Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Dissolving Aluminum Mandrels after Electroforming
adv.
Q. I am trying to dissolve an Al mandrel from a Au/Ni/Au electroform. NaOH takes the Ni through 25 millionths of Au. Any suggestions for an enchant.
Rick Hines- Sitlwell, Kansas
2002
A. 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.
Syracuse, New York
2002
A. 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 aqua regia).
Guillermo MarrufoMonterrey, NL, Mexico
2002
A. 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
2002
A. I would try a caustic solution to dissolve the aluminum or you could try trisodium phosphate ⇦this on eBay or Amazon [affil links] (TSP).
Stephen T. Dowden- Olathe, Kansas
2002
Multiple threads merged: please forgive chronology errors :-)
Q. Hi
I am producing very low quantity copper electroforms with high precision mandrels made with 6061 T6 aluminium and, although I am very happy with the growth I achieve, the problem is that when I erode the mandrel using a 40 grams/litre solution of caustic it turns the copper black. Is there a way of either cleaning the copper with zero etching of the copper or, better still, preventing the black layer being formed in the first place?
Many Thanks
Lee
Shop manager - Cornwall, England
July 15, 2009
A. Try to dissolve the copper oxide in a diluted sulfuric acid. If if does not work good enough, add 1-2 ml/l hydrogen peroxide
In any case, a slight etching of the copper will occur
Sara Michaeli
Tel-Aviv-Yafo, Israel
July 22, 2009
Aluminium mandrel plates out on electroform
Q. I have been electroforming copper onto aluminium formers. In removing the aluminium afterwards I have tried using HCl + Water. I found this to be a little slow and in some, strange cases, it eats the copper that I want left intact.
So I changed this to using Sodium Hydroxide to be used to remove the Aluminium. While this works really fast, when the temperature is set at 70 °C, I found a really unexpected result, the Aluminium seems to plate onto the copper. While this is not pure Aluminium it probably is Al(OH)4 or something like this, according to the chemical reactions. How do I avoid this? Presumably this is not usual.
University of Strathclyde - Glasgow, Scotland
January 9, 2018
Q. We copper plate for the waveguide industry. We are having a problem dissolving the aluminum with caustic. We are getting a hard grey deposit on the part and don't know how to remove it without damaging the inside. Any suggestions?
I can supply pictures. Thank you
- Simpsonville South Carolina
November 24, 2020
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Ed. note: You can send pics to mooney@finishing.com for posting here.
A. In both cases above you are making sodium aluminate which, as one of you said, is hydrolysing to Sodium Hydroxide (read "concrete"). This can be softened and probably rinse off if you will add a chelator to the NaOH. The chelator may be sodium gluconate or sodium hyptonite (?spelling?). All known suppliers of aluminum etch products include a chelator in their product, so use a prepared product instead of plain caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] .
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
November 24, 2020
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Ed. note: googling locates sodium glucohyptonate as a well-recognized chelator, maybe that's the spelling? :-)
A. Personally I would use the caustic (sodium hydroxide) dissolution method - acid dissolution of aluminium can be tricky and messy. I would suggest using about 25% (w/v) caustic at about 35 °C, but be careful, because the reaction is exothermic and will get hotter. That is why I suggest a relatively low temperature! Also, remember that for every 10 °C increase in temperature, the reaction rate approximately doubles. Secondly, make sure the electroform is well agitated in the caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] , as the reaction between caustic soda and aluminium can create a grey mushy hydroxide that will inhibit further dissolution. The extent of the mushy hydroxide is partly dependent on the aluminium alloy used as the mandrel. If you wish to use a chelating agent, try EDTA - one of the most widely used general purpose chelating agents. I have never used it in this context, but it makes sense!
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
November 30, 2020
adv.
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