Following are the methods which are usually adopted to prevent the corrosion of ferrous metals: 1. Coal Tarring 2. Electroplatings 3. Embedding in Cement Concrete 4. Enamelling 5. Galvanizing 6. Metal Spraying 7. Painting 8. Parkersing 9. Sherardising 10. Tin Plating and Terne Plating.
Method # 1. Coal Tarring:
In this method, the iron is dipped in hot coal tar so that a ‘film of coal tar sticks to the surface and this film protects the iron surface from atmospheric actions leading to the corrosion.
As die appearance of the treated surface is objectionable, the method of coal tarring is usually adopted on iron used in substructure i.e., for work, below ground or at places where appearance is not of much importance.
Method # 2. Electroplatings:
In this method, a thin layer of chromium, cadium, copper or nickel is laid on the surface of ferrous metal with the help of electric current and by employing the principle of electrolysis. The surface to be protected is made the cathode and the metal to be deposited is made the anode.
When the electric current passes through a solution between two electrodes, the solution breaks into ions which deposit on the cathode and the iron is thus electroplated. The surface so formed is smooth and the process produces very bright shining surfaces.
Method # 3. Embedding in Cement Concrete:
If steel is embedded in cement concrete, as in case of reinforced cement concrete construction, it is not affected by corrosion. The cement concrete should however be properly laid and cured so that it does not contain voids or cracks. There should also be enough cover of concrete on the steel surface.
Method # 4. Enamelling:
In this method, the surface of iron is enamelled i.e. provided by a smooth brilliant surface by melting a suitable flux on it. For this purpose, the iron piece is cleaned properly and a flux on the surface is melted by placing it in a muffle furnace.
The first layer consists of borax with cobalt or nickel oxide and a small quantity of limonite i.e., hydrated ferric oxide. It is then coated with a second layer of more fusible glaze to give the desired colour to the surface. This method is used for ornamental iron works, sign boards and domestic utensils.
Method # 5. Galvanizing:
In this method, the ferrous metal is thoroughly cleaned by removing scales. The surface is then treated with dilute solution of HCl and after washing, it is dipped in a bath of molten zinc. The thin layer of zinc grants protection against corrosion.
Method # 6. Metal Spraying:
In this method, the ferrous metal is covered with a spray of vaporized aluminium, lead, tin or zinc. The equipment includes a pistol, a bundle of wires of coating metal, compressed air, oxygen and a suitable fuel gas.
An oxy-hydrogen flame is produced inside the nozzle of pistol and it results in the melting of wire of coating metal. This molten metal is forced by the compressed air and it is deposited on the iron surface under high pressure at ordinary temperature.
The surface of the material to be treated must be thoroughly cleaned before the process is started. The spraying is simple and it gives a thin film of uniform thickness. It is a useful method for the protection of existing works already in service.
Method # 7. Painting:
In this method, the surface of ferrous metal is covered with a layer of suitable paint. This is the most common method of protecting metal surfaces from corrosion.
The paints of various types are available in the market for use in different environments like fresh water, sea water, corrosive action of gases, etc.
Before applying the paint, the surface should be thoroughly cleaned and made free from scales, oil, grease, dust, etc. The paint may be applied with brushes or it may be filled in a pistol and sprayed on the metal surface. The painting should be preferably done in dry weather when the humidity is low. If the metal surface to be painted is preheated, the adherence of paint increases.
Method # 8. Parkersing:
In this method, the article to be treated for corrosion is immersed for a period of about an hour or so into a hot water bath of a chemical known as the Parco. The insoluble phosphates are formed on the surface of article due to chemical reactions and these phosphates keep away the moisture.
Method # 9. Sherardising:
In this method, the article to be treated for corrosion is cleaned with all foreign deposits by washing it with acid solution and then with clean water. It is then dried and covered with dust of pure zinc. It is then heated to a suitable temperature of 250°C to 450°C in an air-tight steel box.
The zinc melts and it combines with metal and forms a protective layer on the metal surface. The surface so formed is very durable and can be easily polished. This method gives better protection than the method of galvanizing.
Method # 10. Tin Plating and Terne Plating:
In this method, the ferrous metal is thoroughly cleaned with the help of dilute solution of acid and it is then dipped in a bath of molten tin. For perfect coating, the metal is to be passed through successive baths of molten tin. The coated metal piece is then finished off by passing through rollers in a grease bath to squeeze off the surplus tin.
The durability of utensils for food and equipment for dairy can be considerably increased by tin plating. The process can also be done with the help of electricity.
The method of terne plating is similar to tin plating except that the lead-tin alloy is used for coating instead of pure tin.