Aluminium, to protect itself from the action of atmospheric agents, is naturally covered with a thin film of oxide, this layer of Al2or3 has a more or less regular thickness of the order of 0.01 microns on the recently pickled metal and can reach 0.2 or 0.4 microns on metal that has remained in an annealing furnace.
In order to be able to weld, it is necessary to previously remove this layer by chemical or mechanical procedures.
Artificially much thicker oxide films with characteristics different from those of the natural layer, more protective, can be obtained by chemical and electrolytic procedures. The anodizing process allows the formation of layers in which the thickness can, at will, be from a few microns to 25/30 microns in protection or decoration treatments, reaching 100 microns and more by surface hardening processes, this is hard anodizing.
If a vat is filled with water made conductive by the addition of a small amount of acid, base or salt and if in this electrolyte, there is a cathode (negative pole), unassailable (nickel or lead) and an aluminium anode, there is a detachment of hydrogen in the cathode and no detachment in the anode.
It is observed, on the other hand, that the aluminum anode has been covered with an alumina film. The oxygen from the electrolytic dissociation of water has been used to oxidize the aluminum of the anode; hence the expression “anodic oxidation” previously used and now replaced by the term “anodized”. The nature of the electrolyte is of paramount importance over the phenomena that develop on the anodic surface.
Two types of anodic reactions can be pointed out, which present variants:
The nature of the base metal (unalloyed aluminium of various purities and alloys) is of paramount importance in terms of the results achieved and the means to be used to obtain them.
It is necessary to remember the following two essential factors from the theory of the formation of porous oxide layers:
If a piece of aluminium is oxidised in a solution which has a dissolving action on the alumina layer, it is observed that the intensity of the current, for a given voltage, decreases very rapidly but immediately stabilises at a higher level. After the first few seconds of electrolysis, a real barrier layer is formed, which tends towards the limit value of 14 A/V.
The oxide formed in this state consists of an anhydrous alumina, in an amorphous state (Al2or3), and it has been discovered in recent times that this layer is made up of a stack of juxtaposed hexagonal cells, in which, to be more precise, the center will be amorphous alumina that is not very resistant to acids, while the periphery is formed by crystalline alumina that is very resistant to acids. A multitude of attack points then appear on the surface of the barrier layer as a result of the dissolution effect of the film by the electrolyte that occurs in the center of the alumina cells and constitutes the beginning of the pores.
Each point of attack can be considered as a source of current from which a field of spherical potential is to develop; the ions that are presented to the oxide separation supply the nascent oxygen that transforms the corresponding portion of the metal sphere into oxide; simultaneously, the dissolution action continues to manifest itself at the base of the pore, tending to decrease the thickness of the barrier layer in which it is prolonged; the pore deepens, the ions penetrate preferentially, produce heat and tend to favor dissolution, thus producing a hemispherical advance front of the cell that develops, therefore, from the outside to the inside of the metal from the bottom of the pores.
Among the different anodizing systems we select, we highlight two of the most commercial: protective anodizing and hard anodizing.
The diagram of an anodizing process starting from a profile or a sheet could be represented by following the following steps:
Others result in a chemical combination with aluminum, such as metal complex-based dyes, diazo dyes, and basic dyes. The latter require treatment with collagen substances and are rarely used because their resistance to light is weak.
It is used for general applications that want to be colored and that are not exposed to the weather.
Used in general applications that require solid colors and are going to be outdoors.
For architectural applications, it is essential to fill in very pure water. Practically with demineralized and even deionized water. The most commonly used procedure for demineralization is anion and cation exchange with special ion-changing resins. It is a question of achieving a double exchange of ions (installation of two bodies) and not of a simple softening of the water which, by transforming the insoluble elements into soluble salts, runs the risk of producing bodies harmful to the quality of the clogging or fixing. The temperature of the water is given by the boiling temperature (in practice 97 to 100º C) so that hydration occurs very slowly when in contact with the water molecules at low temperature. The pH of the bath is advisable to keep it between 5.5 and 6.5. The readjustment is done by means of soda, soda ash or sulfuric, acetic and boric acids.
Anodizing can result in layers that are considerably harder than the classical ones (and in particular harder than those obtained in sulphuric-oxalic medium) in a pure sulphuric medium, provided that the dissolution percentages are reduced to an extremely small value, sufficient to allow the passage of ions into the pores, which become very fine channels. These results are obtained by anodizing at a very low temperature (0º C) in an electrolytic medium of 10 to 15% sulfuric acid, with a strong current density (3 A/dm2). The voltage, which will initially be 10 V, can be as high as 80 to 100 V depending on the nature of the alloy. Energetic shaking with effective cooling is necessary. Very thick layers can thus be obtained at a rate of 50 microns/hour. The layers that are currently achieved are around 150 microns, depending on the process and the alloy. The hardness of these layers is comparable to that of hard chrome, their resistance to abrasion and rubbing is considerable. Its use for mechanical parts is becoming more and more widespread due to the greater knowledge of aluminium, its mechanical characteristics and its new applications. Since these are generally parts with tight dimensional tolerances, it is necessary to take into account, in machining, the growth of dimensions, which can reach 50% of the effective thickness of the layer.
All alloys are susceptible to hard anodizing, except those containing copper, because copper tends to dissolve despite the low temperature and disturbs the treatment.
Hard layers are obtained at the cost of a decrease in flexibility, which limits their use to those applications in which they will not suffer significant thermal shocks, because the film would break under the effect of strong expansions.
These layers are not susceptible to being clogged (fixed) with boiling water for the same reasons. They can, on the contrary, be impregnated with fatty bodies and lubricants.
_Propiedades of hard anodizing | Among others, we can highlight the following:
To preserve the full capacity of alumina, it is necessary to use an electrolyte with weak chemical activity at low temperatures, which limits the redissolution of the film formed. Hard anodizing is typically applied on alloys with limited alloy contents. The Anesdur system allows to obtain layers greater than 150 microns with aluminum alloys containing:
_Hasta 6% Mg (Magnesium)
_Hasta 5% Cu (Copper)
_Hasta 8% Zn (Cinc)
_Hasta 13% Si (Silicon)
Due to the thick layer that can be achieved with this procedure, as well as the mechanical characteristics of the layer, parts that have worn out due to some defect can be recovered.
Alloys that have a good suitability for anodizing are perfectly defined on the corresponding pages. It is very important when selecting the material for hard anodizing, to check the part to be machined and to select the alloy also based on its characteristics and mechanical strength.
