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In the analysis of the post-transition corrosion regime of Zircaloys in a water environment it has typically been assumed that the cycles occurring after the first cycle (pretransition) could be approximated by a constant rate. Recently, a long-term testing of Zircaloy-4 has shown this assumption to be nonconservative. Post-transition rates are seen to increase with increasing weight gain or time until a steady-state post-transition rate is achieved. The corrosion of Zircaloy-4 in a water environment is, therefore, now seen to have a transitory regime in addition to the pretransition and steady-state post-transition regimes. The mathematical description of this transitory regime is shown to fit the following: corrosion rate, R = A · ΔW + B, which results in a general equation for weight gain during the transitory regime of ΔW = (B/A) · (eAt - 1). This formulation provides a more accurate description of long-term Zircaloy-4 corrosion for use in engineering design than assuming post-transition kinetics based on a linear fit to the cyclic regime following the first cycle.
pretransition, transitory, steady-state post-transition, water, Zircaloy-4, corrosion, long term corrosion, mathematical description
Lead engineer, Corrosion and Test Analysis, Knolls Atomic Power Laboratory, General Electric Co., Schenectady, N.Y.