Mechanisms of stress corrosion cracking for iron base alloys in hot Ca(NO₃ )₂ solution

The stress corrosion cracking (SCC) susceptibilities of a series of iron base alloys were evaluated in a 2M Ca(NO3)(2) solution at a temperature of 90 degreesC. The inclusion distributions in each material were established using quantitative metallography and energy dispersive X-ray analysis (EDX). Electrochemical and microcorrosion measurements were also performed. Cracks were predominantly associated with pits that initiated at ruptures in the oxide film at potentials more anodic than -25 mV or by dissolution at sulphur containing inclusions at potentials more cathodic than -25 mV. Reducing the inclusion content and/or the activity of the inclusions in iron base alloys moves the minimum potential for cracking to more anodic potentials but does not eliminate SCC. The addition of sulphate to the test solution increased the susceptibilities of the alloys. At the time the work was carried out, the authors were in the Materials Division, Department of Mechanical, Materials and Manufacturing Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne N E1 7RU, UK. Dr Zhou is now at the Energy Institute, Pennsylvania State University, 222 Academic Project Building, University Park, PA 16902, USA. Manuscript received 8 July 1998; accepted 25 August 2000. (C) 2000 IoM Communications Ltd.