We present here some important results investigating aluminum as an effective surface dopant for increased oxidation resistance of zircaloy nuclear fuel cladding. At first, the transport behavior of aluminum into reactor grade zircaloy was studied using simple diffusion couples at temperatures greater than 770 K. The experiments revealed the formation of tens of microns thick graded Zr-Al layers. The activation energy of aluminum in zircaloy was found to be ~175 kJ/mol (~1.8 eV), indicating the high mobility of aluminum in zircaloy. Subsequently, aluminum sputter-coated zircaloy coupons were heat-treated to achieve surface doping and form compositionally graded layers. These coupons were then tested in steam environments at 1073 and 1273 K. The microstructure of the as-fabricated and steam-corroded specimens was compared to those of pure zircaloy control specimens. Analysis of data revealed that aluminum effectively competed with zircaloy for oxygen up until 1073 K blocking oxygen penetration, with no traces of large scale spalling, indicating mechanically stable interfaces and surfaces. At the highest steam test temperatures, aluminum was observed to segregate from the Zr-Al alloy under layers and migrate to the surface forming discrete clusters. Although this is perceived as an extremely desirable phenomenon, in the current experiments, oxygen was observed to penetrate into the zirconium-rich under layers, which could be attributed to formation of surface defects such as cracks in the surface alumina layers.
Bibliographical noteFunding Information:
The authors would like to acknowledge ATI Wah Chang for supplying the zircaloy. Support and student funding was provided by Oak Ridge National Laboratory and the Nuclear Regulatory Commission, respectively. We also would like to acknowledge the contributions of Mr. Manuel Umanzor and Mr. Yonathan Kasey to the successful completion of the experiments. In addition, the Virginia Microelectronics Center and Nanocharacterization Center at Virginia Commonwealth University also assisted with parts of this work.
© 2016, ASM International.
- corrosion and wear
- failure analysis
- heat treatment
- non-ferrous metals