TECHNICAL FIELD

This invention relates to the provision of coatings on substrates, which coatings may be useful, for example, as protective coatings for the substrate or for carrying catalytically active material.

BACKGROUND ART

It is known to provide a coating of a refractory oxide on a substrate by contacting the substrate with a sol of the refractory oxide followed by drying to convert the ^* sol to a gel to give a gel-coated substrate, and optionally firing. For example, the specification of our U.K. Patent No. 1 490 977 (corresponding to U.S. Patent No. 3 957 692) describes, inter alia, contacting an aluminium bearing ferritic alloy substrate, either oxidised or unoxidised, with a boehmite sol, followed by drying to convert the sol to the corresponding gel and firing. Also, the specification of our West German OLS No. 2 64.7 702 (corresponding to U.S. . Patent Application Serial No. 733,152 of 18th October I976) describes, inter alia, carrying out a similar process but using an alumina sol which has been made by dispersing in water an alumina prepared by flame hydrolysis. In each of the above instances, the alumina coatings, both in the gel and in the fired form,are particularly suitable as carriers

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of catalytically active material, such as a platinum group metal, in catalysts. The sols used in each of these instan comprise aggregated colloidal primary particles.

DISCLOSURE OF INVENTION

We have now found that gel coatings may be produced from sols, which coatings have a lower porosity and higher density than the aforementioned coatings, and that such gel coatings are convertible to ceramic coatings of low porosit and high density even after relatively mild heat treatment. This may be done by using sols comprising unaggregated colloidal primary particles, or aggregated colloidal primar particles with additional components to occupy the gaps in the aggregated particles.

Thus, the present invention provides, in one aspect, a method of providing a substrate with a gel coating, characterised in that the substrate is contacted with a sol of a refractory material and capable of being converted to gel of the refractory material, the bulk density of the gel being at least kθ% , preferably at least 45%, of the theoretical density of the refractory material, and the sol is converted to a gel to provide the substrate with the gel coating.

The invention also provides a substrate carrying an adherent coating of a gel of a refractory material, wherein the density of the gel is at least kθ% of the theoretical density of the refractory material.

The refractory material in the sol and the gel of our invention is present in the form of a precursor of the refractory material itself, such as a hydrated form of the material in the case of an aquasol or gel produced therefro

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Such a precursor alwε.ys gives the material itself on firing.

Preferably, the gel-coated substrate of our invention is fired to give a substrate with a ceramic coating of the refractory material itself, which ceramic may have a bulk density which is at least βθ% of the theoretical density of the refractory material. It should be noted that, whilst it may be possible to produce such dense ceramic coatings by prolonged heat treatment of known gel coatings, our dense ceramic coatings may be produced by heat treatment under much milder -conditions.

It should be noted that, in some cases, there may be chemical interaction at the interface between the gel and the substrate thereby giving rise to an interposed layer.

By 'bulk density' in this specification is meant the average density of the material inclusive of the matrix and open and closed pores. By 'theoretical density ¹ is meant the density of the refractory material as such, i.e., the density of the material in the absence of any cavities, pores or the like.

It should be noted that the density of a gel which has been dried at an elevated temperature may, in some cases, be somewhat less than that of a gel which has been dried at ambient temperature due to loss of water on drying. The bulk density values in our invention are to be taken to relate to a gel when dried at ambient temperature, whether actually or notionally.

It should be further noted that determination of densities of very thin layers such as the coatings of our invention may be difficult. The density values given in the examples of this speci ication have been carried out

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on bulk materials, i.e. on gels and ceramics in the absence a substrate, since determination of densities of such materi is relatively simple experimentally. We have not carried ou density determination of the material in a coating as such. We cannot therefore be entirely certain that densities of th coatings will be the same as those of bulk materials derived from the same sols though we see no reason why they should n be substantially similar and, if anything, we would expect t coatings to have higher densities.

