Coating processes – Heat decomposition of applied coating or base material
Reexamination Certificate
2000-06-12
2003-04-01
Beck, Shrive P. (Department: 1762)
Coating processes
Heat decomposition of applied coating or base material
C427S376200, C427S376300, C427S376400, C427S380000, C427S419400, C427S419600, C423S700000, C423S716000
Reexamination Certificate
active
06541066
ABSTRACT:
This invention relates to thin ceramic layers applied to non-porous or coarse-porous ceramic or metallic substrates.
Non-porous ceramic layers are often applied as glazing to ceramic substrates or as enamel to metallic substrates. The aim of applying the layers is generally protection or embellishment.
The present invention is directed especially to the use of thin ceramic layers in the protection of metals and alloys. Primarily, this involves screening against the attack of the metal or the alloy by reaction with carbon-containing molecules in gas form. It has been observed that upon exposure of metal and alloy surfaces to carbon-containing gas molecules, such as methane or higher hydrocarbons or a mixture of carbon monoxide and hydrogen, metal or alloy particles disappear from the surface. As a result, the thickness of the metal or the alloy can decrease rapidly, giving rise to fracture in equipment working under increased pressure. In cases where work is not done under pressure, the loss of metal or the alloy can cause leakage. In other cases, the exposure of metals or alloys to hydrocarbons at increased temperature leads to the deposition of a relatively dense layer of carbon on the metal or alloy surface. This can give rise to clogging and is therefore undesirable. Before clogging occurs, however, the heat transfer from the metal or alloy wall to a gas stream is found to decrease strongly.
In many technically important cases, such as, for instance, in naphtha cracking plants, a significant reduction of the heat transfer is unallowable, since it results in a strong decrease of the capacity of the plant. The plant must be stopped and the carbon layer must be removed, for instance by oxidation. In general, this occurs by reaction with oxygen or with steam.
Technically, it is of great importance to protect metal or alloy surfaces against loss of metal or alloy particles, or against the deposition of carbon layers, by the use of a suitable coat. According to the present state of the art, the application of such a protective layer has been found not to be properly possible. It has been attempted, by starting from an aluminum-containing alloy, to apply a protective aluminum oxide layer to the metal surface. An example of such an alloy is Fecralloy®. In practice, however, a layer formed in such a way was found not to protect the metal surface sufficiently. An additional drawback of Fecralloy® is that this alloy, like other aluminum-containing alloys, cannot be welded.
In general, the fact that aluminum-containing alloys cannot be welded is a drawback of alloys that are resistant to oxidizing gases at highly increased temperature. A second object of the present invention is therefore the provision of non-permeable, oxidation-resistant ceramic layers on metals of good weldability. In that case, the metals or alloys can first be brought into the desired form by welding, whereafter the protective layer is applied.
In this connection, the invention is directed especially to rendering metal gauzes resistant. In catalytic reactions at (strongly) increased temperature, because of the intrinsic high reaction rate, no large catalytically active surface area per unit of volume is necessary. The surface area of a metal gauze is sufficient. In the oxidation of ammonia to nitrogen oxide in the production of nitric acid, use is therefore made of platinum or palladium gauzes, to which often slight amounts of other precious metals have been added. A major drawback is that the precious metal disintegrates during the catalytic reaction. Initially, use was made of a gauze of gold, arranged under the platinum gauze to catch the platinum particles. Later, a platinum gauze was used to catch the small platinum particles formed. In that case, no platinum-gold separation is needed to recover the platinum. If, in fact, no catalytic reaction proceeds over the platinum, the platinum does not disintegrate. To increase the productivity of nitric acid factories, it is highly attractive to work with pure oxygen instead of air; also at increased oxygen pressure the productivity increases strongly. However, because of the greatly accelerated disintegration of the precious metal gauze at higher oxygen pressure, this has not been found possible so far. Applying the precious metal in finely divided form to a stable gauze would enable an important improvement of the nitric acid process. This process has not been fundamentally improved since the invention by Ostwald at the end of the nineteenth century.
In the Andrussow process, in which, at temperatures above about 1000° C., ammonia is allowed to react with methane to hydrogen and hydrogen cyanide, also a noble-metal gauze is used. In this case too, a stabler metal gauze is of great significance. Finally, processes are currently being worked on, to produce synthesis gas, a mixture of carbon monoxide and hydrogen, by contacting a stream of methane and pure oxygen with a catalyst, for instance platinum, at temperatures above 1000° C. Such processes too could highly advantageously utilize precious metals applied in finely divided form to stabilized metal gauze.
Obviously, according to the state of the art of enameling, much research has already been done on the application of protective ceramic coatings to metal and alloy surfaces. In general, the conditions and the chemical composition necessary to accomplish a good bonding to the metal are known. However, it has been found to be very cumbersome to apply an enamel having a softening point or melting point that lies at a high temperature with a homogeneous chemical composition as a thin uniform layer to metal or alloy surfaces. According to the present state of the art, it is also cumbersome to accurately set the chemical composition of the protective ceramic layer. This is an important objective of the invention.
In the use of porous ceramic layers on solid surfaces, an object can be the protection against too high a temperature of the metal or the alloy upon exposure to a high-temperature gas stream. Considered in particular in this connection are gas turbines, where the metal or the alloy exhibits too slight a mechanical strength at the desired high temperatures. In that case, use can be made of a porous layer of a thermostable material which, through an effectively low heat conductivity of the porous layer, leads to a temperature profile over the porous layer such that the temperature of the metal or the alloy does not exceed a particular limit value. Firm anchorage of such a porous layer, when used in gas turbines, is obviously an important condition.
A second use contemplated by the invention is the use of a porous layer applied to a solid surface as catalyst. According to the prior art, such layers are applied to solid surfaces by using so-called dip coating techniques. The surface to be covered is immersed in a suspension of the catalytically active material and the surface is removed from the suspension at an empirically determined speed. Depending on the viscosity and the other properties of the suspension, a layer of the catalytically active material of a certain thickness then deposits on the substrate. For the production of exhaust gas catalysts, this method is presently used on a large scale. Used as substrates are, virtually exclusively, ceramic monoliths. To date, however, no successful attempts have been made to modify the dipcoat or washcoat process such that firmly anchored catalytically active layers can be applied to metal surfaces.
According to the prior art, a high-porous layer exhibiting better bonding can be applied to ceramic and metallic surfaces by starting from solutions of silicone rubber or the titanium-containing equivalent thereof. This is described, for instance, in U.S. Pat. No. 5,472,927. By dipping or by spin-coating, a thin layer of such an elastomer can be applied to the surface to be covered. Pyrolysis of the thin layer of the elastomer resulting after drying then leads to a high-porous layer of a ceramic material. A so prepared layer of silicon dioxide maintains the porosity up t
Beck Shrive P.
Cleveland Michael
Universiteit Utrecht
Weingarten Schurgin, Gagnebin & Lebovici LLP
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