Localized attack in an otherwise resistant surface produces pitting corrosion. The pits may be deep, shallow, or undercut, as shown schematically in Figure 1.4. The stainless steels and nickel alloys with chromium depend on a passive film for corrosion resistance and are especially susceptible to pitting by local breakdown of the film at isolated sites. A deep pit causing wall penetration and leaking is shown in Figure 1.9.