Initiation of Pitting and Agents Causing Pitting

Pitting corrosion is initiated by chemical and mechanical damage to the protective film, local damage to coatings and the presence of non-metallic inclusion.

The major species responsible for pitting is chloride ion. In general all halogen ions like sulfate, and heavy metal ions present in sea water may induce pitting. Pitting occurs only in the presence of oxygen. In petroleum industry, pitting failures are caused by CO₂ and H₂S.

Mechanism of Pitting

At the anodic sites (within the pit) oxidation occurs as M M^z+ + ze. The anode forms the site of the pit.

At the cathodic sites, reduction of oxygen takes place as O₂ + 2H₂O + 4e^- 4OH^-.

Due to continued oxidation at the anode, positive ions (M^z+) are accumulated and excess positive charge is established in the pitted area. These positively charged ions attract the negative Cl^- ions present in the electrolyte in which the metal is exposed.

M⁺ + Cl^- MCl

The metal chloride reacts with the electrolyte (water) to produce metal hydroxide and hydrochloric acid as

MCl + H₂O MOH + HCl

The acid produced dissolves the metal. The pitting process is auto catalytic i.e. once it starts, it continues to accelerate with time and it only ends when the metal is perforated (see figure below).