Why is the element oxygen corrosive?

Answer 1

Oxygen gas (O2) is a pretty good oxidant (in fact, the word "oxidize" comes from "oxygen"). This is because oxygen is fairly electronegative. When bonded to itself, it forms a purely covalent bond. In contrast, when bonded to many other elements, in particular to less electronegative metals, oxygen is able to have more electron density for itself. So oxygen gas will react with other elements, such as iron, and then rust or corrode them. Other very corrosive elements are F2, Cl2, and Br2, all for the same reasons -- they are very electronegative, and by combining with something other than themselves, they are able to gain more electrons. Also note that burning is the same process as rusting, although we don't think of it that way! Both are just something combining with oxygen. Compare the two reactions: Rusting of iron: 4Fe + 3O2 --> 2Fe2O3 Burning of methane: CH4 + 2O2 --> CO2 + 2H2O In fact, burning is just oxygen combining with the fuel (whether it be wood, oil, gas, coal, or anything else) to form products in which the reactants are oxidized. So oxygen is in fact quite reactive with many things. Usually to get things to burn, you need a spark to get the reaction burning though (there is an activation barrier to the reaction, but it is an very exothermic reaction). Rusting is also quite exothermic, but it is kinetically slow.

Answer 2

Oxygen itself is not flammable or combustible; but a combustion doesn't exist without oxygen - the oxidizing agent. Also oxygen can corrode metals by oxidation.