68. NCERT Exemplar XXXXI

 

Iron oxidizes in the presence of atmospheric oxygen and water to form iron oxide di-hydrateiron hydroxide tri-hydrateiron oxide mono-hydrateiron oxide tri-hydrate, known as rust. It peels off, exposing fresh iron surface for further rusting. While, aluminium oxidized in the presence of atmospheric oxygen to form a thin, transparent and non-water-soluble layer of aluminium oxalatealuminium hydroxidealuminium dioxidealuminium oxide, which shield the metal from further reductioncorrosionoxidationrusting and maintains its metallic shine.  Hence, the oxidation of iron to rust is a problem for structural engineers, but aluminium oxide is not.