Using epoxy with aluminium

[Adrian Harris]

There was a posting recently in the uk.rec.crafts.engineering newsgroup from someone having problems with epoxy joints failing when gluing aluminium.

This was the method described in one of the replies:

Clean the aluminium first until water will wet the surface completely.
Clean a stainless steel wire brush the same way.
Then when the brush and aluminium are absolutely dry apply the epoxy.
After the aluminium is covered with the epoxy wire brush the surface by putting the brush through the epoxy.
This removes the oxide layer which prevents good bonds. This layer is
quite thin and the little particles that get mixed into the epoxy
won't have much of a weakening effect.

I've underlined the important bit, which is to score through the oxide layer beneath the epoxy, so that the glue is sicking to the base metal and not the oxide. This is exactly the procedure recommended when using the various aluminium brazing sticks, so that the brazing material bonds to the base metal, bit I'd never heard it recommended for just gluing before.

With thanks to Eric for his permission to cross post this.

Adrian.

[Stephen White]

Great Adrian, might hijack that for the Knowledge Base? Stephen

[Fabrice Le Roux]

Hi Adrian,

Interesting process, but I can't help feeling that several different issues and their solutions are being confused in this method.

The "water break free test" following cleaning (acid or alkali based) simply confirms the absence of hydrophobic films and contamination, not oxide. However the thorough scrubbing of the aluminium with detergent and a Scotch-Brite pad will result in the removal of both. In Aerospace, because of the size of the parts being cleaned, etchants and chemical brighteners are sometimes employed, but these have to be thoroughly cleaned off too. However, once clean, aluminium can be left at room temperature, certainly for twelve hours, in some cases for forty-eight hours, before re-oxidation even starts to be a consideration.

The stainless steel brush idea seems to have been added to the mix from the Aluminium welding process and I totally defer to others here on that subject ! However the use of a stainless brush or even an old screwdriver to break up the surface oxide to "tin" the joint, is required because the application of a heat(via blowtorch) instantaneously generates an oxide layer, even (especially!) on previously cleaned surfaces. This does not happen with epoxy at room temperature.

Going over the area where you are about to apply epoxy glue with a clean stainless brush will improve the mechanical adhesion (as opposed to chemical bonding) by keying/scoring the surface. Always a good idea. Scrubbing the glue into the surface, however, will only trash the brush, and potentially introduce contaminants and air bubbles into the glue layer, possibly weakening it.

The recurrent advice heard over the years about epoxy glue is:
A) Employ a mechanical support, a hidden pin or screw when glueing small things to big things (especially exterior details),
B) Slower (non Rapid)epoxies are stronger.
C) Where possible put the assembly, if materially suitable, in an oven for a couple of hours (max 150C) as this reduces the curing time hugely while significantly increasing the chemical bonding.

Warning: putting tank parts in a domestic oven may also significantly shorten your marriage, and/or lifespan.

Not saying Eric's tip does not work, just that it may work for different reasons than those described.

cheers,
Fabrice