Dr. Dmitri Kopeliovich
Pitting corrosion is an electrochemical oxidation-reduction (redox) process, which occurs within localized holes (cells) on the surface of metals coated with a passive film.
Pitting corrosion is considered much more dangerous than uniform corrosion since its rate is 10-100 times higher.
Pitting corrosion is highly accelerated if chloride, sulphate or bromide ions are present in the electrolyte solution.
Stainless steels and other metals forming a passive oxide layers on their surfaces (Aluminum alloys, Copper alloys, chromium) in electrolytes and atmosphere are sensitive to pitting corrosion.
Mechanism of pitting corrosion is similar to that of Crevice corrosion: dissolution of the passivating film and gradual acidification of the electrolyte caused by its insufficient aeration (Oxygen penetration).
Stages of pitting corrosion:
Anodic reactions inside the pit:
Fe = Fe2+ + 2e- (dissolution of iron)
The electrons given up by the anode flow to the cathode (passivated surface) where they are discharged in the cathodic reaction:
1/2O2 + H2O + 2e- = 2(OH-)
As a result of these reactions the electrolyte enclosed in the pit gains positive electrical charge in contrast to the electrolyte surrounding the pit, which becomes negatively charged.
The positively charged pit attracts negative ions of chlorine Cl- increasing acidity of the electrolyte according to the reaction:
FeCl2 + 2H2O = Fe(OH)2 + 2HCl
PH of the electrolyte inside the pit decreases (acidity increases) from 6 to 2-3, which causes further acceleration of corrosion process.
Large ratio between the anode and cathode areas favors increase of the corrosion rate.
Corrosion products (Fe(OH)3) form around the pit resulting in further separation of its electrolyte.
Means of pitting corrosion control: