Question

Corrosion of metals is sometimes harmful as well as useful . Explain with pictures

Answer 1

Corrosion is an electrochemical process whereby metals are converted to oxides, hydroxides, carbonates or sulfides by reaction with ambient moist air that also might contain CO2, H2S etc.

Corrosion affects all metals except noble metals (gold, platinum), because metals are quite reactive towards oxidation.

When iron corrodes, it forms a layer of rust (oxides and hydrous oxides). Rust does not adhere to the iron very well, and atmosphere can penetrate underneth the layer of rust and attack fresh iron metal layers beneath. This is why a piece of iron left out in the open will rust through and through. Painting or putting on a protective coat of silcone can slow down the corrosive degradation of iron. Stainless steel is an alloy of iron that contains other materials (carbon, nickel) that essentially disrupts the corrosion reaction and stops it.

Aluminum and Copper form oxide layers that adhere to the metal, and prevents the atmospheric corrosion from penetrating below the oxide layer. This protects the metal beneath the outer corroded layer. If you clean the aluminum or copper surface, the freshly cleaned surface will lose its shine and will tarnish fairly quickly, but the corrosion does not penetrate to layers below. Aluminum and copper have self-protecting corrosion mechanism, but other metals (Fe, Ni etc.) do not have this.