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October 11, 2008

Metal Spraying

Protective metal coatings can also be sprayed into cleaned and roughened article in the form of molten metal droplets, which congeal and bond immediately on impact. The article does not become unduly heated (even paper can be metal sprayed) and the coating does not wet or alloy with the basis metal, so that the adhesion of the coating is only mechanical. The coating is only an aggregate of flattened quenched droplets, interspersed with oxide films and some porosity so that is very brittle.

Nevertheless the process has the special advantage over most of the processes so far described, that it can be applied to structures of any size and, if necessary, in situ. It is thus specially suitable for the protection of ships hulls, structural steel work, bridges and other civil engineering steel parts. The steel surface must be freed from all scale and rust before coating; this is usually done by shot-blasting or sand-blasting, using compressed air. This preparation also roughens the surface and facilities the mechanical adhesion of the coating.

Metal spraying is chiefly for applying zinc or aluminum onto steel. Three main methods are used. The most popular and most highly developed utilizes a hand held or machine-held pistol in which a wire of the coating metal is continuously fed into an oxyacetylene flame, where it is melted, and the droplets of molten metal are impelled on the work by compressed air. In another method, the coating in the form of powder is blown through a melting flame onto the work by the compressed air. In a third method, pre melted metal from a reservoir is atomized and blown onto the work piece. The rate of deposition is quite fast, but only a small area is covered at one time. Coatings of at least 0.002 in, and usually of 0.005 in, are applied; such thick coatings are necessary because the deposit is rather porous. The outer surface is rough and unattractive in appearance but is an excellent base of subsequent paint coatings. The porosity is less harmful than in thinner for it soon seals itself with corrosion product.

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