The influence of the composition of Zircaloy-type alloys has been evaluated previously with respect to mechanical properties (1973 International Conference Zirconium in the Nuclear Industry, ASTM STP 551) and to nodular corrosion (1985 International ASTM Conference on Zirconium in the Nuclear Industry, ASTM STP 939). The present study extends this previous work to include uniform corrosion, which is of interest for cladding tubes and spacer grids of fuel assemblies. A variety of ingots with compositions both within and outside of the ASTM specification range for Zircaloy-2 and Zircaloy-4 have been tested. The material was fabricated into strips using a conventional fabrication procedure, which incorporated an intermediate beta quench followed by working and annealing in the upper alpha-range. The alloy elements covered the following ranges: tin: 0.2 to 1.7%; iron: 0.05 to 0.53%; chromium: 0.04 to 1.05%; and nickel: 0.003 to 0.046%. In addition, oxygen, carbon, silicon, and phosphorus were varied over the range of standard Zircaloy contents. Uniform corrosion was studied out of pile by long-time autoclave testing in pressurized water at 350°C (380 to 840 days) and in high pressure steam at 400°C (180 to 397 days). These data supplement testing previously reported for these same alloys in steam at 500°C under static and refreshing conditions (Strasbourg, 1985, ASTM STP 939). Electron microscopic examinations (SEM and TEM) have been used to characterize the intermetallic precipitates to clarify the effect of the various elements. The results of the 350 and 400°C testing show that the time to transition from the cubic to linear rate increases and the post transition rate decreases with decreasing tin and carbon and increasing silicon content. Oxygen and phosphorus have not shown an influence in the range studied. The effect of iron and chromium was more complex than for these other elements; the effect was also different in nature between water and steam tests. Markedly accelerated corrosion was seen at Fe + Cr concentrations below the limits of the Zircaloys (⩽0.15%) especially in steam. This composition effect is similar to that previously reported for the same alloys tested at 500°C. High Cr/Fe ratios often resulted in high corrosion even at moderately high levels of both elements. The effect of iron and chromium can be correlated to type, size, and frequency of the intermetallics.
Previous investigators have reported the separate influences of chemical composition and heat treatment on the nodular and uniform corrosion of zirconium-based alloys. However, many of these compositions lie outside the allowable Zircaloy ranges required by most nuclear fuel specifications. The current study shows that the corrosion performance of the Zircaloys can be optimized by adjusting chemical compositions within current ASTM ranges, and examines the influence of composition on response to heat treatment. It is believed that this approach will yield improved corrosion performance and results in a product that can be introduced for immediate nuclear service application. To investigate the effect of tin, iron, chromium, and nickel on the nodular corrosion of Zircaloy-2, a special 5500-kg ingot was triple melted. The composition was intentionally varied within a controlled portion of the ASTM range along the ingot length. Specimens of the ingot with different compositions were hot worked and beta quenched at a common size. Strip specimens (0.67 mm thick) were fabricated using three process schedules spanning the annealing parameter (∑Ai = ∑ti, exp — QIRTi) range applicable to current production practice for either tubeshell or strip material. Corrosion weight gains were obtained from a 500°C static autoclave test. A statistically significant reduction in 500°C weight gain was found as (1) the percentage nickel and iron increased; (2) the percentage of tin decreased; and (3) the accumulated annealing parameter decreased. A “controlled composition” is defined where nickel and iron are adjusted towards the upper ASTM composition limits and tin is adjusted towards the lower limit. The 500°C weight gain of controlled composition Zircaloy-2 is basically insensitive to accumulated annealing parameter over the range of 6.2 × 10−21 h to 1.1 × 10−17 h. The effect of beta quench rate on 500°C weight gains was also determined for Zircaloy-2 controlled composition material. Weight gains varied less than 5 mg/dm2 over a range of surface cooling rates from 18 to 36°C/s. Analysis of the effect of beta quench rates on the uniform and nodular corrosion of Zircaloy-4 was performed. Uniform corrosion post-transition rates are not significantly affected by beta quench rate for 360, 400, and 420°C autoclave tests. During 450°C testing, unrecrystallized microstructures of Zircaloy-4 promoted retention of tight, adherent oxide layers for much longer exposures than fully recrystallized microstructures. 500°C static autoclave weight gains are inversely proportional to the logarithm of the cooling rate for Zircaloy-4.
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