Coloration of the anodic layers
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
There are three main ways of coloring aluminum:
Self-colored anodizing: the constituents of the alloy are incorporated into the oxide during its formation, giving it a more or less marked color depending on the substrate (pale yellow for an alloy 1100 to black for an alloy 6061, for example). The electrolytes used consist of organic acids, often in combination with sulfuric acid. The
current densities are high. Industrially, self-coloring has given way to other coloring processes.
Chemical coloring: the anodized part is immersed in a solution containing organic or mineral dyes.
The control of the bath is essential for the reproducibility of the color (agitation, temperature, pH, immersion time, concentration ...). The dyes are absorbed by capillary action. Thus the chemical coloring only concerns the upper part of the oxide layer, which explains their poor resistance to light and weathering.
Electrolytic coloring: this coloring consists of depositing metallic particles in the pores, producing colors ranging from champagne to bronze to black (more or less dark), thanks to the phenomenon of light diffraction on the metallic particles deposited at the bottom of the pores.
This coloring is carried out by electrolysis powered by alternating current, by reducing one or more metal cations in the barrier layer. The first electrolytes used industrially were based on nickel and cobalt; they are currently replaced by tin-based electrolytes. The colored and sealed layer offers an excellent resistance to
ultraviolet rays and to bad weather.
SUBSTRATE:
Alloys of the 2000 series can only be colored by chemical coloring.
Restrictions to cast alloys / alloys with high % Si (9%)
VARIANTS:
A variant to electrolytic coloring is also used industrially: interferential coloring.
It consists in a modification of the anodic layer, allowing to widen the range of color (from grey to green,
and possibility to have yellow and red).
Different methods of modification of the anodic layer exist:
A first anodization is carried out in a sulfuric medium and a second one which aims at modifying the structure of the oxide at the barrier layer. This second anodization can be carried out in the same bath as the first with modification of the current or in another bath (phosphoric acid for example). The standard electrolytic coloring that follows, allows to obtain varied and durable colors.
Changes in the oxide can occur after conventional sulfuric anodizing, in the coloring bath itself, involving complex current forms.
APPLICATIONS :
Decoration
Tracking/identification
Optical and thermal (black coloring)
Applications: building, architecture, jewelry, aeronautics, armament ...
SELECTION CRITERIA - LIMITATIONS :
The anodizing conditions must be rigorous (agitation, temperature...) otherwise the colors will not be uniform and the reproducibility will be poor.
The shade and uniformity will depend on the choice of alloy and its manufacture.
An anodic layer that is too thin will not be conducive to darker shades.
After staining, it is necessary to make a sealing to fix the color.
Staining can have a negative effect on corrosion resistance.
Note that hard anodizing (OAD) and chromic anodizing (OAC) can naturally color the oxides.
ENVIRONMENTAL IMPACT:
Cobalt sulfate and nickel sulfate classified as CMR
Implementation
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