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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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Page 1: Emily Gaul- JCE

7/24/2019 Emily Gaul- JCE

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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~ticalheno omen on of thin-

layer interference (iridescence). Using a common dc power

B U D D ~ V

with current-limiting ca~abi lit ies. conductive

aq ko bs 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 a t 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

thisJournal,

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 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 o various colors) of the

incident light. I n ti tanium 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 nterference o light waves. Based on an illustration by Stuart

Hamill.

Figure2. (rigM)A itanium 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

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Oxveen. which is eenerated a t

the

itanium anode bv the

oxldstke breakdown of water, subsequently co mb in ek th

the metal to form titamurn dioxideno

As

shown in Table

1

he 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 ,il 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 oxidefh nd 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 w6 re 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 ith each other (light wave crests

from one d a c e meet wave

troughs

mm ;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 coloringin 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 DUe

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

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

flgure3. n 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

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

Redpurple Green Brass Gold

80 Bronze ull gold Copper

85 Gold purple Green Pale Orange

90

~ o s e Plum rose OIange gokl

95 Daricgreen Magenta Purple pink

100 Gray Blue masenta Purple

105 Gray Greemse Purple

110

Green Blue

120

Greedpurple Turquoise

125

Greenlpurple Turquoise

130

erald reen 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

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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 a s blue-

green from one angle and as gold from another.

Thermal versus nodic 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

ogdef ilm varies~proportionately

i th

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 ai e less sati sfack l'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 i s recommended to avoid reactions of the

chlorides present in tap water, particularly a t 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 a n 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 i s 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 dn 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 i ts 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 ~pu lo us lyleaned. 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.wil1be 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 hould 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

sm 66 268.

3. Seeley,W.A.

MFAThesls UniveraifyofKansas

982isvailablehmReactiveMetals

Studio,see fmtnote 11.

4 Untca~ht, emIry

Conmpl~

nd kchhalagy;

Doubleday:Garden City.

ea York

1982;

r

23.130.

5. Nassau K The Physicsand Chemistry ofCo lor;Wiley-Interscience: New

York

983

Chapter 12 p 2N.

'React ve Meta s S l ~ a.

PO

Box 870. Clarma e Z 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