7/24/2019 Emily Gaul- JCE http://slidepdf.com/reader/full/emily-gaul-jce 1/3 Coloring Titanium and Related Metals y Electrochemical Oxidation Emily Gaul Department of Science and Mathematics, Columbia College, 600 South Michigan Ave, Chicago 60605 1996 The idea of coloring metals through electrocution in- trigues my visual arts students. Anodizing titanium and the related metallic elements niobium and tantalum is a novel means of illustrating electrochemical principles as well as demonstratine the o~tical heno omen on of thin- layer interference (iridescence).Using a common dc power BUDD~V with current-limiting ca~ abi lities. conductive aqkobs electrolyte and tita&m-metal, one can obtain a wide range of iridescent oxide colors on the surface of the metal by simply varying the a pplied voltage. For example , titanium metal is colored purple at 15 V and bronze at 50 V Similar effects can be obtained by substituting niobium or tantalum for titanium. Anodizing is a useful companion experiment to elec- troplating. Both are electrolytic and require an applied voltage, but whereas in electroplating a metal ion in the electrolyteis reduced onto th e surface of the cathode made of the same or different metal, in anodizing the metal anode forms an oxide first on the exposed surface and then oxidizes inward. Previous articles in this Journal, have dealt with anodiz- ing aluminum 1,2). ulfuric acid electrolyte and ir pro- vide the oxygen, which reacts with the aluminum to form its oxide, alumina AI20J. The electrolytically formed alu- mina gives a porous, spongy surface on the aluminum metal, which, when rinsed of th e sulfuric acid, will readily absorb organic dye. Besides providing a means to color the metal, anodizingis important in industrial applications in providing a more corrosion-resistant coating for alumi- num. In titanium anodizing, a much thinner transparent oxide layer of the metal is formed and colors result, not from the oxide layer absorbing added dyes as with aluminum, but rather from the effect of the thin oxide layer interfering with wavelengths (corresponding o various colors) of the incident light. In titanium anodizing the voltage is varied to obtain a variety of colors useful for the artist. The volt- age range is higher and the applied current lower tha n in aluminum anodizing 3,4). Titanium, niobium, and tanta- lum have been used by metalworkers in th e arts for their iridescent coloring when electrochemically or thermally anodized. The electrochemical reactions are as follows: Cathode 4p 4K 2H2 (reduction) Anode: 2~0-to2 4H 4e- Ti + 0 TiO, (osdatim) Figure 1. (above)Thin-layer nterference o light waves. Based on an illustration byStuart Hamill. Figure 2. (rigM)Aitanium vessel spun from flatsheet at high heat; the finish is the oxides that formedduring the process (see Table 1 for color-temperature relationships. Vase and photo by Bill Seeley, Reactive Metals Studio. 176 Journal of Chemical Education
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