Electrocoloring of anodized aluminum: Part 1—Using aliphatic carboxylic acids

Electrocoloring of Anodized Aluminum: Part 1 Using Aliphatic Carboxylic Acids by M. Reda Gad Abu E1-Magd, Chemical Engineering and Pilot Plant Dept., National Research Center, Dokki, Giza, Egypt

' n previous work, simultaneous anodizing and coloring of aluminum .have been obtained in one step, a process called "integral color anodizing," which produces oxide layers only in a restricted color range. The disadvantages are that the intensity of color depends on the thickness of the film, and the process is difficult to carry out due to the need for high electric current, high voltage, and long treatment time, all of which are relatively expensive. For these reasons, a further range of colors has now been made possible by electrolytic coloring technology in which a corrosion-resistant oxide film is produced by a conventional sulfuric acid anodizing, and coloring follows in a second step with the use of an electrolyte containing inorganic metallic salts, x The anodized aluminum comprises one electrode, and graphite or aluminum is the other. The oxide or hydroxide precipitated within the pores colors the film brown, bronze, blue, yellowish gray, or black, depending on the salt solution and the alternating current voltage employed.2"3 Pfanhauser 4'5 indicated that uniform colors of greater depth could be obtained by using a cotmterelectrode of the same metal as the metallic salts in the coloring bath. This article describes the various effects of both monocarboxylic and dicarboxylic acids on the properties of electrocolored anodized layers in the presence of silver nitrate salts.

l

EXPERIMENTAL

Both monocarboxylic and dicarboxylic acids were used as a complexing agent for silver in the electrocoloring solution. From the first category, formic, acetic, and propionic acids were chosen, and oxalic and citric acids were selected as representatives for the second. Silver nitrate concentration was kept constant at 0.1 g/L. Appearance, color of coating, anode material, adhesion, thermal stability, coating thickness, pH value, electrode potential, electrical conductivity, alternating cur52

rent voltage, concentration of carboxylic acids, and structure of the deposit were investigated. Test specimens were cut from 0.1-mm thick aluminum having a total surface area of 20 cm 2, with a hole punched for hanging. Chemical analysis of test specimens indicated 94.66% aluminum and 5.17% silicon. Pretreatment was as follows: clean with acetone and calcium carbonate, and chemical polish. A cold and hot rinse was used after each of the above steps. At the beginning, anodizing was carried out in bath containing 10% sulfuric acid solution and 10 VAc for 10 minutes without agitation. A second step was practiced in order to electrocolor the anodized aluminum layer. This was undertaken using the bath given in Table I. Anodizing in sulfuric acid gives a colorless and transparent coating. Using an electrolyte solution with 0.1 g/L silver nitrate without addition of carboxylic acids, the color obtained was often weak and patchy. The carboxylic acids6'7 were employed in accordance with their dentate number and classified into (1) aliphatic mono£arboxylic acids, namely: formic, acetic, and propionic acids as monodentate and (2) aliphatic dicarboxylic acids, namely: oxalic acid as a bidentate and citric acid as a tridentate. The concentration of these acids was varied from 1.0 to 5.0 g/L in an electrolytic solution of coloring bath that contained anodized aluminum serving as the cathode, and graphite, as a counterelectrode. Coating weights were determined by weighing test specimens before the plating cycle and after coloring deposition, rinsing, and drying. Table I. Eleetrocoloring Bath Silver Nitrate

O.1 g/L

AC voltage pH value Temperature

5.0, 7.5, 10.0, and 12.5 4.7 20°C 15 rain

Coloringtime Anode material

Graphite electrode

© Copyright Elsevier Science Inc.

