Two major factors that affect corrosion behavior of Zircaloy-type alloys have been evaluated in a PWR cladding development program ongoing since the early eighties. One of the important parameters was found to be the alloying concentration and distribution of transition elements.

The influence of transition elements (Fe, Cr, and V) was studied on corrosion coupons and experimental fuel rods. Isothermal long-time corrosion tests were performed out-of-pile (1 to 3 years in 300 to 370°C) pressurized water and 400°C steam with and without LiOH additions) and in PWRs (up to 6 years at 310 to 335°C). Experimental fuel rods were irradiated in PWRs up to nine annual cycles.

The experiments show a large reduction of corrosion rate with increasing content of transition elements in all environments. Above a certain transition element content no further improvement in corrosion is observed. The test results clearly suggest the use of transition element concentrations higher than specified by ASTM for Zircaloy-2 and -4. The beneficial effect of transition elements increases with increasing exposure time. The most significant effect of corrosion behavior was seen after six years of PWR exposure, especially at high temperatures.

Different transition elements influence corrosion differently, depending on the environment. Out-of-pile in 350 to 370°C pressurized water and 400°C steam, a beneficial effect is exhibited only for Fe, but not for Cr and V. In PWR and in diluted LiOH autoclave tests, all three transition elements reduce corrosion.

for Zr-based alloys with transition element (TE) concentrations meeting the ASTM Zircaloy-4 specification range, the fraction of corrosion hydrogen picked up by the metal increases with increasing corrosion resistance. Zr-based alloys with enhanced transition element concentrations (above the ASTM limits) exhibit reduced hydrogen pickup fractions.