|4. Forms of Corrosion|
4.4 Pitting Corrosion [2/3]
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 Mz+ + 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 (Mz+) 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).