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Metal Spraying

Metal spraying is a process to iron and steel for applying protective coatings. It consists in spraying particles of the molten metal on to the surface to be treated and can be used with most of the common metals and various alloys. The coating metal, in the form of a wire, is fed into a spray gun in which it is melted by the combustion of a fuel gas e.g., a mixture. of oxygen and acetylene.

A spray gun (shown in Fig.1) comprises two rollers which are powered by an air turbine (driven by compressed air) or an electric drive and which feed the wire through the central part of a special nozzle; The gas at the nozzle is ignited; the wire is melted on emerging from the nozzle; the molten metal is atomized by compressed air and is projected at high velocity against the surface to be coated. In another type of spray gun (not illustrated) the wire is melted in a combustion chamber in the head of the gun. Although the particles of molten metal are cooled instantly, the impact causes them to adhere firmly to steel surface, provided that it has been cleaned and roughened thoroughly, as by machining or by sandblasting. Special measures may have to be taken in certain cases to ensure good bonding of the sprayed metal to the steel surface.

A special adhesion-promoting intermediate layer may be provided, the steel component may be preheated, or a subsequent heat (after spraying) may be applied. A new process which is known as fuse bond can be implemented in which the surface is roughened by low-voltage electric arcs. This method is generally used in cases where the metal to be sprayed cannot be drawn into a wire, e.g., hard alloys or has a high melting point.

Metal spraying is a versatile technique, which assures portability and flexibility. This technique can be useful in the field to steel bridges, storage tanks etc. Zinc spraying is the best choice for this type of structures. Spraying is used not only for protection against corrosion but also for building up surfaces. e.g., for reconditioning worn or damaged parts, for filling holes and cavities, and for the application of friction surfaces to bearings. In case of metallized coating it can be built up to any reasonable thickness & can be filled and polished.

Zinc metal spray applied by gas combustion or electric arc process will provide a long life coating of 10 to 25 years. Zinc is suitable for protection up to 200C. This process is often specified in preference to galvanising for its ease of application without the need for drilling of drain/vent holes and to avoid the risk of distortion. For Example: Clifton Suspension Bridge, Zinc metal sprayed and painted in 1955 and still in excellent condition. Steel or hard-alloy coatings are used as wearing surfaces; for instance, light-alloy pistons can be surfaced with a sprayed steel coating. In the electronics and telecommunication industries, metallic coatings are applied to nonmetallic materials to make them electrically conductive.

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