Sciencemadness Discussion Board

Stripping nickel and copper from zinc without damage

My name is Luka - 1-6-2014 at 12:18

Dear friends, I tried to search everywhere, but still without result.
I have some metal components from my loved older electric guitar, which are die cast zinc or (according to manufacturer) hardened zinc. This zinc material is clasiccaly copper-plated and nickel-plated on top.
There was a corrosion from sweat-acids caused by previous owners, which penetrated through plating and created bubble effect. I sanded this corrosion and buffed the surface. Now, on some tiny places, there is exposed zinc, somewhere little of copper, the rest is mostly nickel.
My problem is, that lots of electroplating factories I asked in my country - cannot guarantee stripping down to the basic zinc. Plating of clean material is ok, but they say that stripping without damage is lottery.
Could somebody help me, if there is some solution, which can strip the copper and nickel, leaving zinc intact?
I found info here about lime sulfur (calcium polysulfide) and tried it on one part I have extra (sanded a little bit). Works great, this liquid is dissolving the copper leaving zinc intact, but makes nothing with nickel...
I am amateur about chemistry, but good overall technician with tools, never risking and keeping safety on workplace.
Is there anything I can do, to strip these platings down in common utility-room?
Please, excuse my mistakes in English, I am still learning.
Thank you for any answer in advance.
Luka.

[Edited on 1-6-2014 by My name is Luka]

violet sin - 1-6-2014 at 20:36

my first thought, why strip it all off? could you etch it very quickly to accept a plating. then copper plate it all light/medium to cover the bare zinc. then nickel plate the whole thing with a good layer, and buff out the surface irregularities with 1500 grit auto sandpaper? alternately, if you nickel plating bath produces high tensile strength plating, buff the bumpy copper plated piece prior to the hard nickel layer.

does the piece have to be dimensionally precise for music purposes, or just flawlessly plated for appearance? the idea is if nickel is plated on top of copper for adherence, then copper should* be fine over nickel as well? haven't checked the validity of that statement though. I can't think of anything easy off the top of my head that would prefer to take off copper and nickel but not zinc. in fact I think it would probably be a whole lot easier to get the zinc out of the copper/nickel shell... https://www.youtube.com/watch?v=D3dKOEWhQSk youtube vid: dissolve the zinc core of a penny while leaving the copper surface.

[Edited on 2-6-2014 by violet sin]

My name is Luka - 3-6-2014 at 06:55

Thank you for your reply.
In electro-plating factory - they say that there is better to get all plating off, even copper, because during new plating, old copper could separate from basic material on some places.
Of course there is no problem to strip nickel, if the layer is same everywhere. They already recoated some similar parts for me, but they changed only last layer and basic zinc was allways closed in "protective" copper coat. But if you sanding/buffing some corrosion, the result is - that you have some stripped basic zinc, somewhere copper and mostly nickel on top.
And for new re-coating, I need to safely strip the nickel (biggest problem), copper (now i know how) - leaving basic zinc material intact. For information - these parts are bridge and saddles for strings, so they have complicated shapes with a lot holes and tiny threads. So I think that only way to strip it - is sinking to some liquid, which cleans plating, without attacking zinc.
For example - i found that nickel and copper dissolves in several different acids, but which one is the best, for protecting the zinc object?

Zyklon-A - 3-6-2014 at 07:43

My advice, dissolve the nickel in a non-oxidizing acid - hydrochloric acid is likely the best. This will take a very long time, other acids do it faster, but either they're harder to get, or they will oxidize the copper, which you don't want.
Be sure that the entire nickel layer is gone. [Make sure to dispose of the nickel chloride solution safely, nickel salts are very toxic and can be carcinogenic]
Next, dissolve the copper layer in calcium polysulfides solution, as you found your self.
The reason I suggest this procedure, is because I don't know of any acid that will dissolve nickel and copper, but not zinc. Zinc dissolves much easier in acids then either of the other metals. So as soon as the layers of Ni and Cu have even one small hole, the zinc will be destroyed while still encased mostly. That would be bad obviously.
Also, Make absolute certain to wash off ALL acid before treating with calcium polysulfide solution. It reacts with even dilute acid to produce extremely toxic hydrogen sulfide gas! If you smell rotten eggs, run!
Hope this helps.
[EDIT] Typo, and:
Well, I guess nickel (II) chloride isn't quite as toxic as I thought, still not something to play around with, per say. Just wear gloves, splash goggles and try not to drink the solution.
Quote:
Originally referenced here
Nickel(II) chloride is irritating upon ingestion, inhalation, skin contact, and eye contact. Prolonged exposure to nickel and its compounds have been shown to produce cancer.



