June 7, 2008

Choice of Surface Treatment

Metallic coatings have the advantage of being hard and strong and of resisting moderately high temperature; they are thus fairly robust. They can usually be chosen to give a high degree of protection against most types of corrosive environment, excluding strong acids and chemicals. Where low cost is important and good protection is required, without much emphasis on appearance, hot dipped coatings are cheap and very effective. Where a neat, highly polished surface is required costs are much greater and electroplating is pre-eminent. Both hot dipping and electroplating evolve immersion of the articles into a liquid held in a container, so the maximum size of object which can be treated is limited. Metal coatings can be applied by metal spraying to objects of any size, if necessary in situ.

Vitreous enamel coatings can only be applied to a restricted number of metals which withstand the high temperature of application. As they are composed in effect of glass, they are highly resistant to most chemicals, but are extremely brittle and can only be applied to relatively rigid article.

Paint systems can be applied to articles of any size; if necessary to selected areas the hardest stoved paints are damaged relatively easily, are not very heat resistant and are seldom absolutely impermeable to water. Although they are fairly cheap as regards raw materials, paints are expensive and wasteful to apply.

The choice of coating is also dictated to some extent by the basis metal or alloy, iron and steel predominate and as they are so readily and disfiguringly corroded, they need special protection. Copper alloy, such as brass and nickel silver, are much less corrodible, but are more expensive and thus justify a more costly finish. The lower melting point metals and alloys of head, tin and zinc obviously cannot endure a hot process. Zinc, which is used very extensively in the form of zinc base die-casting, is a very active metals and needs effective protection.

Aluminum is a special case, since both the metal and its alloy have a very high resistance in the uncoated condition to atmospheric corrosion and to water, and do not readily accept coatings of other metals or of paints. This natural resistance is a consequence of an anodic film which forms automatically on the metal.

Alone of the cheaper industrial alloys, stainless steel does not need further protection, but its full appearance can only be brought out by suitable polishing, which is made expensive by the hard and tough nature of the material.

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