Electrolytic Coloring and Corrosion Resistance

Question:

Does the electrolytic coloring of aluminum cause corrosion if tin is used to produce the desired color? J.A.

Answer:

The electrolytic coloring process itself does not cause corrosion to anodized aluminum. I will make the assumption that you are really asking if anodic coatings electrolytically colored with tin are more sensitive to corrosion "in the field" than coatings colored with other metal salts or than clear anodic coatings. There is not a "yes" or "no" answer to this question. The mechanism of depositing tin metal, or other metals, in the anodic pore and the chemistry involved is quite complex. The tin sulfate electrolyte is now the most popular choice for inorganic electrolytic coloring of anodized aluminum. However, virtually any metal that is electropositive to aluminum may be used. Some are better than others, that is, they are more viable as a commercial finish. Here is a partial list of some metals that can be used to color anodic coatings along with the colors they produce.

Nickel, tin and cobalt-brown/bronze to black

Copper-red

Silver-gold to brown

Lead-black

Iron-brown

Manganese-gold

Chromium-green

Selenium-red

Zinc-brown

Cadmium-dark brown

Gold-purplish

Tungsten-bluish

Only tin, nickel and cobalt are used commercially. The different metals have different degrees of light fastness. Tin, nickel and cobalt (or combinations of these) are best while copper, silver and selenium are among the least light fast. Tin, nickel and cobalt all have good corrosion resistance. Copper and silver have been shown to be less corrosion resistant.

Corrosion resistance of the finish is mainly a function of the quality of the anodic coating, control of the coloring cycle and the quality of the seal. Bronze colors produced with tin, nickel or cobalt all have good corrosion resistance. Black colors produced with tin, if not carefully controlled, can be difficult to seal. The reason for this is found in the control of the coloring cycle. Simply stated, the tin is deposited at the base of the anodic pore. In a typical commercial or architectural anodic coating of 15-20 microns (0.6-0.8 mil), the thickness of the tin deposit normally may be from 2-10 microns. Lighter colors have less tin; darker colors have more. If the coloring process is not under control when producing a black color, tin metal may be deposited to nearly the same thickness as the anodic coating or more. In this case the tin actually "spills" onto the surface of the coating. It then becomes virtually impossible to seal the coating properly. You can see that the anodic coating would be subject not only to the possibility of corrosion but to much reduced light fastness. When electrolytically colored anodic coatings using tin, nickel or cobalt are produced under carefully controlled production conditions and sealed properly, corrosion resistance is usually not an issue.