I would like to suggest a look at the chemistry of aluminum as a
backdrop for this problem.

Aluminum is a very electropositive metal. It releases a great deal of
energy upon oxidation and as a finely divided powder a componet of
thermit bombs. (Al/Fe2O3 -> Al2O3, Fe). The only reason Aluminum doesn't
cause an explosion when put into water is that Al2O3 is a very tough
oxide (sapphire)which halts further oxidation.

Aluminum is electropositive relative to Hydrogen so Aluminum placed in
water will have an Al2O3 oxide coating instantly.

Aluminium oxide is amphoteric, that is it acts like a base in acid and
an acid in base. So the Al2O3 will dissolve in a solution that isn't
close to pH 7. It will dissolve in Ph 5 one way and pH 8 in another way.

Finally, the Al+3 cation is a tremendous concentrated charge, so the
Al+3 doesn't really exist by itself as a species in aqueous solution
like Na+. It has shells of OH- and polarized H2O. I think this means
sliminess.

So I see three barriers to electroplating.

1. Al2O3 oxide as a barrier to current flow.
2. Electroplating solution slowly dissolving the Al surface.
3. Perhaps a coating of aqueous Al+3 slime on the Al2O3.

I don't know the electrochemistry (there is a better technical term, but
I can't remember it) of Al2O3 on electrodes, but it might be with
sufficient negative potential to reduce Au you might instead generate Al
anion complexes dissolving the Al2O3 surface.

I suspect that attempting to electroplate Al is similar to attempting to
electroplate Magnesium. I don't really know though.

SOLUTION:

I wonder if electroplating can be done in a nonaqueous solution that has
sufficient polarity or ionicity to solubilize a Au salt and an absence
of groups that might oxidize aluminum (No OH groups etc.) . There is a
new field of ionic solvents and they are used in electrochemistry.

A talk with some chemists might be useful.

Ed


-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Ravi Shankar
Sent: Thursday, January 08, 2009 12:34 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Au electroplating on Al substrate


Hi Denis Petrov,

               I came across a similar situtation before and yes, Au was
not electroplated on to the Aluminum substrate... in my experiment, a
sulfite based Au plating solution was used, and even
after 1 hour,   au was not plated on al substrate... i guess it may be
becos of adhesion related issues, since Au was getting plated on Cr
substrate (however, surafce quality of plated gold on Cr substrate was
poor).... interested to know the exact reason, if anyone can explain..