Intergranular Corrosion

Intergranular corrosion consists of localized attack along the grain boundaries of the metal.

Sensitization to this attack in stainless or nickel alloys is caused by precipitation of chromium-rich carbides in the grain boundaries at a temperature low enough that a chromium-depleted zone forms. This precipitation most commonly occurs from the heat of welding. It may also result from a slow cool after annealing or from prolonged exposure to intermediate temperatures, roughly 850–1470 F (450–800 C), in service.

A most effective means of combating intergranular corrosion is to restrict the carbon content of the alloy. In the stainless ‘‘L’’ grades 0.03% maximum is considered sufficient. High chromium and molybdenum additions, as in AL-6XN, also reduce the chance of intergranular attack.

Another approach is to add columbium or titanium to tie up the carbon, the same as is done to resist PASCC. 20Cb-3 stainless steel takes both approaches, being melted to low carbon as well as having a columbium addition.

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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