Post 438 – by Gautam Shah
Metals are coated for many different purposes, and with metals, alloys, non metal substances (carbon, phosphorus, sulfur, selenium, and iodine) and polymeric materials (plastics, elastomers, etc.). Metal coatings provide specific surface properties, like, rust inhibition, preventing tarnishing by weather, improve surface conductance or resistance, for surface alloying, for surface hardening, for pseudo metalizing (providing appearance or behavioural qualities of another metal), for surface alteration like adding gloss or textures, providing insulation, wear resistance, and for imparting colours. Some of the Coatings are discussed here.
CONVERSION COATINGS are chemical surface treatments which provide temporary corrosion protection to the surface and create an appropriate substrate for subsequent treatments or coatings. There are two main processes: Phosphating and Chromating.
● Phosphate coatings are mainly applied to ferrous and zinc, and to some extent for aluminium, tin and cadmium metals. It is mainly used as a pre-treatment for painting work of automobiles and sheet metal components. The applied coating is thin and porous. It offers mechanical keying to the paint film. The electrical insulation or inertness restricts the corrosion breaks. The phosphating treatment consists of immersion, brushing or spraying of hot dilute ortho-phosphoric acid solution. Similarly Zinc phosphate coatings are also used.
● Chromating or chromate coating is depositing a chromium oxide layer over a metal surface to enable the metal to react with the oxide layer. The process is used, mainly to increase the corrosion resistance of metals such as aluminium, magnesium, tin, zinc and cadmium, and also increase the tarnish resistance of copper and silver. Pure Aluminum is very soft, so its alloys (mixtures) with copper and other metals were used to make a lightweight but strong parts for aircraft. Chromium metal coatings were used to reduce the corrosion. It is used for hardware items and tools.
DIFFUSION COATINGS are also known as cementation coatings. A part of the coating interacts with the substrate and forms an alloy with it. Cementation coating process is very similar to carburising of iron to produce a surface-hardened steel (iron is heated with carbon particles for the diffusion to occur). Common diffusion processes are Aluminizing (calorising), Chromising and Sherardising (zinc cementation coating). There are other diffusion coatings such as Siliconising and Borating. Hot dip galvanizing, tinning and terneplating also involve alloying between coating and substrate, but the process differs from that of diffusion coating. Diffusion coatings are created by, the gas-phase at high temperatures by exposing the substrate to a volatile (usually halide) compounds of the coating metal, and through a solid-phase by packing the substrate metal in a container with the powdered coating metal and heating them together.
● Aluminizing can be performed by both solid state and pack diffusion. Aluminizing may also be performed by heating ferrous metals in an inert, or reducing atmosphere containing dry gaseous aluminium chloride at 700-1100°C. It is also possible to produce aluminium diffusion coatings by coating the ferrous surface with aluminium by hot dipping (or with aluminium powder) and then heat treating to permit diffusion to occur.
● Chromising is application of chromium on Steel, for enhanced oxidation, corrosion and wear resistance. For gas phase chromising the articles are heated in a powdered mixture of chromium, alumina or kaolin and an ammonium halide in a hydrogen atmosphere at 1000-1100°C. It is also possible to chromise steel by heating it in a fused salt bath of chromium chloride, chromium metal, barium chloride and sodium chloride in an argon atmosphere.
● Sherardising is zinc diffusion coating of steel. Sherardising is applied on nuts, bolts, screws and chains. The articles are packed into a drum containing blue zinc powder (zinc containing 5-8% zinc oxide) and alumina or sand, the drum is heated to 360-430°C and rotated. Sherardised coating is a hard matt grey finish of uniform thickness.
● Galvanizing is a molten zinc dipped coatings on steel. Steel can also be coated by zinc through electro plating. Such a coating, if it is cut or scratched, then the zinc flows to decay preferentially to the steel and provide continuous protection.
VAPOUR DEPOSITION coatings are formed, by ‘condensation of metal vapour originating from molten metal, high voltage discharges between electrodes (cathode sputtering), or from chemical means such as hydrogen reduction or thermal decomposition (gas plating) of metal halides’ A thin specular coating can be achieved on metals, plastics, paper, glass and fabrics by means of vaporization.
● Siliconising is carried out by heating steel in contact with silicon carbide powder in an atmosphere of silicon tetra-chloride vapour. It case hardens the steel for high surface hardness and wears resistance. Siliconising is done to refractory materials.
METAL CLADDING and PLATING are used for coating, cladding or plating a metal surface with another metal. Hot-dipped coatings of low-melting metals are used for steel articles. Electro plating is used for plating Zinc, Nickel, Gold, Silver, Chrome, Tin, and Nickel-Cobalt alloy through Barrel plating, Rack plating, Strip plating processes. Electro plating nominally provides a very glossy surface, but by reversing the current in electrodes during the final stage a matt finish can be achieved. Other processes very close to this category include Electroless plating, hot dipping, metal spraying, powder spraying, vacuum metalizing and anodizing.
VITREOUS ENAMEL COATINGS often called ceramic coating are glassy but noncrystalline surfacing. A slip is applied by dipping or flow coating, and fired or sintered to form a vitreous coating. Dry enamelling is used for castings, such as bathtubs. ‘The casting is heated to a high temperature, and then dry enamel powder is sprinkled over the surface, where it fuses.’
POWDER COATINGS use polymeric materials such as acrylic, polyester, and epoxies, which are heated and sprayed or sprayed over a heated object. Powders are also applied to electrically charged materials which are attracted to and adhere to the substrate until it can be transported to an oven.