Pitting is localized attack at a point where the passive film can no longer protect the stainless steel. Pits are typically narrow at the surface, are often much deeper than they are wide and may open up under the surface to occupy significant volume. As a result of the corrosion reaction, the environment trapped inside the pit becomes more aggressive than the bulk solution outside the pit and the pit propagates, rapidly becoming deeper. If the stainless steel is thin, as in a heat exchanger tube, through-wall penetration and leaking may occur. Pits become visible to the naked eye, but it typically takes many hours or days of corrosion before they develop to that point. The metal surface around a pit often becomes rust-stained. High chlorides, low pH, and high temperature make the environment more aggressive and make pitting attack more likely. Stainless steels with increased contents of Cr, Mo, and N have increased resistance to the initiation of pitting.
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