'When you galvanize steel you put a barrier between the steel surface and the environment. Galvanizing isn't just zinc coating, it is a metallurgical bond between the two different types of metal.'

If the galvanized steel coating is damaged in any way you would expect the metal underneath to rust, but this is not the case. In fact something all together different happens, an action called sacrificial protection. This is where the zinc itself oxidizes to protect the metal underneath. To understand this process you have to look at the galvanic series of metals, which shows that zinc is closer to the electronegative end than both steel and iron. This simply means that zinc is a far more active metal and is the first one to corrode. It is this corrosion of the zinc that protects the metal underneath by filling the hole in the coating with zinc oxides.