WO1999023050A1

WO1999023050A1 - Fusion-cast refractory article for glass melting furnaces provided with a noble metal coating

Info

Publication number: WO1999023050A1
Application number: PCT/GB1998/003103
Authority: WO
Inventors: Duncan Roy Coupland; Mark Laurence Doyle; Paul Matthew Williams
Original assignee: Johnson Matthey Public Limited Company
Priority date: 1997-10-30
Filing date: 1998-10-16
Publication date: 1999-05-14
Also published as: GB9722901D0; AU9452298A

Abstract

A fusion-cast refractory part can be successfully coated with a noble metal especially platinum, by preparing the surface by etching using e.g. HF, preferably a gel etchant, to remove a portion of a siliceous phase, or by partial reduction in the case of α/β alumina, followed by flame spraying to deposit at least 50 microns thickness. Excellent adhesion of the coating is achieved.

Description

FUSION-CAST REFRACTORY ARΗCLE FOR GLASS MELΗNG FURNACES PROVIDED WITH A NOBLE METAL COATING

The present invention concerns improvements in coatings, and more especially concerns the protective coating of fusion-cast ceramics. 5

Slip-cast and sintered ceramic refractories are used extensively as parts for the handling of aggressive materials such as molten glass. Such sintered ceramic refractories are generally manufactured by forming a dense slurry of the refractory oxide, optionally in the presence of inorganic binding agents, casting the slurry in a mould and sintering the 0 resulting cast item. Such refractories are considered to be of low density, and generally exhibit 15-20% porosity. Chemically, such refractories are mixtures of two or more of silica, alumina and zirconia, although other oxide components such as magnesia may be present.

5 We have shown that such refractories may be given additional protection by coating with platinum using a flame spraying method (see EP 0 559 330). However, there exists another class of refractory materials known as fusion-cast refractories which possess very different physical and physico-chemical properties, despite the fact that in chemical composition they are very closely related to low density slipcast and sintered refractories. 0 Fusion-cast refractories are very dense, possessing at most 3% porosity, and exhibiting a hard, smooth external surface. The chemical constituents are generally fused by arc-welding using graphite electrodes, and cast into moulds or flowed onto surfaces. The fusion-cast refractories find widespread use as furnace lining blocks and channel blocks and in furnace and reaction vessel linings (sometimes called "glass-lined vessels") and generally exhibit 5 improved resistance to corrosion or erosion compared to low density refractories.

Although fusion-cast refractories exhibit high performance in use, under many conditions they are still prone to attack and ultimate destruction. For example, in glass- melting furnaces, even fusion-cast refractories are subject to attack at or above the line of 0 molten glass. The lifetime of such components is determined by temperature, glass-type and the amount of glass processed. Damage to these refractories can lead to the need for shut- down of the furnace and loss of production. Because of the smooth, low porosity surface of fusion-cast refractories, it has generally been found to be impossible to satisfactorily coat such materials with protective materials such as the noble metals. Methods of keying the surface of low density refractories, such as grit blasting, machining, de-greasing using solvents or traditional acid etching using mineral acids, are ineffective to provide a suitable surface for bonding noble metal coatings to fusion-cast refractories. The deposition of a plasma-sprayed intermediate layer of ceramic oxide has been suggested as necessary in the coating of "hard, high density refractories", prior to plasma flame spraying with platinum, in US 4,159,353. The reason for utilising such an intermediate oxide layer is that "neither grit blasting or chemical etching such hard dense refractories has sufficiently roughened the surface to allow an adherent mechanical bonding of the platinum to the refractory". Another characteristic of such materials which makes adhesion of any protective coating problematical is their tendency to exude a glassy phase during exposure to high temperatures. To our knowledge, there has never been any successful, commercial, method of platinum coating fusion-cast refractory parts directly, without an intermediate oxide coating.

It is known to etch conventional glass using hydrofluoric acid (HF) for decorative purposes. It is also known that alkalies attack certain glasses. It is known to treat "smooth refractories such as fused Si0₂ or high-Al₂0₃ with a water solution of a single fluoride salt to improve protection against attack by molten Al or Al alloys" (Light Met., 1986, 2, 797-9). Such treatment is not, however, followed by any additional coating of a noble metal. Etching is also used in microelectronics to form structural elements; for example, HF etching of silicon or phosphosilicate glass (J. Electrochem. Soc. 142 No 1, 237).

