Anodizing - Coloring Niobium, Titanium and Tantalum
This process is called "anodizing" because the piece being colored forms the anode electrode of an electrical circuit. It is similar to electroplating except that the cathode electrode is treated in electroplating rather than the anode.
The metal is immersed in a solution that contains electrolytes (acids, bases or salts) which allows an electric current to pass through the solution onto the niobium. As the electricity flows through the solution on to the metal, a layer of oxide a few atoms thick forms on the metal's surface via electrolytic passivation.
I use TSP (tri-sodium phospate, commonly used in cleaning walls and floors) in my solution. This passivates the metal by removing previous impurities on the surface of the metal and dissolving the any oxide layer present.
This oxide layer generates color by creating an interference effect with visible light. The oxide layer, while very thin, is much stronger than the underlying material. I obtain specific colors by controlling the amount of voltage through the piece. Rainbow or multicolor effects are gained by turning up the voltage as the item is removed from the anodizing bath. Single colors are done with full immersions at the correct voltage for the color desired.
I use niobium over the more commonly known titanium because of the problems with removing titanium oxide.
Upon exposure to oxygen titanium forms a very hard thick oxide layer which must be removed prior to anodizing. If it is not removed colors are dull and the higher voltage colors (teals, greens) are impossible to obtain. The process of removing titanium oxide is usually done with strong acids and as little exposure to oxygen as possible. Niobium needs a much gentler deoxidation process (stripping) that is more suitable to my workshop environment.