Pitting corrosion

Post-examination should reveal the local cathode, since it will remain impervious to the corrosion attack.

Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped

Pitting is initiated by:

a. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)

b. Localized damage to, or poor application of, a protective coating

c. The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions.

Theoretically, a local cell that leads to the initiation of a pit can be caused by an abnormal anodic site surrounded by normal surface which acts as a cathode, or by the presence of an abnormal cathodic site surrounded by a normal surface in which a pit will have disappeared due to corrosion.

In the second case, post-examination should reveal the local cathode, since it will remain impervious to the corrosion attack as in the picture of an aluminum specimen shown on the right. Most cases of pitting are believed to be caused by local cathodic sites in an otherwise normal surface.

Apart from the localized loss of thickness, corrosion pits can also be harmful by acting as stress risers. Fatigue and stress corrosion cracking may initiate at the base of corrosion pits. One pit in a large system can be enough to produce the catastrophic failure of that system. An extreme example of such catastrophic failure happened recently in Mexico, where a single pitin a gasoline line running over a sewer line was enough to create great havoc to a city, killing 215 people in Guadalajara.

Some definitions

Pitting: corrosion of a metal surface, confined to a point or small area, that takes the form of cavities. *

Pitting factor: ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. *

Pitting resistance equivalent number (PREN): an empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels. It is expressed as PREN = Cr + 3.3 (Mo + 0.5 W) + 16N.

Numerous pages of this web site refer to pitting corrosion:  Aloha incident, Corrosion monitoring, Human errors, Inhibitors, Internal corrosion,  Magnesium, Maintenance, Pit shapes, Passivation, Pipeline,   Standard, Stress corrosion cracking, Swimming pool, Theory Numerous pages of the Corrosion Doctors Web site discuss passivation related topics: Beer, Biomaterials, Blocking, Calcareous deposits, Electrochemical noise, Electrode passivation, Galvanized, Inhibitors, Iron, Nickel aluminum bronze, Oxidizers, Passivation layer, Passive curve, Passivity, pH, Pickling, Pitting, Potentiodynamic polarization, Rouging, Stainless steels, Steel, Stress corrosion cracking, Surface contaminants