How does pitting corrosion occur

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 pit in a gasoline line running over a sewer line was enough to create great havoc to a city, killing people in Guadalajara.

Some definitions: Pitting: corrosion of a metal surface, confined to a point or small area, that takes the form of cavities. Register for a course. Check your grades. Get Standards. Get Reports. Join a Standards Committee. Career Center. Our History. Board of Directors. Pitting corrosion is usually found on passive metals and alloys such aluminium alloys, stainless steels and stainless alloys when the ultra-thin passive film oxide film is chemically or mechanically damaged and does not immediately re-passivate.

The resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal. ASTM-G46 has a standard visual chart for rating of pitting corrosion. The shape of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the pit shape, the pit size, and the pit depth of penetration can be determined. Mechanisms of Pitting Corrosion What causes pitting corrosion?

Chloride is particularly damaging to the passive film oxide so pitting can initiate at oxide breaks. The environment may also set up a differential aeration cell a water droplet on the surface of a steel, for example and pitting can initiate at the anodic site centre of the water droplet. In most cases, both the environment and the material contribute to pit initiation. What are the factors influencing pitting corrosion? Sufficient aeration supply of oxygen to the reaction site may enhance the formation of oxide at the pitting site and thus repassivate or heal the damaged passive film oxide - the pit is repassivated and no pitting occurs.

An existing pit can also be repassivated if the material contains sufficient amount of alloying elements such as Cr, Mo, Ti, W, N, etc.. These elements, particularly Mo, can significantly enhance the enrichment of Cr in the oxide and thus heals or repassivates the pit.

How to evaluate the resistance of an alloy to pitting corrosion? In general, when pitting occurs on a freely accessible clean metal surface, a slight increase in corrosivity of the environment will cause general or uniform corrosion. Pitting on clean surfaces ordinarily represents the start of breakdown of passivity or local breakdown of inhibitor-produced protection.

When pits are few and widely separated and the metal surface undergoes little or no general corrosion, there is a high ratio of cathode-to-anode area, and penetration progresses more rapidly than when pits are numerous and dose together. Pitting is one of the most insidious forms of corrosion; it can cause failure by perforation while producing only a small weight loss on the metal.

Also, pits are generally small and often remain undetected. A small number of isolated pits on a generally uncorroded surface are easily overlooked. A large number of very small pits on a generally uncorroded surface may not be detected by simple visual examination, or their potential for damage may be underestimated. When pits are accompanied by slight or moderate general corrosion, the corrosion products often mask them. Some causes of pitting are local inhomogeneity on the metal surface, local loss of passivity, mechanical or chemical rupture of a protective oxide coating, galvanic corrosion from a relatively distant cathode, and the formation of a metal ion or oxygen concentration cell under a solid deposit crevice corrosion.

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