Surface Technology, 22 (1984) 15 - 20 15

CHEMICAL COLOURING OF ALUMINIUM

S. JOHN, A. PERUMAL and B. A. SHENOI

Central Electrochemical Research Institute, Karaikudi 623006 (India)

(Received August 15, 1983)

Summary

In addition to anodizing, which is a very familiar process, there have been a number of conversion coatings available for aluminium over the years. In this paper a chemical colouring process is described wherein colours ranging from grey to black, yellow and brown are produced. An alkaline ammoniacal solution containing triethanolamine and metallic carbonates is used at temperatures of 80 - 90 °C.

1. Introduct ion

Of the four well-known non-ferrous metals, namely aluminium, copper, lead and zinc, aluminium is the most important and widely used, even though it is the last of the four to enter into the service of man. Because of its high strength-to-weight ratio, good corrosion resistance, better form- ability and its good architectural characteristics, it has found innumerable uses in electrical applications, transportation, housing, packaging and can- ning, and engineering industries. Aluminium has a naturally bright and attractive surface appearance. For a great many applications it is entirely suitable without the necessity for further finishing. However, many proce- dures and methods have been developed to satisfy our varied requirements. It is possible to alter the reflectivity, appearance and colour of bare alu- minium by means of a variety of chemical and electrochemical methods. Although aluminium can be anodized and dyed almost any conceivable colour or t int through proper application of the appropriate organic dye after anodizing [1 -3] , in the last few decades significant progress has been made in the single-step colouring of aiuminium by chemical t reatment methods [4, 5]. In this paper the development of a simple chemical col- ouring solution to produce different shades for decorative applications is reported.

Chemical conversion coatings based on oxides, chromates or phosphates on aluminium are well known and mainly serve as a base for paints, lacquers and enamels [6 - 11]. Some of the well-known processes such as the modi- fied Baur-Vogel process produce iridescent shades which can be coloured with organic dyes, while the chromating process gives a yellow colour and the

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chromate -phospha te coating gives a green colour. Black coatings may be obtained on aluminium by dipping it in a hot solution containing molyb- dates or ant imony chloride [12]. Another solution contains permanganate [1] and a further solution utilizes a nickel salt containing potassium thio- cyanate [13]. The range of colours produced is limited and is usually iri- descent (with the exception of the chromate -phospha te coating); this makes the coatings suitable for indoor decorative applications. In this study, sparingly soluble metallic carbonates in an ammoniacal solution containing tr iethanolamine were utilized to produce different colours. The solubilities of the metallic carbonates used in the study are given in Table 1.

TABLE 1

Solubility of metallic carbonates at 20 °C [ 14 ]

Carbonate Solubility (g (100 g H20 ) l)

FeCO3 0.072 NiCO3 0.090 MnCO3 0.040 CuCO3 0.030 ZnCO3 0.070 PbCO 3 0.014

2. Experimental details

2S aluminium alloy panels of size 75 mm × 50 mm were degreased with tr ichloroethylene, etched in 45 g NaOH 1-1, rinsed, cleaned in 15 vol.% nitric acid and finally rinsed. The panels were immersed in a chemical colouring solution containing the following: (NH4)2CO3, 20 g l - l ; metallic carbonate MCO a (M - Zn, Pb, Cu, Mn, Ni, Fe), 0.2 g 1-1 ; tr iethanolamine, 2 - 5 ml 1 - l . Laboratory grade reagents were used. The pH was adjusted electrometrically using ammonium hydroxide or acetic acid. The metallic carbonates were added after a slurry had been made. Deionized water was used for all the studies and the temperature was maintained by using a constant- temperature water bath. After the panels had been coloured, they were rinsed and dried, and the colour was evaluated visually.

