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Corrosion Fundamentals

Pitting Corrosion

Passive metals, such as stainless steel, resist corrosive media and can perform well over long periods of time. However, if corrosion does occur, it forms at random in pits. Pitting is most likely to occur in the presence of chloride ions, combined with such depolarizers as oxygen or oxidizing salts. Methods that can be used to control pitting include maintaining clean surfaces, application of a protective coating, and use of inhibitors or cathodic protection for immersion service. Molybdenum additions to stainless steel (e.g. in 316 stainless) are intended to reduce pitting corrosion.

Corrosion Product Accumulation

Rust Bubble on Cast Iron

(Courtesy of

The rust bubbles or tubercules on the cast iron above indicate that pitting is occurring. Researchers have found that the environment inside the rust bubbles is almost always higher in chlorides and lower in pH (more acidic) than the overall external environment. This leads to concentrated attack inside the pits.

 Pitting Corrosion Diagram

Similar changes in environment occur inside crevices, stress corrosion cracks, and corrosion fatigue cracks. All of these forms of corrosion are sometimes included in the term "occluded cell corrosion."


Tube Split Due to Pitting Corrosion

Pitting Corrosion of Tube Wall

Pitting corrosion can lead to unexpected catastrophic system failure. The split tubing above left was caused by pitting corrosion of stainless steel. A typical pit on this tubing is shown above right.

Sometimes pitting corrosion can be quite small on the surface and very large below the surface. The figure below left shows this effect, which is common on stainless steels and other film-protected metals. The pitting shown below right (white arrow) led to the stress corrosion fracture shown by the black arrows.

Pitting Corrosion of Tubing Wall

Stress Corrosion Cracking Caused by Pitting

A complete discussion of this corrosion is contained in Steven J. McDanels, "Failure Analysis Of Launch Pad Tubing From The Kennedy Space Center," Microstructural Science, Vol. 25, 1998, ASM International, Materials Park, OH, pp. 125-129.