pH and potential measurements were carded out using a digital pH meter. Potential measurements were observed using a graphite electrode (as a counterelectrode) against a saturated calomel electrode (SCE) at room temperature (20°C). pH, potential, and conductivity values were taken after addition of carboxylic acids to the coloring bath at various concentrations. The structure of the deposits was investigated using a scanning electron microscope. A d h e s i o n Test Adhesion can be classified into a minimum of three categories: excellent, intermediate, and poor. The intermediate class is most often subdivided into the classes of good and fair. A destructive method has been developed for determining the quality of adhesion-bending. The test specimens were bent repeatedly through an angle of 180 ° until rupture occurred. Following fracture, no detachment of the coating shall be possible by probing with a sharp instrument. If the edge of the ruptured plating from the basis material could be observed, adhesion was considered unacceptable. Also, cracks in the basis metal or plating shall not be considered as failure unless accompanied by flaking, Peeling, or blistering. T h e r m a l C y c l i n g Test A thermal cycling test or thermal stability is used to determine both the coating's ability to withstand high temperarares and its adhesion with the base metal. This test was carried out by heating the panels in an electric oven to 300°C within 30 minutes, maintaining them at that temperature for the next 8 hours, and then allowing the panels to cool at room temperature overnight. This test was carried out for 12 consecutive days. Any resulting discoloration indicates the poor thermal stability of the coating due to thermal decomposition of the coating constituents. METAL F I N I S H I N G

• MARCH 1997

Table II. Effect of Monocarboxylic Acids on Appearance and Color of Coating Formic Acid AC Voltage

Acid Concentration (g/L)

Appearance

Color

Acetic Acid Color Uniformity Appearance

Propionic Acid

Color

Color Uniformity

Appearance

Color

Color Uniformity

5.0

1.0 2.0 3.0 4.0 5.0

W W W S B

PB SB WB WB WB

N,I N,I N,I N,[ H,R

W,P W,P W,M W,M S

WB WB WB WB WB

N,I N,I N,I N,I N,I

W W W W S

WB WB WB WB WB

N,I N,I N,I N,I N,I

7,5

1.0 2.0 3.0 4.0 5.0

W S S B B

SB MB PB MB MB

N,I N,1 H,R H,R H,R

W,P W,M S S S

WB WB WB SB PB

N,I N,I N,I N,I H,R

W W

B

WB PB SB SB SB

N,I H,I H,I H,I H,R

1.0 2.0 3.0 4.0 5.0

S B B B B

MB AB NB CB LB

N,I H,R H,R H,R H,R

W,M W,M S B B

WB PB PB SB SB

N,I N,I H,R H,R H,R

W S B B 8

WB PB SB B B

N,I H,R H,R H,R H,R

1.0

B

2.0 3.0 4.0 5.0

B B B B

DB LB AB LB DB

H,R H,R H,R H,R H,R

S,P S,M B B B

PB PB NB LB LB

N,I H,R H,R H,R H,R

S B B B B

SB NB LB LB LB

H,R H,R H,R H,R H,R

10.0

12.5

S S

Appearance: B = bright, smooth; M = mottle; P = patch; S = semibright,smooth;W = weak. Color of coating:AB = amber bronze; CB = canary bronze; DB = dove bronze; LB = light bronze; MB = medium bronze;NB = near bronze; PB = pale bronze;SB = slightly bronze; WB = weakerbronze. Color uniformity: H = homogeneous;I = irregular; N = nonhomogeneous;R = regular.

I n f l u e n c e of C o l o r

RESULTS AND DISCUSSION Appearance

The f'mal appearance of the electrolytically colored article is related to the stwface finish of the article before the porous layer of alumina is formed. On a brilliant or glossy specimen, the color effects obtained by the process are brilliant, whereas on a dull or matte object, the color effects are dull or matte. The effects of the different coloring process variables upon the anodized colored layer is depicted in Tables II and III. When the concentration of acids is increased beyond the optimum level, a spotted appearance due to islands of thick oxide layer are observed. 8'9 Oxalic and citric acids are relatively weak and have low solvent power, giving the color and appearance shown in Table III. The molar susceptibility (XM) of carboxylic acids 6 shown in Table IV indicates that lower XM (e.g., formic as monocarboxylic acid and citric as dicarboxylic acid) gives better appearance. At an increased voltage of 12,5, pits and milky strains were obtained. Increasing coloring time to 30 minutes had no appreciable effect, but mottling and blistering were obtained. METAL FINISHING