3. Results and discussion

3.1. Influence o f metallic carbonates Preliminary experiments with various concentrat ions of (NH4)2CO 3

showed that a concentra t ion of 20 g 1-1 is essential for the formation of good uniform coloured coatings. Higher concentrat ions (greater than 35 g l -l ) lead to a dull finish and the coating is muddy and streaky, whereas

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lower c o n c e n t r a t i o n s (less t han 10 g 1-1) lead to in te r fe rence colours . T r i e t h a n o l a m i n e (2 - 5 ml 1 - l ) was also f o u n d to be essential fo r the fo rma- t ion o f u n i f o r m co l ou red coat ings.

The ca rbona t e s o f zinc, lead, copper , i ron, manganese and nickel were used in c o n c e n t r a t i o n s o f 0.1 - 1.0 g 1-1 and the co lou r ob t a ined a f t e r im- mers ion fo r 10 min at 90 °C is shown in Tab le 2. Yel low, golden yel low, grey and b r o w n colours were ob ta ined . When mix tu r e s of ca rbona te s were i n c o r p o r a t e d in the so lu t ion , deep shades such as b lack were p r o d u c e d . Table 3 shows the co lou r ob t a i ned with m i x e d salts. The co lou r of the film can be a t t r i bu t ed to the p resence o f insoluble c o m p o u n d s of me ta l and a l u m i n i u m f o r m e d as a resul t o f reac t ion with the solut ion.

TABLE 2

Colour produced with different metallic carbonates

Carbonate Colour

ZnCO3 Brownish yellow PbCO3 Medium yellow CuCO3 Yellowish grey MnCO3 Dark yellow NiCO3 Golden yellow FeCO3 Brownish grey

TABLE 3

Colour produced with combinations of metallic carbonates

Carbonate combination Colour

CuCO3 + NiCO3 Black FeCO3 + CuCO3 Brownish black ZnCO3 + CuCO3 Greenish yellow

3.2. Influence o f immersion time The i m m e r s i o n t ime has a great inf luence on the co lou r deve loped .

Within 5 min the coat ings s ta r t t o f o r m and are usual ly l ight in shade com- pa red wi th t he da rke r co lours o b t a i n e d b y p ro long ing the t r e a t m e n t t ime. Table 4 shows the co lou r o b t a i n e d fo r var ious i m m e r s i o n t imes for a solu- t ion con ta in ing 20 g o f (NH4)2CO3 1-1, 2 - 5 ml o f t r i e t hano l amine pe r li tre and 0.2 g of NiCO3 1-1 at 85 °C. A p r o l o n g e d t r e a t m e n t impairs the qua l i ty o f the coat ing .

3.3. Influence o f pH A m m o n i a or acet ic acid was used to adjust the pH. S t rong alkalis m u s t

never be used as t hey dissolve a l u m i n i u m as well as the coat ing. The p H was var ied b e t w e e n 6 and 11. Higher p H values (pH > 10) lead to a dull

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TABLE 4

Influence of immersion time on the colour obtained using 0.2 g NiCO31-1

Immersion time (min) Colour

3 Interference colours 5 Light yellow

10 Golden yellow 15 Dark golden yellow 20 Dark yellow 45 Dark yellow (matt finish)

streaky finish, whereas low pH values (pH < 7) lead to interference colours. Hence pH 8 - 10 is found to be the optimum. Because of evaporation losses at the operating temperature (above 80 °C) a cont inuous addition of ammo- nia is required to maintain the pH.

3.4. Influence o f temperature The colouring temperature was varied from 70 °C to the boiling point

of the solution. An increase in the solution temperature accelerates both the rate of format ion of the film and the rate of attack on the metal surface. This can result in a change in the colour of the coating. The temperature of the solution should be maintained between 80 and 90 °C to ensure con- sistent results. A lower temperature of the bath (less than 70 °C) leads to interference colours.

3.5. Influence o f solution agitation Agitation of the colouring solution accelerates the reaction and pro-

vides a more uniform film formation. Uniform coatings were produced in this s tudy with air agitation, whereas a non-agitated solution gave a streaky finish.