[Edited on 3-6-2014 by Zyklonb]

WGTR - 3-6-2014 at 14:43

Usually either you grind the plating off mechanically, or strip it under positive potential in dilute sulfuric acid. Either method will destroy the zinc substrate if you go too far, or if you are just unlucky.

Anyway, the nickel is the hardest thing to remove. After this is the copper, which is soft and usually not very thick. In any case, all corroded bits of the substrate must be completely removed. Depending on the exact alloy, sometimes the corrosion has to be drilled out, or scraped out with a knife. The old nickel should be removed (or at least etched very well), since it is difficult to re-activate, but the old copper is less of a problem.

When I copper plate aluminum pot metal alloys, the bare part is activated in dilute sulfuric acid after cleaning, and rinsed thoroughly in DI water. Within seconds, the part is put in "hot" into a cyanide copper plating bath. After a few minutes the part is pulled out and inspected. If there are any bits of corrosion, these are carved out with a knife, and the part is put back into the plating bath. Eventually I end up with good coverage. At that point I add solder to the damaged areas. The excess solder is filed away, and the surface sanded smooth. Then the part is degreased, and then put back into the cyanide copper to get full coverage. After rinsing, acid copper is used to build up the copper thickness, and then the part is sanded smooth and polished. At that point I usually hand it off to someone else for the other plating steps.

Zinc is reactive, and reacts with both acids and bases. If aluminum or magnesium is added to the alloy, then this problem is multiplied times 10. Nickel and copper are inert by comparison. This is the main problem. Also, dilute cyanide copper strikes do not form immersion deposits on zinc or aluminum.

If you want a higher-quality part, then try to reproduce it with a better metal. Clean the part up, make a mold with it, and get the part recast.

[Edited on 6-3-2014 by WGTR]

My name is Luka - 13-6-2014 at 13:32

Thanks a lot for your answers guys, I will try your recommendation soon. By chance, I have discovered one thing (I don´t know if it could help). I dipped one piece to hot water with baking soda - and on the bare zinc places only - there is micro thin layer of some hard, almost glossy, grey oxidation now. Copper and nickel are still same pollished, zero difference.. Let me ask silly question - could this oxide layer help a little to protect zinc against acid, when stripping nickel? Just asking, I am curious.
Or it´s a bullshit and rather obvious confirmation, that the zinc alloy is the weakest point of whole part, as you told? :D

Zyklon-A - 13-6-2014 at 13:57

I'm not sure what the 'oxide layer' is. But I highly doubt it will help prevent acid from reacting with it.

AJKOER - 14-6-2014 at 00:38

Quote: Originally posted by My name is Luka  
Dear friends, I tried to search everywhere, but still without result.
I have some metal components from my loved older electric guitar, which are die cast zinc or (according to manufacturer) hardened zinc. This zinc material is clasiccaly copper-plated and nickel-plated on top.
There was a corrosion from sweat-acids caused by previous owners, which penetrated through plating and created bubble effect. I sanded this corrosion and buffed the surface. Now, on some tiny places, there is exposed zinc, somewhere little of copper, the rest is mostly nickel.
.....


Actually, not sweat-acids, just plain moisture and salt. The latter form a electrolyte to complete an electrochemical reaction. Your instument being Zn, Ni and Cu in air then forms a galvanic cell!

Research so called Zinc air batteries, for example.

One possible solution to avoid future galvanic corrosion, apply a thin layer of oil to limit O2 exposure. Consider employing an inert gas like CO2. Also, store in a case with a drying agent to reduce moisture. Wearing gloves could limit NaCl exposure.