Aluminum anodizing 

Aluminum color anodizing

Among various staining methods anodizing – color anodizing – electrolytic method (two-stage staining, electrolytic coloring) is an, certainly, the most interesting.

Electrolytic coloring anodised aluminium

Main characteristics, by which this method has advantages:

  • uniformity of color;
  • coloring of thin anodic coatings;
  • staining speed;
  • cost;
  • resistance to light;
  • corrosion resistance.

Electrolytes based on tin sulfate

The most widely used electrolytes in electrolytic coloring of tin sulfate. These electrolytes are given all the shades of "bronze" – from light to dark and further - to black (picture 1). Moreover, with the help of modern DC converters with them, you can get, for example, these colors, as a "champagne" and "stainless steel".

floral-anodiriovannyy-aljuminij-bronzecvetnoe-anodirovanie-aljuminiya-chernyyFigure 1 – Basic color when elektrookrashivanii anodized aluminum
in an electrolyte based on tin sulfate
(the names of the colors are conditional, colors may differ from actual colors on profiles)

The electrolytic method has replaced the old and expensive method of integral coloring, when the color, for example, bronze, obtained using high voltage in electrolytes based on organic acids simultaneously with obtaining an anodic coating (one-stage dyeing).

Unlike integral coloring electrolytic coloring gives the possibility of obtaining the spectrum of colors and shades, not only in different electrolytes, but also in the same electrolyte, but with a different waveform of the electric current / voltage, which is cheaper and easier.

The effect on anode layer elektrookrashivanie

The surface layer, which is subject to staining, comprises:

  • electrically conductive aluminum base;
  • Barrier oxide layer thickness 50-2000 angstroms (that is, approximately 2000 times thinner than the oxide layer) and
  • a porous oxide layer thickness 10-20 m.

Elektrookrashivanie very sensitive to the chemical composition of a painted aluminum alloy, for example, to the content of such elements, as iron and silicon. Chemical composition of the aluminum alloy can affect the porosity of the anodic oxide layer, and, thereby, the tone of the resulting color.

However, anodizing conditions have an even greater impact, since tin is deposited on the bottom of the pore sizes and high yield at the bottom of the same quantity of deposited tin plating layer different thicknesses. This leads to different conditions for the scattering of light and, Consequently, different tones of color. Further, since the rate of deposition of tin ions within the pores depends on the dimensions of their cross-section, the uniformity of the porosity of the surface to be painted is a very important factor for the formation of a uniform color.

The uniformity of the barrier layer plays an important role in ensuring the formation of a uniform coating color. Since the barrier layer is an electrical insulator, it is an energy regulator for the reaction elektrookrashivaniya. Therefore, differences in the thickness of the barrier layer result in differences in the deposition rate and, thereby, a non-uniform coloration.

Electrocarsing factors: chemical, physical and electrical

The characteristics of the barrier layer and the pore size of the anodic coating depend on the following factors:

Chemical factors:

  • electrolyte type and concentration
  • type additives and their concentration
  • pH
  • possible contamination

Physical factors:

  • electrolyte temperature
  • The electrical resistance of the electrolyte
  • bath size and linkage with profiles
  • The total surface area, which is painted

Electrical factors:

  • The type and the supplied current waveform
  • Interval applied voltage
  • Material type cathodes
  • Schematic design cathode system in terms of electric current along the sample phase offset with profiles
  • The distortion of the current wave shape as compared with the shape of the stress wave
  • Lines of magnetic induction electric fields

Deposition of tin in the anode pore

Basic patterns and facts:

  • Electrostaining is a linear process from light tones to black (Figure 2).
  • The dye - metallic tin - is located immediately above the barrier layer (Figure 3).
  • Tin anode pores - a metal tin with a tetragonal atomic structure.
  • A layer of tin in the pore grows perpendicularly pores base.
  • Color anodizing coating arises due to the phenomenon of light scattering depends on the thickness and density of the deposited layer.
  • For forming black required thickness of the deposited tin in the anode pore about 7-8 m.
  • The amount of metal tin in the pores: from 5 mg / dm2 for light bronze to 25 mg / dm2 to black.

osazhdenie-olovaFigure 2 - Kinetics of electrolytic deposition of tin

The linearity of the kinetics of electrocoloration of aluminum is expressed by the following formula:

D = v · t,

Where

  • D - the amount of precipitated tin, which is proportional to the color tone for the same anodic coating;
  • v - speed of electro-coloring;
  • t - duration elektrookrashivaniya

anodnaya-poraFigure 3 – tin deposition in the pores of the anodizing coating

The mechanisms of deposition of tin in the anode pore

The deposition of tin in the anode pore occurs as a result of the operation of two mechanisms: electrolytic and chemical.

The electrolytic mechanism of tin deposition on aluminum oxide is written as:

Sn2+ + 2e → Sn°

The second mechanism is the reaction of tin with atomic hydrogen by a redox reaction

Sn2+ + 2H° → Sn° + 2H+

Participating in this reaction atomic hydrogen is released at the cathode in accordance with the reduction reaction

H+ + e → H°.

elektrookrashivaniya speed

The electrostaining rate (v) is a function of the concentration (C) of stannous in solution:

v = KC

obviously, that an increase in the concentration (C) of stannous Sn2+ It makes it possible to reduce the duration of staining in one or another color. However, it follows from this formula, that an increase in the coloring rate can also be achieved by increasing the reaction rate (K) instead of increasing the concentration of stannous.

As mentioned above, tin reduction reaction depends on the rate of reduction of H+. With an increase in the concentration of hydrogen ions (i.e., pH) the rate of tin reduction is increased. At the same time, the increase in the acidity of the bath has a limit: if the concentration of sulfuric acid exceeds 30 g / l, it may cause a defect in the form of delamination of the anode coating.

From divalent to tetravalent tin

Typically, in the electrolyte on the basis of specific stabilizing additives tin sulfate are present in order to prevent spontaneous oxidation by dissolved oxygen of divalent tin ions Sn+2 tin ions to tetravalent Sn+4. Tetravalent tin ions Sn+4 undergo irreversible hydrolysis, which is accompanied by precipitation of sludge. The result is a spontaneous and irreversible loss of useful content stannous Sn+2:

SNSO4 + The ½2 + 3H2O → Sn(OH)4 + H2SO4

Therefore, the concentration of stabilizing additives is important, because they slow down the reduction of the divalent tin concentration below the limit value, below which the rate of deposition of tin in the pores of the anode layer is significantly slowing.

Source: W. D. Barba and F. Vincenzi (Italtecno). Доклад на Aluminum Extrusion Technology Seminar, Chicago, 2004.