Corrosion of metals can be prevented in many ways. Some commonly used methods are
(i) By surface coating
(a) By applying, oil, grease, paint or varnish on the surface.
(b) By coating/depositing a thin layer of any other metal which does not corrode. For example, iron surface can be protected from corrosion by depositing a thin layer of zinc, nickel or chromium on it. Copper/brass can be protected by coating it with a thin layer of tin. Tinning of brass utensils is a very common practice in our country.
(c) By Galvanization : Prevention of corrosion of iron by Zn coating.
(ii) By connecting metal to a more electropositive metal : As long as the more electropositive metal is there, the given metal does not get corroded. For example, iron can be protected from corrosion by connecting it to a block/plate of zinc or magnesium. This method of corrosion protection is called cathodic protection.
(iii) By forming insoluble phosphate or chromate coating : Metal surfaces are treated with phosphoric acid to form an insoluble phosphate. Formation of a thin chromate layer also prevents the corrosion of metals.
(iv) Using anti – rust solutions: Solutions of alkaline phosphates and alkaline chromates are generally used as anti – rust solutions. For example, iron articles are dipped in boiling alkaline sodium phosphate solutions, when a protective insoluble sticking film of iron phosphate is formed.
Tips & Tricks
?When two or more ions compete at the electrodes then the ion with higher reduction potential gets liberated at the cathode while the one with lower reduction potential at the anode.
?Cell constant is determined with the help of conductivity bridge, where a standard solution of KCl is used.
?If the external EMF is slightly more than the actual EMF, the current will flow into the cell and reverse reaction takes place.
?Identification of cathode and anode is done by the use of galvanometer.
?KCl / NaCl / NH4Cl etc., can not be used in the salt bridge of a cell containing silver salt as one of the electrodes as Cl– ions form a ppt. of AgCl with silver ion.
?Weston cell is a common example of standard cell. The emf of a standard cell does not change with temperature.
?In Appolo moon flights, H2 – O2 fuel cell was the source of energy and drinking water.
?Conductivity water is the highly purified water whose on conductance is very small. It is prepared by the demineralisation of ordinary water by passing through cation and anion exchange resins.