Coatings and Protective Layers

Galvanizing (Zinc Plating) 

The surface of iron or steel is coated with a thin layer of zinc to protect it against corrosion. There are two processes: Hot Dip galvanizing and electro-galvanizing (cold process). In the $former process, the article is cleaned in a bath or acid and then dipped in molten zinc, the surgical of which is covered with a layer of ammonium chloride (sal- ammoniac). This produces a skin of zinc alloyed to the steel. A small quantity of aluminium is generally added to the molten zinc. The amount of zinc coating on galvanized sheets varies from 300 to 750grams/sqm, both sides inclusive.  Electro-galvanizing is a process of zinc plating similar to other forms of electroplating. The coating produced by the electrolytic process is an improvement upon that produced by the hot process and the film of zinc although ample to protect from oxidation is porous and inferior to that produced by the hot dip process. With the hot galvanizing process, ductility and tensile strength of fine gauge wires or sheets are reduced.  Zinc will not adhere on a dirty surface or scaly places and will blister or leave out uncoated patches. 

Sheradising (Dry Galvanizing) 

Is a process for coating articles with zinc by packing them in zinc dust and heating to 300 deg. Celcius.  The articles to be coated are first dipped in an acid bath.  The zinc combines with the surface of the metal at a temperature below the melting pint of zinc which slightly hardens the metal superficially. The coating is very durable and can be polished.  This is also known as cementation (coating) process. 

Metal Spraying 

A coating of molten zinc, tin, lead or aluminium, etc. is given by means of a spray-gun under high pressure at ordinary temperature.  The surface of the material to be treated must be scrupulously clean. It is a useful method for the protection of works already in service. 

Nickel Plating 

Iron, steel, copper, brass, bronze, lead, aluminium are often nickel plated by immersing the articles in a solution of nickel cyanide and sulphuric acid.  Nickel is readily deposited over the surface by electro- plating.  Before nickel-plating, the article, if not made of copper or brass, usually requires preliminary copper plating before nickel can adhere to that satisfactorily. In copper plating, a copper sulphate solution is generally used. 

Chromium Plating 

Chromium plating is more difficult than nickel plating as special precautions are necessary in the process.  Chromium, if plated sufficiently thick, gives a very hard surface and is therefore employed for coating wearing parts of soft steel articles, which is done at a less cost, instead of using heat-treated high alloy steels. Due to its low co-efficient of friction. chromium is used for coating the wearing parts of pins, brushes, etc. 

Chromium is superior to nickel and has brighter luster than nickel and does not tarnish and will stay indefinitely while nickel may turn black in some atmospheres.  For chromium plating iron or other ferrous metals the articles are nickel plated first which has applied over a base coat of cadmium, otherwise they do not give a good finish and there is a tendency for peeling of the chromium.  When chromium plate is applied to brass or other non-ferrous metals, it is applied over a base coat of nickel. 

Chromium plating can usually be distinguished as it has a bluish cast as compared with nickel which has a white colour and a little more of a yellow tinge. Chromium is also softer and more easily scratched than nickel.  Cadmium plating has a dull grey colour. To test to ascertain if plating is chromium or nickel, hang a plated sample in a ten percent solution of hydrochloric acid and if the plating is chromium it will be dissolved in about five minutes Lime but if the plating is nickel, the acid will have little effect.

Tin Plating 

Tin plating is a process similar to hot galvanizing, tin being used for coating instead of zinc. Sheets of black iron or other base metal are first dipped in a four to eight per cell solution of sulphuric acid to remove any rust, then washed and dipped in a flux of zinc chloride or ammonium chloride and then passed through a bath of molten tin.  The protection afforded to iron or steel (against corrosion) by till is less effective than is that of zinc. 

Terne Plating 

Terne plating is similar to tin plating, the difference being, lead-tin alloy is used instead of pure till for coating. Pure lead cannot be coated on iron.  Gasoline tanks, metal furniture-food containers are terne plated. 

Electro-Plating 

Metallic coatings such as nickel plating, chromium plating, copper or zinc plating are the common methods of electroplating, and the coatings are given on the principles of electrolysis.  The negative electrode is known as the cathode and the positive electrode is the anode. The article to be electro-plated is made to form a cathode while the positive electrode consists of a piece of metal which is lo be plated on the article. The process produces very bright surfaces.