In applications of our invention, thin coatings (i.e. o the order of microns) are generally adequate, which distingu our coatings from coatings obtained from paints, glazes, enamels and plasma sprayed coatings. The coated substrates our invention have a number of valuable applications depende upon the substrate and refractory material chosen. Thus, th coatings may be used, for example, to confer oxidation resistance to the substrate, as a pre-coat on the substrate carrying subsequently applied catalytically active material, to inhibit carbon deposition in certain environments. A mor detailed discussion of such applications will be provided hereinafter.

The general role of the coating is to confer a high deg of protection to the substrate by virtue of its high density and low porosity. The coating therefore isolates the substr from its environment thereby protecting it from attack by gaseous species in the environment. Also, when the coating carries an additional layer such as of catalytically active material, the latter is protected from attack by the substra such as when the substrate contains mobile metal ions. Furt more, the coating may be catalytically active in its own rig

It should be noted that the sols used in the present invention need not necessarily comprise colloidal particles one refractory material only. Thus, they may be 'mixed' sol comprising colloidal particles of more than one refractory

material. Also, the sols may contain additional components dispersed in the liquid medium of the sol, for example, in solution in the liquid medium.

A preferred way of carrying out the method of the invention is to use, as the sol, a dispersion of substantially unaggregated colloidal primary particles of the refractory material in a liquid medium. Because of the lack of aggregation, such sols are readily convertible, on drying, to dense, low porosity gels as required in the present invention, i.e., the primary particles can readily 'pack down' to a dense, low porosity structure upon drying and firing. Such sols are known in the art and examples include certain sols of refractory oxides such as a CeO sol described at page 3 line 4.9 of our U.K. Patent Specification No. 1 34-2 '8 3 and at column 3 line 63 of our corresponding U.S. Patent Speci ication No. 3 76 57L Also, the conditioned slurry specifically mentioned in Example 3 of each of these specifications may be diluted with water to give such a sol, and the gel specifically described in the same example may be redispersed in water to give such a sol. Also, the gel specifically described in Example 5 of each of the above specifications may be redispersed in water to give such a sol. Other examples of sols which may be used in the present invention are a ZrOo sol as described in our U.K. Patent Speci ication No. 1 l8l 794 (corresponding to our U.S. Patent Speci ication No. 3 518 0 0) , a i0 ₂ sol as described in our U.K. Patent Specification No. 1 412 937, a Si0 ₂ sol believed to be made by hydrolysing sodium silicate and sold commercially by Monsanto under the trade name of 'Syton' , and ThOo sol made for example by thermally denitrating hydrated thorium nitrate at not more than 90 C and dispersing the product in water.

The particle sizes of the colloidal particles in the sols are typically in the range of 20 A to 500 A, for example 0 A to 200 A. It should be noted however, that the above exemplified sols are not necessarily of equal utility in the applications of the present invention, i.e., some sols may be better than others for specific applications.

The preferred sols above may, if desired, contain components additional to the unaggregated primary colloid particles. For example, they may contain colloidal particles comprising loose aggregate structures of primar particles, wherein the colloidal particles have been made by dispersing primary-particles, made by a vapour phase condensation method such as flame hydrolysis, in water an as described in the speci ication of aforementioned West German OLS No. 2 647 702. Such additional components, fo example Al 0o, may be used to provide the coatings in our invention with other desired properties such as improving their ability to cause further layers to adhere thereto.

Alternatively, the sols used ±n the method of our invention may comprise colloidal particles which arc aggregated, but where the sols contain additional compone dispersed therein which substantially fill the gaps in th aggregated particles so that the sols give rise to a dens gel coating according to the invention when converted to a gel. Such additional components may, for example, comp salts in solution in the liquid medium of the sol and of sufficient concentration for the ions of the salt to substantially fill the gaps in the aggregated colloidal particles. A preferred example of such a sol is a sol comprising components which when dried to give a gel and subsequently fired are convertible to a glass-based coati Such a sol may comprise, for example, a SiOo sol containi aggregated colloidal particles and w-hich contain addition components, in solution, which are capable of reacting together and with the SiO _o on firing to give a glass-base material. Such components may include, for example, solu borates, and soluble Li and Na salts in solution in the s The SiOo sol may, for example, be a sol made by dispersin water SiOo which has been made by a vapour phase condensation method such as flame hydrolysis and to which

reference has already been made herein. It should also be mentioned, however, that coatings comprising glass-based materials may be provided according to our invention using sols comprising substantially unaggregated colloidal primary particles, such as the abovementioned 'Syton' SiOo sol. Glass-based materials include, for example, conventional glasses and also glass-ceramics.

