Brown Anodized Powder

Question:

We anodize aluminum and use a 5 percent sodium dichromate seal afterwards. However, much of the work develops a brown powder thats hard to remove. Why is this?

Answer:

Your troubles are likely in the anodizing operation rather than the seal. High bath temperatures, excessive concentrations of sulfuric acid, poor agitation, and elevated current densities all may cause a lack of coating integrity. This leads to a soft, "fluffy" anodic film that can manifest itself as a brown discoloration after sealing.

Importantly, too, watch out for contaminants in the seal bath. These hot solutions should be made up with deionized (or at least demineralized) water to avoid calcium, chloride, and other contaminants.