In connection with industrial furnaces, in particular those used in the fabrication of metals, such as flash smelting furnaces, blast furnaces and electrical furnaces, massive cooling elements are used, which are typically made of copper. Operating conditions are extreme, so that copper is subject to powerful corrosive stress caused by the furnace atmosphere or even contacts with the melt. For example the copper corrosion caused by the oxidizing and sulphating reactions in an SO2 atmosphere may at worst lead to material depletion of up to tens of millimetres from the corroded surfaces.

A cooling element is usually composed of a frame made of copper or copper alloy, in which a network of cooling channels is formed for the circulation of the cooling medium.

WO publication 01/63192 describes a cooling element intended for furnaces in the metallurgical industry, where the surface section of the copper cooling element has a refined steel coating formed with a diffusion joint. The coating is only on part of the frame surface of the element. The diffusion joint is formed with at least one intermediate layer, which is either of chrome or nickel. When nickel is used as the first intermediate layer it can be formed on the surface of the layer for instance by electrolysing or in foil form. If several transmission layers are used, the second layer comprises an activator, with a melting point lower than that of pieces to be joined and which achieves the necessary reduction of temperature to create the joint. The activator is mainly composed of silver (Ag) and/or tin (Sn), or silver and copper (Ag+Cu) as alloy or compound, aluminium and copper (AI+Cu) or tin and copper (Sn+Cu). The joint surfaces with the intermediate layers are pressed together

and at least the joint area is heated to e. g. a temperature range of 600- 850°C.

A cooling element made of copper or copper alloy is also described in JP patent application 58-147504, where a coating made up of several layers is formed on the surface. First a nickel alloy layer is added to the copper surface by electroplating. A chrome coating is further dispersed on the nickel alloy surface. The chrome coating can be obtained by electroplating, spraying or some other coating technique. When the piece is heat-treated, diffusion layers are formed between the layers.

The cooling elements described above are without doubt excellent and long- lasting in practice, but in both cases the formation of the coating on the surface of the element is a process that demands many stages. In addition, when using refined steel coatings they cannot be chosen purely on the basis of the desired corrosion properties, because the composition of the kinds of refined steel that are commercially available is a limiting factor.

The purpose of this invention is to attain a method for the formation of a coating on at least part of a metallurgical furnace cooling element in a simple way with thermal spray coating. This can be used to achieve a coating on the surface of a cooling element, which is durable in furnace conditions. The corrosion properties required in different applications vary and the method according to this invention extends the freedom of choice of material with regard to corrosion properties considerably. The advantage of a coating formed with thermal spraying technology is that it can be made on the surface of the element either in the production stage or at the location of the cooling element, without the necessity of removing the element from the furnace. A cooling element comprises a frame section of copper or copper alloy, into which a cooling channel network is made for the circulation of the cooling medium. Coating at the location is made on the surfaces that will come into contact with the furnace atmosphere.

The invention is based on the concept where a highly corrosion-resistant coating is attached to a cooling element mainly of copper, using thermal spray technology. The coating is composed of substantially iron and/or nickel based metals and the coating forms a metallurgical bond with the element.

In addition, molybdenum, chrome, cobalt or silicon can among other things be used as an alloying material in the coating and they can be aione or together, as metals or various alloys. The alloying components can also be as separate particles in the composite structure for example in the form of carbide or oxide. The coating can also be formed of special alioys such as Ni50Cr50, commercial superalloys or various combinations of all and/or some of the above. The principle is however the same i. e. the coating forms a metallurgical bond with the copper of the element. Thus the coating is still highly heat conductive but the corrosion properties are better than in a copper element.

The essential features of the invention are mentioned in the claims.

Typical alloying elements in the sprayed alloys, which are added to the basic coating, include Cr, Co, Mo and even Si. Small amounts of other metals can also include into the alloying elements. In particular it has been observed that nickel and molybdenum form sulphate quite slowly in the S02-S03 atmospheres typical of flash smelting furnaces, for example. On the other hand it has been found that alloying elements such as chrome and silicon promote the growth of a passive oxide layer on the coating material. In both cases the durability of the coating is increased. Using of copper or copper alloy as a coating is not sensible since the cooling element itself has been manufactured of copper or copper alloy and adding of said coating gives no additional advantage.

In this application the word copper also applies to alloy materials with a copper content of at least 50%. Before spraying the coating to the surface of

the cooling element, one transmission layer is possibly added to the surface, preferably of tin or a tin-dominant alloy. Hereafter in the text for the sake of simplicity we shall refer only to tin, but the term also covers other tin- dominant alloys. For example a tin layer prepared on the copper in advance is attached well metallurgically to the substrate and tin as an active metal reacts with the chemical elements in the coating, so that tin promotes the generation of a tight joint.

The shape and size of a cooling element depends on its use. One preferred application of the invention is that the element is what is termed a cooled chute element especially for conveying melt. In this case the coating layer can be arranged e. g. for the part of the surface that comes into contact with the melt.

Of the thermal spraying techniques available, in practice at least techniques based on gas combustion have proved practicable. Of these, High Velocity Oxy-Fuel (HVOF) spraying is based on the continuous combustion at high pressure of fuel gas or liquid and oxygen occurring in the combustion chamber of the spray gun and the generation of a fast gas flow with the spray gun. The coating material is fed into the gun nozzle most often axially in powder form using a carrier gas. The powder particles heat up in the nozzle and attain a very high kinetic speed (several hundreds of metres per second) and they are directed at the piece to be coated.

In ordinary flame spraying, as the mixture of fuel gas and oxygen burns it melts the coating material, which is in wire or powder form. Acetylene is generally used as fuel gas due to its extremely hot flame. The coating material wire is fed through the wire nozzle with a feed device using a compressed air turbine or electric motor. The gas flame burning in front of the wire nozzle melts the end of the wire and the melt is blown using compressed air as a metallic mist onto the piece to be coated. The particle speed is in the range of 100 m/s.

Thermal spraying techniques melts the surface material and since the molten droplets of the coating to be used have a high temperature, a metallurgical bond is generated between the copper, tin and coating material in the coating of the cooling element. If necessary a separate heat treatment can be carried out after spraying, which promotes the phase changes occurring in the diffusion reaction at the interface, thus safeguarding the creation of a lasting joint.

Typically, but not necessarily, at least one intermediate layer is brought to the joint area in the form of a foil. Using a tin layer with a thickness of for instance 5-10 um has achieved extremely good quality joints. Tin layers can be formed in many ways as in the prior art, by tin coating through heating, electrolytic coating e. g. by producing a foil on the surface as an intermediate layer or actual coating by thermal spraying directly on the surface point.

When the coating of the cooling element is performed at the production stage the coating can be performed with several techniques like HVOF-techniques or flame spraying. When the coating is preformed at the cooling element location both said spraying techniques are applicable since the size of the equipment needed are such that they are movable and can be used also in furnace conditions.

Example A copper cooling element of a flash smelting furnace being L-shape was coated at the production stage. An intermediate layer of tin having thickness of 5 microns, were first formed on the surface of the cooling element. After that, the coating was formed to the element by using HVOF-techniques. The coating material was as a fine-grained powder and its composition was as follows : 62% Ni, 1% Co, 5% Fe, 21% Cr, 9% Mo, 0,5% Mn and 0,5% Si. The layer thickness of the coating was in the average 0.6 mm. The investigations pointed out that a metallurgical bond was formed between the coating and the element.