The method of our invention may be carried out very simply, for example .by immersing the substrate in the sol, removing and drying to convert the sol to the corresponding gel, optionally followed by firing if a non-gel ceramic coating. is desired. Thus, a substrate of complex shape may readily be treated to provide a coating. Also, a coating of controlled thickness may be produced, typically, 1 "J ^" m or less, so that significant dimensional changes are avoided, even if more than one coating is provided.

The substrate in the invention may be either metallic or non-metallic, though we prefer the former since protective coatings are more often required for metallic substrates. Thus, metallic substrates, such as steels, may be protected from oxidative attack by the present invention. An example of a metallic substrate which may be used is an aluminium bearing ferritic alloy such as an alloy of Fe, Cr, Al and Y, a specific example of which is an alloy having proportions by weight of up to 20% Cr , 0.5% to 12% Al, 0.1% to 3% Y, and the balance Fe. Such alloys are known to be very useful substrates in catalysts for the treatment of the noxious constituents of motor vehicle exhause gases (see, for example, the specification of our U.K. Patent No. 1 471 138 and of our corresponding U.S. Patent No. 3 920 583) _* However, such alloys owe their oxidation resistance in the exhaust gas treatment application to the presence of an A1 ₂ 0~ barrier layer, preformed on the

alloy by oxidising at elevated temperatures, for example, by heating at about 1000 C in air, typically for 8 hours. This preforming step may, however, constitute an expensive step in the production of a catalyst. We have found that it may be dispensed with by using the present method, for example by using the abovementioned CeO sol and a firing temperature in the range of 500 C to 800 C for a much shorter time, typically 1 minutes, which gives a highly- satisfactory barrier layer for inhibiting diffusion of metallic ions from the substrate to the surface, and for preventing diffusion of gases and liquids towards the substrate. A catalyst may then be- prepared by applying a catalytically active material, such as a platinum group metal, to the coating, for example, in combination with a high surface area refractory oxide such as A1 ₂ 0 as described in the specification of our aforementioned West German OLS No. 2 647 702.- The Ce0 ₂ coating in such a case acts as a temporary protective barrier until such time as alumina is generated from the alloy -during use of the catalyst.

The present invention also has application in situations where it is desirable to alter the surface chemistry of a metal and thereby eliminate certain undesira chemical effects. One such effect is the deposition of carbonaceous layers on steel surfaces which are exposed to hydrocarbon-containing environments. This can occur, for example, in chemical plant such as plant for the therma cracking of hydrocarbons where the formation of carbonaceou deposits on heated steel cracker tubes gives rise to an undesirable insulation effect. Also, carbonaceous deposits can occur in nuclear reactors, such as the Advanced Gas Cooled Reactor (known in the art and referred to hereinafte as the 'AGR' ) where stainless steel fuel cans are exposed

