Stripping Type III (Hardcoat) Anodic Coatings

Q. Sometimes we have trouble stripping sealed or even unsealed Type III coatings completely off the substrate when parts have to be rerun. Parts that are left in the caustic etch bath for too long are often unusable because they become undersized, too matte or both. Occasionally, we find that when parts are removed from the etch bath, some of the coating has been stripped while some still remains in patches. Even if these patches can eventually be removed, it takes so long that more metal is etched away in the areas that were successfully stripped than in the areas where coating remained. This leaves high and low areas on the stripped parts and renders them scrap. Usually, this is only a problem with coatings thicker than one mil (25 microns).  Is there an effective technique for completely stripping Type III (hardcoat) anodic coating from parts on the first try while removing as little substrate as possible? –J.C.

A. The hardcoat coating is usually thick and dense, and a sealed coating makes the task of stripping it even more difficult. However, there is a pretty simple way to strip such coatings that works most of the time.

If you have a desmut tank with a strong acid bath, soak the parts to be stripped in the acid bath for 30 to 45 minutes. This breaks the seal, softens the coating and makes it much easier to strip. After the acid soak, rinse the parts and then strip them in the etch bath until the coating is completely removed. It helps to keep the caustic etch bath around 140^°F (60^°C). Temperatures lower than 130^°F (55^°C) usually won't work. Refining this technique may take some practice to determine an appropriate soak and strip time for different coating thicknesses. Keep at it until you find the most effective times. Hopefully, you will be pleased with the results.

As an alternative to a desmut bath, you can also use an anodizing bath to presoak parts. No matter which bath is used, keep in mind that the extra step of soaking parts could cause a slight disruption in production.