The invention relates to a body of the porous type, in particular suitable for use in a corrosive environment with a high temperature. The invention also relates to a process for the manufacture thereof. Such bodies are known per se in the form of ceramic bodies which are porous to the extent that they offer a relatively low flow resistance to gases flowing through.

A known application of such bodies takes place in catalytic processes. For this purpose, the ceramic body comprises channels which typically have a honeycomb structure. Catalytic materials are provided on the- wall of the internal channels of the ceramic body. When a suitable selection of the catalytic materials is made, exhaust gases of motor vehicles and flue gases, for instance, can be purified. Also, such bodies can be used as a burner bed.

However, a drawback is that such ceramic bodies have a low thermal conductivity and a slight mechanical strength. A low thermal conductivity often leads to an insufficient removal of the heat of reaction in catalytic reactions with a great positive thermal effect. As a consequence, the catalytic materials become too warm and may start sintering so that the catalytic activity of the body will decrease. Also in catalytic reactions with a negative thermal effect a good thermal conductivity is often desired to supply heat of reaction. Another drawback of ceramic bodies is that they are difficult to attach to a metal object in order to obtain an end product or intermediate product, since ceramics cannot be welded or soldered.

Attachment of a ceramic body to a metal object moreover involves the further drawback that ceramics and metal have different coefficients of expansion, so that, at high temperatures, great stresses are produced between the metal object and the ceramic body, which may lead to fractures in

the ceramic material or disintegration of the metal object and the ceramic body.

The object of the invention is to provide a porous body which does not have the above-mentioned drawbacks. To this end, according to the invention, the body is composed of a sintered material, substantially comprising a metal or a metal alloy provided with at least one diffusion coating at least at or near a surface or a partial surface of the body.

Because the body is partly composed of a sintered material substantially comprising a metal or a metal alloy, an excellent thermal conductivity is obtained. Also, such a body can readily be attached to a metal object without giving rise to the above-mentioned drawbacks in situations of extreme temperatures. Moreover, the sintered material provided with a diffusion coating is insensitive to a corrosive environment with a very high temperature.

A sintered material is known per se and is composed of metal particles, typically in the form of grains, which undergo a change through a pressure or temperature increase, such that the particles cement together as a result of softening or melting of the surface layer of the particles. The most important characteristic of a diffusion coating is that particular elements of the coating, such as Al and Cr, provided on the surface of the material to be coated, have at least partly diffused into the material and thereby change the chemical and physical properties of the material in a positive sense. Typically, an alloyed, often intermetallic surface layer is formed consisting of the inwardly diffused element and the sintered material. This will involve the formation of a metal oxide of the inwardly diffused element as a surface layer. In accordance with the invention, this does not only change the above-mentioned properties of the portion of the sintered material, i.e., the sintered particles, on which a diffusion coating is provided, but also the properties of the entire body.

European patent application 0.410.200 describes a sintered metal on which optionally, through vaporization, catalytic material may further be provided. Because according to this European patent application, no diffusion coating has been applied, the end product does not have the advantages described hereinabove in connection with the invention.

German patent application 3.642.839 describes a body of a sintered material which is provided, in the vicinity of its edges, with a synthetic material which, after a heat treatment, diffuses into the pores of the sintered material. However, such a coating is not a diffusion coating and is intended to strengthen the sintered material.

According to a process in accordance with the invention, the body is made up of particles substantially comprising a metal or metal alloy by means of the sintering process and the particles of the body are provided with at least one diffusion coating.

It is particularly surprising that it proves to be possible to provide a diffusion coating on the sintered material with techniques that are known per se, in such a manner that the sintered material is well-protected against a corrosive environment by the diffusion coating. The body thus obtained is suitable to be used in corrosive processes in an environment with a very high temperature of, for instance, above 500 degrees centigrade and even of up to more than 1,200 degrees centigrade. Up to the present, the skilled person has always sought a solution to the problem of corrosion at a high temperature in the selection of the basic material (typically ceramics) of the porous body. Up to the present, diffusion coatings and diffusion coating processes have been used exclusively in non-porous, solid bodies.

It has been found that the porous body according to the invention is particularly suitable to be used in a corrosive environment at extremely high temperatures. In addition, it is surprising that the body according to the invention has good

porous properties which contribute to the possibility of using the body in various products and processes.

