Critical analysis of alloy 600 stress corrosion cracking mechanisms in primary water
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I.
INTRODUCTION
THE intergranular stress-corrosion cracking (IGSCC) of Alloy 600 steam generator tubing is a problem of great importance in pressurized water reactors (PWRs). Though numerous articles have been published during the last two decades (for a review, see References 1 and 2), the mechanisms that control IGSCC in nickel-base alloys are still under controversy. Three kinds of mechanisms are usually proposed: dissolution/oxidation mechanisms, in which the crack grows by dissolution/ oxidation of the material; hydrogen embrittlement mechanisms, in which the fracture results from a decohesion due to hydrogen accumulation in zones of maximum triaxial stresses; and corrosion-enhanced plasticity mechanisms, t3,41 which predict that a brittle fracture occurs as a result of local interactions between corrosion and plasticity. More recently, creep mechanisms tS~ and internal oxidation mechanisms t6j have also been proposed. The implication of various parameters in two or three kinds of mechanisms makes the assessment of the involved mechanism(s) rather difficult. The well-known deleterious role of hydrogen in aqueous solution on IGSCC of Alloy 600 in primary water tT,SJ can be explained by the classical hydrogen mechanism of decohesion. But it can also be explained by a dissolution mechanism in which hydrogen weakens the protective properties of the oxide film and favors dissolution or by a plasticity model, if one assumes that hydrogen enhances the plasticity of Alloy 600, as it was shown on nickel. I9j Moreover, some parameters can influence both dissolution/oxidation and plasticity so that the experimental results can be interpreted simultaneously in terms R. RIOS, Ph.D. Student, is with Electricit6 De France, Les Renardi~res, and with the Laboratoire de Mrtallurgie Physique, Bhtiment C6, Universit6 de Lille I, 59655 Villeneuve d'Ascq Cedex, France. D. NOEL, Head of Corrosion-Chemistry Section, and O. de BOUVIER, Research Engineer, are with Electricit6 De France, Les Renardi~res, Route de Sens, 77250 Moret-sur-Loing, France. T. MAGNIN, Professor, is with Ecole Nationale Suprrieure des Mines de Saint-Etienne, 158 Cours Fauriel, 42023 Saint-Etienne, Cedex 2, France. Manuscript submitted April 14, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
of both mechanisms. Table I summarizes the effects of parameters that are supposed to play a role both on dissolution/oxidation and plasticity. Some of them are well-known facts, and the others are interpretations made from experimental results. For example, chromium in solid solution decreases the dissolution rate but also the dislocation motion (thus the creep rate[5]). The beneficial influence of intergranular carbides can be due to a stress relaxation at the grain boundaries ull or a decrease in intergranular sliding, ~ but it can also be understood in a dissolution model: carbide precipitation decreases the amount of carbon in solid solution, and then increases the dissolution rate. u~ The key role of strain rate, which, rather than stress, governs the stages of cra
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