The invention relates to a method according to the preamble of claim 1 for coating a part used in an oil refinery unit with a layer of a nickel alloy material. The invention also relates to a coated part according to the preamble of claim 9 for use in an oil refinery unit.

Hydrofluoric acid (HF) is an important basic chemical commonly used in the indus- try. For instance, in alkylation units of oil refineries this acid is used as a catalyst.

Hydrofluoric acid attacks most metals and metal alloys in a corrosive manner. The rate of corrosion is affected, among other factors, by the concentration, temperature and moisture content of the acid. The corrosion rate may additionally be accelerated by air and oxidizing metal ions that often occur in hydrofluoric acid as impurities.

Construction materials commonly used in the handling of hydrofluoric acid are carbon steels and, in particularly demanding cases, nickel alloys of which particularly well known is a nickel-copper alloy marketed under tradename Monel 400. In fact, carbon steel parts have generally been replaced in applications most severely subjected to corrosion by similar components made from a nickel-based alloy. However, nickel alloys have a limited use in plural sites due to their high, even hundred-fold price and weaker structural properties in regard to carbon steels.

To improve their corrosion resistance, carbon steel parts may also be protected by a coating of a nickel alloy material. Herein, the coating is applied to the surface of the base material generally by spraying, welding or dip-coating, the latter taking place by dipping the part to be coated into a bath of molten coating material. A disadvantage of these methods is that the coating layer contains pores via which the corrosive medium can penetrate to the surface of the base material, whereby local corrosion at the points subjected to the corrosive material may proceed extremely rapidly. To provide an unpenetrable coating layer, it must be made thick with the inevitable consequence, that the coated part often requires machining after the application of

the coating layer. Hence, conventional coating methods are often awkward in the application of a sufficiently solid and thin coating layer.

It is an object of the present invention to provide an entirely novel type of coating method for applying a coating of a nickel alloy material onto the surface of a part employed in an oil refinery unit.

The goal of the invention is achieved by means of forming the coating layer with the help of a laser coating method wherein the nickel alloy coating material is melted on the base material surface with the help of a laser beam. When necessary, a separate intermediary layer can be applied between the base material and the coating material layers if these two materials are not inherently metallurgically compatible with each other.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the part according to the invention is characterized by what is stated in the characterizing part of claim 9.

The invention offers significant benefits.

The coating layer applied by means of laser coating is solid and free from pores because the operating parameters of laser coating equipment can be effectively controlled during the entire coating process. This makes it possible to produce a coating layer free from pores, whereby the corrosion resistance of the coated part is improved substantially over coatings applied by conventional thermal coating methods.

In the following, the invention is described in more detail with reference to the ap- pended drawing illustrating a laser coating apparatus and a part being coated.

The part 1 to be coated is a part intended for use in an oil refinery unit, such as the alkylation unit thereof, wherein the part will become in contact with a corrosive medium such as hydrofluoric acid (HF). Such parts are, e. g., flanges used in piping.

Typically, the base material 2 of part 1 is carbon steel which in itself is not sufficiently resistant to very moist hydrofluoric acid. Obviously, the base material 2 of part 1 may also be any other material typically used in the oil refinery industry such as structural, alloy or stainless steel.

To improve its corrosion resistance, the part 1 is coated in laser coating equipment 5 with a coating layer 4 of a nickel alloy material such as a commercially available nickel-copper alloy known as Monel 400. In nickel-copper alloys, the basic compo- nent is nickel having copper and trace amounts of other elements alloyed therewith.

In Monel 400, for instance, the composition is about 64 % nickel (Ni), about 31 % copper (Cu) with trace amounts of iron (Fe), chromium (Cr), silicon (Si), carbon (C), sulfur (S) and manganese (Mn). Nickel-copper alloys have a high corrosion resistance against hydrofluoric acid and silicon hydrofluoric acid as well as salt water, high-concentration chloride solutions. Furthermore, they are resistant to alkaline solutions having a concentration less than 50 %.