to a hydrocarbon-containing coolant gas. Here, carbonaceous deposits deleteriously affect the heat exchange balance between the fuel cans and the coolant thereby causing over¬ heating. We have found that the present invention, when applied to the fuel cans, can bring about substantial reductions in carbonaceous deposition under the above circumstances. It should be noted, however, that, for this 'AGR' application, we have indications that it may be desirabj.e to provide the steel with a first oxide coating by, for example, providing a preliminary oxidising treatment (e.g. heating in air at 800 C for 1 minutes) before providing the coating according to ■ the present invention. Examples of steels which may be used in the 'AGR' as the fuel can material and which are suitable for coating according to our invention are Cr bearing austenitic steels, for example, stabilised by Nb , a particular example of which . is the so-called "20/25" steel which contains 20% Cr, 25% Ni, about 0.1% Nb and the balance iron, wherein the proportions are by weight. The role of the coating in the inhibition of carbonaceous deposition may be twofold. Firstly, it may act to isolate the substrate from the environment, thereby preventing certain constituents in the substrate from catalysing chemical reactions giving rise to carbonaceous deposition. Secondly, the coatings may themselves act catalytically in processes which prevent carbonaceous deposition. The a orementioned Ce0 ₂ sol is particularly advantageous in this respect.

The coatings of our invention may be provided with additional constituents in order to achieve particular aims or properties. The a orementioned provision of glasses on substrates is an example of this. Also, for example, coatings with controlled electrical properties may be provided on electrically conductive or non-electrically conductive substrates.

ijU A OMPI

A number of ways of carrying out the invention are described in detail in the examples below.

Examp1e 1

Preparation of CeO sol

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3.5 kg of cerium IV hydroxide (99.5% purity) ex Rhone Poulenc (2.48 kg oxide, 0.210 kg NO ~) were mixed with 7 1 of demineralised water and O.58 1 of 8 M nitric acid (tota o slurr'- volume 9«6 1) and the stirred slurry heated to 8θ

.0 over a period of 2h and maintained at 8θ~85 for 1 h. The pH reached at equilibrium was <1. The slurry (HNO /CeO.. : 0.32) was allowed to cool overnight ( 16 h) . The supernate was syphoned off (6.76 1) and analyaed for acidity (0.28M) nitrate (0.5 M) and oxide content (8.0 g/l) . A sufficient volume of water (2.5 1) was added to the settled condition slurry residue in order to give a non-chalking colloidal dispersion (sol) and the new total volume measured (5»35 1 The sol was then analysed for density (1.42 g/cc) , oxide content (462 g/l) nitrate (0.8 M : N0.-/Ce0 ₂ = 0.29) .

0.2 ml of a 20% polyvinyl alcohol solution were added per 100 ml of a Ce0 aquasol prepared as above and adjuste to a concentration of 100 g of Ce0 ₂ per 1, and also a few drops of a 1% solution of BDH Nonidet (Registered Trade Mark) P4θ wetting agent.

Oxidation Protection of Steel

A specimen of an austentitic stainless steel containi 18% Cr by weight, 8% Ni by weight, and a small amount of Ti (the 'so called' 18/8/Ti steel) was immersed in the CeO,, aquasol prepared as above. The specimen was removed

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and dried to convert the Ce0 ₂ sol coating to a Ce0 ₂ gel coating. The specimen was next fired at 85O C for 5 minutes to give a Ce0 ₂ coated steel product.

When the above product was heated for 12 hours in o air at 850 C, it remained ductile and exhibited a smooth, continuous surface. Its characteristic X-ray spectrum, generated by 20KV electron bombardment, was dominated by

Cr. Also, when the product was further heated for 12 o hours in air at 1000 C, it showed no severe deterioration.

By way of comparison, an untreated specimen of 18/8/Ti stainless steel was heated for 12 hours in air at 85O C. The specimen became brittle and was observed to be covered with a discontinuous, poorly adherent oxide layer. Its characteristic X-ray spectrtim, generated as above, was dominated by iron (oxide).

Example 2

Prevention of Carbonaceous Deposition under AGR Conditions

Specimens of 20/25/ b stainless steel were provided with Ce0 ₂ coatings as described in Example 1. The coated specimens were stacked on a steel rod and placed in a test rig in a materials testing reactor (known as 'DIDO' ) and exposed at a temperature of 650 C for 1200 hours at a dose

-1 rate of 1 W.g to recirculate C0 ₂ gas containing 2% CO,

350 vpm CHΛ flowing at 40 litres/hour at a pressure of 6θO psig. At the completion of the exposure period, the

Ce0 coated specimens were observed to be substantially free from carbonaceous deposits. In contrast, uncoated specimens of 20/25/Nb steel which had been subjected to identical conditions were observed to be covered with a dark carbonaceous layer.

