Correlation Between 400°C Steam Corrosion Behavior, Heat Treatment, and Microstructure of Zircaloy-4 Tubing

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Abstract
The uniform general corrosion behavior of Zircaloy-4 has been determined in long-term autoclave tests at 400°C and modelled using the annealing parameter A concept. The cumulative effect of subsequent heat treatments in the alpha phase field after beta quenching can be given by A = ∑i ti exp (Q/RTi) where ti is annealing time, Ti is the temperature (degrees Kelvin), Q is the activation energy, and R is the gas constant. The corrosion weight gains at prolonged corrosion testing were determined and plotted as a function of the annealing parameter. The resulting curves can be used to predict the corrosion behavior. It was found that a minimum A value, that is, heat treatment combination, has to be exceeded in order to obtain good corrosion properties since a sharp transition from bad to good corrosion behavior occurred. Detailed microstructural investigations using mainly electron microscopy techniques were performed on material with known corrosion behavior both before and after corrosion tests. The microstructures were characterized, and intermetallic particle size distributions were measured. Special care was taken in order to measure the particle size distributions on the tube outer surfaces, that is, where waterside corrosion actually occurs. No correlation was found between intermetallic particle sizes and the corrosion behavior: It is postulated that the matrix solute concentration is more important for the corrosion properties and that silicon and perhaps iron have the largest beneficial effects.

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