The coating layer may also be nickel or some other nickel alloy material such as a nickel-chromium (Cr), nickel-molybdenum (Mo), nickel-chromium-molybdenum or nickel-chromium-molybdenum-copper alloy. Typical compositions of the most commonly used nickel-based alloys are listed in the following table. Type Tradename Ni % Cr % Mo % Fe % C % Cu % Ni Nickel 200 99. 5 0. 08 Nickel 201 99.5 0.01 NiCuMonel 400 64 2 31 NiCr Inconel600 72 16 8 Incoloy800 33 21 45 0. 1 NiMo Hastelloy B-2 68 28 0. 01 NiCrMo Inconel625 60 22 9 5 0.1 Hastelloy C-276 56 16 16 5 0.01 Hastelloy C-4 64 16 15 3 0.01 Hastelloy C-22 55 22 13. 5 4 0. 0015 NiCrMoCu Hastelloy G-3 45 22 7 19.5 0.0015 2 Incoloy 825 42 22 3 30 0. 02 2 The laser coating equipment 5 comprises a laser gun 6, wherefrom coating material in pulverized form is applied with the help of a carrier gas to an area of the part 1 premelted by the laser beam 7. Prior to coating, the surface of part 1 can be machined to a desired roughness and cleaned in order to improve the adhesion of the coating material 4 thereto. The feed of the coating material in the laser gun takes place coaxially about the laser beam 7, whereby the flow of the coating material with the shielding gas surrounds the laser beam 7. Instead of using pulverized coating material, the coating material may also be introduced to the working area of the laser beam 7 as a sheet or wire. The shielding gas is carbon dioxide or argon. The surface of the part 1 being coated is moved relative to the laser beam 7 and/or the laser beam 7 is moved over the part 1 relative to the surface being coated. The part 1 to be coated may have a rotation-symmetrical or planar shape, for instance. Rotation- symmetrical parts 1 can be coated when mounted, e. g., on a lathe.

The scanning speed of the laser beam 7 over the surface of the part 1 being coated is advantageously 100 to 1000 mm/min, most advantageously 230 to 270 mm/min. As the heat imported by the laser beam 7 is primarily absorbed by the part 1, the melted coating material 4 solidifies rapidly with the progress of the coating process. Due to the low total amount of thermal energy and fast cooling rate used in the process, the

coating layer 4 becomes pore-free. Since the laser beam 7 melts only a small area on the surface of part 1, the cooling thereof does not cause a major shrinkage of the coating layer 4. Moreover, the shielding gas protects the molten coating material 4.

The width of a coating layer strip 4 applied in a single sweep of the beam is deter- mined by the distance of the focus point of the laser beam 7 relative to the surface 2 being coated. Typically, the width of a single strip of the coating layer 4 formed by moving the laser gun 6 and/or the surface to be coated is 2 to 3 mm. The thickness of the coating layer 4 formed by the laser coating method is typically 0.1 to 4 mm, most advantageously about 1 mm. When necessary, a greater number of coating layers 4 can be superposed on each other.

The center axis of the laser beam 7 is aligned substantially perpendicular to the envelope surface of the part 1 to be coated. In the context of the present invention, the term envelope surface is used in making reference to the surface delineating an ideal shape of the part being handled wherefrom the actual shape may differ due to, e. g., manufacturing tolerances, wear and other deformations.

The coating material must be metallurgically compatible with the base material 2.

The mutual compatibility may be improved by a proper choice of additives in the coating material or through altering its composition. The qualities achievable by means of the coating are determined by the application, base material, coating material and process parameters used.

If the material of the coating layer 4 and the base material 2 are not metallurgically compatible, cracking of the coating layer 4 may occur. To prevent this, it may be advantageous to form onto the surface of the base material 2 an intermediary layer 3 of a material that is metallurgically compatible with both the base material 2 and the material of the coating layer 4. Since the intermediary layer 3 thus remains protected between the base material 2 and the coating material 4, it cannot be attacked by the corrosive medium. The intermediary layer 3 can be deposited using, e. g., the laser coating method or by spraying. Next, the intermediary layer 3 is machined to a desired roughness and cleaned to improve the adhesion of the coating material 4

thereto. If the base material 2 is carbon steel and the coating material 4 is a nickel- copper alloy, the intermediary layer 3 may in certain cases be necessary between the surface of the base material 2 and the coating layer 4. Herein, it is possible to make the intermediary layer 3 from some other type of a nickel-based alloy, such as a nickel chromium or nickel-chromium-molybdenum alloy.