studies on anodizing of aluminium in alkaline electrolyte using alternating current

Surface Technology, 26 (1985) 207 - 216 207

STUDIES ON ANODIZING OF ALUMINIUM IN ALKALINE ELECTROLYTE USING ALTERNATING CURRENT

S. JOHN, V. BALASUBRAMANIAN and (the late) B. A. SHENOI

Central Electrochemical Research Institute, Karaikudi 623006, Tamil Nadu (India)

(Received January 3,1985)

The work reported in this paper concerns a study of the anodizing of aluminium in alkaline electrolyte baaed on borax (sodium tetraborate) using alternating current. Coating thicknesses of 11 pm were produced from 50 g 1-i borax solution the pH of which is adjusted with sodium hydroxide to a value of 10.5 at 65 “C!, using a current density of 1.5 A dmP2. The effect of bath composition, temperature, current density, anodizing time and pH on the rate of build up of oxide coating and their relation to anodizing voltage and coating ratio were studied.

1. Introduction

Anodic films are formed on aluminium alloys in aqueous and non- aqueous solutions by treatment of an aluminium anode in an appropriate electrolyte. The properties of the films formed depend on the type of electrolyte used [l - 51. Thus, in neutral solutions a highly resistant anodic film whose thickness is proportional to the applied voltage is formed. How- ever, in a number of acid solutions (sulphuric, phosphoric, chromic and oxalic) another type of anodic oxide is formed. In contrast to the barrier layer type of film, the thickness of this oxide does not depend solely on the voltage applied, but rather on the duration and magnitude of the current. In these acid solutions the resistance of the anodic film rapidly attains a constant value, which is low relative to that of a film formed in a barrier layer forming solution.

Some recent publications have shown that porous oxide films much thicker than the usual barrier layers may be grown in alkaline electrolytes [6, 71. Yoshimura et al. [8] reported the formation of anodic coatings on 99.5% aluminium by using alternating current in 0.1% - 0.4% sodium hydroxide to which 5% - 14% sodium glycolate was added. The thickest and hardest coatings of 9.5 pm were obtained by electrolysis for 30 - 60 min at 3 - 6 A dm2 in 0.3% - 0.4% sodium hydroxide containing 12% sodium glycolate and using a stainless steel counterelectrode. The same

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authors [9] also investigated a.c. treatment in 0.1 M sodium hydroxide to which various oxidizing agents had been added: potassium permanganate, ammonium vanadate, potassium dichromate, potassium persulphate, hydrogen peroxide and potassium ferricyanide. The thickest and hardest coating was obtained in 0.1 M sodium hydroxide solution containing 0.05 M potassium permanganate.

Bugoyavlenskiy [lo] determined the optimum temperature and concentration for anodizing in carbonate and found that best results were produced using 5% sodium carbonate at 30 “C for 25 - 30 min at 0.5 A dmP2 and a voltage of 100 - 110 V. Aluminium anodized in alkaline solution was used as a base for electroplating. Aluminium was treated in a bath containing 40 g 1-r sodium carbonate at 45 V, using alternating current, at 40 “C for 10 min prior to rinsing and plated using standard baths [ 11.

Neufeld and Ali [ 111 reported the formation of thick porous coatings in electrolytes containing borax at pH 9 - 11. By operating in the range of 60 - 80 “C they obtained coatings on 99.5% aluminium which did not differ radically in sealing, corrosion resistance or abrasion resistance from conventional films formed in sulphuric acid. These authors also used 5% solutions of sodium carbonate, sodium silicate, sodium citrate and sodium potassium tartrate but in no case were they able to produce similar thick porous coatings.

This paper presents the results of an investigation on the production of porous anodic oxide films on aluminium using alternating current in alkaline electrolytes for decorative and protective applications. The authors have already reported a.c. anodizing in sulphuric acid for decorative applications [12] and a.c. hard anodizing for functional applications [13]. The present work is a continuation of these studies on the use of alternating current for anodizing as it offers advantages such as (a) low capital invest- ment on electrical equipment, (b) both electrodes are aluminium and hence anodized simultaneously and (c) electrolyte volume is most efficiently utilized.

