On the black oxide colour of zirconium alloys

Tubes of four zirconium alloys, used as cladding materials in light water nuclear reactors, were oxidized in an autoclave to produce oxide layers 1, 2 and 9 μm thick. The reflectance of the tubes was measured using a UV–vis–NIR spectrophotometer equipped with an integrating sphere detector. The 1 and 2 μm oxide tubes had a black appearance and a reflectance of only 10–12% in the visible region, whereas a dark grey appearing 9 μm oxide tube had 13–17% reflectance. The low reflectance was interpreted in terms of localized surface plasmon resonance in metallic particles embedded in the oxide layer, having a mean size of 40–70 nm.