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1997

The color could be changed according to the time of treatment and the applied AC voltage. The color of the coating and color uniformity are shown in Tables I1 and Ill. It was found that when the operating AC voltage and concentration of acids have low values, weak or slightly bronze colors start to appear in the coating. By increasing the concentration further, these effects are enhanced so that amber or light bronze coatings can be produced. This has been done frequently in order to obtain hard coatings, and the color effects in these cases have always been regarded as quite incidental or even a nuisance. Little thought has been given to securing uniformity of color. From a practical point of view, the whole surface of the specimen is eventually covered with an initial light color followed by an amber (or jet) bronze. Schering patents ~°'11 mention that oxalic acid seems to have the right anodizing characteristics when used alone for producing coatings thick, hard, and colored, but not colored quite deeply enough. Finally the colors produced are restricted to the weaker and near bronze. The ionization constant of carboxy-

lic acids are shown in Table IV. 6 It seems that lower ionization constant (e.g., formic as monocarboxylic acid and citric as dicarboxylic acid) gives dove, amber, and light bronze colors. I n f l u e n c e of A n o d e Material

Appearance and color of coating dec veloped in the case of formic acid (concentration at 5.0 g/L), in a given length of time (15 rain) and applied AC voltage (10 V) at room temperature (20°C), depend on the anode material as shown in Table V. The electrical resistivity and crystal ionic radius of the elements lead, iron, and copper, are shown in Table g. 6'7 Higher electrical resistivity and also higher crystal ionic radius gives a bright appearance and amber or fight bronze color. A d h e s i o n Test

The adhesion was satisfactory and can be improved by heating at 750°F (400°C) for 10 minutes. The coatings produced were found to possess good adhesion to the substrate. Exfoliation, blistering, and peeling were not experienced. T h e r m a l Stability

It is interesting to note that no discoloration of the coating was observed 53

Table III. Effect of Dinocarboxylic Acids on Appearance and Color of Coating Citric Acid

Oxalic Acid Acid Concentration (g/L)

Appearance

Color

Color Uniformity

Appearance

1.0 2.0 3.0 4.0 5.0

W,M W,M W,P W S

WB WB PB PB PB

N,I N,I N,I N,[ H,I

W W W W

WB WB PB PB

N,I N,I N,I N,I

S

MB

H,I

7.5

1.0 2.0 3.0 4.0 5.0

W,M W,P S S B

SB SB MB MS SB

N,I N,I H,R H,R H,R

W S B B B

WB SB NB NB SB

N,I H,R H,R H,R H,R

10.0

1.0 2.0 3.0 4.0 5.0

S,M S B B B

NB NB CB LB LB

H,R H,R H,R H,R H,R

S B

B

SB CB AB DB AB

H,R H,R H,R H,R H,R

1.0 2.0 3.0 4.0 5.0

S,P B B B B

NB NB LB AB LB

H,R H,R H,R H,R H,R

B . B B B B

MB AB DB LB DB

H,R FI,R H,R H,R H,R

AC Voltage 5.0

12.5

B B

Color

Color Uniformity

Appearance: B = bright, smooth; M = mottle; P = patch;S = semibright,smooth;W = weak. Color of coating:AB = amberbronze;CB = canary bronze;DB = dove bronze; LB = light bronze;MB = medium bronze;NB = near bronze; PB = pale bronze;SB = slightly bronze; WB = weakerbronze. Color uniformity: H = homogeneous;I = irregular; N = nonhomogeneous;R = regular.

after the thermal cycling test for 12 days, and the coatings can be used in high-temperature environments. Effect of C o a t i n g T h i c k n e s s The thickness of the coating obtained for different concentrations of carboxylic acids and different AC voltage with coloring times for 15 minutes at constant room temperature (20°C) is given in Table VI. Results show that the maximum coating thickness is of the order of t4.2 g/m 2 in the case of formic (as monocarboxylic acid) and

13.0 g/m 2 in the case of citric (as dicarboxylic acid) at 12.5 V and 5 g/L acid concentration, which are sufficient for decorative applications. Curves of coating thickness as a function of acid concentration at different AC voltage are illustrated in Figures 1 to 4. Coating thickness increased with an increase of acid concentration at constant AC voltage. For any set of conditions, the color can be used as a quality check on the thickness. The uniformity in both thickness and color is improved if the

aluminum is treated with a slight etch prior to the conversion coating. In the case of the severely heat-treated and cast materials, a good etch is almost essential to good coating. 12 Effect of pH The normal pH range of the coloring bath without additives or complexing agents is 4.7. To get a steady rate of deposition, essential factors consist not only of the need for a properly prepared bath and accurate temperature control, but also proper pH. When a

' Table IV. Molar Susceptibility 6, Ionization Constant 6, and Ionic Conductance 7 of Complexing Agents Complexing Agent