Although it is known to use sintered bodies in catalytic and combustion processes, such bodies are not suitable for use in a corrosive environment with a very high temperature. Under the influence of a corrosive environment with a high temperature, such a sintered body will inevitably start oxidizing. As a consequence, the sintered body will lose its catalytic activity. Moreover, such a body will lose its porous properties because the spaces between the sintered particles will be filled with metal oxides. As a consequence, for these reasons, too, the body is not suitable to be used in catalytic, filtering and combustion processes with a high temperature. In this connection, by porous body is meant a body composed of a porous basic material. This involves that the material itself is porous. In general, the porous properties of the material are at least practically equal in all directions, so that in this respect, the material and more particularly also the porous body can be referred to as "isotropically porous". Such porous sintered metal can be obtained by adjusting the size of the particles of which the material is composed and the sintering temperature to each other in a manner known per se. To reach a high porosity, water-atomized metal powders are preferably started from. The diameter of these powders is between 200 and 1,200 micrometers, depending on the desired porosity and pore size of the sintered body. As a starting material, rustproof steel, for instance of the type 316 L, may be selected, the sintering temperature being between 1,100 and 1,400 degrees centigrade. The porosity obtained is between 50% and 80%, the pore diameter being between 50 and 400 micrometers.

European patent application 0.450.897 describes a body composed of a sintered material. Within the body, channels (honeycomb structure) are provided, extending in the longitudinal direction of the body. Through these channels, a

gaseous medium to be treated may for instance be conducted. Here, however, the sintered material itself is not porous.

Preferably, the sintered material is provided with grains substantially comprising metal or a metal alloy. The grains are particularly suitable for being formed into a body having good porous properties. According to a particular aspect of the invention, the sintered material is provided with fibers substantially comprising a metal or a metal alloy. The size of the grains or fibres is not bound to any limit. A fibre can be defined as a particle having a length-width ratio greater than five. In all other cases a grain is involved.

According to one aspect of the invention, at least those particles of the sintered material that are under the influence of a corrosive environment with a high temperature are provided with a diffusion coating.

In particular, practically all sintered particles of the sintered material are at least practically completely provided with a diffusion coating. This renders the object particularly suitable for use in a catalyst or in a filter, because in that case a corrosion-aggressive, high-temperature gas to be treated flows through the entire body and, therefore, all sintered particles have to offer resistance to the corrosive properties of the gas. If the sintered material provided with a diffusion coating is used in a catalyst, the diffusion coating will at least partly be covered with a catalytic material. Such a catalytic material proves to adhere well to the diffusion coating.

According to a particular embodiment of the invention, particles of the sintered material that are located at and directly below the above-mentioned surface or partial surface of the body are provided with a diffusion coating.

According to the invention, the body is also suitable for use as a burner bed. In that case, the body preferably comprises a diffusion coating on and near the partial surface of the body where the flame is formed, because particularly this portion of the body will be under the influence of the

high combustion temperature of a gas. A burner bed according to the invention is suitable for use at temperatures running up to more than 1,200 degrees centigrade. As has already been mentioned above, it is known to use a body formed from a sintered metal in a combustion process. However, such a burner bed is completely unsuitable for use in high-temperature combustion processes, as the body will lose its porous properties due to oxidation.

According to the invention, the metal or the metal alloy comprises at least one element from the group Fe, Νi, Cr or Cu. The metals or alloys obtained from these elements are relatively cheap and particularly suitable for being sintered. Hence, the body will have technically superior properties at economically attractive costs. After a diffusion coating has been provided in the sintered porous body, this body will also exhibit superior behavior at high temperatures.

According to the invention, the diffusion coating preferably comprises at least one element from the group Al, Cr, Si, Pt, Pd and Rh, while in particular at least one of the last three elements is used as an addition to the first three elements. The advantage of these elements and alloys is that they are excellently capable of offering resistance to a corrosive environment at a high temperature.

If, for instance, a sintered material substantially comprises Fe or Νi, Al may be selected as diffusion coating element. However, it is also possible to select Cr or Si as coating material. If the body consists of Νi and the coating element of Al, a diffusion layer consisting of ΝiAl, Νi ₂ Al ₃ or Al ₂ 0 ₃ can be formed, depending on the conditions under which the coating element is added. After heating under oxidative conditions, an Al ₂ 0 ₃ layer is formed. The addition of Cr to the Al diffusion coating further increases the resistance of the body to corrosion, in particular to corrosion in a sulfurous environment. In addition, Pt or other noble metals such as Rh and Pd can be added to further increase the resistance to corrosion.

