There are several methods for protecting 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 any of and following methods:
(i) y painting the surface.
(ii) by coating the surface with a thin .film of oil or grease
(iii) by electroplating iron with some non-corrosive metal such as nickel, chromium, copper, etc.
(iv) Lamination with plastics
All these methods do not involve any oxidation or reduction. Hence, these are called non-redox methods. However, in this type of protection, if scratches or cracks appear in the protective layer then surface of iron may get exposed. In this region, moisture and oxygen may come in contact with iron and rusting starts. This rusting extends beneath the protective layer and eventually peels off the protective layer.
2. Sacrificial Protection. In this method, surface of iron is covered with a layer of more active metal like zinc. This active metal loses electrons in preference 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 is present, even the nearly uncovered surfaces of iron do not get rusted. In this way, scratches in the protective layer of this type are not harmful. This type of protection is called sacrificial protection.
Zinc metal is generally used for protecting iron and the process is called galvanization. Galvanized iron sheets maintain 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, C02 and moisture in air. .
Zinc, magnesium and aluminium powders dissolved 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 in preference to iron. The iron surface, acts as cathode. This method, therefore, is also called cathodic protection of iron. Underground water pipes or tanks made of iron are protected by connecting them to more easily oxidisable metal as shown in Fig. 35.2.
4. Use of Anti-rust Solutions. The alkaline solutions of some phosphate and chromate salts act as anti-rust solutions. For example, when iron articles are dipped into the boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the articles from rusting. The alkaline nature of solutions decreases the availability of H+ ions which facilitate the oxidation of iron to Fe2+. /
Why steel is preferred to iron in many instances?
Steel is an alloy of carbon and other metals like Cr, Ni, Mn, etc. Pure iron is soft whereas steel is much harder and has high tensile strength. Moreover, iron is more prone to rusting but the presence of other non-corrosive metals in steel make it quite resistant to corrosion/rusting. These qualities make steel more useful as a structural material as compared to iron.