Color aluminum anodizing

Among various staining methods anodizing – color anodizingэлектролитический метод (двухстадийное окрашивание, electrolytic coloring) является, certainly, the most interesting.

Aluminum electrolytic color anodising

Main characteristics, по которым этот метод имеет преимущества:

  • однородность цвета;
  • окрашивания тонких анодных покрытий;
  • скорость окрашивания;
  • стоимость;
  • сопротивление воздействию света;
  • 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" – от светлой до темной и далее – до черного цвета (рисунок 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
(названия цветов условны,
цвета на экране могут отличаться от фактических цветов на профилях)

The electrolytic method has replaced the old and expensive method of integral coloring, when the color, for example, bronze, получают с помощью высокого напряжения в электролитах на основе органических кислот одновременно с получением анодного покрытия (одностадийное окрашивание).

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, состоит из:

  • электропроводной алюминиевой основы;
  • Barrier oxide layer thickness 50-2000 ангстрем (то есть примерно в 2000 раз тоньше оксидного слоя) и
  • 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.

Факторы электроокаршивания: химические, physical and electrical

Характеристики барьерного слоя и размеры пор анодного покрытия зависят от следующих факторов:

Химические факторы:

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

Физические факторы:

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

Электрические факторы:

  • 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

Основные закономерности и факты:

  • Электроокрашивание является линейным процессом от светлых тонов до черного цвета (рисунок 2).
  • Краситель – металлическое олово – располагается сразу над барьерным слоем (рисунок 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.
  • Количество металлического олова в порах: от 5 mg / dm2 for light bronze to 25 mg / dm2 to black.

osazhdenie-olovaFigure 2 - Kinetics of electrolytic deposition of tin

Линейность кинетики электроокрашивания алюминия выражается следующей формулой:

D = v · t,

Where

  • D - the amount of precipitated tin, которое пропорционально тону цвета для одинакового анодного покрытия;
  • v – скорость электроокрашивания;
  • 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

Осаждение олова в анодную пору происходит в результате работы двух механизмов: электролитического и химического.

Электролитический механизм осаждения олова на оксиде алюминия записывается в виде:

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

Скорость электроокрашивания (v) является функцией концентрации (С) двухвалентного олова в растворе:

v = KC

obviously, что увеличение концентрации (С) двухвалентного олова Sn2+ It makes it possible to reduce the duration of staining in one or another color. However, it follows from this formula, что увеличения скорости окрашивания можно добиваться также и путем повышения скорости реакции (К) вместо повышения концентрации двухвалентного олова.

As mentioned above, tin reduction reaction depends on the rate of reduction of H+. С повышением концентрации ионов водорода (то есть, рН) скорость восстановления олова возрастает. At the same time, повышение кислотности ванны имеет предел: если концентрация серной кислоты превышает 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.

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