METAL PROTECTION

Abstract

Metals for use in glass-making furnaces and which are susceptible to oxidation at furnace operating temperatures, especially iridium or molybdenum, are protected by applying at least (200) microns thickness of a coating formed of metal oxide particles in a metal oxide matrix. Oxidation, measured by weight loss, is significantly reduced.

Description

The present invention concerns improved metal protection, and more especially concerns the protection of metals from oxidation at elevated temperatures.

The quality of glass products is markedly affected by the characteristics of the vessels in which the glass is originally melted. High volume, relatively low quality glass is easily obtained by melting in ceramic refractory vessels and furnaces, with the best of these providing excellent resistance to the corrosive nature of the molten glass, and hence good longevity. This is important since the failure of the refractories is usually the instigator of expensive furnace rebuilds, and anything which can inhibit this corrosion is potentially valuable. Platinum claddings and other forms of platinum coatings have been introduced to protect the most critical parts of the furnace, to improve longevity, and glass quality. In the ultimate scenario the whole furnace is protected with platinum and the very highest quality glass, with very low inclusion count, is produced.

The drive for excellence in respect of glass quality is nowhere greater than for glasses requiring optical transparency characteristics, such as LCD and flat glass. Inevitably furnaces constructed using platinum or platinum alloys are intrinsically very expensive. Moreover, the processing temperatures in the melting section of such furnaces are very close to the melting temperature of the metal containment. Although alloying platinum with rhodium and other platinum group metals can give some considerable increase in useable temperature range, this is insufficient to be practicable. It is very desirable therefore to use an alternative metal or alloy, with a higher melting point, higher strength, better creep resistance and excellent resistance to the molten glass corrosion. Molybdenum, rhenium, tantalum, niobium and tungsten all have these basic characteristics, but each has the fatal limitation that it oxidises rapidly at elevated temperature, in some cases as low as 450° C. Indeed as the temperature is raised above this, the rate of oxidation increases until by 1000° C. the metals can be considered to be “burning”.

Iridium has similar physical and mechanical characteristics but is much less susceptible to oxidation attack than the aforementioned metals, such that oxidation does not become significant until at least 1200° C. Indeed for short periods of time, iridium car used unprotected in air at temperatures as high as 2200° C., although for durability over several years the oxidation rate would still represent a concern.

Thus the problem to be solved is how to decrease the oxidation rate of iridium, or indeed other high melting temperature metals, and their alloys, to achieve a desirable length of life at very high temperature when exposed to oxygen-containing atmospheres. It is known to use inert atmospheres to protect iridium vessels operating at high temperatures.

Certain metals, including molybdenum, have been coated with ceramic by flame or plasma spraying, but such coatings have limited protectiveness in practice. Indeed, such coatings tend to be inherently porous and do not resist ingress of air/oxygen well. Molybdenum which has been flame or plasma sprayed with alumina and/or zirconia has to be protected from air with platinum fabrications with any free space meticulously expunged of air. It can be protected with silicide layers produced by hydrogen reduction of silane, but these are suited only to the short times required for set up of electrodes prior to immersion in glass.

The present invention offers, at least in its preferred embodiments, a method for restricting the rate of oxidation of iridium and its alloys and to a lesser degree similar metals and alloys, at temperatures of at least 1600° C. and in some cases as high as 1800° C.

Accordingly, the present invention provides a protected metal, having a substrate selected from the group consisting of platinum group metals, molybdenum, rhenium, niobium, tungsten and alloys of any of such metals, comprising, on a surface exposed to an oxygen-containing atmosphere, a continuous composite coating of at least 200 microns thickness, especially of 500 to 1000 microns, which coating comprises metal oxide particles in a metal oxide matrix.

