METALLIC COATINGS

Metal or inorganic coatings are considered permanent coatings. The base metal (on which coating is provided) should have a higher melting point than the coating or plating metal. A large variety of metal coatings may be derived from various plating metals such as lead, copper, tin, nickel, zinc, cadmium, silver, gold, chromium, brass or bronze. The following methods are commonly used in providing metallic coatings.

1. Hot dipping
2. Electroplating
3. Galvanizing Tin plating
4. Metallizing
5. Sputtering

Hot Dipping

The process consists of a thorough cleaning of base metal by acid or other methods, followed by proper fluxing (usually with zinc ammonium chloride), after which the base metal is immersed in bath of molten coating metal. It is a cheaper method and hence most commonly used. Aluminium, lead, tin and zinc are the commonly used coating materials. Examples of hot-dip process are aluminium, zinc and tin coated on to steel and lead and zinc alloys on to non-ferrous metals.

Electroplating

Electroplating is the most widely used method. The four essential elements of a platin process are cathode (job to be plated), anode (plating metal), electrolyte (salts of the plating metal in solution) and DC current. As the current leaves the anode (plating metal) and migrates through the electrolyte to cathode (job), ions are deposited on cathode. The process of plating depends, to a large extent, on the existence of metal ions in the bath of plating metal. These ions carry +ive charge from anode (plating metal) and move towards negatively charged metals (job) or cathode where they meet the job surface and after loosing their charge, they deposit on the cathode as metal.

Zinc plating provides galvanic resistance to steel and atmospheric corrosion. It is the cheapest coating.

Cadmium plating is costlier but provides greater protection against corrosion in saline atmosphere.

Tin plating is especially resistant to tarnishing and their oxides are neither toxic nor they have objectionable taste. Tin plating is used on tin cans, kitchen ware, and food containers.

Copper plating on steels protects them from corrosion and gives high decorative value.

Chromium plating is known for its decorative and protective properties as also for high wear resistance. Chromium plates have high reflectivity giving attractive bright appearance.

Plating of plastics: Plated plastics knobs, handles, buttons, etc. are very common these days. In the plating of plastic parts, acrylonitrile-butadiene-styrene and polypropylene plastics are most commonly plated first with silver or gold to form a base for another coating with a thin layer of nickel or copper, which is finally followed by a layer of chromium. Other platings include lead plating, silver plating, gold plating, etc.

Galvanizing

Galvanizing makes use of an electrochemical action for providing a coating of highly corrosion resistant metal on the surface of another metal. The process is largely used for depositing zinc on iron and steels as corrosion resistant coating. It is a low cost process. A number of techniques are used to deposit zinc on a metal surface and these are hot dipping galvanizing, flow galvanizing, electroplating galvanizing and cold dip galvanizing.

Tin Plating

Tin plating or tin coating is a hot dip coating process mainly used for providing a coating of tin on sheet metal. The cleaned sheet to be treated is dipped in the molten tin bath which also contains zinc chloride floating over the tin bath. In the process when the job is dipped in the bath, a layer of zinc chloride is first deposited as a primer coat to later help in depositing the layer of tin. Tin coated sheets are passed through a set of rollers to ensure uniformity of the coating.

Metallizing

Metallizing is the process of impregnating a metal surface with another metal or metal compound. The two most commonly used methods of metallizing are:

(i) Metal spraying or spray metallizing or plasma spraying

(ii) Vacuum metallizing

Metallizing is extensively used for building up worn-out parts, protection from corrosion, and developing wearing surface (hard facing). Metal coated clothes and papers are used in electrical condensers.

Metal spraying or spray metallizing The metal to be deposited may be in the form of wire (known as wire metallizing) or powder (known as powder metallizing). In wire metallizing, the process is accomplished by using a spray gun that automatically feeds the wire through its nozzle surrounded by an oxy-acetylene flame. The gas flame melts the wire. A blast of compressed air breaks the molten metal into globules and also sprays it on to the job surface. These globules spread over the entire surface. and freeze there to form a deposited layer. Several such layers may be laid one over the other to build up to the required thickness. In powder metallizing, the coating material is in powder form and fed to the spray gun where a gas flame melts the powder. Compressed air is then used to atomize and spray the molten metal on the job surface. Plasma spraying makes use of plasma heat to melt the metal powder, and the plasma jet is used to carry molten metal to the job surface.

Wire metallizing process is more commonly used because of ease in handling of wire (than powder) and the manipulation of gun. However, powder metallizing gives faster speed and denser deposits.

Wire metallizing and powder metallizing are used for spraying coating materials such as carbon steel, stainless steel, brass, zinc, lead and bronze, but for providing coatings of metals such as copper, tungsten, chromium, and refractory metals (carbides of tungsten or titanium), and oxides of titanium, chromium, the technique of plasma spraying is considered more effective.

Vacuum metallizing

Vacuum metallizing is a vapour coating process and any metal (even non-metals) can be coated on a base metal by this process. The process is used to deposit very thin film of coating material. Vacuum metallizing is a process whereby metals and non-metals are deposited under high vacuum on to plastics, metals, glass, paper, textiles, etc. Both a small amount of coating material and the job are placed in a high vacuum chamber and the coating material is electrically heated to a temperature so that it vaporizes and the vapours leave the surface in atomic form and get condensed and deposited on the job surface. The components are kept rotating slowly so that their entire surface is exposed to vapours. After metallizing, the objects are dipped in top coat lacquer. A wide range of colours can be produced by dipping the objects in dye solutions, which colour the top coat.

The advantages of vacuum metallizing are as follows.

(i) It can place a thin film of metal on non-conductive material.

(ii) The deposit may be exceedingly thin, less than five millionths of an inch (0.005 mils) yet it completely conceals the colour of the object.

(iii) It is much less expensive than electroplating.

(iv) The corrosion resistance of lacquered metal surfaces is high.

See More: Mechanical Cleaning

See More: Adhesive Bonding Technologies

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