Corrosionpedia explains Corrosion Resistance
Usually the resistance to corrosion is expressed in terms of corrosion rate and measured in units of millimeter per year or "mils." These measurements for resistance to corrosion attack are taken in a particular environment at defined operating conditions, pressure, temperature and fluid velocity.
Besides intrinsic corrosion resistance, the resistance to corrosion of a metal can be increased by applying different methods such as cathodic protection, coating, painting and applying a corrosion inhibitor. There are no materials that are resistant to all corrosion in all environments. Materials must be matched to the environment they will experience. Corrosion resistances for metals are estimated and data used to check the sustainability of a metal in a particular environment.