May 31, 2008
Metal Surface Protection
Metals are essentially artificial and unstable materials, that is to say, they are not found as such in nature (excepting gold and copper) and they tend, under the influence of the weather, waters and similar corrosive exposure, to revert back to a non-metallic state. It is convenient for reasons of cost, strength and ease of working to manufacture metallic articles without too much regard to their external appearance or to their behavior against corrosion, and then to deal with these purely surface characteristics by a subsequent treatment, which may therefore be designed in general as surface treatment of metals. Since iron and steel are by far the most commonly used metals, and also unfortunately among the most prone to rust, decay and corrosion, they figure very largely, but not by any means exclusively, in the general subject of the surface treatment of metals.
The main purpose of the surface treatment of metals are predominantly twofold, with emphasis on one or the other to different degrees in different cases, viz:
1. To improve the appearance
2. To improve resistance to corrosion, tarnish or staining under the conditions of use which are foreseen.
There are other objectives, each important in its own sphere, but these together are only of minor importance compared with the above main purposes. Examples of these minor objectives are to provide resistance to scaling by heat, to facilitate other subsequent surface treatments or to change the surface hardness or other surface properties, or even to change the overall dimension.
The surface treatment of metal almost invariably consist of applying a coating of some kind to the metal, and thus providing an external skin which can be selected solely for its appearance and corrosion resistance. If such a coating can be applied without any discontinuities, and if it is firmly adherent to the behavior of the coating. However, such external coatings must inevitable be thin, unless the whole character of the article is changed. In the normal course of use these coatings therefore entail the risk of penetration through to the basis metal by mechanical damage or flexing; the effect of the small areas of basis metal thus exposed can't be ignored, not only because of the local corrosion of the exposed metal, but because of the risk that the remainder of the coating may be undermined and thrown off.
In the choice of surface treatment for any particular metallic article, many factors must be considered, such as cost, type of corrosive environment to be withstood, and compatibility with special materials such as foodstuffs or washing materials. In very many cases an additional factor of continuing good appearance must be considered. Metal surfaces, especially satisfying appearance. It is obviously appropriate to provide a metallic article with a metallic coating of higher corrosion resistance and more attractive appearance, so that the whole system is a unity. But very few metallic coatings are absolutely resistant to corrosive influences, and these few are the most expensive. In particular, metallic coatings are not completely resistant to some of the many chemical substances now involved in everyday life, such as washing chemicals, fruit acids and many foodstuffs. Nor do they provide much opportunity to introduce colors, including white, which may be desired to conform to decorative schemes or fashions. Thus non-metallic coatings, such as vitreous enamels, paint and lacquer, are applied to metallic articles for decorative as well as protective reasons.
Before any surface coating can be applied it is essential that a clean, sound surface must be provided on the article to be coated. In particular, scale and oxide, grease and dirt remaining from the manufacturing operations, must be removed; the surface usually must be smoothed or even polished. It may also need to be prepared physically or chemically to ensure satisfactory adhesion of the coating. These preparatory treatments are an integral part of the surface treatment process, and obviously have to be applied at the end of the main manufacturing process. For this reason these cleansing, protective and preliminary decorative treatments are almost invariably applied as the final stage of the manufacturing process of a metallic articles, to such an extent that the processes are often collectively referred to under the term 'metal finishing'. In the cases of hot dipping tinning and hot dipping galvanizing of sheet steel the protective films are applied to the semi-fabricated material; here, however, the subsequent process are limited to cutting, bending and joining; moreover, decorative effects are subordinate to economics.