2. Experimental details

Test specimens of size 75 mm X 50 mm with a centre tap for making electrical contact were treated in the following manner: (a) degreased with trichloroethylene; (b) etched in a solution of sodium hydroxide 50 g 1-l for 1 min; (c) rinsed in water; (d) desmutted in 20% nitric acid for 30 s; (e) rinsed in water; (f) anodized in an electrolyte containing 50 g 1-r borax (sodium tetraborate) and sodium hydroxide to pH 10.5, at a temerature of 65 “C; (g) rinsed in water; (h) coloured with organic dyes; (i) rinsed in water; (j) sealed.

Distilled or deionized water was used for solution preparation. All the chemicals used were of laboratory reagent grade. The current was supplied from a 270 V, 30 A single phase (50 Hz) autotransformer. Both electrodes

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were suspended using titanium jigs. The electrolyte was heated using a water bath. Air agitation was provided from a low pressure air compressor. Test panels were prepared at different temperatures at times from 5 to 60 min. Temperatures of 55, 65, 70 and 80 “C were employed in this study. These panels were used for thickness and coating ratio measurements.

The coating ratio, i.e. the weight of oxide film to the weight of metal con- sumed, was determined to measure the degree of solvent action of the electrolyte on the oxide film. For the determination of coating thickness of the oxide film a stripping solution of 20 g 1-l chromic acid and 35 ml 1-l phosphoric acid was used at 90 “C.

After the parts were placed in the electrolyte and the power turned on, the current was raised slowly to the prescribed current density. During anodizing the voltage was raised slowly throughout the anodizing cycle to maintain the pre-established constant current density. The decay of current at a constant voltage was also studied.

3. Results and discussion

3.1. Influence of the concentration of borax on the bath voltage The voltage-time curves obtained during anodizing in various concen-

trations of borax at 65 “C are shown in Fig. 1. At high borax concentrations a lower anodizing voltage is required to maintain a constant current density of 1.5 A dm-* due to the higher conductivity of the electrolyte. The 10 g 1-l and 2.5 g 1-l b orax electrolytes produced a pitting attack on the anodized surface.

1 IO 20 30 40 50 60

TIME IN MINUTES

Fig. 1. Variation of anodizing voltage with time for various borax concentrations: current density, 1.5 A dm*; pH 10.5; temperature, 65 “C.

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IO 20 30 40 50 60 IO 20 30 40 50 60

TIME IN MINUTES TIME IN MINUTES

Fig. 2. Variation of anodizing voltage with time for various temperatures: borax concentration, 50 g 1-l; current density, 1.5 A dmP2; pH 10.5.

Fig. 3. Variation of anodizing voltage with time for various current densities: borax concentration, 50 g 1-l ; pH 10.5; temperature, 65 “C.

3.2. Influence of temperature on the anodizing voltage Figure 2 shows the relationship between the anodizing voltage and time

at various temperatures in 50 g 1-l borax solution. As expected, at higher temperatures the voltage necessary to maintain a constant current density is lower than at lower operating temperatures. At high temperatures (greater than 75 “C) the operating voltage falls because of the etching action of the electrolyte. The optimum anodizing temperature was found to be 60 - 70 “C.

3.3. Influence of current density on the anodizing voltage The next variable studied was the operating current density. Figure 3

shows that as the current density increases so does the operating voltage. Current densities greater than 2.5 A dme2 result in streaking and pitting of the coating and hence the current density~ was restricted to 2 A dmW2 in this study. The optimum current density was chosen as 1.5 - 2.0 A dme2.

3.4. Influence of pH on the anodizing voltage Electrolyte pH was found to affect the growth of anodic films consid-

erably. Figure 4 shows the variation of voltage with time at different pH values. From the figure it is evident that lower voltages are required at higher pH values. However the coatings produced at pH values above 11 are usually streaky and pitted. Hence the optimum pH was chosen as 10.5. At higher pH values (11.5) the voltage drops due to the etching action of the electrolyte.

211

I I

IO 20 30 40 50 60

15 c

IO 20 30 40 50 60


Fig. 4. Variation of anodizing voltage with time at various pH values: borax concentration, 50 g 1-l; current density, 1.5 A dm”; temperature, 65 “C.

