Stripping Hard Anodic Coatings

Q: Our company has an anodizing vendor that recently had some problems with our hardcoat anodized (Type III) parts. Portions of these parts are masked with a liquid maskant and then the parts are hard anodized. The problem was leakage around the masking, resulting in poor quality parts. The anodizer claims they can chemically strip the hardcoat and re-anodize the parts. Is it possible to strip hardcoat anodized coatings? B.C.

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A: Any anodic coating on aluminum can be stripped off. Some are more difficult than others, however. Type III (hardcoat) is the most difficult. The thicker the coating, the more difficult it is to strip. What I am seeing from your description is this: The masking has leaked and the anodize has “bled” onto a surface that is supposed to be free of anodic coating. The area that has “bled” is irregular in shape. Now the anodizer is saying, “No problem, it can be stripped off.”

Technically, the anodizer is correct. However, you may not be satisfied with the results. The anodizer can use either the standard etchant of sodium hydroxide, or a “non-etch stripper.” Even if all the coating is removed uniformly, the area that was anodized where it was not supposed to be will be somewhat rough and possibly uneven with the non-anodized surface around it. This is because as the anodic coating forms it actually penetrates the aluminum. The ratio of penetration to buildup varies from 1/3 penetration to half and half. So, if the coating thickness is two mils, once the coating is stripped off, the area that had anodic coating could be roughly one mil, or more, “smaller.” However, in stripping, the unanodized area will start to etch before the substrate that is under the anodic coating. It’s anyone’s guess as to how the surfaces will look.

The bottom line is that you could end up with an uneven surface on the surface that was supposed to be masked. Nothing short of resurfacing with a cutting or grinding tool will make the surface even and smooth if this happens. Obviously, if grinding or cutting that surface will put it out of dimensional tolerance, then you have a problem. So the two best ways to deal with this situation are: 1) Cut or grind the surface with the anodic leakage, or 2) Live with the leakage. Try a test piece to see how it reacts.

A good practice to follow when stripping all heavy anodic coatings is to soak the parts, or the loads that are to be stripped, in a strong acid bath such as the deoxidizer or the anodize for up to 45 min, rinse and then strip the coating off. This method helps produce much more even coating removal than going directly to the stripping (etching) bath.