SUR-FIN ‘95 . I

New Attract ive Finishes for Architectural and Decorative Anodized Aluminum

Walter Dalla Barba ltaltecno Srl, Modena (Italy)

ABSTRACT This paper reports on two new technologies for producing attractive new finishes on anodized architectural aluminum, which are the subject of patents.

The first of these represents a revolutionary approach to the production of a fine uniform finish on architectural extrusions, which reduces caustic soda consumption by up to 75% and minimizes the problems associated with effluent treatment and sludge removal. As well as these advantages, the technique developed eliminates defects such as die lines, scratches, etc., thus improving yields and reducing operating costs.

A second major development is the development of a new electrolytic coloring system by means of which grey, blue, green, red and purple can be obtained from the same electrocoloring bath. This multi-colour process modifies the barrier layer with a specially developed current source and selectively “electrodeposits” tin in the pores of the oxide film.

Whereas the “interference colors” previously developed required a second phosphoric acid anodizing step, the new technique uses only a sulfuric acid electrolyte. In conjunction with a computer controlled power source for the coloring process, the reproducibility of the colors obtained is of a very high order.

Using the above developments as complementary processes attractive uniform matt shades are obtained which differ from those normally produced by conventional anodizing and coloring methods, or by painting or powder coating.

1 . In t roduct ion

By means of this paper I will introduce two new technologies for anodized aluminum which can be used separately or in combination in order to obtain a very interesting final surface effect.

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The first technology is an innovative new way t o obtain a very fine. matt surface before anodizing by means of a mechanical pretreatment based on a patented shot blasting technique. The second part of the paper introduces a new technology for obtaining new electrolytic colors such as blue, green, grey and brick red, which are outdoor resistant. The combination of the new mechanical system as a pretreatment and the new electrolyte colors after the anodizing step result in new unique color shades which give a "lacquered matt" effect t o extruded profiles or to any anodized aluminum object.

Before the actual anodizing treatment, the aluminum material must undergo degreasing, etching and deoxidizing processes. Usually degreasing is carried out in a solution containing alkaline salts and surface-active agents. This solution does not produce any chemical etching but only softens the dirt and the oils on the surface. The degreasing treatment is therefore followed by a pickling or etching process in caustic solution with appropriate additives.

The first basic degreasing treatment is completed during the first 10- 30 seconds of the second treatment in which the complete cleaning of the surfaces is brought about. A longer time, usually up t o 20 min., considerably influences the surface finishing, as the longer the immersion time, the greater the chemical etching produced.

In Italy, brushed finishings produced with special equipment are usually requested.For this finishing the etching tank immersion time is approx. 3-5 min. in order to maintain the characteristic "scratched" effect. On the other hand, in many European countries the so-called "matt" finishing is preferred. This is usually produced in an etching solution of the following composition: Free caustic soda: 70-100 g/l Dissolved aluminum 100-1 50 g/l

Temperature: 55-65 "C Stabilizing additives 10-20 g/l

Immersion time: 15-20 min.

Even though excellent additives have long been known for their capability of maintaining the perfect stability and fluidity of the solution, preventing the regular elimination caused by the formation of sludges (or encrustation) of aluminum hydroxide, the operating costs of this treatment are high due t o the following factors: a)the high quantity of soda consumed;

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b) the problems connected with the treatment of waste waters caused by the entrainment of great quantities of dissolved aluminum.

In view of these problems and in order t o overcome the objective difficulty of eliminating extrusion defects (stripes, scratches, etc.) found on the aluminum materials, a mechanical treatment has been developed with a subsequent brief dip into an etching solution.