The invention also provides a method of protecting a metal substrate selected from the group consisting of platinum group metals, molybdenum, rhenium, niobium, tungsten and alloys of any of such metals surface from oxidation in an oxygen-containing atmosphere at elevated temperature, comprising applying to said metal substrate a coating of metal oxide particles together with a metal oxide matrix or a metal matrix precursor.

The metal oxide particles are suitably of varying sizes, suitable diameters being diameters in the range of 50 to 100 microns, but this is not believed to be critical (although size can have an effect on ease of coating application). As well as conventional particulate of approximately spherical shape, the invention includes the use of ceramic fibres as an alternative to conventional particulate or in admixture with conventional particulate. The metal oxide may be one of or, preferably, a mixture of, alumina, silica and zirconia. Such particles may be mixtures of individual oxides or may be mixed oxides. The particles may contain one or more other metal oxides including one or more of iron oxide, one or more rare earth metal oxides, magnesia, titania and hafnia. Other components may also be present, providing these are beneficial or do not significantly adversely affect the stability of the particles under the operating conditions. Preferred particle compositions in our initial trials contain from 45 to 90% by wt Al₂O₃, 10 to 45% by wt of SiO₂, and less than 1% by wt of Fe₂O₃ and other metal oxides, from a total composition of 100%.

The metal oxide matrix should be physically compatible with the metal oxide particles, such that the coating has no physical cause for degradation during use. Desirably, the matrix is such that during its formation, there is a chemical bond between the particles and the matrix. The preferred matrix precursor is a silicate-based solution such as an aqueous solution of sodium silicate. Other precursors, or other components, are also to be considered.

A ceramic paste or slurry containing particles and matrix-forming material may be prepared to allow easy application to the metal surface to be protected using a spatula or trowel. In refined versions of the invention, some compositions may be applied by brushing, spraying, dipping or even a combination of these. Additionally some forms of the coating may be applied in two parts, by which an aqueous component of the matrix-forming precursor is deposited by spraying and brushing and the oxide components by sprinkling or stucco treating. In this latter embodiment, multiple layering is generally required to build the correct minimum coating thickness. This process is very reminiscent of the process technique used from preparing shell moulds for metals casting, e.g. jewellery and gas turbine blades.

After deposition of the coating on the metal, it is desirable to dry the coating in a controlled manner to reduce or eliminate any formation of gas bubbles. Initial tests indicate that firing of the coated metal is not essential, but may be desirable to develop the full hardness and strength of the coating. Such firing should be carried out at a temperature of at least 700° C.

It is, of course, desirable to ensure that the coating is continuous and non-porous. In certain circumstances at least, the application of two, three or more coatings is desirable. In certain circumstances, if there is damage caused to the protective layer, for example during thermal cycling, it may be possible to apply a repair layer of oxide.

Suitable slurries for use in the invention are available commercially, and they have been marketed for joining, protecting and repairing ceramics by companies such as Sheffield Refractories (Jonsett H A) and Fortafix Ltd. (Fortafix). Such slurries have not previously been used, to the best of our knowledge, for such an application of protecting metals of the type that this invention is concerned with, and their particular suitability to protection of iridium, molybdenum and similar metals at high temperature had not previously been recognised.

The invention will now be described by reference to the following Examples.

EXAMPLE 1

A variety of ceramic slurries were prepared or purchased and applied to iridium coupons which had previously been grit blasted and cleaned sonically. Each slurry was applied by painting in two phases, with each phase being allowed time to dry under ambient conditions before being turned and the remainder painted. Two complete layers were applied to each sample coupon, but the final thickness was determined by the nature of each mixture such as viscosity.

The coated coupons, together with an unprotected control coupon of iridium, were placed flat on upturned high purity alumina tube to allow for maximum airflow around the samples, inside a muffle furnace. The sample coupons were then heated at 1200° C. in air for 336 hours, and allowed to cool.

