That Sticky Feeling

Question:

We operate a Type III (hard anodize) bath which operates at the following conditions: 15% sulfuric plus 1% oxalic, dissolved aluminum less than 7.5 g/l, 40F, aluminum cathodes, air agitation, solution is filtered. We have noticed that hardcoated, unsealed parts exhibit a sticky feeling after processing. What is the cause of this stickiness? How can it be eliminated? R.V.

Answer:

Virtually all anodized, unsealed parts feel sticky to some degree or another. Probably the best example of this is on parts with relatively thick, hard anodic coatings. Many years ago in some shops this was often used to judge whether or not the anodic coating was sealed (obviously, before the days of QS 9000). If it felt "sticky," it wasn't sealed. Good sealing will eliminate the stickiness. However, many hard anodized coatings are purposely not sealed in order to preserve maximum coating hardness. The sticky feel does not affect the quality of the hard anodic coating in any way.

I am not sure why unsealed anodic coatings are sticky. Perhaps it has something to do with the fact that sealing "closes" or "plugs" the pores of the coating. If not sealed, the "open" pores of the coating feel "sticky." Would anyone care to shed some light on this? Comments are welcome.