Matters to remember when electrolytic coloring

Yesterday I explained in short how the various coloring processes work on anodized aluminum. This post will be about electrolytic coloring of anodized aluminum, the process, things to be aware of and problems.

Today the electrolytic coloring process is one of the major methods to color anodized aluminum. One of the main advantages over other coloring processes is the light fastness of this process. After Anodizing in sulfuric acid forming a porous oxide layer, the parts are rinsed thoroughly and then followed by the AC coloring step. During the coloring process metal salts are deposited at the bottom of the pores. Tin is the most stable one introduced in the 1970´s to improve the coloring process due to a better throwing power compared to the nickel and cobalt.

One of the most important effects in electrolytic coloring is the throwing power of the coloring bath. The throwing power of an electrolyte indicates how the distribution of metal is found on the surface. The process uses an AC sinusoidal waveform. In the cathodic part of the process metal is deposited at the bottom of the pores. In tin coloring the amount of metal deposited is within the range of 5 – 20 mg/dm² of anodized aluminum surface.

The color range is from light bronze to dark black when using tin baths. The color is controlled by voltage and time, and the process time ranges from seconds up to 10 minutes

Courtesy of AAA Anodizing and Metal Finishing, Inc.

Specialized rectifiers have been build to perform this process as smooth as possible with the highest reproducibility.

One important note about equipment is to remember to use aluminum racks and not titanium racks. The reason for this is the difference in electrochemical behavior of the titanium and aluminum. This will lead to zones around the contact points with irregular interference coloring. Use of plastic clamps is widely used because of this, and also, to get an adequate color, the parts need more space between them.

Courtesy of AAA Anodizing and Metal Finishing, Inc.

Racking parts for electrolytic coloring should be more spacious than when racking for anodizing because of a shielding effect from other parts.

A couple of well-know defects can happen when electrolytic coloring an anodized oxide layer.

Spalling, surface deposit, uneven coloring and white spots. Spalling is due to hydrogen generation within or beneath the barrier layer The loose deposit found on the surface is mainly due to the low pH and the high throwing power, so the deposit is on the outside of the oxide layer instead of the inside. Both of these are caused by high Voltage and very long coloring time.

Uneven coloring occurs when parts are to close to each other. A lot of work is done to increase the throwing power of the electrolyte to overcome this problem.

White spots will be explained in one of the next posts.

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