GB2059440A - Treatment of refractory substrate prior to electrodepositing metal - Google Patents

Abstract

Articles suitable for use at high temperatures and/or in oxidising environments are produced by applying to the substrate a first sensitising metallic deposit and then applying a further metallic deposit by electrodeposition from either a fused salt electrolyte or an aqueous electrolyte.

Description

SPECIFICATION Composite articles This invention relates to the production of metal/refractory composite articles, and in particular provides a novel process for applying a metallic layer or deposit onto an electrically non conducting refractory substrate, the resulting article being suitable for use at high temperatures and/or in oxidising environments such as those obtaining in handling molten glass.

It is well known in the art of glass manufacture to use apparatus comprising a molybdenum core carrying a layer in the form of a coating or cladding of platinum. Apparatus constructed from such material has a relatively short service life due to oxidation in molten glass, however, unless care is taken during apparatus manufacture to exclude air from the molybdenum/platinum interface. This may be at least partially achieved by the application of an intermediate barrier layer, for example of alumina.

Certain components used in glass manufacture have complex shapes and it has proved difficult to apply a coating or cladding of platinum to cores for such components with sufficient integrity to exclude air to an acceptable standard.

It is therefore one object of the invention to provide a novel process for applying a layer of platinum to an electrically non-conducting refractory substrate. This would be of significant benefit in the manufacture of components having complex shape and for use in the manufacture of glass but would also be of benefit in applying coatings of platinum or other metals to refractory substrates having any shape, whether complex or not. It is therefore a broader object of the invention to provide a process for applying a metallic layer to an electrically non-conducting refractory substrate, the resulting article being particularly suitable for use at high temperatures, typically those obtaining in handling molten glass.

By "metallic" we mean to include both metals and alloys.

We have now found that a metallic layer may be applied to an electrically non-conducting refractory substrate by first applying a thin sensitising layer of metal to the substrate and then applying a further layer by electrodeposition.

According to the invention, therefore, a process for applying a metallic layer to an electrically nonconducting refractory substrate comprises applying to the substrate a first sensitising metallic deposit and then applying a further metallic deposit by electrodeposition.

The further metallic deposit may be applied from a fused salt electrolyte or from an aqueous electrolyte.

The technique is especially useful for the application of layers of platinum but other metals of the platinum group, for example iridium and ruthenium, or base or other precious metals which can be electrodeposited, such as aluminium, or alloys, may be applied by the process. In general, fused salt electrodeposition gives thick (greater than about 25 yam), pore-free deposits and aqueous electrodeposition gives less thick deposits and the technique is especially applicable for depositing platinum on molybdenum. The sensitising metal or alloy need not, in order to achieve an electro deposit of the required thickness, be the same as the electrodeposited metal or alloy, but it is preferred in order to provide the required high temperature oxidation resistance.Alternatively, an alloy of the metal subsequently to be electrodeposited, e.g. of Rh/Pt where the electrodeposited layer is Pt, may be applied as the sensitising deposit.

The sensitising deposit may be applied by known techniques, for example, by reducing a suitable salt of a metal or by applying the metal by flame-spraying or by applying a coating of a metal-containing ink.

The sensitising deposit must necessarily be capable of withstanding high temperatures encountered in the subsequent electrodeposition step, particularly when using a fused salt electrolyte. One preferred method is to apply a dispersion of one or more metals in an alcohol, the metal being prepared by reduction of an oxide of the metal in a solution containing an alcohol and/or one or more aldehydes. An example of such a technique, specifically for the application of a deposit of platinum black, is described in our British patent number 1,147,563, in which a dispersion of platinum oxide (Adam's catalyst) in a C2-Cs aliphatic alcohol is reduced with formaldehyde to platinum black which, when applied to a substrate and dried, coalesces and forms a coherent and adherent metallic film.

However, other methods may be used provided that the resulting metallisation provides a deposit which contains a minimum amount of impurities, is firmly bonded to the refractory substrate and is sufficiently electrically conductive to render it suitable for electrodeposition before it is dissolved by the electrolyte. Two or more applications may be made to build up a sufficient thickness and, depending on the precise method of applying the sensitising deposit, a firing or baking step may follow one or each of the applications. Suitable deposit thicknesses are 0.1 to 1 5 cm, preferably 1 to 5 ym, for example 2.5 ym.

