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 °C and immersion test in a steam at 400 °C, 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 μm thick with 1.3–2.5 wt% Nb, depending on laser beam power. The alloyed layer was composed of a mixed structure of α-Zr and β-Zr phases with the β-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 β 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 °C, a result of the formation of β-Zr phase and Zr hydrides.