2.Description o f the Process

The extruded material is conveyed directly to the mechanical treatment described below without any intermediate stage. This treatment is carried out by means of a special machine which "shotblasts" the aluminum sections. Special turbines with a suitable and adjustable layout provide for the launching of metal shots of appropriate size capable of covering the complete surface to be treated. By means of an accurate adjustment, both the section bar forward feed and the quantity and force of the shotblasting can be regulated. The effect obtained is a completely uniform mechanical etching, as the special arrangement of the turbines ensures the treatment of grooves and recesses on the visible parts of the sections, for which the "grain size" depends on the size of the shots used. For industrial use, a finer finishing has met with grater interest. The aluminum material thus treated is ready for the traditional anodizing process, because any shots present have been eliminated by means of the special outlet devices with which the machine is equipped. A sieving system eliminates any shots that are too small. The machine is constructed according to the highest safety standards to prevent excess noise, dust and any other problem regarding the safety and health of the operators. The machine itself can be equipped with an automatic infeed/offload system so that the operator's function is limited t o the simple control of the operating parameters.

The treated material as described above is conveyed t o the anodizing stage in a completely traditional manner. In fact, the sections are hooked in the usual way and conveyed t o the alkaline degreasing tank, from where they pass on to the etching tank. 2 to 5 minutes immersion time is sufficient to obtain a correct finishing very similar t o the above-mentioned "matt" finish and is very common in most of Europe. After subsequent washing and neutralization, the material is conveyed to the anodizing stage and then to the coloring stage in the usual manner, as the etching treatment does not have any significant

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influence on the formation of the anodic oxide layer and (possible) coloring by immersion or by electrolysis.

Considering the reduced etching treatment time, the above-mentioned problems relating t o the etching composition and to the use of special additives for: a) the complexing of dissolved aluminum; b) the attainment of a well-leveled finishing even with a high quantity of heavy metals dissolved (iron, zinc, nickel), can show us various interesting aspects. With a treatment time as indicated above, the dissolved aluminum concentration will tend to stabilize around 70 g/l (hence a t a value approx. half of that indicated for the production of the matt finishing by chemical treatment alone). It is therefore possible t o assume a formulation of additives enabling the recovery and recycling of the soda solution by the precipitation of aluminum as aluminum hydroxide according to the Bayer process. The composition of the etching solution becomes the following:

Free soda: 50 g/l Dissolved a I u mi n u m 50 g/l Leveling additives 15 g/l Temperature: 55-65 "C Treatment time: 3-5 min.

For this type of solution a system is available on the market for the recycling of the solution itself by means of aluminum elimination. However, i is not the case to go into further detail on this well-known process. Those interested in further details may refer to other publications (1, 2 ) . The treatment for the recovery and recycling of etching solutions is very useful t o maintain a low aluminum content in the etching solutions and to reduce i ts entrainment into the waste waters. Considering the operating conditions of the tank, this becomes a useful suggestion, but not a serious necessity as with the above- mentioned solutions of high aluminum content. The lesser the aluminum content entrained into the waste waters, the lesser the costs of water treatment and quantity of sludges produced.

3 .Economic Considerations

If we compare the conventional chemical etching treatment to the mechanical one proposed above, the most evident fact which emerges is the different "consumption" of metal aluminum, i.e. the different loss in the weight of material treated.

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. The use of the described process considerably reduces operating costs for production of a uniform matt finishing and is even capable of covering certain extrusion defects (stripes, scratches, etc.). (See table 1)

Second Part

The following process enables us to obtain the colors grey, blue and green by means of a selective coloring method already described in a previous work (1). A suitable electrolytic treatment is capable of directing the settling of the metal particles in order to produce various colour shades. We have also tried to give an explanation of the colour results according t o light scattering phenomena. From a practical point of view, only colorings giving colour shades from light bronze to black are possible. The use of different metal cations (Ni, Co, Sn) do not substantially change the colour produced. Only by using special ions such as copper and selenium, can we obtain colorings other than bronze, but these are not conventionally produced due t o both physical appearance and quality. The settling o f metallic copper particles inside the oxide pores produces colorings ranging form pink to garnet and black. This may however trigger off corrosion phenomena. Recently, colouring by interference has been proposed, but there seems t o be considerable difficulties regarding reproduction of the colours obtained (usually bluish). Since the demand for new colours - in combination with improvements in electronic methods - could be an excellent starting point for attempting t o find new ways of colouring anodised aluminium, our lab. has carried out complex research, which has opened up for new colourings such as grey, blue and green.