Molar Susceptibility (× 10e)

Ionization Constant

Ionic Conductance (mho-cm2/equiv)

19.9 31.5 43.5 48.9 60.0

1.77×104 1.75x 10s 1.34 x 105 1.70×105 5.40 × 10~

54.6 40.9 35.8 70.2 40.2

Formic acid Acetic acid Propionic acid Citric acid Oxalic acid

Table V. Appearance, Color, Electrical Resistivity, 6 and Crystal Ionic Radius 7 of the Anode Material Anode Material Graphite Lead iron Copper

54

Appearance

Color

Electrical Resistivity (microhm-cm)

Crystal Ionic Radius (,4)

Bright Semibright, patchy Semibright, blistery Semibright, pits

Light bronze Near bronze Slightly bronze Weaker bronze-red

1,375 20.6 9.7 1.67

2.60 120 0.76 0.72

METAL FINISHING

* MARCH

1997

Table VI. Effect of Carboxylic Acids on Coating Thickness of Aluminum Coating Thickness (g/m2) AC Voltage

Acid Concentration (g/L)

Formic

Acetic

Propionic

Oxalic

Citric

5.0

1.0 2.0 3.0 4.0 5.0

10.5 11.2 11.7 12.0 12.1

10.0 10.6 11.0 11.3 11.4

9.2 9.8 10.3 10.6 10.8

8.6 9.0 9.4 9.7 9.9

9.0 9.6 10.0 10.3 10.4

7.5

1.0 2.0 3.0 4.0 5.0

11.0 11.7 12.3 12.7 13.0

10.5 11.2 11.7 12.1 12.3

10.0 10.8 11.4 11.8 12.0

8.9 9.5 10.0 10.4 10.6

9.7 10.5 11.0 11.4 11.6

10.0

1.0 2.0 3.0 4.0 5.0

11.9 12.5 12.8 13.0 13.1

11.0 11.7 12.3 12.6 12.7

10.7 11.4 11.9 12.2 12.4

9.4 9.8 10.2 10.5 10.7

10.4 11.0 11.5 11.9 12.1

12.5

1.0 2.0 3.0 4.0 5.0

12.5 13.1 13.6 14.0 14.2

12.0 12.5 13.0 13.3 13.5

11.6 12.2 12.7 13.0 13.2

10.0 10.5 11.0 11.5 11.8

11.4 12.0 12.5 12.8 13.0

complexing agent is added to the coloring bath, chemical reduction causes a pH drop during coloring. The effect of pH values for a coloring bath containing different concentrations of complexing agents is shown in Table VII. The influence of pH on the rate of deposition can be represented by the relation: rate = F (1/pH). The rate decreases as the pH increases. Figure 5 is a graphical representation of these data. Effect

of Electrode

Potential

Typical standing potentials (at 20°C)

tions of complexing agents in tin coloring solution are given in Table VII. Both coating thickness and potential in the electrolytic solution were affected by coupling of different complexing agents with anodized aluminum. The coating thickness was accelerated by decreasing pH and increasing the potential. A maximum coating thickness (14.2 g/m2) was obtained in the case of fomaic acid at + 302 mV versus SCE. As can be seen in Table VII, the standing potential is increased with complexing agent concentration.

Effect of Electrical C o n d u c t i v i t y The electrical conductivity for the coloring bath containing different concentrations of complexing agents is shown in Table VII. It is clear that the electrical conductivity of the electrolyte (K) is increased by the addition of carboxylic acids. It should be noted that the results obtained were consistent to those previously reported. 13,14 It was found that for deposits having a high coating thickness of 14.2 g/m 2 (formic acid), when the pH was 1.81, a high conductivity of 4.3 × 10 -3

for test specimens at different concentraI

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15.0

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•

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Acid concentration, g l L

Figure 1. Effect of acid concentration on coating thickness for 15-min coloring time, 5.0 VAc, 20°C, and 0.1 g/L silver. METAL FINISHING

10.0

8.0

,

5.0

1997

11.(

Figure 2. Effect of acid concentration on coating thickness for 15-min coloring time, 7.5 VAc, 20°C, and 0.1 g/L silver.

