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Passivated Aluminum... Will electroplate be adhesive?

Hello! I am intrigued by this site and thankful for the help it's provided me thus far.

Project: Electroplating Gold with the substrate being a pair of reflector bowls from an automobile headlight. The bowls are meant to reflect the light output by a metal halide lamp. Composition is aluminum.

Purpose: Apply a new coating that allows for maximum reflectance. Current coating is a Vapor Deposited Aluminum.

Question: If I am to remove the native oxides from the aluminum with a halogen salt formula, such as vinegar [in bulk on eBay or Amazon], will the adhesion be stronger than the aluminum that hasn't been "treated?" In other words, does it matter in the case of plating over the oxide? Will the positively-charged ions and metal within the solution displace the oxide layer in their route to seek electrons?

Question: How long does it take for the oxidation to re-form after being removed?

If you see flaws in my project or plans I would appreciate the constructive criticism.

Evan Leonard
Student/Hobbyist (Auto enthusiast, college undergrad) - Fort Collins, Colorado
March 24, 2011

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Well, further research has provided me the answers to my own questions...
Hydrogen Embrittlement is what I spoke of and it needs addressed properly.

Yes, it does matter whether you plate over oxides due to the problems that arise with blistering and cracking if hydrogen is trapped within the metal even after it has been removed as it is nearly instantaneous in re-formation on aluminum alloy or stainless steels.

My quest proceeds...

Evan Leonard
- Fort Collins, Colorado, USA
March 25, 2011

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April 4, 2011

There appears to be a few problems with your plan.
Firstly, hydrogen embrittlement is not a problem with aluminium. There are no halides in vinegar (acetic acid [on eBay or Amazon]) and this would be a poor deoxidiser for aluminium.
Electroplating onto aluminium requires specialist pretreatment - normally zincate - and is not a kitchen sink operation.
Possibly more important is your choice of gold. Gold is an excellent reflector of infra-red radiation (heat) but a poor reflector of visible light. You may not see much at night but could easily cook with the focused beam.
I would look for a specialist supplier to re-aluminise your mirrors. It is the best solution and why the original manufacturer used it.

Geoff Smith
Hampshire, England