Metallic corrosion is the process of either stealing or donating electrons, when the electrons are stolen or donated, the metal atoms will dissolve in the water. Salt water is a the main conductor, so the rate of corrosion increases dramatically. When mgo board in high humidty enviroment(80%),inside chloride iron will dissolve and move toward water,when it contact with metal,unavoidale corrosion will happen on its surface.