Aluminium is a very complex metal to treat due to its chemical ductility, a characteristic that also makes it a metal of great value and increasingly used.
There are different ways to treat aluminium, based on the alloy, production costs and final use destination.
Being a very reactive material with air, on the surface there is always a thin irregular layer of natural aluminium oxide which preserves it over time. To properly treat aluminium, it is necessary to remove this layer and replace it with a regular oxide layer that increases its performance and uniforms its nuance.
For manufactured goods intended for painting, it is possible to apply a single acid degreasing/pickling treatment based on surfactants, sequestrants and fluorinated agents (such as ammonium bifluoride), which cleans and creates a surface suitable for paint adhesion. For non-paintable products, intended mainly for domestic and decorative use (windows, handles, etc ...), it is necessary to use meticulous treatments in series.
Degreasing is the first step, essential for de-oiling and removing dirt; neutral or alkaline degreasers, based on salts and surfactants, can be used.
The oily and greasy contaminants must be perfectly emulsified and eliminated, otherwise you could run into serious surface defects during the following steps: whitish spots, clouding (spots due to a different time of contact with the alkaline agent) and opaque streaks.
A good degreaser should possibly be non-foaming in order to facilitate mechanical agitation and avoid the formation of foam which would also be dragged into the etching finishing tank.
This process follows the degreasing, without a necessary rinsing. This operation gives to the aluminium a uniform and opaque appearance
repeatable over time, with undoubted advantages from an aesthetic and economic point of view as it allows to avoid mechanical brushing.
The etching product is a mixture of caustic soda and additives designed to reduce the aggressiveness of the soda itself, as well as to avoid the precipitation of aluminium compounds and to reduce the roughness of the treated surfaces.
The reaction between aluminium and soda produces aluminium oxides and hydroxides, in stoichiometric balance with free soda which, if not respected, can easily cause precipitates and clouding of the bath (with consequent creation of waste) and etching or corrosion defects.
There are additives that hold the chemical reaction in greater stability, while helping in the chelation of metals (such as zinc) that can compromise the final result.
In general, it is necessary to pay attention to the gas formation (pay attention to the quantity and morphology of the treated pieces), to the temperature of the solution which must be between 50-60 ° C and the concentration of free soda which must remain between 40-60 g/L.
Since there is a high consumption of soda, it is necessary to chemically analyze the bathroom daily.
After the etching phase, the pieces are carefully rinsed in two tanks in counter-current cascade and immersed for no less than two minutes in an acid solution for pickling.
Historically, nitric acid at about 300 g/L is used to neutralize the alkaline residues from the etching finish and to dissolve and remove the alloying agents from the aluminium surface.
Nitric acid does not attack aluminium so the pieces can be left in immersion for a very long time without problems.
The contraindication in the use of this product lies in the increase of nitrates in the waste water and the possible entrainment of nitric acid in the oxidation tank with consequent pollution. Alternatively, sulfuric acid solutions can be used with the addition of particular additives that reduce their aggressiveness and at the same time improve the cleaning of the product.
It is carried out in acidic tanks (usually based on sulfuric acid) where, by means of an electric current, the aluminium (that acts as an anode) is oxidized. Precise voltages, acidity and temperatures (around 20 ° C) determine the final characteristics of the aluminium surface.
The electrolytically obtained oxide layer is composed of about 80% of aluminium oxide and 20% of aluminium sulphate. The process is exothermic and the tank must be cooled by means of heat exchangers.
The treatment continues by rinsing, neutralization in bicarbonate and coloring (adsorption or electrolytic). Adsorption coloring can be organic or inorganic. Organic colors (pigments) are used almost exclusively for small metal parts. The inorganic colors mainly create gold and bronze finishes. For gold, ammonium iron oxalate is used, while for bronze, cobalt acetate and potassium permanganate baths are alternated, until the desired shade is reached.
Electrolytic coloring is an electrodeposition of colloidal metal in the oxide pore and is the most commonly used for architectural profiles. The process employs specific voltages and cycles of alternating and direct current; thanks to computerized systems it is possible to manage productivity correctly and quickly.
To color are used tin sulphate or copper sulphate, in acid solution of sulfuric acid and stabilizers. Depending on the parameters, colors ranging from dark bronze to black can be achieved. The oxide layer does not affect the hue of the colors, but if there were discrepancies in thickness the uniformity would be affected.
Also, in this case, imbalances between the components of the electrolytic solution or incorrect temperatures can generate defects in the treatment and precipitation of salts. In any case, it is advisable to provide for discontinuous filtration and clarification of the solution.
Sometimes mixed colors are carried out, between electro and organic color; for example, by overlaying a blue color on a very clear base of tin sulphate electro-color, a light-resistant green color can be obtained.
It is the last operation of the whole cycle; it seals the still open pore giving the product its characteristic resistance to atmospheric corrosion.
The process can be carried out hot or cold. For hot fixing, the operating temperature is maintained above 96°C, the pH between 5.5-6.5 and the immersion time of 3 minutes for each micron of thickness. Demineralized water is compulsorily used, with the addition of special anti-dust additives (they buffer the pH and chelate pollutants).
Cold fixing (also called by impregnation) is carried out with an aqueous (demineralized) solution of nickel fluoride at a temperature between 20-30°C, a pH of 5.5-7.0 and an immersion time of 1 minute for each micron of oxide thickness.
Unlike the hot system where the sealing of the pore occurs by hydration of the oxide layer, the fixing by impregnation involves the precipitation of nickel in the pore and, by subsequent hydration of the nickel ion, the sealing of the pore itself. The hydration reaction of nickel is rather slow, it is therefore recommended, after rinsing in running water, a further passage (conditioning) in a tank of non-demineralized water, containing nickel sulphate, at a temperature of 50-60°C to a time of about 10 min.
Checking the cold fix solution is more complicated as the pH and concentrations of nickel and fluorides will have to be checked.