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Short-term autoclave tests are currently being used by some as a selector for nodular corrosion, a phenomenon which is observed in boiling water reactors. The effects of composition have been evaluated previously with respect to mechanical properties. The present study extends this earlier work on chemical effects to include corrosion performance in 500°C steam, one of the predictor tests. Fifty-three ingots having compositions within as well as outside the specification range for the Zircaloys were melted and fabricated into plate and strip using a conventional fabrication procedure that incorporated an intermediate beta quench followed by working and annealing in the upper alpha region.
The roles of various elements, singly and in combination, on 500°C corrosion were examined systematically. The alloy elements covered the following ranges: tin, 0.02 to 5.14 wt%; iron, 0.04 to 0.57 wt%; chromium, 0.04 to 2.76 wt%; and nickel, 0.003 to 0.046 wt%. In addition, carbon, silicon, aluminum, and phosphorus were evaluated over the range of standard Zircaloys.
The frequency and severity of nodule formation were generally related to the weight gain for both static and refreshed autoclave tests. The transition elements chromium, iron, and nickel exhibited the most pronounced effects, with the effect of nickel being dominant. High weight gains are generally observed at low concentrations of iron, chromium, and nickel, and low weight gains are observed at high concentrations of these elements. The composition of Zircaloy is in the transition region between the two extremes. Equations were developed describing this relationship.
Heat treatments that affect the distribution of the intermetallics have a significant effect on the corrosion properties of Zircaloy. Process temperatures in the high alpha-range generally lead to high weight gains for Zircaloy. For beta quenching the results depend on the specific conditions of the quench and of the test performed. Testing in static steam resulted in low weight gains for all beta treatments studied, while testing in refreshed steam led to different results depending on quenching rate. In the refreshing autoclave tests high weight gains are observed at low quenching rates from the beta.
The best results are obtained by beta quenching at a minimum rate of above 5 K/s. For materials processed in this manner low weight gains persist even after long time annealing in the high alpha-range and sensitivity to nodular corrosion develops in Zircaloy only at temperatures close to the alpha/beta-transition.
zirconium, zirconium alloys, Zircaloy, corrosion, nodular corrosion, autoclave tests, heat treatment, quenching, phase transformation, chemical effects, impurities, second-phase, microstructure
Kraftwerk Union AG, Erlangen,
Kraftwerk Union AG, Erlangen,
Teledyne Wah Chang Albany, Albany, OR
Consultant, Teledyne Wah Chang Albany, Albany, OR