Anodizing
Iron or carbon steel metal exfoliates when oxidized under neutral or alkaline micro-electrolytic conditions; i . e . , the iron oxide ( actually ferric hydroxide or hydrated iron oxide , also known as rust ) forms by anoxic anodic pits and large cathodic surface , these pits concentrate anions such as sulfate and chloride accelerating the underlying metal to corrosion . 
<triplet> rust <subj> iron oxide <obj> subclass of