anodising titanium

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Coloring Titanium and Related Metals by Electrochemical Oxidation Emily Gaul Department of Science and Mathematics, Columbia College, 600 South Michigan Ave, Chicago 11 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~abilities. a 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 applied voltage. For example, titanium metal is colored purple at 15 V and bronze a t 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 electrolyte is reduced onto the 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). Sulfuric acid electrolyte and air 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 the sulfuric acid, will readily absorb organic dye. Besides providing a means to color the metal, anodizing is 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 (correspondingto 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 than in aluminum anodizing (3,4). Titanium, niobium, and tanta- lum have been used by metalworkers in the 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 interference of light waves. Based on an illustration by Stuart Hamill. I Figure 2. (rigM)A titanium 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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Page 1: Anodising Titanium

Coloring Titanium and Related Metals by Electrochemical Oxidation Emily Gaul Department of Science and Mathematics, Columbia College, 600 South Michigan Ave, Chicago 11 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~t ica l heno omen on of thin- - layer interference (iridescence). Using a common dc power BUDD~V with current-limiting ca~abilities. a 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 applied 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 electrolyte is reduced onto the 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). Sulfuric acid electrolyte and air 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 the sulfuric acid, will readily absorb organic dye. Besides providing a means to color the metal, anodizing is 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 to 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 than in aluminum anodizing (3,4). Titanium, niobium, and tanta- lum have been used by metalworkers in the arts for their iridescent coloring when electrochemically or thermally anodized.

The electrochemical reactions are as follows:

Cathode: 4 p + 4K +2H2 (reduction)

Anode: 2~0-to2+4H++4e-

!Ti + 0, + TiO, (osdatim)

Figure 1. (above)Thin-layer interference of light waves. Based on an illustration by Stuart Hamill. I Figure 2. (rigM)A titanium vessel spun from flat sheet at high heat; the finish is the oxides that formed during the process (see Table 1 for color-temperature relationships. Vase and photo by Bill Seeley, Reactive Metals Studio.

176 Journal of Chemical Education

Page 2: Anodising Titanium

Oxveen. which is eenerated at the titanium anode bv the oxldstke breakdown of water, subsequently combinekth the metal to form titamurn dioxide no, As shown in Table 1, the thickness of the oxide formed is &rectly to the applied voltage (3,4).

Thin-Layer Interference Colorine titanium electrochemicallv is a vivid wav to

illustrate-thin-film interference. Irid&cence due to thin- laver interference is also exhibited bv o~als , oil slicks. soaD bkbbles, ancient buried glass, rainbow bout, Ymood rkgsG, mother of pearl, and pigeon and peacock feathers. Unlike colorants such as dyes and pigments, which operate by se- lective absorption of certain wavelengths of light, in irides- cent coloring selective wavelengths of light are interfered with by the thin oxide fh, and the color obsenred will vary with the angle of viewing (5).

The colors result fmm interference of reflectedlieht fmm thin transparent oxides, as shown in Figure 1, w6re part of the light of anv eiven waveleneth of color is reflected bv the fir; outer &&ace and of the light throwh the outer surface and reflects off the inner metal surfa& If two reflections of a particular color are a half. wavelength out of ~ h a s e with each other (light wave crests from one d a c e meet wave troughs fmm ;he other), they interfere with each other. When opposite phases meet, the light interference is called "destructive" and the color ob- semed will be white light minus that color giving its com- plementary color.

Converselv. if two waves of the same color or waveleneth are retlectei'.om the inner and outer surfaces where ihe crest of one maichea the crest of the other, the waves are in step or "in phase" and they will constructively interfere or reinforce each other and as a result the dolor will appear - - brighter.

Thus the red coloring in a rainbow tmut or red anodized titanium is due to the thin layer destructive interference of

Table 1. Titanium Heat Oxidation a d Anodized Spectrum (4) Showing the Relation of Film Thickness and Color to Voltage and Temperature of Oxidation

Color

Yellow Brass Purple Violet-blue Purple-blue Light blue Gray DlUe Pale aqua Green blue Pale bronze Pale green Purple Green Rose gold Red purple Bronze Gold purple Rose Dark green Gray

Voltage (dc) Temperature ('C)

371

385

398 41 2

426

440 454

468

482 496

51 0 523

537

551 565

579

593

607

621 635

Film Thickness (w))

0.03

0.035 0.04

0.046

0.053 0.06

0.063

0.066 0.07

0.08

0.95 0.H 0.12

0.13

0.14

0.15

0.16

0.17

0.18 0.19

flgure 3. An anodized niobium sample showing the range of colors with varying voltage. Photo by Bill Seeley, Reactive Metals Studio.