According to the invention, the diffusion coating can be applied by means of processes that are known per se, for instance diffusion via a gas phase, liquid phase or solid phase, more particularly by chemical vapor metallization. In the case of vapor metallization, the diffusion coating is provided via a gas phase. In that case, at least one diffusion coating element is converted into a gaseous compound. This compound mostly consists of a metal and a halogen. The metal/halogen compound can deposit on the sintered material to be provided with a diffusion coating, through breakdown or reduction of the compound or through an exchange reaction with the sintered material. The net result is that the metal of the compound precipitates on the sintered material of the body and subsequently diffuses into the material. Because the sintered material has porous properties, the gaseous compound can precipitate not only on the surface or a partial surface of the body, but also on that surface of elementary sintered particles located inside the body that is located at least near, i.e. at least directly below, the surface or the partial surface of the body. This last is relevant when the body is used as a burner bed, because the elevated temperatures are attained in particular near the flame, i.e., at and directly below a partial surface of the body.

According to a particular embodiment of the invention, it is even possible to provide the entire body, i.e., all or at least practically all sintered particles, with a diffusion coating. For this purpose, the gaseous compound or a suspension is caused to flow through the entire body. As has already been mentioned above, this is particularly relevant in the case where the body is used in filters and catalysts, because in that case a corrosion-aggressive gas to be treated flows through the entire body and, therefore, all sintered particles have to offer resistance to the corrosive properties of the gas. In the literature, known processes for obtaining a diffusion coating are known, for instance, under the names of

^■ pack-cementation' process, 'halogen-streaming' process and 'vacuum-pack' process.

In the 'pack-cementation ¹ process, the sintered material to be provided with a diffusion coating is at least partly enveloped by a powder mixture consisting of an inert filling material, a halogen salt and the coating element. By heating, the halogen salt becomes gaseous and, together with the coating element, forms the gaseous compound mentioned, which will subsequently decompose at the surface of sintered particles so that the coating element precipitates and diffuses into the sintered material. Also, the powder mixture may be provided in the porous body beforehand and heated subsequently.

In 'halogen streaming', no halogen salt is used and instead, a halogen gas is supplied from outside, which forms the compound mentioned with the coating element. Thereafter, the process proceeds as described above. The process is carried out in vacuo if no inert filling material is present. This process is also referred to as the 'vacuum-pack' process. It also proves to be possible to obtain a diffusion coating by applying a covering layer which, after a temperature treatment in an inert medium, diffuses into the sintered particles.

It is also possible to simultaneously carry out the sintering process for obtaining the body and the process for obtaining the diffusion coating, more in particular the supply of the gas mentioned, resulting in a body composed of a sintered material provided with a diffusion coating of one or more elements. It will be understood that other known processes and variants of these processes can be used as well to provide the sintered material or the particles of the material to be sintered with a diffusion coating.

According to another aspect of the invention, the body is characterized in that the body can be obtained by putting together particles substantially comprising a metal alloy by

means of the sintering process, the metal alloy comprising at least one element which is suitable for functioning, after oxidation, as a coating, so that the sintered material can be used at a high temperature. The invention also relates to a process for obtaining such a body.

The alloy MCrAl (wherein M represents at least one metal element) , in particular FeCrAl, known as socalled Fe-CrAlloy alloys, the alloy containing at least 4 wt.% of aluminum, can be used for this. If the body sintered from this alloy is exposed to a high temperature under oxidative conditions, a continuous skin or coating of aluminum oxide will be formed which has a relatively slight rate of growth. Additions to the alloy of rare earth elements such as yttrium and cerium, or the addition of noble metals such as Pd, Pt and Rh lead to an even better oxidation behavior. Owing to the skin or coating mentioned, a sintered porous body thus obtained can also be used at high temperatures and has advantages and possibilities of use comparable with those described above in relation to the diffusion coatings mentioned, because in both cases a protective oxide skin is formed.

Finally, it is observed that the methods mentioned for obtaining a coating or diffusion coating can also be used in combination, resulting in a diffusion coating of one or more elements.