High frequency stage I corrosion fatigue of austenitic stainless steel (316L)
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I.
INTRODUCTION
THE subcritical propagation of cracks in metals during corrosion fatigue (CF) in aqueous environments has received considerable attention. Despite this, the basic mechanisms governing the synergism between the environment and cyclic loading are not completely understood in all systems, particularly for austenitic stainless steels. Difficulties in controlling and separating the interacting processes that occur during CF testing are responsible for much of this confusion. In general terms, higher fatigue crack propagation (FCP) rates in aqueous environments, relative to dry air, are usually associated with localized electrochemical processes at the crack tip. These may generate hydrogen and promote local embrittlement of the metal lattice, or enhance the rate of crack advance by anodic dissolution of the metal. [L2]Hydrogen embrittlement effects are considered to be most important during FCP in high strength steels and aluminum alloys, t2,3,+jDissolution is usually implicated in the CF of austenitic stainless steels, tS-ml together with the rupture of passive films and repassivation processes at the crack tip.15'6'9'ml However, some authors [m do not completely exclude hydrogen effects from contributing to FCP in these steels. This uncertain role of hydrogen is further highlighted by the growing consensus that it may be involved in the stress corrosion cracking (SCC) of austenitic stainless steels, tll'm and it has been shown by fractography that there is a remarkable similarity between transgranular SCC and stage I (crystallographic) C E |131 Mechanistic studies of FCP behavior in aqueous environments are complicated by the possibility of localized differences in chemistry between the crack solution and that of the bulk solution. The crack may behave as a crevice and it CLINTON FONG, Research Engineer, and DESMOND TROMANS, Professor, are with the Department of Metals and Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada, V6T lW5. Manuscript submitted October 16, 1987. METALLURGICALTRANSACTIONS A
is well established that limited mass transport inside crevices leads to localized hydrolysis of metal cations which, in the case of austenitic stainless steels in chloride solutions, causes a decrease in pH within the crevice, t~4'l~'lmHence, potentiostatically controlled experiments that are required to distinguish between anodic dissolution enhanced FCP and hydrogen embrittlement enhanced FCP, on the basis of thermodynamic conditions in the crack that are either favorable or unfavorable for the generation of hydrogen, are complicated by the imprecise knowledge of the solution chemistry within the crack. This problem was recognized earlier by Ford. t17] In reality, the propagating fatigue crack situation differs from a crevice, because the cyclic displacement of the opposing faces of the crack increases the degree of mixing between the bulk solution and the crack solution. Liquid is expelled from the crack during the decreasing load (closing) portion of th
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