Short review of the various coloring processes of anodized aluminum

Before sealing it is possible to color the porous oxide layer. The four ways in which almost all anodized aluminum is colored. Adsorption and Electrolytic are the major coloring processes, Integral coloring is on its way out and Interference is gaining popularity.

1) Adsorption
2) Integral
3) Electrolytic
4) Interference

In Adsorption and Electrolytic coloring anodizing is done as a separate step before the coloring whereas Integral coloring is done during anodizing. The most common used coloring process for outdoor exposure is the Electrolytic coloring due to its high light fastness. This light fastness depends on the metal deposition in the pores.

The drawing below shows the three different types of coloring, a) Adsorption, b) Integral c) Electrolytic.

When Adsorption is used the dying solution can be organic or inorganic. The porous oxide layer is immersed into the solution directly after anodizing. The dye will flow into the open pores filling the outer part of the oxide layer, see a). A final sealing of the layer is done to enclose the dye inside the pores. When using an organic solution almost all colors are possible, however, there are some uncertainties about the light-fastness of some of the colors. One of the companies providing colors for the anodizing industry is Reliant.

Integral coloring (also called one-step coloring) is performed in the anodizing electrolyte, which can consist of organic acids such as oxalic acid, sulphosalicylic acid and others. Here the color of the coating is dependent on the thickness of the oxide layer, varying from light yellow to black. These coatings have a very high wear resistance and hardness. Also variation in alloys are used to exhibit differences in colors of the oxide layer. This type of coloring is normally not used due to the high price of process cost.

The Electrolytic coloring method consists in immersing the anodized part in an acidic solution containing metal salts and then normally applying an alternating current to precipitates the metal ions. This process is also the two step coloring process. The metal salts used can be cobalt, nickel and tin, as the most common ones, giving different coloring tones from bronze to black shades. The color obtained by this method is also very dependent on the thickness of the oxide layer and the alloying elements found in the material.

Modification of the barrier layer or the structure of the pore in the bottom can expand the color spectrum. Interference coloring is one of these coloring processes where an intermediate step between anodizing and coloring is introduced. This process step could be a phosphoric acid treatment to enlarge the pores. The colors obtained ranged from blue to green and pink/red. One of the companies providing the rectifier for Interference coloring is MUNK´s Alu-Spectral.

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