Influences of laser surface alloying with niobium (Nb) on the corrosion resistance of Zircaloy-4

The influence of laser surface alloying (LSA) with niobium (Nb) on the corrosion and mechanical properties of Zircaloy-4 was examined by potentiodynamic polarization test in a chloride solution at 80 degreesC and immersion test in a steam at 400 degreesC, and microhardness test. The results are discussed with structural and compositional variations of LSA layer. LSA on the Zircaloy-4 precoated with Nb produced a Nb-alloyed layer 170-300 mum thick with 1.3-2.5 wt% Nb, depending on laser beam power. The alloyed layer was composed of a mixed structure of alpha-Zr and beta-Zr phases with the beta-Zr phase being increased with the Nb content in the alloyed layer. Microhardness of Zircaloy-4 was increased by LSA with Nb, due to three factors; solid solution hardening with Nb, grain-size refinement by rapid cooling, and transformed beta phase. The significant improvement in the resistance to localized corrosion of laser surface alloyed Zircaloy-4 in a chloride solution was attributed to the combined effects of rapidly cooled fine microstructure and the Nb alloying. However, the corrosion resistance of laser surface alloyed Zircaloy-4 was reduced in steam at 400 degreesC, a result of the formation of beta-Zr phase and Zr hydrides. (C) 2003 Elsevier B.V. All rights reserved.