The Role of Hydrogen in the Mechanism of Stress Corrosion Cracking of Austenitic Stainless Steels in Hot Chloride Media

Abstract

Abstract Previous studies on high strength martensitic stainless steels in sodium chloride solution have attributed crack growth to (1) hydrogen embrittlement at highly cathodic applied potentials, and (2) active path dissolution (to the exclusion of hydrogen embrittlement) at open circuit and anodic applied potentials. A recent re-evaluation of cracking in this system, however, has shown that crack growth at any applied potential can be adequately explained on the basis of a hydrogen embrittlement process. In view of this recent clarification, the question arises whether hydrogen is involved in the cracking of austenitic stainless alloys where this possibility had been previously excluded on a basis similar to that applied to high strength steels. To this end, electrochemical studies have been conducted on AISI Type 304 stainless steel in lithium chloride. The data indicate that although hydrogen is evolved under conditions of corrosion at the free corrosion potential, none is absorbed into the steel during cracking. When hydrogen absorption was promoted by the use of sodium arsenite as a poison, a marked increase in time to failure was observed, contrary to what would be expected if absorbed hydrogen was contributing to crack growth. To help clarify the kinetics of the surface reactions proceeding during cracking, the influence of applied current on time to failure in boiling lithium chloride is described. On the basis of the data obtained, it is concluded that hydrogen embrittlement plays little or no role in the crack propagation process, which is believed to proceed by a strain assisted anodic dissolution mechanism.