The substrate material which it is preferred to use, when making articles for use in the manufacture of glass, comprises molybdenum carrying a layer - that is, a barrier layer - of alumina. We prefer to apply the alumina by a flamespraying technique but this is not essential for satisfactory performance of the process of the invention, having, in fact, a more significant effect of the service performance of the finished component. Proprietary alumina coatings may be used instead or, indeed, for purposes other than making components for use in the manufacture of glass, other refractories altogether may be used as the substrate material. Examples of such refractory materials include zirconia, thoria, beryllia, magnesia and compounds thereof.

Fused salt electrolyes are generally based on a cyanide system, for example a eutectic mixture of salts comprising 53 wt % sodium cyanide and 47 wt % potassium cyanide, which has a melting temperature of about 5200 C. Alternatively, sodium cyanide alone, or mixed with potassium cyanate, may be used. Non-cyanide systems include, for example, a lithium chloride/potassium chloride mixture, which melts at 4500 C. The metal(s) to be electrodeposited are added to the electrolyte either as salts or as consumable anodes. Examples of salts for addition of platinum includes K2PtCI6, Pt(CN)2 and Na2Pt(CN)4 and for addition of iridium include K3lrCl6 and K3lr(CN),.

Mixtures of salts may be added for alloy deposition. The electrodeposition temperature influences the ductility and smoothness of the resulting deposits and control of deposit thickness may be achieved by varying the current density and time. Deposits may be of small (i.e., electroplated) or large (i.e. electroformed) thickness.

Aquous electrolytes comprise aqueous solutions of a salt or salts of the metal(s) to be deposited, with various further additions to the bath as required. One suitable electrolyte comprises Na2Pt(OH)6 at a concentration of approximately 1 5 g.lr1 Pt, to which is added sodium hydroxide and optionally sodium oxalate to relieve stresses in the deposit.Such a bath is typically used at a temperature of 700C and a current density of 0.75A.dm to give a plating rate of 5,um.hr-'. Another suitable electrolyte comprises an aqueous solution of a platinum dinitrosulphato salt H2Pt(NO2)2SO4, which has a pH of 1 .5-2. At a concentration of 5 g.l.-' Pt, a temperature of 30-350C and a current density of 0.5 A.dm-2, a plating rate of 2.5 Mm.hr-' is obtained, and at a concentration of 1 5 g.l.-' Pt, a temperature of 600C and a current density of 2 A.dm-2, a plating rate of 5 ym.hr-' is obtained.

Other known aqueous plating baths may be used to apply other metals such as rhodium, pailadium, silver and gold.

The process according to the invention will now be described by way of example, in which is described the application of platinum from a fused salt electrolyte to a molybdenum rod coated with a barrier layer of alumina, and the application of platinum from an aqueous electrolyte to alumina tubes.

EXAMPLE 1 Samples of a molybdenum rod were coated.

with a film of alumina by a flame-spraying technique to an average thickness of 200,us, the lower and upper limits being 80 ,um and 320 Mm respectively. The rods were then coated with a sensitising layer of platinum which varied in thickness between 0.1 ym and 10 ym. Fused salt electrodeposition of platinum was then carried out at a current density of 1 A.dm-2 by suspending the rods from platinum wire in an electrolyte comprising a eutectic mixture of sodium cyanide and potassium cyanide at a temperature of 5200 C, the platinum being added initially as K2Pt(CN)4.Nominal deposit thicknesses of 25 and 50 yam were applied to similar specimens some of which were then subjected to heat treatment, either at 1 4000C for 48 hours in vacuum or at 11 000C for 2 hours in air, to test the integrity of the platinum coating and the resistance to oxidation of the core.

Examination of the samples indicated that the platinum electrodeposited layer was uniform, continuous and adherent and was unaffected by heat treatment. Microprobe analysis revealed the presence of only the expected elements with no evidence of residues from the deposition process.

