Metal spraying

AppHcation of an adhesion-promoting paint before metal spraying improves the coating. Color-coded paints, which indicate compatibiHty with specific plastics, can be appHed at 20 times the rate of grit blasting, typically at 0.025-mm dry film thickness. The main test and control method is cross-hatch adhesion. Among the most common plastics coated with such paints are polycarbonate, poly(phenylene ether), polystyrene, ABS, poly(vinyl chloride), polyethylene, polyester, and polyetherimide. 

Thermal or Flame Spray Process. The earliest experiments in metal spray used molten metal fed to a spray apparatus, where it was dispersed by a high speed air jet into tiny droplets and simultaneously blown onto the surface of the part to be covered. The metal solidified on contact. Modem processes use a more convenient source than premelted metal. Spray heads using a flame or an electrical arc to melt metal wires or powders directly are much more convenient. These are the only types used on a large scale in the United States. 

Metallizing-also called metal spraying refers to the application of a metal coating to a surface (either metallic or nonmetallic) by means of a spray of molten particles. 

Zinc or aluminum metal spray Sprayed metal coatings are porous and should be sealed after application by applying a sealer coat (i.e. a thin coat such as an etch primer) or a thinned version of the final coating system. Oil-based systems should not be used. Metal spray coatings can have excellent durability without overcoating with paint and, particularly for aggressive conditions it is preferable to leave them with sealer only. 

The metal spraying operation using zinc or aluminium as a protective coating is usually followed by a painting scheme. The choice of sprayed metal and paint scheme depends on the service conditions , but normally this type of system is used on prestige buildings or structures, where longevity is of prime importance and maintenance requirements need to be kept to a minimum. 

The second process (the Schori process) of metal spraying is that in which... 

Deposition by metal spraying can also be used for the reclamation of worn parts in this case, surface preparation is often accomplished by machining, i.e. by cutting a rough thread on the surface or by increasing the surface area of the part by grooving. Such methods. are not, however, normally used in corrosion prevention, except in the case of pump rods, which can be built up with nickel or stainless steel. 

The metals most used for corrosion protection by metal spraying are aluminium and zinc, both of which are anodic to steel in most environments. Physical properties of these coatings are shown in Table 12.6. 

PRINCIPLES OF APPLYING COATINGS BY METAL SPRAYING Table 12.6 Physical properties of sprayed zinc and sprayed aluminium... 

The coatings produced by metal spraying have an unusual structure which is characteristic of the method of formation. They are composed of small particles usually not more than 0-01 mm in diameter which, having reached the surface in the molten condition, have splashed outwards and then solidified. Figure 12.27 (left) shows in section the irregular form of the flattened particles. In transverse section the surface profile is undulating (Fig. 12.27 (right)). 

In all metal spraying processes the particles emerge from the nozzle in a conical stream, and although the particles near the centre are molten, those at the periphery have solidified. In the powder process there are in addition solid particles which have not melted. The solid particles tend to become entrapped in the coating, making it porous. The effect is more pronounced in the powder process owing to the larger number of solid particles present. 

Where the end use of the product is known, there is usually preference to use either zinc or aluminium, both technically and because of the works problems associated with use of an alloy (identification, separation of overspray). However, in some countries (such as the United States) where there has been a recent-surge in anti-corrosion uses of metal spraying, a zinc-15%-aluminium alloy wire has been widely used. The original commercial experience was with 65-35% alloys used in powder form. Both have many of the advantages of the parent metals. At one time, the zinc-5%-aluminium alloy was also of interest. These alloy coatings may prove particularly satisfactory for sprayed coatings on articles where service conditions are not known in advance. 

A low acid polyvinyl butyral-based pretreatment primer ( etch or wash primer are alternative names) is usually advantageous as the first treatment of a metal-sprayed surface before painting. Up to an equal volume of spirit soluble phenolic resin is used as a diluent to the polyvinyl butyral of conventional pretreatment primers. This has an incidental, but particularly valuable effect, in reducing the free acid available to penetrate into the pores of the coating. The modified pretreatment primer is highly water resistant and this helps to avoid damage due to condensation. 

Bailey, J. C., Porter, F. C. and Round, M., Metal Spraying of Zinc and Aluminium in the UK , Proc. 12th Int. Thermal Spraying Conf, Paper 8 (1989)... 

Bailey, J. C. Corrosion Protection of Welded Steel Structures by Metal Spraying, Metal Constr. 15 No. 5, 264-270 (1983)... 

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