The role of hydrogen in stress-corrosion cracking of austenitic stainless steel in hot MgCl 2 solution

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I.

INTRODUCTION

D E S P I T E many investigations, there are conflicting views on the mechanism of stress-corrosion cracking (SCC) of austenitic stainless steels in boiling MgC12 solutions. Numerous mechanisms have been proposed to explain the SCC. The two most important theories are the anodic path dissolution mechanism and the hydrogen-induced cracking (HIC) mechanism. 11,2,3] Evidence for the HIC mechanism is as follows. (1) The solution remaining in a crack tip has a pH value of about 1 during SCC of austenitic stainless steels in boiling MgCI2 solution, which results in a thermodynamic condition suitable for the occurrence of cathodic hydrogen evolution. Tests have indicated that hydrogen gas evolves near the crack tip.t4-vj (2) It is well established that austenitic stainless steels whether stable, e.g., types 310 and 316, or unstable, e.g., types 304 and 321, have undergone hydrogeninduced ductility reduction and fracture, ts-lu It should be pointed, however, that the previously mentioned conditions are only necessary for the HIC mechanism of SCC, rather than sufficient. Condition (1) only verifies that electrochemical conditions at the advancing SCC front are such that hydrogen atoms may enter the specimen during SCC. Condition (2) shows that HIC can occur if the hydrogen concentration in the specimen is high enough. Nevertheless, if hydrogen introduced into the specimen during SCC in boiling MgCI2 solution is below the critical value, HIC will not occur. In addition, previous work has indicated that there was no hydrogen-induced ductility reduction when the test temperature was higher than 150 ~ The temperature of the boiling MgC12 solution used in most of the tests was in the range of 140 ~ - 160 ~ Thus, it is possible that no HIC occurs because of the high temperature, L. QIAO, Postdoctoral Research Associate, and X. MAO, Associate Professor, are with the Department of Mechanical Engineering, The University of Calgary, Calgary, AB, Canada T2N 1N4. W. CHU, Professor, is with the Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, P.R. China. Manuscript submitted July 22, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A

even if a large amount of hydrogen entered the specimen during SCC. In fact, there are many data refuting the HIC mechanism. For example, in a 146 ~ LiC1 solution, cathodic polarization increased the amount of hydrogen evolution but decreased or checked the SCC of austenitic stainless steel. 113~That macroscopical compressive stress-induced SCC of austenitic stainless steel in boiling 42 pct MgCI2 solution was also an important finding to support the anodic dissolution mechanism. [14] The incubation period under compressive stress, however, was 10 to 100 times longer than that under tensile stress, and fractography of SCC under compressive stress is different from that under tensile stress. Is it possible that hydrogen plays a important role in the SCC of austenitic stainless steel under tensile stress? The purpose of this work is to explore the