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Dissolve Al from Au and Ni

I am trying to dissolve an Al mandrel from a Au/Ni/Au electroform. NaOH takes the Ni through 25 millionth of Au. Any suggestions for an enchant.

Rick Hines
- Sitlwell, Kansas

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Muriatic acid (HCl) works fine. Both gold and nickel are inert in chloride environments. Aluminum will etch quickly. I'm not sure I understand the problem with caustic however ... it should work as well.

Milt Stevenson, Jr.
Plating shop technical manager - Syracuse, New York
 

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Hi,

I disagree with the previous answer. Nickel is not inert in chloride environment (less in presence of gold which forms a galvanic cell of around 1.73 Volts). I would recommend 25% sulfuric acid which is less aggressive to nickel (though not to human tissues). There are, of course other alternatives with organic acids but I ignore the correct concentrations or additives. Gold is indeed totally tolerant to all (except AR).

Guillermo Marrufo
Monterrey, NL, Mexico

I don't see how the nickel can come through the gold, however I have seen aluminum redeposit on the gold under certain conditions. If this is the problem you can acid dip in 50% HCl for 1-2 minutes. The aluminum will gas slightly and then reveal the gold underneath and the nickel will appear dull grey.

Russell Richter
- Danbury, Connecticut, USA

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I would try a caustic solution to dissolve the aluminum or you could try trisodium phosphate ⇦ on eBay or Amazon [affil link] .

Stephen T. Dowden
- Olathe, Kansas

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