The anodic oxide coat produced by conventional sulphuric acid treatments has a porous structure, which can be the base for substances capable of colouring. Fig. 1 shows the theoretic structure of the anodic oxide coat. The barrier layer, the thin film between the aluminium and the actual oxide coat is also indicated. Fig. 2 shows the results of absorption colouring, while Fig. 3 demonstrates electrolytic colouring. The basic difference between these two processes is that absorption colouring is a process in which the colorants fill up the outer parts of the anodic oxide coat, while the electrolytic process is based on the deposition of metal particles a t the bottom of the pores. These

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particles have a particular light scattering. As the number of particles increases, the number of light refraction also increases and the colour becomes darker because the intensity of the light emitted will decrease considerably. This physical phenomenon is sketched macroscopically in Fig. 4, a, b, c.

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Fig. 4 a clearly shows that with material of a natural colour there is no particular scattering of the incident light ray, and i ts energy, therefore, does not change significantly. The surface appears a silver colour. If a certain number of metal particles (Ni, Co, Sn) are deposited a t the pore bottom by means of a brief electrochemical process (Fig. 4b), these particles are capable of scattering the incident light rays several times. The energy of the emergent (refracted) ray will therefore decrease. The higher the number of light refraction (i.e. the lower the energy of the refracted ray), the darker the colouring. A bronze colouring of different intensity is therefore obtained. If a high quantity of particles are deposited (so as t o practically fill up the pore) there is so much light refraction that the energy of the incident ray is completely dissipated and that of the refracted ray is reduced t o almost nothing. The colour of the surface will be black. By means of an intermediate electrochemical treatment in phosphoric acid between the actual anodizing and the subsequent colouring stages, the anodic oxide pore bottom is enlarged and the so-called interference colouring (2) is obtained. The colours attainable range from blue t o green and pink (3, 4, 5). In practice, however, it seems that only the colour blue (grey) is sufficiently reproducible industrially.

By means of an intermediate phosphoric acid treatment, the pore alteration indicated in Fig. 5 is produced. The metal particles deposited during the subsequent electrocolouring stage settle a t the bottom of the anodic oxide coat, but due t o the special shape of the pore, an optical interference effect is obtained. On this background, it is assumed possible t o modify the barrier layer and/or the bottom of the oxide coat pores. In this manner it is evident that the colour of the anodic oxide coat itself will change, and possibly the subsequent electrolytic colouring too.

4.0ur Experimentation

Continuing our experiments (6), we formulated a special solution consisting of strong inorganic acids and special additives (organic salts

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. and acids) to be used for an electrolytic treatment subsequent to the conventional anodizing process (7). The anodic oxide coat was produced in a solution of the following type: sulphuric acid 180-200 g/l dissolved aluminium 6-7 g/l current density 1.6 A/dm2 temperature 19 + 1°C thickness 15-20-25 microns

Much attention was paid to the cooling and dissipation of the colour produced in the solution in order to produce a high-quality oxide coat. The anodizing tank was equipped both with the normal cooling system (refrigerator and colour exchanger) and with a micro-bubble air injection system. The high-quality anodic oxide layer already produced was therefore ready t o be treated. For this purpose, a tank similar t o the anodizing tank was then prepared, equipped with the following: a)a cooling system keeping the temperature a t 20°C; b) a micro-bubble air injection system; c) electrodes (aluminium, lead or graphite).