There has been a demand for many years, therefore, for platinum-coated fusion- cast refractory parts, which has been impossible to satisfy with known technologies. The Applicants also failed, in their initial tests, to achieve adequate adhesion of a flame-sprayed platinum coating. Contrary to the teaching of US 4,159,353, however, the Applicants have now discovered that an understanding of the surface chemistry of the fusion-cast refractory can permit successful etching which is adequate to provide exceptionally adherent platinum coatings applied without requiring any intermediate oxide layer.

The present invention provides a fusion-cast refractory part, possessing at least one surface area having an etched surface either from which a portion of a siliceous phase has been removed, or, in the case of fusion-cast α/β alumina, the surface has been partially reduced on which is bonded a coating of at least 50 microns of a noble metal or alloy thereof.

The invention also provides a method of coating a fusion-cast refractory part, comprising etching at least one surface area of said part to remove a portion of a siliceous phase, or, in the case of α/β alumina, partially reducing said surface, to form an effective bonding surface, and subsequently depositing a coating of at least 50 microns of a noble metal or alloy thereof by flame spraying.

The noble metals useful in the present invention are one or more of the platinum group metals, namely platinum, rhodium, palladium, ruthenium, iridium and osmium, and alloys with each other or with base metals. Preferably, the metal is platinum, an alloy of platinum, eg Pt5%Au, Ptl0%lr, Ptl0%Rh, Pt5%Ru, or Pt with up to l%Zr, or grain stabilised Pt or Pd.

The fusion-cast refractory part may be of any of the conventional fusion-cast refractory compositions, incorporating one or more of Si0₂, A1₂0₃, Zr0₂ and Mg0₂, optionally including amounts of other refractory oxides such Cr₂0₃. Preferred fusion-cast refractories are those known as AZS refractories (AZS = alumina/zirconia silica). Other similar refractory parts, such as fusion-cast chromias, may be platinum-coated using the present invention.

In the casting process, zirconia has the highest melting point of the regular components of an AZS refractory, and tends to crystallise preferentially at the surface of the mould. This can result in the surface of the fusion-cast part not being representative of the bulk material, and can be less amenable to etching. It may be desirable in such cases that the surface of the part is machined to expose bulk AZS material; in manufacturing certain parts this was done as part of the manufacturing process.

There are several other methods of surface preparation which may be developed if necessary. One suitable method is to coat or line the mould to provide a platinum coatable, etchable surface, on the as-cast face of the final ceramic. For example, the surface of the mould may be coated, eg by flame spraying, with alumina silicate and/or silica.

Other surface preparation methods amount to a modification of the invention in that no etching may be required because the surface provides adequate keying. For example, the mould surface may be coated with titania or a titania-enriched refractory material. Another variation is to coat the mould with tetragonal zirconia prior to casting. As the molten AZS refractory is poured into the mould, the tetragonal zirconia converts into a higher volume cubic zirconia, causing expansion and micro-cracking. Other mould coatings may prove to be adequate to change the surface composition of the cast refractory part, and either provide a more readily etchable surface or a surface capable of accepting a flame sprayed platinum coating.

We have also discovered that the casting methods may be modified in order to achieve a surface layer with reduced or no zirconia, providing an etchable surface. For example, a low-zirconia AZS material or a silica melt may be spin-cast in the mould, immediately followed by casting a regular bulk AZS material.

Other possible modifications of the casting procedure include vibro-casting by vibration during casting, thus hindering the formation of the ZιO₂ layer at the surface of the cast part.

Some other possible surface modifications which may be considered are: the mould may be pre-heated and solidification controlled to either eliminate or reduce the thickness of the Zr0₂ zone at the surface of the cast part. The narrow Zr0₂-rich zone may readily be machined off. Another possibility is to increase the solidification rate by chilling the mould, thereby creating a surface Zr0₂ zone having a very fine microstructure and reducing the time available for the A1₂0₃ and Si0₂ phases to migrate from the surface. Increasing or decreasing the solidification rate may also be achieved by changing the mould material, as well as by heating or cooling, and it is possible to modify the surface roughness of the mould, providing micro-roughness in the as-cast surface, and thereby improve the adhesion of the noble metal coating.

