The development of stress-corrosion cracks in austenitic Cr-Ni steels

Abstract

Direct-loaded axially stressed specimens (0·160–0·635 cm dia.) of a variety of austenitic Cr-Ni steels were exposed to boiling 42 per cent MgCl 2 solution. The progress of cracking was followed by potential measurements and by removing specimens for examination after suitable periods of exposure. The results are comparable with those earlier obtained on 0·051 cm dia. specimens. The life of a specimen can be divided into an induction period and a period of crack propagation; the latter consists of two stages. The cracks remain fine during the first stage, during which no change in potential can be detected; this stage is relatively more important the larger the specimen and the lower the applied stress, and so was not recognized previously. The cracks open up and become branched during the second stage, which is accompanied by a fall in potential. The relative duration of the induction period and the total period of crack propagation varies considerably with the experimental conditions, particularly the size and form of the specimen and the mode of stressing, but for fully softened axially-stressed cylindrical specimens the ratio of these periods is constant for a given specimen diameter. The practical significance of these observations is discussed briefly. Sufficient corrosion current is available to account for crack-formation by purely electrochemical processes.