Example 3

Catalyst Preparation

A specimen of Fecralloy (Registered Trade Mark) aluminium bearing ferritic alloy of composition by weight 5 of up to 20% Cr, 0.5% to 12% Al, from 0.1% to 3% Y and the balance Fe, was immersed in a Ce0 ₂ sol as used in Example 1, removed and dried to convert the sol to a gel, and fired for a few minutes at 500° to 6θO C to give a Ce0 ₂ coated product, wherein the alloy was observed to 10. have retained its silvery appearance after the firing. (In contrast, an untreated sample of the alloy acquired a golden colour, due to oxidation, after similar firing).

Finely powdered A1 _Q 0„, having a small particle size

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('^ ^• 10 run) and high surface area (-» _' 100 m /g) was dispersed

15 in water to give a sol containing l6θ g Al ₂ 0~/1. A solution of yttrium nitrate of composition 170 g Y„0 equiValent/1 was made up and the sol and the solution mixed in proportions to give a "mixed sol" having 91-5 g

A1 ₂ 0 /I and 0.45 g YgO _^ /l. Polyvinyl alcohol (PVA) and 0 H ₂ PtCl _/ - were dissolved in the mixed sol to give 0.6l g

PVA/1 and 15-5 g H ₂ PtCl ₆ /l (= 6.06 g Pt) in a final sol, which a few drops of Nonide-t P4θ wetting agent were added

The Ce0 ₂ coated alloy was immersed in the above final sol, removed, dried and fired in air at 850 C for 5 5 minutes to produce a catalyst where the Ce0 ₂ coated alloy had a catalytically active coating of Pt carried by AlgO . Standard tests were carried out on the catalys for treatment of motor vehicle exhaust gases and gave almost identical results to those obtained with a 0 catalyst prepared as above but wherein the alloy had been ooxxiiddiisseedd aatt 11000000°CC ffor 12 hours instead of being provided with a Ce0 ₂ coating.

Example 4

Catalyst Preparation

An alumina sol with a concentration of 289 g A1 ₂ 0 /l was prepared as described in Example 3 and yttrium nitrate solution was added to give relative proportions by weight of 99.8% Al 0 and 0.2% ₂ °3 _' °* ^{2 ml} ° ²⁰ °^ ^PVA s ^01" ^ ¹⁰¹¹ per 100 ml of the sol and a few drops ^" of Nonidet P4θ wetting agent were also added. A 10 ml aliquot of the resulting sol was then mixed with 100 ml of a Ce0 ₂ sol, prepared as in Example 1 and containing 26θ g Ce0 ₂ /1, to give a mixed sol wherein the relative propoι*tions by weight were: Ce0 ₂ 89.78%; A1 ₂ 0 ₃ 10.03%; YgO 0.19%.

A specimen of 'Fecralloy' alloy, as used in Example 3 was immersed in the mixed sol, removed, dried and fired for a few minutes at 500 to 600 C. In the coated product, the alloy had retained its silvery appearance, and the presence of the Alr _> 0„, which was porous, was found to assist in the 'keying' of subsequently applied coatings.

Example 5

Preparation of Glass Coatings on a Substrate from Sols

Sodium borate (100 g) was added to water (500 ml) and heated to 60 C to assist dissolution; the pH of the solution was 9-5 and 16 M nitric acid (35 ml) was gradually added to give a solution with pH 1.5. Lithium nitrate trihydrate (110 g) was added followed by sodium nitrate

(3 g) ; no change in pH occurred and the solution (1.14 1) was stable to precipitation at 4 C. The solution was separated into two 570 ml aliquots which were then treated as ollows: -

