Billions of tons of iron products and other metallic products are produced every year. Prevention of corrosion through protective coatings of polymer films, coating with other metals (galvanizing), passivation (by an adhering oxide layer), cathodic protection, use of sacrificial anodes, and so on, constitute a “mini” industry in itself. We will outline the principles involved in these processes.
a) Paint Coatings :
Paints obstruct the mechanisms that sustain corrosion and provide protection against mechanical forces as well as natural ultraviolet radiation. They also provide aesthetically pleasing coloured and glossy appearances. The major components of paints are the binding media (that control the physical and chemical properties of hardened coating), pigments (insoluble filters grounded and mixed with binding media) and solvent for application on the metal surface . The binding media have structural units that contain functional groups that can be used for linear as well as cross linking of the chains. The common ones are alkyd resins (polyesters), amino resins (urea or melamine) with formaldehyde and epoxy resins (whose structural units contain hydroxyl and epoxide groups at two ends). Typical pigments (of particle size range of 0.1 to 50
m) are white pigments (containing TiO 2 ), black pigments (carbon black), red pigments (Fe(III) oxide), yellow pigments (hydrated Fe(III) oxide) and inhibiting pigments (Pb 3O4, CaPbO3, ZnCrO4 and metallic zinc dust). Common solvents are white spirit, toluene, xylene, alcohols, ketones, esters as well as water for solubilization of especially cationic species. If the paint comes off or has defects, the consequences will be disastrous.
b) Galvanizing:
Galvanizing iron is done by coating its surface with zinc. The electrode potential of zinc is –0.76 V which is more negative than Fe/Fe2+ (-0.44 V). Therefore, oxidation of zinc is thermodynamically more favoured than that of Fe. Fe survives over Zn and zinc too survives as it gets coated with a protective layer of a hydrated oxide. Tin can not be used as a galvanizing material as its reduction potential (-0.14 V) is higher and if the tin surface is scratched, it oxidizes iron and corrodes it. Protective layers of oxides protect the metal surface, especially in the case of Al where the reduction potential is very low, -1.66 V. This is also a form of passivation. We can use the Pourbaix diagram to find ut the most suitable conditions for electrodepositing Ni on metal surfaces. The pH range of 4.2 to 6 is most suitable. At lower pH, there is too much hydrogen evolution and at higher pH, Ni(OH)2 gets incorporated into the deposit.
m) are white pigments (containing TiO 2 ), black pigments (carbon black), red pigments (Fe(III) oxide), yellow pigments (hydrated Fe(III) oxide) and inhibiting pigments (Pb 3O4, CaPbO3, ZnCrO4 and metallic zinc dust). Common solvents are white spirit, toluene, xylene, alcohols, ketones, esters as well as water for solubilization of especially cationic species. If the paint comes off or has defects, the consequences will be disastrous.