3.6. Influence o f surface preparation The appearance of the coating produced depends greatly on the initial

surface condi t ion of the metal being treated. The coloured film accurately reproduces the surface condi t ion of the base metal. Thus brilliant and attractive shades are produced on polished aluminium (chemically polished or electropolished), whereas etched and mat t finishes produce dull shades.

3.7. Influence o f top coat The coloured coatings are very thin (of the order of 1 - 4 tzm) and have

less abrasion resistance than anodic coatings. They should not be used without an overcoating of transparent lacquer when resistance to wear and abrasion is required. The coating possesses satisfactory protective properties when it is used in combinat ion with a top lacquer coating.

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3.8. In f luence o f a lumin ium alloy The co lou r is a f fec ted by the presence of al loying e lements in the

aluminium. G o o d colours are p r o d u c e d on a luminium alloys which do n o t con ta in heavy metals and silicon. On commerc ia l ly pure a luminium and 2S alloy the co lou r is un i fo rm and at t ract ive.

3.9. E c o n o m y In compar i son with anodic films, chemical ly co lou red coat ings have

advantages o f e c o n o m y , speed of f o rma t ion and relative s implici ty o f equip- m e n t requi red for the i r p roduc t ion . T h e y are very suitable for p roduc ing cheap imi ta t ion i tems such as bangles, earrings, bracelets , chains and buckles. When small parts are to be co lou red by this t echn ique , t h ey are kep t in a plat ing barrel which is ro t a t ed gent ly as in plating.

4. Conclus ion

Alumin ium can be chemical ly co lou red to various shades o f yel low, b rown, grey and black using an alkaline ammoniaca l so lu t ion conta in ing metal l ic ca rbona tes and t r i e thanolamine . The o p t i m u m condi t ions are as fol lows: (NH4)2CO3, 20 g 1-1; metall ic ca rbona te , 0.2 g l - l ; t r i e thano lamine , 2 - 5 ml l -~ ; pH 8 - 10; t empera tu re , 80 - 90 °C; immers ion t ime, 5 - 20 min.

A c k n o w l e d g m e n t

The au thors ' thanks are due to Dr. K. S. Rajagopalan, Central Electro- chemica l Research Ins t i tu te , Karaikudi , for his kind permiss ion to publ ish this paper .

References

1 S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminium and Its Alloys, Vol. I, Draper, Teddington, Middx., 1972.

2 W. Lewis, The Practical Anodising of Aluminium, McDonald and Evans, London, 1960.

3 A. W. Brace, The Technology of Anodising of Aluminium, Draper, Teddington, Middx., 1968.

4 G. H. Kissin, The Finishing of Aluminium, Reinhold, London, 1963. 5 T. Biestak and J. Weber, Electrolytic and Chemical Conversion Coatings, Warzawa

Publications, Warsaw, 1980. 6 R. M. Burns and W. W. Bradley, Protective Coatings for Metals, Reinhold, London,

3rd edn., 1967. 7 S. Spring and K. Woods, Met. Finish., 79 (6) (1981) 49. 8 L. F. Spencer, Met. Finish., 58 (1) (1960) 58. 9 D. B. Freeman and A. M. Triggle, Trans. Inst. Met. Finish., 37 (1960) 56.

10 W. E. Pocock, Met. Finish., 52 (1.2) (1954) 48.

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11 H. Silman, G. Isserlis and A. F. Averill, Protective and Decorative Coatings for Metals, Finishing Publications, Teddington, Middx., 1978.

12 Br. Patent 1,156,356-7, 1967. 13 N. V. Shanmugam, K. N. Srinivasan, S. John, M. Selvam and B. A. Shenoi, Proc.

Natl. Solar Energy Cony., Allied Publishers, New Delhi, 1982, p. 7.021. 14 N. A. Lange and G. M. Forker, Handbook o f Chemistry, McGraw-Hill, New York,

1967.