Metal finishing Q&As since 1989

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Unsealed anodized aluminum corrosion resistance

Q. Good day. We produce a honeycomb core panel with an anodized frame that is bonded to a plastic skin; after much trial and error we now specify unsealed anodizing which was necessary to improve the bondline. This was a panel that was intended for indoor use, but now we are looking at outdoor applications as well. My question is if the unsealed anodized part will provide equivalent corrosion resistance as a sealed anodized part? This part is cut and machined in several locations, and our extruded implied that our corrosion resistance shouldn't suffer much in these areas because it is the sealant which is the primary corrosion inhibitor.

Best Regards

Rob Maday
Product Design Engineer - New Hope, Minnesota
November 23, 2010

Q. Brian,

The anodizing is specified as Mil A-8625 type II, class 1, unsealed.

Rob Maday [returning]
- New Hope, Minnesota
November 29, 2010

A. Rob, Type II will be more corrosion resistant than Type I, which is a start. The problem is that the anodising unsealed is porous and will have low corrosion resistance. How long this will last in outdoor conditions will very much depend what conditions it will see. Coastal positions will be severely corrosive and you will be lucky if the item will last a week without going furry with corrosion. If it is in the Arizona desert then it will probably last forever. Can you clear lacquer the product after bonding? This will certainly improve corrosion resistance. Brian Terry Aerospace - Yeovil, Somerset, UK December 1, 2010 A. You normally will get better bonding with unsealed anodize than you will with sealed anodize. There are probably a few, but not many , exceptions. Unsealed is not a problem when it is painted or bonded. James Watts - Navarre, Florida December 1, 2010

Q. Brian - That confirms the suspicion that we will have little or no resistance to corrosion effects before cutting the panel. Obviously, we'd like to stay away from any type of secondary coating for cost purposes, but we may have to investigate it. I am going to get some samples going in our salt spray chamber to see what happens.

James - We learned that lesson the hard way with some early production runs. Quite a bit of delamination.

Any idea on a durable clear lacquer that can be hand-applied?

Rob Maday [returning]
- New Hope, Minnesota
December 6, 2010

A. Rob,

I have had experience with clear lacquers from PPG, Akzo-Nobel and Indestructible. I can't really recommend one over the others as these have all been clear lacquers for use in Aerospace with consequent increase in lacquer costs, as we are paying for release against certain specifications. It is worth talking to these companies as they also supply commercial lacquers as well. There are quite a number of paint manufacturers so you may also want to consider companies such as Deft and Randolph as well.

Brian Terry
Aerospace - Yeovil, Somerset, UK
December 8, 2010

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Gluing to anodized aluminum

Q. I need to glue foam to color anodized alum 6063 for outdoor furniture. My questions are, is there a method of sealing the anodize that best allows epoxy adhesion? If not, does the unsealed anodized alum 6063 offer good corrosion resistance? Are there alternatives that would be better choices of alum, 6061? There will be several pieces glued on each article, so spot surface prep is not an option with our volume.
I appreciate any guidance.
Matt

Matt Ayres
Product Design - Manahawkin, New Jersey
May 25, 2016

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Q. Thanks Ted,
We have 12 different bond points on each, unfortunately not a straight bond line. I'm afraid in volume this is likely not feasible. There will be very little stress(pull) against the bond, it's primarily just to keep foam applied to a seat. I know this is vague, but can even a marginally strong bond be created on sealed anodized alum?
Thanks,
Matt

Matt Aytres [returning]
- Manahawkin, New Jersey
May 25, 2016