Acid Etch for Aluminum Extrusion:

Technical, Economical and Environmental Aspects

Alberto Abad Fuentes, Alvarez Schaer, S.A., Valencia, Spain

ABSTRACT – The purpose of the etching process is to reduce the brightness of extruded aluminum, remove or mask extrusion lines and defects, and prepare the aluminum surface for anodizing. The etching process can be carried out physically, for example through blasting, or chemically, in either an acid or alkaline medium.

GENERAL ASPECTS OF ALKALINE ETCH During the alkaline process, the aluminum is dissolved by caustic soda, and aluminate is formed (see reaction 1). The classical alkaline etch can be achieved by the use of two different alkaline baths.

In long-life etch baths, aluminate is stabilized in solution by the use of organic complexors or quelators.

Besides caustic soda and complexors, long-life etch baths contain grain refiners and fluidizers. Very high quantities of aluminum are necessary for the decorative finish to be good.

If instead, the solution is allowed to hydrolyze in a second tank, the used caustic soda could be recovered.

Thus, the overall consumption of caustic soda is low, and small amounts of sludge are generated (see reactions 2 and 3). Nevertheless, this process has not prevailed at industrial levels, because the finish is not good enough. That is why in Europe almost 100 percent of anodizers use long-life etch baths. Reactions 1, 2, and 3 are:

2 Al + 2 NaOH + 2 H2O → 2 NaAlO2 + 3 H2 (1) 2 NaAlO2 + 4 H2O → 2 Al(OH)3 + 2 NaOH (2) 2 Al (OH)3 → Al2O3. 3 H2O (3)

However, the long-life etch process presents some technical drawbacks. The process has a limited capacity to conceal extrusion lines, and is not able to achieve brightness levels under seven to eight percent. Besides, the so-called galvanizing or spangling defect appears when the zinc content in aluminum profiles is more than 0,05 percent. Moreover, the aluminum dissolution rate is very high (80g/m2 to 120g/m2), and therefore, huge amounts of sludge are generated. Finally, the baths of alkaline etch are very viscous, so that sometimes the profiles can hardly be rinsed, and some alkaline residues can remain.

ACID ETCH: TECHNICAL ASPECTS

Aluminum etching in acidic media with fluoride derivatives is well known since the 1930s.[1-4] Aluminum is dissolved in acid media with the aid of fluoride salts. During the process, hexafluoraluminate is generated, and it readily precipitates. The acid etching process also includes etch regulating additives and surfactants. Among the working conditions it is notable that lower temperature and less time are used, compared with alkaline etching.

Furthermore, with the acid etch process the aluminum dissolution rate is ten times lower than with the

alkaline process. The aluminum dissolution rate determines the consumption (average consumption of 60g/m2 to 70g/m2), and it can be modulated according to concentration, temperature, time, and pH.

As aluminum reacts in the bath, the reaction rate decreases. This is because the aluminum surface is

passivated by the deposition of aluminum fluoride (see figure below).

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The layout of an anodizing line using acid etching is essentially like any other anodizing line. After the process of acid etching, there is the possibility of processing the aluminum during 30 to 60 seconds in a bath of alkaline etching, or in an anodizing stripping bath. In this way, the final brightness can be increased up to seven to eight percent brightness. If, on the contrary, the already etched material is processed directly with an acid neutralizer, although the passivate of aluminum fluoride will also be removed, the anodized material will show a brightness level of four to six percent.

It is import to take into account that two materials with different brightness levels will also present a

different aspect after the coloring.

The process of acid etching can better conceal or disguise the extrusion lines than the alkaline etch.

The pictures taken by optical microscope show that with the acid etch, the grain boundaries are less visible than in the case of the alkaline etching, because the aluminum dissolution rate is lower. This is the reason why the acid etch produces a surface with finer aspect (see figure below).

ACID ETCH ALKALINE

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We have conducted a roughness study by interferometric profilometry. A non-etched sample of extruded aluminum (AA6063) presents areas of higher roughness and therefore, lower brightness. These line-shaped areas correspond to the extrusion lines, and are visible because they present lower gloss. In the acid etched sample, the roughness of material is increased more than in the alkaline etched one. A higher overall roughness allows extrusion lines to be more efficiently masked.

We can conclude that acid etch presents many technical advantages compared to alkaline etch. - Lower aluminum dissolution rate - Lower temperature - Shorter process time - The bath is not viscous, so there are no drain problems - Acid etch is not sensitive to the presence of zinc in the bath.

