Coloring Niobium

Electric current in the anodizing bath causes oxygen to separate from the liquid. It combines with niobium to form a layer of niobium oxide (NbO2). The higher the voltage the thicker the oxide formed. The thin oxide is harder than the pure metal, but it can be easily scratched. The oxide is transparent. When light strikes the transparent oxide, some of the rays are reflected off the oxide and some pass through to reflect off the base metal. The rays blend with each other slightly out of phase. They cancel or augment producing color to your eye. The thickness of the layer determines what color is seen. Pumiced, sandblasted and lightly sanded surfaces scatter light and increase the viewing angle making the colors appear more vibrant.