Summary for adding more anodize Discussion
Adding more anodize to an anodized surface is not possible once the coating has been completed, according to Robert H Probert. The question came from Don Tanski, who had close-tolerance Type II anodized parts that came back .0001/.0002 undersize and was worried that stripping might remove too much base metal. Probert explained that sealed anodizing is hydrated aluminum oxide and cannot be built on further. He added that even if sealing were skipped, exposure to air for a few hours would hydrate the surface enough to prevent additional anodizing. He also noted that sealed anodizing can actually be used as a maskant, later partially machined off for heavier hard-coat work. The practical takeaway is that the coating cannot simply be thickened in place, so the part would need another approach rather than trying to anodize over the existing finish.
Q. Can you add to anodizing once the part has been completed. We have some very close toleranced parts that were Type II anodized. We allowed for a total build-up of .0003/.0005 on diameter. When we received the parts they were .0001/.0002 undersize to the finish dimension. I’m afraid to have them stripped because the diameter is only a .0005″ tolerance and stripping may remove base metal. Can I add to the already anodized surface?
Don Tanski
Manufacturer – Chesterland, Ohio, USA
2004
 | A. If you have .2 mils anodic coating, after stripping you’ll lose only 0.1 mils of your original dimension. If you anodized your part for .5 mils you’ll add .25 mils approx. to your original dim. (by experience, at room temp). Anodizing requires more than 24 hours to close the pores, but I never tried re-anodizing. If you have sealed your coating, I don’t believe that it is possible. Good luck
2004 A. No. Sealed anodizing (hydrated aluminum oxide) cannot be added to. Even if the anodizer skipped the sealing, then after a few hours in the air enough hydration (sealing) will take place to prevent additional anodizing. Indeed sealed anodizing is used as a maskant which is partially machined off for hard coat in heavier thicknesses.
2004 |
A. Once the coating is sealed, I’m afraid not. Work closely with your coater on tight-tolerance parts like this. I’d even provide them with a few “dummy” parts to coat, so all can be sure how the dimensions will change after coating. You didn’t say whether it was an ID or OD, but remember, that unless your coater makes a special cathode to go into an ID, typically you will get less coating thickness on an ID.
Marc Green
anodizer – Boise, Idaho
2004
