Pitting Corrosion

The Critical Pitting Temperature System is used to determine the temperature-dependence of pitting corrosion. An electrochemical technique measures pitting while the temperature of a metal sample is varied. The pitting process is electrochemically accelerated, so you can get answers much more rapidly than with traditional exposure techniques.

What do we mean by pitting corrosion? Many electrochemical techniques assume a simplified model in which corrosion occurs uniformly over the entire metal surface. Unfortunately, many real-world systems undergo highly localized corrosion quite unlike the uniform corrosion model. Corrosion occurs in pits formed in the metal or in crevices where metal pieces are joined. The localized corrosion can cause an otherwise corrosion-free metal piece to fail. Stainless steels, high alloys, and aluminum are often subject to localized corrosion problems, especially in the presence of chloride ion or other active pitting agents.

How does a pit form? Pits normally are seen on metals that form passive films. For various reasons, the passive film breaks down in a small region of the metal surface. At this location, the metal anodically dissolves. The chemical products formed by the metal's dissolution hinder regrowth of the passive film, often via a local change in pH. As the metal continues to dissolve in the localized area, a pit forms in the metal surface. This pit traps yet more dissolution products, which further prevent repassivation. As a result, once a pit forms, it is self-sustaining. Pit growth can continue until the pit reaches completely through the metal.

The mechanism of crevice corrosion is very similar. Think of the crevice as a preformed pit trapping corrosion products and preventing repassivation.

There are two voltage parameters that are used to describe pitting behavior. The first is the critical pitting potential. Below the critical pitting potential, pits do not start. The second is the critical repassivation potential. Below this potential, pits that have already formed repassivate.

Localized corrosion phenomena often depend strongly on temperature. Often systems that do not undergo pitting attack at room temperature are pitted if the temperature is increased. The critical pitting temperature is used to describe the temperature at which pit initiation begins.