EXAMPLE 2 Alumina tubes were cleaned thoroughly, fired at 1 2000C for 2 hours, allowed to cool down slowly, washed in methylated spirit and dried in hot air. They were then coated with a sensitising layer of platinum/rhodium alloy from a dispersion in an aliphatic alcohol followed by drying in a current of hot air and then at 9500C for half an hour, the process being repeated 8 times, the sensitising deposit of Pt/Rh having an average resistance of 600 ohms. The sensitised tubes were then plated from an aqueous solution of Na2Pt(OH)6 (1 5g) plus NaOH (5g) made up to 1 litre. Current density was 0.75 A.dm-2, plating time was half an hour, and the final resistance of the coating was approximately 2 ohms at room temperature.

Although the invention has been described by way of example with reference to the production of a platinum-coated alumina substrate, other applications are possible. For example, the invention can be used to apply a layer of platinum to an alumina chip or cylinder in the production of resistance thermometer elements comprising a tortile electrically-conducting path of platinum applied to an insulating substrate. The tortile of meandering pattern may be achieved either by applying a layer of platinum over a substantial surface portion of the suitable substrate and then cutting, for example laser-cutting, platinum away from certain areas to leave the desired pattern in relief or, alternatively, the sensitising layer may be applied through a mask in the desired pattern so that subsequent fused salt deposition applies platinum directly in the desired pattern. A further use of the invention is in the production of furnace elements, in which a filamentary deposit or iridium, for example, is applied to a refractory substrate and further refractory material is then applied over the iridium effectively to enclose, with substantial elimination of air, the filament of iridium in the refractory material. The filament may be used as a furnace element in either the solid or liquid state. Yet a further use of the invention is in the production of crucibles comprising iridum, for example, bounded to a refractory support.

Claims (14)

1. A process for applying a metallic layer to an electrically non-conducting refractory substrate comprising applying to the substrate a first sensitising metallic deposit and then applying a further metallic deposit by electrodeposition from a fused salt electrolyte or from an aqueous electrolyte.

2. A process according to claim 1 in which the metallic layer comprises a metal of the platinum group or other precious metal or an alloy thereof.

3. A process according to claim 2 in which the platinum group metal is platinum, iridium or rhodium.

4. A process according to claim 1 in which the sensitising and further metallic deposits comprise the same metal or alloy.

5. A process according to claim 1 in which the sensitising deposit comprises an alloy and the further deposit comprises a metal, the alloy including the metal of the further deposit as a constituent.

6. A process according to claim 5 in which the sensitising deposit comprises an alloy of platinum and rhodium and the further deposit comprises platinum.

7. A process according to claim 1 in which the substrate comprises alumina.

8. A process according to claim 7 in which the alumina is applied as a layer to an ultimate support comprising molybdenum.

9. A process according to claim 1 in which the sensitising metallic deposit is applied to the substrate by reducing a dispersion of an oxide of the metal in an alcohol, applying it to the substrate and drying.

10. A process according to claim 1 in which the sensitising metallic deposit is applied to the substrate by flame spraying or by applying a coating of a metal-containing ink.

11. A process according to claim 1,9 or 10 in which the sensitising metallic deposit has a thickness of from 0.1 to 15 ym.

12. A process according to claim 1,9 or 10 in which the sensitising metallic deposit has a thickness of from 1 to 5 Mm.

13. A process according to claim 1 in which the electrolyte is a fused salt electrolyte comprising an eutectic mixture of sodium cyanide and potassium cyanide or sodium cyanide alone or mixed with potassium cyanate and the metal or metals to be electrodeposited is/are added to the electrolyte either as salts or as consumable anodes.

14. A process according to claim 1 in which the electrolyte is an aqueous electrolyte comprising an aqueous solution of Na2Pt(OH)6 or H2Pt(NO2)2SO4.

1 5. A process as hereinbefore described with reference to the Examples.

1 6. A metal/refractory composite article when prepared by the method of any of claims 1 to 1 5 and suitable for use at high temperatures and/or in oxidising environments.