Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting is defined as the slow decay of iron metal in presence of air and moisture. This process produce no heat and no light. Also it is very slow process. The process of rusting cannot be called combustion because in this process no heat and light is produced. Due to this reason iron is not considered as combustibIe substance.