Fig. 5. Relation between the anodizing time and the coating thickness at various temperatures: borax concentration, 50 g 1-l; current density, 1.5 A dme2; pH 10.5.

3.5. Influence of temperature of the anodizing electrolyte on the coating thickness

Figure 5 shows the relationship between temperature of the anodizing electrolyte and the coating thickness at a constant current density of 1.5 A dmv2. At lower operating temperatures (less than 70 “C) the thickness of the coating is a linear function of time. In general the thickness of the coating increases with increasing temperature except for those films grown at the highest temperatures (greater than 75 “C).

3.6. Influence of pH on the coating thickness The coating thickness is influenced by the solution pH and the relation-

ship between pH and thickness with anodizing time is shown in Fig. 6. It is

IO 20 30 40 50 60

TIME IN MINUTES

Fig. 6. Relation between the anodizing time and the coating thickness at various pH: borax concentration, 50 g 1-l ; current density, 1.5 A dmP2; temperature, 65 “C!.

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apparent that the thicknesses of the coatings increase with increasing pH except for those grown at the higher pH values (greater than 11). This would appear to be due to excessive dissolution of the coating in the electrolyte at high pH values.

3.7. Influence of current density on the coating thickness Rates of film growth over a range of current densities are shown in Fig.

7. Thicker coatings are formed at higher current densities as expected. As pointed out earlier current densities above 2.5 A dm-* result in streaking and pitting of the oxide coating. For the successful operation of the bath it is very important that the voltage be initially raised slowly and steadily. Otherwise, burning of the coating occurs because of the rapid dissolution of the coating in the electrolyte.

3.8. Variation of current density with time at constant voltage There are two techniques available for anodizing, namely the constant

voltage process and the constant current process. In the constant voltage process, the voltage is slowly raised keeping the current density constant in the first 5 - 10 min and the voltage is maintained at that value. Owing to the formation of the oxide coating, the current density falls. In the case of the constant current process, the current is maintained throughout by continuously varying the voltage and hence the power consumption is high. Agitation of the electrolyte must be good to prevent local overheating and subsequent burning. Figure 8 shows the decay of current at a constant anodizing voltage of 20 V.

3.9. Influence of anodizing time on the coating ratio The coating ratio is influenced by treatment time. The relationship

between time and coating ratio is shown in Fig. 9. The decrease in the coating ratio is approximately linear with time at constant current density.

2.0 A/&t+

1.5 A/dm=

IO 20 30 40 50 60 IO 20 30 40


Fig. 7. Relation between the anodizing time and the coating thickness at various current densities: borax concentration, 50 g 1-l; pH 10.5; temperature, 65 “C.

Fig. 8. Variation of the current density with time at a constant voltage of 20 V: borax concentration, 50 g 1-l; pH 10.5; temperature, 65 “C.

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1 I 1 1

IO 20 30 40 50

TIME IN MINUTES BORAX CONCENTRATION 3/.f

Fig. 9. Relation between the anodizing time and the coating ratio: borax concentration, 50 g 1-l; current density, 1.5 A dm”;pH 10.5; temperature, 65 “C.

Fig. 10. Variation in the coating ratio with borax concentration: current density, 1.5 A dm”; pH 10.5; duration, 30 min; temperature, 65 “C.

The fall in coating ratio with time is explained as follows. The voltage rises as the coating thickness increases and this reflects increasing dissolution of the coating during its growth, associated both with a larger active surface area with progressive dissolution in the pores and increase in the local temperature due to the higher voltage required as the coating grows in thickness.

3.10. Influence of borax concentration on the coating ratio Shown in Fig. 10 is the relationship between concentration of borax

and coating ratio at 65 “C. It is evident from the figure that the coating ratio increases with increase in the concentration of borax. Here again 50 g 1-l borax gave the optimum coating ratio.

3.11. Influence of temperature on the coating ratio The operating temperature of the electrolyte has a pronounced effect

on the coating ratio. From Fig. 11 it can be seen that with increase in temperature the coating ratio increases up to 70 “C and then falls. This is due to the higher solubility of the coating in the electrolyte at higher temperatures.

3.12. Influence of pH on the coating ratio The variation in the coating ratio with solution pH at 65 “C is shown in

Fig. 12. As the pH increases initially, the coating ratio also increases; it subsequently drops at higher pH values because of the etching action of the electrolyte.

