Pitting Corrosion

Pitting is an extremely localized form of corrosion that results in holes in the metal. Although total metal loss may be small, the equipment may be rendered useless because of perforation.

Pitting usually requires a long initiation period before attack is visible. Once a pit has begun, the attack continues at an accelerating rate. Pits tend to grow in a manner that undermines or undercuts the surface. Typically a very small hole is seen on the surface. Poking at this hole with a sharp instrument may reveal a rather cavernous hole under what had looked like solid metal. In effect, a pit may be considered a self-formed crevice. Pitting attack increases with temperature.

Chloride solutions are the most common cause of pitting attack on stainless steels and nickel alloys. The alloying additions molybdenum, nitrogen, and, to some extent, chromium all contribute to pitting resistance. A laboratory measure of resistance to pitting corrosion is the critical pitting temperature, or CPT, which is the highest temperature at which an alloy resists pitting in a given environment. Alloy ranking with respect to chloride pitting resistance would be 304L (0% Mo, poor), followed by 316L (2% Mo), then four austenitics each with about 3% molybdenum, 20Cb-3/825/317L/RA333, the duplex 2205 (3% Mo–0.16% N),

AL-6XN (6.3% Mo–0.22% N), 625 (9% Mo) and C-276 (15.5% Mo). Alloys AL-6XN and higher have sufficient chloride resistance to be used in hot seawater service. The lower molybdenum grades, including 2205 with 3% Mo, are unsuitable for use in seawater.

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STAINLESS STEELS
James Kelly
Rochester, Michigan

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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