Other methods may be used to alter the solidification rate, for example electromagnetic stirring and/or heating, or induction heating during cooling. Other methods are available to the person skilled in casting and surface treatments, and may be used without departing from the present invention.

The etching step may also be carried out in a number of different ways. A first method which is highly effective, but may create production difficulties from health and safety viewpoints is to etch using hydrofluoric acid. Initial optimisation tests resulted in a process involving immersing the ceramic in 48% HF diluted 1:1 with water for 10 minutes, which dissolves sufficient siliceous phase without unduly weakening the ceramic. The adhesion of the subsequently deposited platinum coating was in excess of 10 MPa, in fact adhesion exceeded the capability of the test cell.

A caustic solution, for example boiling KOH, is also effective to remove sufficient siliceous material to provide good adhesion; adhesion results are of the order of 5 to 6 MPa. Alternative chemical etching can be achieved with NaOH and similar materials, including molten salts.

The preferred method, which has been found to give similar adhesion values as

HF etching, is etching using a fluoride gel, which can be made up from, for example, sodium bifluoride and sulphuric acid as essential components. Suitable compositions are commercially available as glass etching compositions. Treatment times are longer than for

HF treatment, and may usefully be in the region of 24 to 72 hours. Suitable treatment times can be ascertained by routine experimental procedures, and will depend upon the grade of ceramic. Adhesion values for the Pt coating deposited onto a thus-etched surface are in excess of 10 MPa. This technique has the important advantage of enabling on-site treatment of refractories prior to on-site coating.

Certain fusion-cast ceramics, especially α/β alumina ceramics, do not possess a siliceous phase which may be removed by such etching procedures, but can be prepared for coating by alternative procedures. For example, a very thin layer of platinum may be deposited on the surface and then the part heat treated in a reducing atmosphere. Deposition may be achieved by sputtering, CVD, electroless and even by rubbing with a piece of platinum. The heat treatment may be at temperatures in the range 600 to 1500°C for 1-5 hours in hydrogen or a mixture of hydrogen and an inert gas. Alkali etching as described above for AZS ceramics may also be used, and other surface modification utilising mould coatings are to be considered.

The noble metal may be deposited on the etched surface in a number of different ways. A preferred method is by combustion flame spraying in a method analogous to that described in EP 0 559 330. Other methods include plasma flame spraying, and high velocity oxy-fuel combustion spraying. Further methods may be developed without departing from the scope of the present invention.

The coating desirably has a thickness of 50 microns up to 2mm (the thickness is probably limited only by the economics). More desirably, the thickness is 50 to 500 microns, suitably about 200 microns.

A post-treatment of the coating to remove or significantly reduce any porosity is desirably included. Such post-treatment is preferably the peening described in detail in

EP 0559330, but other methods including burnishing may be used providing either there is mechanical energy or pressure applied, or the surface is glazed or re-melted by a high energy beam. It should be realised that although excellent results may be achieved according to the invention by etching and coating essentially all exposed surfaces of the fusion-cast ceramic, in many uses it may be sufficient and economical to etch and coat only a proportion of the surface. For example, in glass-melting furnaces, or other furnaces exposed to similarly aggressive conditions, erosion takes place in a relatively small area at the normal line of molten glass. For such furnaces or vessels, the invention permits coating those regions most at risk.

It will be readily understood that the skilled person may apply the present invention by modifying one or more of the specified details whilst still gaining the benefit of the invention.

The invention will now be described by way of Example.

EXAMPLE 1

An etching solution was prepared by diluting 48% HF to twice its volume with water. Samples of commercial fusion-cast AZS ceramic blocks identified as ER1681 and

ER1711 supplied by SEPR were treated with the etching solution for 10 minutes at 25 °C before drying. A coating of 200 microns of pure platinum was applied to the etched surface by combustion flame spraying. The surface was thereafter shot-peened.

The adhesion of the platinum coating was tested by standard tensile testing methods. The samples exhibited failure stresses in excess of 10 MPa. Failure occurred in the ceramic itself immediately adjacent the coating.

The above-described procedure was modified by heating the ceramic blocks after etching .and coating to 1500°C for 72 hours. After cooling, the adhesion of the coating was tested using the same procedure, yielding failure stresses of between 8 and 12 MPa. However, in 3 out of 5 tests, the test had to be terminated before failure of the coating because the adhesion exceeded the capacity of the testing instrument. A comparison with high quality platinum-coated sintered ceramics (ACT™, Johnson Matthey) demonstrated adhesion values in the range 3-5 MPa.

