Question

difference between corrosion and combution​

Answer 1

This is a very intelligent question. Corrosion usually forms metal oxides while combustion can form carbon, hydrogen, or metal oxides. Water is formed as a byproduct in the case of carbon and hydrogen fuels, but not with metallic fuels like thermite. For practical purposes, a reasonable metric is the reaction rate. Combustion generally occurs quicker than corrosion.

Interestingly enough, corrosion and combustion are sometimes coupled:

Na+H2O→NaOH+H2

which leads to hydrogen combustion:

H2+O2→H2O

which generates more water, leading to further oxidation of Na in an endless cycle until Na is fully consumed. Quite beautiful from a chemical perspective, but maybe not as an observer. This is why Li-ion batteries can sometimes explode.