The solution composition was formulated so that: a) the solution had a good conductivity; b)it would not cause any damage to the anodic oxide coat even during

c) it would contain suitable compounds capable of influencing the

Trials were carried out over a long period in order to establish the type of current necessary and the relevant operating parameters. According to the latest data, the best features seem to be the following: * the current must not necessarily increase the aluminium oxide layer,

but, on the other hand, it must not damage the already existing oxide layer;

* a bi-directional current created by the sum of two direct currents of different polarity, appropriately mixed and derived from an alternating current which has been modified or choked in various manners gave the best results;

long dips;

barrier layer.

* the operating voltages ranged from 5 to 25 volts; * processing time depended on the effect required. After this intermediate treatment, a smooth, matt surface coat of a uniform grey colour was obtained, the intensity of which depends on the processing time. The material thus treated could be conveyed to a conventional pore closing treatment or could be treated in an electrocolour solution to darken the colour.

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The workpieces deriving from the intermediate process were dipped . into an electrolytic solution similar t o that used for conventional electrocolours based on metal salts (Ni, Co, Sn), and in this manner a selective and oriented deposition of metal particles inside the aluminium oxide pores was brought about. The result was a grey colouring, even of very dark intensity, which - according t o the processing time - varied and turned into a very interesting bluish colou ri n g . A decidedly important feature of all these experiments was that even after the pore closing treatment, the anodic oxide coat turned out to be coloured in a very uniform manner without any surface alterations, even with very complex samples. Tests implemented in an industrial anodizing plant confirmed the uniformity and reproducibility of the colourings. A theoretic model of what occurred during the intermediate treatment is sketched in Fig. 6 and 7. It is almost certain that during the intermediate treatment the following was produced: a)a thickening of the barrier layer; b)a modification of the pore bottom (they became smaller); c)formation of opaque finish and/or colourings due to:

1 . thickening of the barrier layer; II.formation of special metal compounds inside the oxide coat

structure (similar to those produced during the hard anodizing process).

To explain this theory further we wish to point out that when anodizing is carried out with acids giving a thick barrier layer or reduced porosity, the colouring of the oxide layer is obtained (8). A typical example is the self-colouring anodizing (or integral colouring). Preliminary light solidity tests showed no decoloring or colour alteration, and this demonstrates t o what extent the colour grey is innate in the structure of the coat. The subsequent electrolytic colouring in a metal salt solution causes an evident darkening of the grey colour instead of the formation of the conventional bronze colour shades, typical of classical electrocolouring as explained in the previous chapter. This different behaviour is due to the fact that the modifications made t o the oxide coat direct the metal particle deposition during the electrocolouring stage. The consequent light scattering is therefore completely different than that described above. Thus a grey colour is obtained which has an intensity depending on the time of treatment (i.e. on the quantity of metal particles present in the oxide coat pore). Certainly the different colour is derived from the different type of settling. The modification of the barrier layer and of the pre-bottom is sure to change the electric conductivity of the oxide coat undergoing

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. colouring. The metal particle pattern will also consequently change. This new pattern will change the light scattering, and hence, even with a rather reduced number of metal particles (very short colouring times: 30-90 sec.), the colouring will be very dark (grey-black or grey-dark blue). Blue and green colourings are also implemented if the appropriate variation of the oxide coat is obtained.

Through the appropriate use of current, it is possible to modify the anodised layer in order to produce colourings ranging from grey-blue to mallow green, brick red and purple. The reproducibility of the colour is good provided a suitable computer capable of controlling the operating parameters is available. By making suitable modifications t o the operating parameters of the intermediate treatment (voltage applied, current waveform and application time) it is possible t o produce, in the subsequent electrocolouring phase, a complete range of colours from grey to blue, red and purple.

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Conclusions

In the first part of this paper I have described a new process named “Satmac” by which it is today possible to produce a uniform finish on anodized aluminum; a t the same time it is a big step ahead in the direction of ecology. In fact this type of pretreatment reduces dissolved aluminum concentration and caustic soda entrainment in the waste waters which means a considerable reduction of sludges produced by the waste water treatment plant.