After the heating cycle, the ceramic coatings were removed mechanically, and the coupons were lightly peened to remove any residual ceramic and oxide. The weight losses for each coupon, measured in mg/sq mm, were established, and are shown in FIG. 1. The unprotected coupon lost the most weight, and all the coatings provided some protection. The coupons 6 and 7 showed the least protection; the slurries for these were water-based, and all other slurries contained sodium silicate.

EXAMPLE 2

As part of the experimentation, small coupons of iridium were cut and cleaned. After weighing, each coupon was placed on a bed of ceramic powder within an alumina crucible. The crucible was then filled with the same ceramic powder, so that the coupons were completely immersed. In one case, the best slurry from Example 1 was used to cover the coupon. The crucibles and their contents were placed in a small muffle furnace and the temperature was raised to 1600° C., which was maintained for 168 hours.

The crucibles and contents were removed, and each coupon was carefully recovered. Any residue of ceramic was cleaned by gentle peening, and the coupons wrre reweighed. The weight differences, as a function of surface area, are plotted in FIG. 2. The most effective barrier to weight loss was that of “Slurry 3” from Example 1.

EXAMPLE 3

Three iridium coupons were cut from the same rolled iridium sheet. Each was coated with “Slurry 3” and allowed to dry, two of the coupons were given a second coat, and one of those had a third coat applied. The coupons were then heated in air at 1200° C. for 336 hours, as in Example 1. The results are shown in FIG. 3, and it is clear that multiple coatings improve the protection significantly. Indeed for three coatings, the improvement is an order of magnitude better than the single coating. Thermal cycling of coated metals can demonstrate some spalling of thicker ox but microscopic examination of the surface shows a residual layer of oxide is retained, which continues to give a measure of protection.

EXAMPLE 4

Initial tests were carried out by coating coupons of tantalum and of molybdenum using a single coating of “Slurry 3”, and exposing the coated coupons to air at 1400° C. for 48 hours. The tantalum coupon exhibited catastrophic oxidation, with the increase of volume causing spalling of the oxide layer. However, the molybdenum coupon appeared to be significantly better protected, with some small areas probably coinciding with flaws in the coating. Optimisation of coating characteristics and the underlying metal needs to be carried out. This test indicated, however, that useful short-term protection of molybdenum is certainly possible, for example for installation purposes, or the invention may be used together with a platinum cladding.

Claims

1. A protected metal, having a metal substrate selected from the group consisting of platinum group metals, molybdenum, rhenium, niobium, tungsten and alloys of any of such metals, comprising, on a surface exposed to an oxygen-containing atmosphere, a continuous composite coating of at least 200 microns thickness, which coating comprises metal oxide particles in a metal oxide matrix.

2. A protected metal according to claim 1, wherein the metal substrate is iridium or an alloy thereof.

3. A protected metal according to claim 1, wherein the metal substrate is platinum or an alloy thereof.

4. A protected metal according to claim 1, wherein the metal substrate is molybdenum, rhenium, niobium, tungsten or an alloy thereof.

5. A protected metal according to claim 1, wherein the metal oxide matrix is derived from a silicate salt.

6. A protected metal according to claim 1, wherein the metal oxide particles comprise from 45 to 90% by wt Al2O3, 10 to 45% by wt of SiO2, and less than 1% by wt of Fe2O3 and other metal oxides, from a total composition of 100%.

7. A protected metal according to claim 1, wherein the coating has been formed from two or more layers of coating.

8. A method of protecting a metal substrate selected from the group consisting of platinum group metals, molybdenum, rhenium, niobium, tungsten and alloys of any of such metals surface from oxidation in an oxygen-containing atmosphere at elevated temperature, comprising applying to said metal substrate a coating of metal oxide particles together with a metal oxide matrix or a metal oxide matrix precursor.

9. A method according to claim 8, comprising the application of two or three layers of the coating.

10. A method of making glass comprising using a protected metal according to claim 1.

11. A protected metal according to claim 1, wherein the continuous composite coating is between 500 and 1000 microns in thickness.