To carry out the acid etching process just a simple installation is needed. An acid etching tank has to be made of polypropylene, since stainless steel is not resistant to the corrosiveness of the employed solution. It also requires a heating system, and a filter press to remove the generated sludge. It is recommended to use a recirculation system, which allows the agitation of the tank. It is preferable to introduce a mechanical stirring, not by air, to minimize the release of gases produced during the process (see figure below)

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One possible solution for the installation is to use a tank divided into two parts, a main tank and a sub-tank. The sub-tank will be designed with slope, so that it works as a decanter for sludge. This sludge will pass through the filter press, and then the bath returns already filtered to the tank. ENVIRONMENTAL ASPECTS

The most important aspect from the environmental point of view is the sludge generated. In the case of acid etching, the generated sludge is ammonium hexafluoraluminate. In the alkaline etching process, sludge is mainly composed of hydrated aluminum hydroxide. It is important to note that the sludge from alkaline etching contains a large percentage of water, whereas in the case of the acid etching, the water content in the sludge is low. This fact, together with differences in the aluminum dissolving rate between the two processes, explains the different amount of sludge generated in both processes (see figure below).

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(NH4)3AlF6 Al(OH)3 .xH2O

0102030405060708090

100

Acid etching Alkaline etching

% DRY WASTE

% WATER

When 2,4 tons of aluminum (approximately 1000m2) are processed by alkaline etching, one ton of sludge is generated and has to be managed. Acid etching produces only 90Kg of sludge per 1000m2 processed.

Moreover, sludge obtained in the acid etching process can be transformed into a substance of considerable

industrial importance, cryolite. So sludge has an industrial interest, and an economic value (see figure below).

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2,5 TON Aluminium = 1000 m2

Acid etching10 g/m2 Al

Alkaline etching100 g/m2 Al

240 kg of Al2O3 dry1000 kg of sludge Approx. 75% of water

70 kg of (NH4)3AlF682 kg of sludgeApprox. 15% of water

90 % reduction

Waste management Revaluable waste into cryolite

Wet sludge can react with a sodium salt in an ion exchange reaction to form cryolite, and an ammonium salt. From an environmental point of view it is interesting that the obtained sub-product, the ammonium salt, is also useful. When using sodium chloride to react with the sludge, we obtain ammonium chloride, but when using sodium phosphate we get ammonium phosphate, a well-appreciated industrial fertilizer.

Cryolite obtained from industrial sludge has a high-quality granulometry and chemical composition (see

figure below).

Cryolite has many industrial applications, such as providing primary and secondary aluminum, and the

production of ceramic, glass, and agricultural insecticides.

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ECONOMIC ASPECTS One important factor to take into account is the power consumption. Working temperature is higher in the alkaline etching; however during production, none of the processes requires prolonged heating, because the processes are exothermic. But there is a difference in the inactivity periods. Whereas for the acid etching, the tank can get cooled at room temperature, for acid etching the temperature has to be kept at 40ºC. The savings due to this fact would be around $6 per 1000m2 (see figure below).

The aluminum loss occurring during the etching process is ten times as low for acid etching as for alkaline etching. This fact is not relevant in the case of sales of anodized aluminum, based on surface or theoretical tons. But when the sale is made on the basis of real tons, the economic difference is so important that it justifies the change to acid etching itself.

Another point to be considered is the rejections caused by the inability of the alkaline etch to mask some extrusion defects or other problems derived from the alkaline bath, like draining problems, or the so-called spangling defect. These problems will be reduced and in some cases eliminated, and therefore the rejection rate will decrease. A decrease of 0,5 percent in rejection will reduce the cost of processed aluminum by $30 to $40 per 1000m2.

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As regards the cost of chemicals, in the case of alkaline etch, it has to be taken into account the caustic soda itself, and the long-term additive used. For the acid etching, besides the product AC 10, it should be considered a small soda and additive consumption. The overall cost of chemicals is higher for acid etching, as the fluoride salts are more expensive. Finally, the management of less quantity of waste leads to significant savings (see figure below). In a global calculation, the acid process turns out to be between 10 to 15 percent more expensive (see figure below).

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0

50

100

150

200

250

Chemical products

Waste management

Total Cost

acid etch

alkaline etch

However, when adding the energy consumption differential and the higher rejection rate of the alkaline process, we would have equivalent prices.

In conclusion:

- Acid etching is technically better than classic alkaline etching. It can achieve lower brightness, finer look and better finishing, hiding extrusion lines;

- Acid etching process is technically simpler and easier to manage. It is not necessary to control

aluminum quantity in the bath;

- Acid etching process has a lower environmental impact, thanks to the lower sludge formation, and the possibility of revaluating the sludge.

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