There are numerous methods for shielding metals from corrosion (iron from rusting). Some of these methods are being discussed as follows:
1. Barrier protection; in this method, a barrier film is introduced between iron and atmospheric oxygen and moisture. Barrier protection can be achieved by nay of the following methods:
(i) By printing the surface.
(ii) By covering the surface with a thin film of oil or grease.
(iii) By electroplating with some non-corrosive metal such as nickel, chromium, copper etc.
In this type of protection, if scratches or cracks appear in the protective layer then surface of iron may get expressed. In this region, moisture and oxygen may come extend in contact with iron and rusting starts. This eventually peels of f the beneath off the protective layer.
2. Sacrificial protection: in this technique, surface of iron is protected with a layer of more active metal like zinc. This active metal loses electrons in reference to iron and hence, prevents the rusting of iron. However, the covering metal gets consumed in due course of time, but so long as it prevent even the nearly scratches in protective layer of this type are not harmful. This way of protection is called sacrificial protection.
Zinc metal is normally used for protecting iron and this process is known as galvanization. Galvanized iron sheets maintained their shine due to the formation of a thin protective layer of basic zinc carbonate ZnCO3. Zn(OH)2 due to the reaction between zinc, oxygen, CO2 and moisture in the air.
Zinc, magnesium and aluminium powders to dissolve in paints can also be applied as protective layers. The well known aluminium paint contains aluminium powder suspended in varnish.
3. Electrical protection; this is also a case of sacrificial protection. In this method, the exposed surface of iron is protected by connecting it to some more active metal such as magnesium. The other metals which can be used for this purpose are aluminium, zinc etc. the more active metal acts as anode and loses electrons preference to iron. The iron surface, perform as cathode. This method therefore, is called cathodic protection of iron. Underground water pipes or tanks made of iron are protected by connecting them to more easily oxidisable metal.
4. Use of anti-rust solutions: the alkaline solutions of some phosphate and chromate salts as anti-rust solutions. For example, when iron articles are dipped into the boiling and strongly alkaline solution of sodium phosphate, a protective insoluble protects the acticles from rusting. The alkalines nature of solutions decreases the availability of H+ ions which facilitates the oxidization of iron to Fe2+.