The preparation and tests were repeated with AZS fusion-cast ceramics supplied by Refel and Carborundum, with essentially identical results.

EXAMPLE 2

Tensile samples of ER1711 and ER1681 refractories were prepared in the manner described in Example 1 except a sulphuric acid/sodium bifluoride etching gel was used instead of the HF. The etching times were between 24 and 72 hours. In all cases the adhesion of the coatings exceeded the capacity of the testing equipment having withstood tensile stresses in excess of 1 OMPa.

EXAMPLE 3

An AZS ER168 refractory block was prepared for coating in the manner described in Example 2. Platinum was deposited by a proprietary flame spray, followed by shot peening; it was then subjected to a high temperature (1500°C in air) soak for a total of almost 6 months, which included four excursions back to room temperature for examination.

The block, as expected, exuded heavily, but the coating remained firmly attached and intact.

EXAMPLE 4

A sample of ER1681 was prepared as Example 2 and the coating/ceramic interface was tested in shear stress at 1400°C to assess the effect of exudation on the interfacial integrity. This confirmed that adhesion was maintained even with the refractory exuding substantially. EXAMPLE 5

A sample of Refel 1532 refractory was prepared as described in Example 2 and successfully coated with platinum over a 5x5cm area, with four free edges, using air plasma spraying. The adhesion was again excellent and enabled the coating to resist considerable mechanical efforts to force lifting at the free edges.

EXAMPLE 6

A sample of ER1681, 65x21x11cm size, was prepared as described in Example 2.

It was coated over the 11x65cm face by a proprietary platinum flame spray technology followed by shot peening (Johnson Matthey ACT™ technology) and placed on trial with the coated face in contact with flowing opal glass at a temperature in excess of 1200°C. After 3 months the protection was being maintained without reported problems.

EXAMPLE 7

A part of commercial Jargel M α/β alumina supplied by SEPR was etched by rubbing the surface with a platinum wire to deposit a very thin layer of smeared platinum on the surface. The part was then heat treated at 1000°C for 3 hours in a H₂/N₂ atmosphere.

After cooling, the part was flame spray coated with 200 m platinum, shot peened and subjected to adhesion testing. The adhesion values averaged 4.7 MPa.

Claims

1. A fusion-cast refractory part, possessing at least one surface area having an etched surface from which a portion of a siliceous phase has been removed or, in the case of fusion- cast ╬▒/╬▓ alumina, the surface has been partially reduced, on which surface is bonded a coating of at least 50 microns of a noble metal or alloy thereof.

2. A refractory part according to claim 1 , wherein at least the surface of the coating is substantially non-porous.

3. A refractory part according to claim 1 or 2, wherein the coating thickness is 50 to 500 microns.

4. A refractory part according to claim 3, wherein the coating thickness is about 200 microns.

5. A method of coating a fusion-cast refractory part, comprising either etching at least one surface of said part to remove a portion of a siliceous phase, or, in the case of said surface being ╬▒/╬▓ alumina, partially reducing said surface, to form an effective bonding surface, and subsequently depositing a coating of at least 50 microns thickness of a noble metal or alloy thereof by flame spraying.

6. A method according to claim 6, wherein etchan is carried out using a gel, paste or cream.

7. A method according to claim 6, wherein an etchout is used which is aqueous or liquid.

8. A method according to any one of claims 5 to 7, wherein the part is an AZS refractory and etching is carried out using hydrofluoric acid, a composition comprising .an acid and a fluoride or an alkali.

9. A method according to claim 5, comprising the additional step of treating the surface of the coating to render it substantially free of pores.

10. A method according to claim 9, comprising shot peening, burnishing or glazing or re-melting the surface by a high-energy beam.

11. A method according to any one of claims 5,9 or 10, wherein the part is an ╬▒/╬▓ alumina and partial reduction of the surface is effected by treating the surface with platinum or a platinum compound and subsequently heating said surface in a reducing atmosphere.

12. The use of a coated fusion-cast according to any one of claim 1 to 4, in a glass- melting furnace or exposed to molten glass.