of Na 2 SO 4 + 0.1 mole/l K 2 SO 4 , in such a way that the dried salt load was in the range of 1.0 to 1.5 mg/cm2.
• the hot corrosion test conditions were as follows:
• the corrosion resistance of the coating was demonstrated by the evolution of the specific weight gain and the microstructure of the sample. The results after 360 hours of exposure are listed in Table 3.

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

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Cited By (8)

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• 1985
  • 1985-11-07 EP EP85810521A patent/EP0184985A3/fr not_active Withdrawn
  • 1985-12-06 JP JP27485385A patent/JPS61179892A/ja active Pending

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