Metal will rust if it oxidizes. Oxidation is the interaction of oxygen molecules and all the different chemicals it may contact which can include metal, for example. It is when there is a loss of one or more electrons when two substances chemically interact. It can be a destructive process when metals rust or when fruit is spoiled. When iron rusts, a brittle brown substance called iron oxide forms. When copper rusts, a green coating on the upper surface forms called copper oxide. When metal is exposed to air and water, the oxygen will interact with the metals. However, if a shipwreck has a coating of mud over the hull, it will slow down the process of oxidation considerably. Destructive oxidation can't occur if the oxygen can't penetrate to the surface of the ship because of the coating of mud.