Rust (also commonly called oxide or rust), a substance that forms on metallic iron exposed to humidity, is a brown substance - whose hue varies from yellowish to reddish, depending on the composition - that corrodes its surface until it pierces it.[1]​[2][3]​ It is a mixture in variable proportions of the following iron (III) hydroxides and oxides:

Pieces deeply penetrated by rust present a structural problem, since rust has much lower mechanical properties than unoxidized metal, so rust represents a loss of resistant section that can lead to the collapse of metal structures.