Rusting is a result of a process called oxidation. Oxidation takes place when oxygen molecules come in long term contact with certain metals.
The main catalyst for the rusting process is water which can penetrate easily, iron or steel structures which appear solid but can be easily penetrated by water molecules. After some time water which has hydrogen atoms combine with other elements which cause more metal to expose. The presence of sodium, like in the case of seawater, this process is hastened. Meanwhile, oxygen atoms combine with the atoms of the metal to form a destructive oxide compound. This makes the metal structure brittle. While pieces of steel or iron which are thick, are able to maintain their integrity even if rust occurs on their surface but thinner pieces are more prone to rusting. Four conditions are required for steel to rust. First there should be a metal, in this case, steel, where the rusting process will take place. Secondly, there should be a cathode, where the reaction will take place. Then there should be a cathode where the reduction reaction takes place and then there must be a conducive, continuous liquid path known as electrolyte, for example, water, humidity or sea water. In absence of any of these conditions, steel will not rust. An unbroken (smooth) surface will prevent the electrolytes from connecting with the cathode and anode and therefore steel will not rust unless the surface of the metal is broken.