A unified mechanism of stress corrosion and corrosion fatigue cracking
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I.
INTRODUCTION
A
widely accepted mechanism of stress corrosion cracking (SCC) under static tensile load has eluded experimentalists and theoreticians for decades. Less effort has been devoted to corrosion fatigue cracking (CFC) under cyclic load. The relationship, if any, between SCC and CFC is presently unclear. The purpose of the present paper is to describe a mechanism to account for previously known and recently revealed facts concerning these two forms of metal failure. The keystone of the proposed mechanism is the experimental evidence that anodic dissolution or corrosion relieves strain hardening in many, if not most, metals and alloys. It will be demonstrated that relief of strain hardening at the crack tip surface reduces the fracture stress and facilitates nucleation of brittle cracks. The rupture of passive films is essential to concentrate the anodic dissolution at the crack tip rupture sites. Cyclic loading has also been observed to relieve strain hardening and increase plastic creep in many alloys at ambient temperature. Crack tip relief of strain hardening, caused by a combination of corrosion and cyclic load, can account for CFC without any critical anion such as C1- and for accelerated CFC at lower cyclic frequency. Recent experimental evidence is reviewed in the initial section of this paper to provide the necessary background, followed by a description of the proposed mechanism and finally by discussion of further experimental data which support the mechanism. Whenever possible, results are cited involving SCC of austenitic stainless steel in hot chloride solutions, a system which has been the subject of extensive investigation and exhaustive review. II.
EXPERIMENTAL BACKGROUND
A. Film Rupture
Many recent discussions ~-5 of the SCC mechanism have emphasized film rupture and repassivation as key elements. DENNY A. JONES is Professor and Chairman, Department of Chemical and Metallurgical Engineering, University of Nevada-Reno, Reno, NV 89557. Manuscript submitted September 21, 1984.
METALLURGICALTRANSACTIONS A
One measure of repassivation kinetics is given by the straining electrode method originated by Hoar and co-workers. 6'7 A straining electrode is maintained at or near the original corrosion potential with a potentiostat. Fresh surfaces formed by plastic strain and consequent film rupture have more active free corrosion potentials and are anodically polarized when maintained potentiostatically at the original corrosion potential. Such a test thus results in an anodic current transient which is related to repassivation kinetics and resistance to SCC. Results of Jones et at. 8 using the straining electrode method are shown in Figure 1. The ratio i*/i, potentiostatic anodic current during straining i* divided by the anodic current i without straining, is plotted in Figure 1 vs time, which is proportional to strain in the constant strain rate test. A significant increase in anodic current did not occur until plastic deformation had developed above the elastic limit. Plastic strain of the
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