(a) flame hydrolysed silica powder (93 g) was gradually added to a first aliquot with stirring; to maintain the sol in a fluid state it was necessary to add further water (100 ml). 0.2 of a 20% PVA solution per 100 ml of the sol and a few drops of Nonidet P4θ wetting agent were also added. The sol contained I83 g/l total oxides and was stable to coagulation for several weeks;

(b) to the second aliquot of the nitrate solution, a proprietory silica sol (SYT0N-X30) (250 ml) contain 34θ g/l Si0 _? was added to give a total oxide concentration of 159 g/l. 0.2 ml of a 20% PVA solution per 100 ml of the sol and a few drops of

Nonidet P4θ wetting agent were also added. After mixing for minutes the C and found to be thixotropic, e.g., within a few hours the sol assumed a jelly-like condition but when gentl agitated it regained its former fluidity.

Each of the sols produced in (a) and (b) above was tested as follows. A specimen cf 'Fecralloy' alloy, as us in Example 3, "was partly immersed in the sol, removed, dri and fired for a few minutes at oOO C. In each case, the portion of the alloy which had been immersed retained its silvery appearance, whilst the portion which had not been immersed had acquired a golden colour, due to oxidation.

Exa p1e 6

Bulk Densities of Gel and Fired Products Obtained from Sol

Samples of refractory oxide sols, usable in the prese invention, were dried to the corresponding gel form and the bulk density of each resulting gel measured by known Hg immersion techniques. The gels were then fired to give

the non-gel ceramic form of the oxide and the densities measured in all cases. The results are summarised in the table below where the densities are given as a percentage of the theoretical density of the refractory oxide.

Bulk Densities (as % of the theoretical density of the anhydrous oxide)

Sol Gel (after drying Fir ed G el ( firing at ambient teπro era tur e in tem er ure) par en" these s )

Ce0 (prepared as in 6 7 ⁰ / ^/ -o (8oo°c)

Ex. 3 of UK Patent Specification No. 1 3 2 893)

Si0 ₂ ('Syton ¹ sol) 77 . 0/

• O 67% (500 c)

Zr0 ₂ . • n kJo //0 94% (870°c) Ti0 ₂ 5 % 96% (8oo°c)

Si0 ₂ ( + LiNO + N ₂ B^0 ₇ 50 . .3% 87% (700°C)

NaNO (as prepared in Example 5 (a)

If the densities of the gels are considered as percentages of the theoretical densities of the appropriate hydrous oxides rather than of the final anhydrous oxides as- used above, the values are considerably higher, e.g., the Zr0 ₂ gel density is 87- % of the theoretical density of zirconium hydroxide.

Also, as mentioned herein, the density of a gel which has been dried at an elevated temperature may, in some cases, be somewhat less than that of a gel which has been dried at ambient temperature. For example, the above ^{" ~"} ZrOg. gel, if dried at an elevated temperature, was found to have a % bulk density of 48.8% of the theoretical densit;, of the anhydrous oxide.

Example 7

Coating of Mild Steel

A sample of mild steel was immersed in a Ce0 ₂ sol prepared as in Example 1 and containing additionally a water soluble silicone. The concentrations were: CeO _p. 37-5 g/l; silicone3.5 g/l. The sample was then removed and dried to convert the sol to a gel. The silicone was provided because CeO _o sol itself may be sufficiently acidic to attack mild steel.

The gel coated sample was then fired at 200 C for 10 minutes. This gave a ceramic coating which was found to improve the resistance of the mild steel to atmospheric corrosion and w-hich was capable of acting as a primer for a subsequently applied paint layer.

It should be noted that the above firing temperature is substantially lower than those of our preceding examples.

This is because mild steel is liable to oxidise at high firin temperatures before the applied coatings have densifiod and can provide protection.

Example 8

Coating of Mild Steel

The procedure of Example 7 was repeated but using, inste of the silicone containing Ce0 ₂ sol, proprietory silica sol (SYT0N-X30) of concentration 20 g/l. The results were substantially similar to those of Example 7.