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Anodizing

To protect itself from the action of atmospheric agents, aluminium is naturally coated with a thin oxide film. This layer of Al2O3 has a more or less regular thickness of around 0.01 microns on recently stripped metal and can reach 0.2 or 0.4 microns on metal that has been in an annealing furnace.

To be able to weld it is necessary to remove this layer beforehand by chemical or mechanical procedures.

Rust films artificially much thicker and with different properties from those of the natural layer can be obtained by chemical and electrolytic processes and are more protective. The anodising process makes it possible to form layers in which the thickness can, by choice, range from a few microns to 25/30 microns in protection or decoration treatments, reaching 100 microns and more by surface hardening processes. This is known as hard anodising.

PRINCIPLE OF ANODISING

If a tank is filled with water made conductive by adding a small amount of acid, base or salt and if in this electrolyte, there is a cathode (negative pole), unassailable metal (nickel or lead) and an aluminium anode, hydrogen detachment on the cathode and no detachment on the anode may be observed.

However, it may be observed how the aluminium cathode is covered in an alumina film. The oxygen from the electrolytic dissociation of the water has been used to oxidise the anode aluminium; hence the term "Anodic oxidation" previously used and now replaced by the term "Anodising". The nature of the electrolyte has a major impact on the phenomena that occur on the surface of the anode.

Two types of anodic reactions can be pointed out, which have variants:

  1. In electrolytes that have no solvent action on the oxide layer, a highly adherent and non-conductive film is formed. The film is formed until its electrical resistance is so high that it prevents the circulation of the current towards the anode. A layer called a "barrier layer" is then formed.
  2. In electrolytes that have a solvent action on the oxide layer, if the metal itself is dissolved and if the reaction products are soluble in the electrolyte, no oxide layer is formed. In a sulphuric medium, the anodizing process is the most widely used due to the economic operating conditions, the satisfactory results obtained and the means used to obtain them.

The nature of the base metal (unalloyed aluminium in various purities and alloys) has a significant impact on the results that are achieved and the means to be used to obtain them.

1. DISSOLUTION OXIDATION MECHANISM

It is necessary to remember from the theory of formation of porous oxide layers, the two following essential factors:

  1. Growth of the oxide layer from the outside to the inside as a result of a pure electro-chemical phenomenon, from which a certain speed of oxidation Vo results.
  2. Dissolution of the oxide layer as it is formed by a purely chemical phenomenon (naturally related to the electrical phenomenon), from which a certain dissolution speed Vd results.

2. FORMATION OF POROUS LAYERS

If a piece of aluminium is oxidised in a solution that has a solvent action on the alumina layer, it may be observed that the intensity of the current, for a given voltage, decreases very rapidly but then stabilises at a higher level. After the first few seconds of electrolysis, a true barrier layer is formed, which tends to have a maximum value of 14 A/V.

The oxide formed in this state consists of an anhydrous alumina, in an amorphous state (Al2O3) and it has been discovered in recent times that this layer is constituted by a stack of juxtaposed hexagonal cells, in which, more precisely, the centre will be amorphous alumina with little resistance to acids, while the periphery is formed by crystalline alumina that are highly resistant to acids. A multitude of attack points then appear on the surface of the barrier layer as a result of the effect of dissolution of the film by the electrolyte which occurs in the centre of the alumina cells and which constitutes the start of the pores.

Each point of attack can be considered as a source of current from which a spherical potential field will be developed; the ions that are presented for oxide separation supply the nascent oxygen that transforms the corresponding metal sphere portion into oxide; simultaneously, the action of dissolution continues to manifest itself at the base of the pore, tending to decrease the thickness of the barrier layer in which it extends; the pore deepens, the ions penetrate preferentially, produce heat and tend to favour dissolution, thus producing a hemispherical advance front of the cell that therefore develops from the outside to the inside of the metal from the bottom of the pores.

ANODISING SYSTEMS

Among the different anodising systems that we select, we highlight two of the most commercial ones: protection anodising and hard anodising.

1- PROTECTION ANODISING

The scheme of an anodising process starting from a profile or a plate can be represented by following the following steps:

  1. Surface preparation of the material by
    • a_Polishing with a cotton brush
    • b_Sanding with sandpaper
    • c_Brushing with a metallic brush
    • d_Other types of polishing
  2. Degreasing
    • a_Acid for frosting, brushing, etc.
    • b_Alkali for polishing
  3. Washing with running and continuously regenerated water
  4. Stripping (with 5% caustic soda in water at 45-50°C) or stripping for direct matt finishing or chemical polishing or electrolytic polishing
  5. Washing with running and continuously regenerated water
  6. Neutralising (60ºBe nitric acid at 50% in water at room temperature)
  7. Washing with running and continuously regenerated water
  8. Anodic oxidation (20% sulphuric acid in water at a temperature of 19-20ºC and direct current at 1.5 A/dm2)
  9. Washing with running and continuously regenerated water
  10. Dyeing | Due to its porous structure, the oxide layer formed in sulphuric medium resembles textile fibres and, like them, it can be dyed by means of special dyes derived from textile industry dyes. As the layer is transparent, the brightness of the base metal is transmitted and aspects can be obtained that no other treatment by paint or varnish is capable of equalling. The dyes used for colouring aluminium can be organic or mineral.
    • a_Organic dyeing | Many types of dyes can be used for this treatment. They differ especially in the absorption mechanism. Most are absorbed by capillary action. In particular, these are the groups of acid dyes and dyes known as substantive, such as those of alizarin and indigo dyes.

