The control of catalytic poisoning and stress corrosion cracking

Abstract

When a structural metal is stressed in a hydrogen environment, the metal may crack at stress levels much lower than its normal strength. This embrittlement is caused by hydrogen atoms or ions rather than hydrogen molecules. The catalytic dissociation of hydrogen molecules into atoms or ions takes place at dislocation sites on a metal surface. The termini of dislocations on a metal surface are sites of localized high energy. There are “poisons” which will retard or even stop catalytic processes. The toxicity of a poison depends on the binding energy of the foreign atom to a dislocation terminus. Cracked specimens made of 4340 steel were tested in hydrogen gas and its mixture with SO 2, CS 2, CO 2, N 2 and Ar. Both SO 2 and CS 2 are very “toxic” and can stop a running crack. The toxicity of CO 2 is moderate. n 2 and Ar have no noticeable effect on a running crack.