Although the anodization process is one of the most common, it must be performed with professionalism and expertise.
The workpieces to be machined are positioned on special hooks of aluminum or titanium and the first processing step occurs in the degreasing tank, containing an industrial degreasing detergent at a temperature of about 70° -80° C. The purpose of this step is to eliminate any trace of organic matter such as oils and fats from the surface. The next step is chemical satinizing, the treatment used to give the surface a uniformly matt-satin finish. The satinizing solution consists of caustic soda (40-60 g/l) with additives necessary to etch the material and give it a very refined finish surface that is aesthetically pleasing. The solution temperature is between 50°-60° C.
The material to be anodized, after the treatments of alkaline degreasing, chemical satinizing and related washes in clean water, is immersed briefly in the so-called acid neutralization tank. The purpose is to eliminate the insoluble hydroxides which have appeared on the surface of the workpiece following the alkaline treatments.
At this point, the workpieces are immersed in an anodic oxidation bath. This process consists of dipping the aluminum workpieces in tanks containing sulfuric acid, connected to the positive pole (anode) of a suitable current generator. Its volume should theoretically be 3 liters/ampere of current propagated in the liquid. At the sides of the tank there are the cathodes (negative poles made of lead or aluminum alloy). The work piece, once immersed, is subjected to an electric potential difference between the anode and the cathode, where the sulfuric acid has the function of conducting the current to the workpiece itself. From this moment, the workpieces begin to oxidize chemically by opening the existing pores of the material surface where, over time, the thickness of the oxide will form (in depth), which is expressed in microns (thousandths of a millimeter).
The Italian standards UNI 10681-2010 (ex UNI 4522-66) currently provide 4 classes of oxide thicknesses:
– 5 microns for architectural parts for internal use, not subject to frequent manipulations;
– 10 microns for architectural parts exposed to rural or urban environments with regular maintenance (non-industrial);
– 15 microns for architectural parts exposed to industrial or marine environments;
– 20 microns for architectural parts exposed to industrial or marine environments (reinforced layer).
(More than 20 microns may form the so-called super-oxide, giving the surface a strong roughness and a layer of dust which can be removed, but the surface remains shaded and with uneven finish defects).
The choice of the thickness must be agreed with the user, and depends on the level of environmental aggressiveness where the material is to be placed. The aluminum oxide is transparent, porous, dielectric and becomes almost inert to weathering after having undergone an appropriate fixing treatment. The quality and properties of the anodic oxide depend on the variables that can affect the characteristics, namely:
– Concentration of sulfuric acid (180-200 g/l)
– Current density (1 ampere/dm2)
– Duration of treatment (according to the required microns)
– Bath temperature (18° -20° C)
– Dissolved aluminum content in acid (18-20 g / l)
– Agitation of the bath (with air)
– Presence of extraneous impurities on the surface.
Our oxidation system is equipped with computers for setting and controlling microns requested by the client. At the end of the process, the oxide thickness is measured with a feeler gauge to verify that the thickness is as determined (sample checked).
The final step is the fixing: The durability over time of the material subjected to anodic oxidation depends, in an essential way, on the effectiveness of this treatment. The principle, on which it is based, consists on closing the pores of the anodic oxide layer by filling and stabilizing through chemical reactions using appropriate substances. Generally, the composition of the fastener is formed of additives. In recent years, the European standards, to certify the quality of the finished product, imposed a wash in softened water at a temperature between 50°-60° C as the final treatment in order to improve the fastening of the anodic oxide.
Please note: The aforementioned is based on practical experience and information taken from industry-specific documents.