Corrosion & Environmental Degradation
Table of Contents
Corrosion Pitting
· Pit corrosion is an autocatalytic process in which the corrosion products promote further corrosion reactions. This is shown for a metal in an oxygenated NaCl electrolyte. The pit is anodic and the metal surface is cathodic.
· The localized production of positive metal ions in the pit gives a local excess of positive charge which attracts the negative chlorine ions from the electrolyte to produce charge neutrality. 
· The pit contains a high concentration of MCl molecules which react with water to produce HCl, the metal hydroxide, and H+ ions, accelerating the corrosion process.
· In the pit, the oxygen concentration is essentially zero and all of the cathodic oxygen reactions take place on the metal surface outside the pit. The pit is anodic and the locus of rapid dissolution of the metal.