In the second part I have presented a new electrolytic technology for anodized aluminum based on the principle of selective electrodeposition of metals like nickel, cobalt and mainly tin. This technology offers the anodizers and architects new durable anodized colors like blue, grey, green and brick red which open new markets to anodized aluminum mainly known by architects for the only one “bronze” shade available today electrolytically. If the “Satmac” mechanical process is used as the pretreatment process before the anodizing and the coloring steps with the new Greylox and Multicolour processes, new extremely interesting color shades are obtained, with a less metallic minor effect, diffferent from the conventional anodizing or powder coating shades. These final effects can be described as similar to enamels or glazes and can be considered a third way between anodizing and painting effects.

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. TAB. 1 ECONOMIC COMPARISON BETWEEN THE TWO PROCESSES BASED ON A PRODUCTION OF 4000 M2

PARAMETER Measurem Conventio Mechanic Diff. A-B ent Unit n a l al &

Etching Chemical ~

Etching -

65.7 -65.7 ~

Electric energy consumption Shot consumption Consumption of parts subject to wear

1 79.2 -1 79.2 58.4 -58.4

Manpower costs (2 w x 8 h) 320 -320 Caustic soda consumption US$ 500.0 75 +425 Caustic soda (*) additive US$ 133.3 20 +113.3 consumption

Sludge (-1 disposal cost US$ 180.0 66 +114 Aluminum (A) loss cost US$ 1,493.3 233.3 +1,260 Total difference (A-B) US$ 1,308.4

Sulfuric acid cost US$ 31.6 12.2 +19.4

NOTES on Table 1 (*)Consumption of additives has been estimated to be 10% of the

caustic soda quantity. When used industrially, if the entrainment of alkaline substances is not high, their neutralization is brought about by means of the acid waters of the subsequent rinsing in the anodizing tanks. But for more concentrated solutions, this is not sufficient.

(-) In many European countries, the cost of disposal of anodizing sludges is approximately 0.15 US$ per kg (as long as their heavy metal content does not exceed the limit established by law).

(A)Values are obtained from the data indicated in paragraph 4. The aluminum loss with a conventional system, for a production of 4,000 m2, is the following:

160 x 4.000 = 640 kg 1,000

1,000

while the loss with the mechanical + chemical system 25 x 4.000 = 100 kg

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BIBLIOGRAPHY

1 )Wernick, R. Pinner, P.G. Sheasby: The surface treatment and finishing

2)A.W. Brace, P.G. Sheasby: The technology of anodizing aluminium, Z M

3)E. Strazzi, A. Da Pieve: Alu '91 Conference Proceedings, pgs. 295-324,

4)W. Dalla Barba: ibidem, pgs. 325-365 5)T. Sato: Advanced metal finishing in Japan 1980. Tecnocrat, Tokyo 6)C.T. Speiser: Trans. Inst. Met. Fin. 1980, pgs. 58, 121-1 27 7)G. Mast, C.T. Speiser: Aluminium 1980, pgs. 1 1,56,710-712 8) Japanese Patent 54-85 1 37 (1 977)

of aluminium and its alloys. Sth ed., 2"d Vol.

ed. Technicopy Ltd., Stonehouse

Interall, Modena

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1 I

Aluminium

Fig. 1 Theoretical Stnctllre of anodic oxide layer

i

Ispregnation

! i I ;

Anodic oxide layer

colouring

Anodic oxide J Lj Eazrier

i Aluminium f

layer

Layer

layer

FIG. 2 Absor2tion colouring

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F I G .

Barrier

Aluminium

F I G . 3 Electrolytic colouring

Alumini i lm

4 L i g h t scacterinc effect

a) Natu ra l s i l v e r 1 colour

Light bronze c 3 1 ouring

l a y e r

c) Black colouring

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oxide layer

Barrier layer

A l u m h i um

FIG. 5 Anodic oxide layer producing colouring by interference

Al umlni um

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Anodic oxide layer

FIG. 7 Light scattering effect on a c m d i t i o n e b and subsequently e l e c t r x o l o u r e d layer

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