Electrolytic coloring is a two step process, as mentioned in an earlier post. After anodizing, to form a porous aluminum oxide layer, the second step is to deposit metallic salt into the pores.
According to the book written by Arthur Brace, "The technology of Anodizing Aluminium". the following reactions take place during the cathodic period of the AC wave to convert the metallic ion into deposited metal.
The metal ions will the be reduced at the cathode (parts) to metal.
This reaction is accompanied by hydrogen evolution and is the reason for the "spalling" effect, where the hydrogen destroys the barrier layer and causes disruption of the oxide layer from the aluminum.
During the anodic part of the coloring process, depending on pH, a new porous oxide layer will be formed.
The sulphosalycylic acid is a stabilizing agent, which is added to prevent Sn(II) (stannous sulphate) to oxidize to Sn(IV) (stannic sulphate). These additives reduce rather than eliminate the oxidation.
What normally causes white spots on anodized material?
White spots, when found on electrolytically colored parts, are mainly caused by the spalling effect.
Qualital has found white spots because of the porosity in the anodic layer of oxide with partial detachment of the same and resulting "bare mass" of the metal below.
Courtesy of QualitalThis is caused by alignments of coarse crystals of intermetallic compounds, parallel to the longitudinal direction of the extruded bar of departure. These crystals were formed during the casting of extrusion billets and their line shape was determined during the manufacturing process.
A closer look at the microstructure of the material (alloy EN AW-7075 T6) before electrolytic coloring characterized the number of alignments by coarse crystals of intermetallic compounds. (Magnification X 170 - without acid attack).
Courtesy of Qualital
What would you recommend as an appropriate spacing for electro-colored extrusions?
Appropriate spacing is totally dependent on the profiles but some main issues to take care of are to keep the distance between the part and the counter-electrodes as constant as possible, to keep the current density as even over the load as possible and keep the parts separated from each other, so none are in the shadow of the other.
What produces the best results, ac or dc in electro-coloring?
The use of DC (direct current) is mainly used for nickel-based electrolytes. These electrolytes are very sensitive regarding the amount of sodium and some of them need an interruption in the cycle to discharge.
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