3.13. Influence of current density on the coating ratio The effect of increasing the current density is to speed up the rate of

growth. This effect of current density will last the whole course of the

214

M I I

9.0 9.5 10.0 10.5 II.0 11.5

TEMPERATURE ‘C PH

Fig. 11. Variation in the coating ratio with temperature: borax concentration, 50 g 1-l; current density, 1.5 A dm”; pH 10.5; duration 30 min.

Fig. 12. Variation in the coating ratio with pH: borax concentration, 50 g 1-l; current density, 1.5 A dm”; duration, 30 min; temperature, 65 “C.

0.5

t L I I I

I.0 1.5 2.0 IO 20 30 40

CURRENT DENSITY A/d* TIME IN MINUTES

Fig. 13. Variation in the coating ratio with current density: borax concentration, 50 g 1-r; pH 10.5; duration, 30 min; temperature, 65 “C!.

Fig. 14. Variation in the current density (in amperes per square decimetre) with time at a constant voltage of 20 V at various temperatures: borax concentration, 50 g 1-r; pH 10.5.

anodizing process and has a pronounced effect on the coating ratio. The relationship between current density and coating ratio is shown in Fig. 13. At higher current densities a steep rise in the coating ratio is obtained. This rise can be explained as being due to a decrease in the solution rate within the pore channel owing to the build up of ‘solution products. Increase in the coating ratio at higher current densities is invariably associated with a steeply rising voltage with time. In practice this condition presents serious dis- advantages because of the generation of heat due both to the passage of current and to the exothermic nature of the reaction. As dissolution of the coating takes place at the base of the pores because of the higher local temperatures prevailing in those regions, efficient agitation is required.

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3.14. Relation between the current density and temperature The temperature of the electrolyte affects the current density when the

constant voltage process is adopted. Figure 14 shows the variation of the current density with time at different temperatures when the anodizing voltage is kept at 20 V. From the figure it is evident that the current density falls with time. Again at higher temperatures a larger amount of current flows for a given anodizing voltage.

3.15. Colouring process The oxide coating is coloured with (a) organic dyes, (b) inorganic pig-

ments and (c) electrolytic colouring using suitable solutions for decorative applications. Multicoloured name plates and designs are produced by suitable masking and bleaching techniques. There is no difference between the colours produced using this method and conventional sulphuric acid anodized coating excepting that the coating produced using alternating current is a little brighter due to the efficient cleaning action of the evolu- tion of hydrogen gas at the electrode surface.

3.16. Influence of alloy composition Alloy composition significantly affects the formation of oxide coating

because of the difference in behaviour of alloys in the electrolyte. Alloys containing copper and silicon are difficult to anodize using this technique.

4. Conclusion

Anodizing aluminium was carried out using alternating current in 50 g 1-l borax with pH adjusted to 10.5 with sodium hydroxide at 65 “C at a current density of 1.5 A dmP2. In general the coating thickness increases with increasing pH and temperature except for those films grown at the highest temperatures and pH values. Coating ratio also increases with pH and current density. The coating produced is similar to the conventional sulphuric acid coating excepting that the coating is a little brighter. The coating is coloured with organic dyes or inorganic pigments or by electrolytic colouring using suitable solutions for decorative applications. It should be noted that the capital cost of an a.c. anodizing installation in comparison with a conventional d.c. anodizing plant is lower because of the simpler electrical equipment.

Acknowledgment

The authors wish to record their sincere thanks to the Director, CECRI, Karaikudi 623006, for his kind encouragement during the study and permis- sion to publish this paper.

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References

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10 A. F. Bugoyavlenskiy, Zh. Prikl. Khim. (Leningrad), 20 (7) (1947) 613. 11 P. Neufeld and H. 0. Ali, Trans. Inst. Met. Finish., 47 (4) (1969) 171. 12 V. Balasubramanian, S. John and B. A. Shenoi, Surf. Technol., 19 (1983) 293. 13 S. John, V. Balasubramanian and B. A. Shenoi, Met. Finish., 82 (1984) 9.

studies on anodizing of aluminium in alkaline electrolyte using alternating current

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