i. One of the properties of iron is that it reacts with oxygen in the presence of moisture and gets corroded by forming rust(Fe2O3.x H2O).
ii. Air contains oxygen and moisture. Hence, iron readily rust when exposed to air.
iii. Steel (stainless steel, not mild steel) is an alloy of iron having the property to resist corrosion including rusting.
Hence, stainless steel, though it contains iron, does not undergo corrosion.