      Others create a chemical combination with aluminium, such as dyes based on metal complexes, diazo dyes and basic dyes. The latter require treatment with collagen substances and are not often used because their resistance to light is weak.

      They are used for general applications that are to be dyed and that are not exposed to the weather.

    • b_Electrolytic dyeing | Electrolytic dyeing,for this treatment metal oxides are used, which become fixed in the pores of the oxide layer and are highly resistant to light and heat.

      Used in general applications that require solid colours and that will be outdoors.

  11. Washing with running and continuously regenerated water
  12. Clogging or Fixing | Experience shows that a 20 micron oxide layer, formed on aluminium and not clogged, disappears in a few hours in a decinormal solution of nitric acid. The same perfectly clogged layer does not experience practically any loss after 1200 hours of immersion. In reality, one of the main characteristics of the alumina formed in the clogging or fixing is its resistance to acids.

    For applications in architecture, it is essential to clog with very pure water. Practically with demineralised water and even deionised. The most commonly used procedure for demineralization is anionic and cation exchange with special ion exchange resins. The aim is to achieve a dual ion exchange (installation of two bodies) and not simple water softening which, by transforming the insoluble elements into soluble salts, runs the risk of producing bodies that are harmful to the quality of the clogging or fixing. The temperature of the water is given by the boiling temperature (in practice 97 to 100°C) in order that hydration occurs very slowly on contact with water molecules at low temperature. It is advisable to keep the pH of the bath 5.5 and 6.5. The readjustment is made by means of soda, sodium carbonate or sulphuric, acetic or boric acid.

2- HARD ANODISING

With anodising, considerably harder layers than classical ones (and in particular harder than those obtained in a sulphuric-oxalic medium) can be obtained in a pure sulphuric medium, as long as the dissolution percentages are reduced to an extremely low value, enough to allow the ions to pass through the pores, which become very fine channels. These results are obtained by anodising at a very low temperature (0°C) in an electrolytic medium of 10 to 15% sulphuric acid, with a strong current density (3 A/dm2). The voltage, which will be 10 V at the start, can reach 80 to 100 V depending on the nature of the alloy. Vigorous stirring and efficient cooling are necessary. Very thick layers can be obtained at a speed of 50 microns/hour. The layers that are currently available are around 150 microns, depending on the process and the alloy. The hardness of these layers is comparable to that of hard chromium, its resistance to abrasion and friction is considerable. Its use for mechanical parts is becoming increasingly widespread due to greater knowledge of aluminium, its mechanical properties and its new applications. Since these are, in general, parts with narrow dimensional tolerances, it is necessary to take into account, in the machining, the increase in the dimensions, which can be up to 50% of the effective thickness of the layer.

All alloys are susceptible to hard anodising, except those containing copper, because copper tends to dissolve despite the low temperature and disrupts the treatment.

Hard layers are obtained at the expense of a reduction in flexibility, which limits their use to applications in which they will not suffer significant thermal shocks, because the film would break under the effect of sudden expansion.

These layers are not likely to be clogged (fixed) with boiling water for the same reasons. However, they may be impregnated with fatty and lubricating bodies.

_Properties of hard anodising | Amongst others we can highlight the following:

  • _Resistance to abrasion since alumina is an extremely hard body, a little less hard than a diamond. Which allows it to have a greater surface wear resistance than many types of steel.
  • _Electrical resistance. Alumina is an excellent quality electrical insulator, superior to porcelain, this insulation depends on the porosity; in addition, it is greatly affected by the presence of impurities in the metal and is generally characterised by measurement of the breakdown voltage.
  • _Chemical resistance. The anodic layer effectively protects the base metal against the action of numerous aggressive media. For this reason it is increasingly used in marine and industrial environments to protect certain parts against corrosion.
  • _Secondary porosity or more or less pronounced opening at the entrance of the pores due to the effect of dissolution of the bath. This porosity is mainly manifested on the outside of the film and is used in dyeing and impregnation techniques.

ANESDUR PROCEDURE

To preserve the full capacity of the alumina, it is necessary to use an electrolyte with low chemical activity at low temperatures, which limits the re-dissolution of the formed film. Hard anodising is normally applied on alloys with limited alloy content. The Anesdur system makes it possible to obtain layers greater than 150 microns with aluminium alloys that contain: 
_Up to 6% Mg (Magnesium) 
_Up to 5% Cu (Copper) 
_Up to 8% Zn (Zinc) 
_Up to 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 been worn out due to defect can be recovered.

The alloys that have good anodising ability are defined in full in the relevant pages. It is very important when selecting the material for hard anodising to verify the part to be machined and select the alloy also according to its properties and mechanical resistance.