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
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