8.0i

I

1.0

I

2.0

I

3.0

Acid concentration,

I

4.0

I

5.0

g/L

Figure 3. Effect of acid concentration on coating thickness for 15-min coloring time, 10.0 VAc, 20°C, and 0.1 g/L silver. 55

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1

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Figure 4. Effect of acid concentration on coating thickness for 15-min coloring time, 12.5 VAC, 20°C, and 0.1 g/L silver. mho/cm was obtained. These results can be explained by the free mean path of conducting electrons of silver metal.

S t r u c t u r e of the D e p o s i t Scanning electron micrographs of the colored coatings demonstrate the incorporation of silver metal (or metal oxide) in the pores of the anodically oxidized film of aluminum, providing a wide range of color tone. Laser 15 has suggested, in the case of silver, that color is due to finely dispersed metal particles. Sandera 16 gives evidence for the presence of embedded metal particles in all the common coloring metals.

Figure 5. Effect of coating thickness on bath pH for formic acid at 12.5 VAC.

The ionic conductance of complexing agents is shown in Table IV. 7 Electron microscopy of the specimens revealed the presence of pores, some broader (Figs. 7 and 8) (as in case of propionic and oxalic acids, which have low ionic conductance) than others [formic and citric acids (Figs. 6 and 9), which have high ionic conductance and maximum coating thickness]. Electron micrographs give direct observation that metal deposition eventually occurs in most pores of the original film but that the nucleation and growth take place apparently randomly, so the pores do not fill unifomaly. This is consistent with previous work. 17

References 1. Narasimhan, N.R. and E. L. Banjza, Metal Finishing, 71(1):32; 1973 2. Caboni, V., Italian Patent 339,232; 1936 3. Wernick, S. and R. Pinner, The Surface Treatment and Finishing of Aluminium and Its Alloys, 4th ed., vol. II, Robert Draper, Teddington, England; 1972, pp. 507, 546-554 4. Pfanhauser, L., German Patent 741,753; 1940 5. Pfanhauser, L., Norwegian Patent 69,930; 1944 6. Weast, R.C., Handbook of Chemistry and Physics, 52nd ed., The Chemical Rubber Co. Ltd., Boca Raton, Fla.; 1971-72, pp. El14-118, D122, E 141 7. Dean, J.A., Lange's Handbook of Chemistry, 12th ed., McGraw-Hill Book Co., New York; 1979, pp. 6-34, 3-120 8. Scott, B.A., Transactions of the Institute of Metal Finishing, 43(3):1-8; 1965 9. Csokfin, E, Transactions of the Institute of Metal Finishing, 41:51-56; 1964 10. Schering, A.G., German Patent 657,902; 1938 11. Schering, A.G., German Patent 664,240; 1938 12. Groshart, E.A., Metal Finishing, 82(6): 69-70; 1984 13. Gedde, O.C., British Patent, 1,311,716; 1969 14. Sumimoto Chemical Co., German Patent 2,441,261; 1973 15. Lasser, L., Aluminium, 48:169; 1972 16. Sandera, L., Aluminium, 49:533; 1973 17. Doughty, A.S. et al. Transactions of the Institute of Metal Finishing, 53(2): 53; 1975 MF

Table VIL pH Value, Standing Potential, and Electrical Conductivity for Different Concentrations of Complexing Agents Complexing Agents Acid Concentration g/L

Formic

pH value

1,0 2.0 3.0 4.0 5.0

2.25 2.10 1.95 1,86 1.81

2.43 2,40 2,30 2.10 1,80

2.87 2.77 2.67 2.57 2.51

2.32 1,93 1.87 1.75 1.62

2.75 2.64 2.54 2.41 2.32

Standing potential, mV vs SCE

1.0 2.0 3.0 4.0 5.0

+289 +292 +296 +299 +302

+275 +280 +285 +287 +289

+252 +256 +260 +262 +265

+290 +297 +303 +309 +319

+252 +258 +260 +271 +279

Electrical conductivity x 10 3, mho/cm

1.0 2.0 3.0 4.0 5.0

2,8 3.2 3.7. 4.1 4.3

1,7 2.1 2.7 3.2 3.6

0.48 0.56 0,62 0.75 0.84

2.0 3.1 4.2 5.1 6.2

0.44 0.63 1.00 1.40 1.86

Parameter

Monocarboxylic Acids Acetic

Propionic

Dicarboxylic Acids Oxalic Citric

SCE, saturatedcalomelelectrode.

56

METAL FINISHING

° MARCH

1997