Electric arc spraying

The distributed anodes used in Florida are principally electric arc sprayed zinc, a few tenths of a millimetre thick, or zinc metal mesh (Figure 7.5). 

Thermal sprayed zinc - Zinc is flame or electric arc sprayed onto the concrete surface and a direct connection made to the reinforcement. It can be used as sprayed on marine splash or tidal applications. In drier locations a humectant solution of hygroscopic salts can be applied. Over 50,000 m has been applied, mainly in Florida. 

During electric arc spraying, an electric arc between the ends of two wires continuously melts the ends while a jet of gas (air, nitrogen, etc.) blows the molten droplets toward the substrate at speeds of 30 to 150 m/s. 

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. 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

Although at the time of his early inventions Dr. Schoop envisaged that an electric arc could be used to produce the molten metal for spraying, forty years passed before the method became commercially important. Then, in Germany, Russia and Japan tools were made based on the arc. Although in Japan alternating current is used, the noise is nearly intolerable and elsewhere direct current from motor generators is employed. The fundamental idea is simple two wires, carefully insulated from each other, are advanced to meet at a point where an arc is formed, immediately in advance of a Jet... 

The vision of the late W E Ballard ensured that this chapter is still very relevant but there have been some changes in emphasis. Of the spraying techniques, the electric arc process is now generally more economical in operation than oxy-fuel gas processes. The rate of spraying is proportional to the current used and several types of equipment are available ranging from 200 to 1 000 A capacity. For manual operation, pistols using currents up to... 

James, D. H., Thermal Spraying by the Electric Arc Process , Metallurgist and Materials Technologist, IS, 85-90 (1983)... 

Wire flame spraying Electric arc flame spraying Powder flame spraying Plasma flame spraying Hypersonic flame spraying... 

The same method may be applied to solids when they melt without decompn. Spraying may also be applied to solns of solids in volatile liquids, such as acetone, alcohol, etc. Solids may also be diminuted to very small size by the methods used for ptepn of colloids such as by using a colloid mill or a homogenizer, as described in Ref 1. Another method for prepn of small solid particles is electric atomization, which is achieved by passing an electric arc between electrodes of pure metal in distilled water contained in a vessel made of practically insol material. This method produces a stable colloidal soln of the metal used as the electrodes (Ref 3)... 

The reaction gas is cooled to ambient temperature in a water spray column, and the quench water that condenses is fed back in a closed cycle to the electric arc reactor. The gas is then compressed and subjected to a sequence of scrubbing operations with selective solvents [i.e., methanol, octane, and A/ -methyl-2-pyrrolidone (NMP) in that order]. The solvents are regenerated by low-pressure stripping with gas fractions from the production process. This removes mainly the polyunsaturated hydrocarbons. 

For solvent-sensitive plastics where paint systems are unsuitable, zinc arc spraying is sometimes used. In this method, the metal is melted by an electric arc and sprayed as droplets by compressed air onto the part to be coated. Consequently, only materials with very high melting points can be used. The method gives a hard, dense coating of good conductivity, but it requires expensive equipment. 

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