Table 2. Comparison of Colors Produced at Given Voltages on Titantlum, Niobium, and Tantalum

Voltage (dc) Titanium Color Nmblum Color Tantalum Color 5 Yellow Yellow

10 Brass Bra% 15 Purple Plum Brass 20 V~olet-blue Vmlet-blue Yellow 25 Purple blue Sky blue Purple 30 Ught Mue Blueish gray Blue violet 35 Gray Mue Light gray blue Bluesiiver 40 Pale aqua Green gold Sky blue 45 Green blue Orange gold Silver blue W Pale bronze Rose Silver 55 Pale green Blue purple Silver 60 Purple Green blue Silver 65 Green Sea green Pale yellow 70 Rose gold Gold green Yellow 75 Red purple Green Brass Gold 80 Bronze Dull gold Copper 85 Gold purple Green Pale Orange 90 ~ o s e Plum rose OIange gokl 95 Daric green Magenta Purple pink

100 Gray Blue masenta Purple 105 Gray Greemse Purple 110 Green Blue 120 Greedpurple Turquoise 125 Greenlpurple Turquoise 130 erald Green Yellow green 135 Pale Green Pea Green 140 Silver Green Silver green 145 Blue silver Pale yellow 150 Silver Yellow

Volume 70 Number 3 March 1993 T i 7

Page 3: Anodising Titanium

blue-green, or cyan, which is the complementary color of red. The color that will be observed will vary with the thickness of the oxide layer (Table 11, which varies directly with the voltage or temperature used to produce it (3, 4). The light reinforcement or interference differs with per- spective; hence a person will see a peacock feather as blue- green from one angle and as gold from another.

Thermal versus Anodic Coloring The thin oxide films can be generated on titanium by re-

action of oxygen with metal by either of two methods: ther- mal or heat oxidation and electrolytic oxidation or anodiz- inn (3, 4). During thermal oxidation. the thickness of the ogdefilm varies~proportionately with time and the tem- perature of the metal. Colors within the blue and gold range are obtained by heating titanium metal with a pro- pane flame. Colors resulting from higher temperature are obtained by heating with an acetylene torch or placing in a kiln (see Fig. 2.)

Oxidation by electrochemical anodization has the advan- tage over thermal oxidation in that the voltage, hence the film thickness. can be more accuratelv controlled (Fie. 3). In addition niobium and tantalum aie less satisfackl'y colored bv heat but exhibit an even wider ranee of interfer- ence colo& than titanium when wlored elec&chemically as shown in Table 2.

Procedure Titanium can be anodized in any wnducting electrolyte

such as Dr. Pepper, Epsom salts, or ammonium sulfate. The best results were found using trisodium phosphate.

Approximately 200 mL of 10% solution of trisodium phosphate (a detergent base available in most hardware stores) is added to a 250-mL Pyrex glass or plastic beaker. Deionized water is recommended to avoid reactions of the chlorides present in tap water, particularly at the higher voltages. Anodizing reactions should be run at room tem- perature.

The cathode is a 6- x 2 314411. strip of 26-gauge titanium with an attached tab to conned to the external leads. The cathode. with an extrndine tab. is wraDDed around the in- side of the beaker, and coiered by a &ip of plastic mesh (such as used for needlework) to prevent its touching the anode. A2- x &in. strip of 26-gauge titanium or niobium or thinner tantalum foil is used as the working anode.

The greater the purity of the metal and the cleaner the surface, the more brilliant the colors exhibited. If the metal is industrial grade it can be cleaned as follows:

Scrub with 320-grade followed by 400400 grade silicon ear- bide paper followed by steel wool and steel wool and detergent, then rinse with acetone to remove grease, oil, or salt residue.

Titanium must be acid-etched to reach its greatest color ~otential. Niobium (which is s h i ~ ~ e d in a ~rotective ~ l a s - tic) and tantalum need only be &&eased before use.'Any metal intended for use as jewelry should first be cut to shape with its edges well filed. It can he flattened with a rubber or rawhide mallet.

A low current-limiting 0-200-V dc power supply'is con- nected in series with a voltmeter to the cathode (-) and anode (+I. Electroplating power supplies do not provide the higher voltages and lower currents required for titanium anodizing.

Alligator connectors should be sc~pulously cleaned. The electrodes are then placed in the electrolyte except for the connecting tabs.

To prevent a short circuit the electrolyte should not come into direct contact with the leads from the power sup- ply, nor should the two electrodes come in contact with each other when the voltage is on. Rubber gloves must be worn at all times and work should never be done on a metal table.

The voltage can be varied to produce a range of thin layer interference colors as shown in Table 1. Only the part of the metal that is in contact with the electrolyte.wil1 be an- odized. Colors are obtained almost immediately. Students may mask portions of the reactive metal with electrical in- sulating tape, and then unmask portions of the tape as they work from high to low voltages, thus creating an image. The final ~roduct should be rinsed in deionized water to remove tke trisodium phosphate. The thin layer. which is easily scratched, can be protected by spray acrylic.

Literature Cited 1. Doe1tz.A. E.;Tharaud, S.;Sheehan,W. F. J. Chem. Edue. 1988.60, 156157. 2. Blstt, R. G. J. c h e m ~ d u c ism,66,268. 3. Seeley, W. A. MFAThesls,UniveraifyofKansas, 1982isvailablehmReactive Metals

Studio, see fmtnote 11. 4. Untca~ht, 0. JemIry Conmpl~ end lkchhalagy; Doubleday: Garden City. Nea York,

1982; rp 723.130.

5. Nassau, K The Physics and Chemistry ofColor; Wiley-Interscience: New York, 1983, Chapter 12, p 2 N .

'React ve Meta s S l ~ a 0. PO Box 870. Clarma e AZ 86324 s a SoJrce lor tnese rnalerla s I tanurn nloo Lm, tanta8Jrn (101 only,. and anodizing power supplies, as well as the thesis cited inref. 3.

178 Journal of Chemical Education