Q & A: Impregnation After Anodize

Q: I have some cast aluminum (355 T6) parts in question that have been anodized but have issues with porosity. Can impregnation be done on these parts to correct the condition in an anodized state? Would we need to re-anodize after impregnation?

A: The reason you’re seeing porosity after anodize is that during processing the pores adsorbed the anodize electrolyte — namely sulfuric acid. And if your parts are dyed, we’re sure the acid is leeching the dye, making the pores even more noticeable. The parts can be impregnated-post anodize ... however, this type of approach is less effective (if not frowned upon) as: (1) the entrapped electrolyte and chemicals inhibit the impregnant resin from penetrating as deep as it might have otherwise penetrated and (2) impregnating post-anodize entraps those acid ‘corrodents’ in the part and should they be released later on, this can result in significant corrosion. That said, they can be impregnated after anodize but we wouldn’t recommend it. Better to strip, impregnate and re-anodize, but there may be dimensional consideration that would be impacted. We would also recommend a mask for bushing holes or threaded features to prevent them from being stripped.