Chemical removal/stripping of chrome or copper-nickel-chrome plating from aluminum

A. Hi Robert. The good news is that decorative "chrome" plating is actually always heavy nickel plating followed by a thin flash of chrome plating. So, if the existing plating is in good shape, all you actually have to do is remove the chrome via a quick dip in about 20% HCl and rinsing well, and you'll be left with that slightly yellower nickel plating.

Although the acid is quite dangerous, and requires goggles ⇦this on eBay or Amazon [affil links] and rubber gloves ⇦this on eBay or Amazon [affil links] as a minimum, once you slowly, slowly, neutralize it with baking soda [in bulk on eBay or Amazon [affil links] (allowing for the almost endless foaming), the stuff you have left can be dried in the sun on newspaper then put in the garbage or even flushed down the toilet; this does not generate hexavalent chrome, only trivalent. (Note: this instruction is for an individual consumer; if you were running a business this stuff would probably be regulated).

The bad news is in two parts: first the chrome is there not just for the looks but for corrosion resistance, so you'll lose some corrosion resistance -- periodic waxing will help.

Secondly, if the existing nickel plating is not in good shape, you will find it nearly impossible to 'activate' that old nickel plating for good adhesion. A plating shop might attempt it with, among other things a well controlled Wood's Nickel Strike, but even they might be unsuccessful and have to chemically strip the existing nickel plating. Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. For Cr - 200 g/l HCl, for Cu and Ni - 300 g/l HNO3.

A. Vijay Kumar!
If you have to strip copper nickel and chrome on aluminum substrate:

1. First you have to strip chrome by using hydrochloric acid or reverse current in electrocleaner solution.

2. After stripping chrome, rinse the parts in water then transfer to nitric acid for stripping nickel and copper.

A. Hi Steve,

You are fortunate to have that copper beneath the chrome, otherwise it would be even harder. The following advise is for short runs only. For ongoing operation consult a local supplier of plating chemicals ...

Put the part in Hydrochloric (muriatic) acid, this will readily strip the chrome and slightly etch the copper. Then rinse it and put it in nitric acid. This will dissolve the copper and not the aluminum. Use caution, safety equipment & ventilation. and dispose according to local rules.

A. Hi Gino. If you see a copper layer between the aluminum and chrome, or know there is one, Guillermo's advice sounds good (if you've had the appropriate safety training). If no copper, keep reading please.

Regards,

A. Preferably get a proprietary product from a metal finishing supplier. If you must use nitric acid, try at 70% by vol and float a 30 cm thick film of light parafin oil on the surface to minimize fumes.

Add a little hydrogen peroxide H₂O₂ to your nitric acid to eliminate the noxious red NO₂ fumes created by the nitric acid reaction with Ni & Cu. The peroxide converts the NO₂ back into nitric acid before it can escape from the surface. For an illustration of the effects of nitric acid on Cu & Al, see:
http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v44-24-2.html

A. Hydrogen Peroxide Stripping is a much safer process.

A. Hello...

You can use 50% Nitric acid (%Volume), 1-2 min. for stripping it. But you must observe it during stripping (lift it 20-30 seconds later).

Good Luck.

A. Dear,

You cannot strip the chrome plating from die-cast aluminum by any acid solution because by this method you will destroy the aluminum alloy. So let us knew if the finish is nickel chrome or not; if it is, you can strip chrome by anodic degreasing in mild alkaline.

A. Hi, Ben. Decorative chrome is always a heavy layer of nickel plating followed by a thin flash of chromium (please see our Introduction to Chrome Plating). The chrome is removed easily and almost instantly (but not necessarily safely) by a dip in muriatic acid ⇦this on eBay or Amazon [affil links] , but the nickel is harder to remove. Metalx (Ronatec.us) [a finishing.com supporting advertiser] offers their B-9 line of strippers for this purpose.

In general, though, this is a job you farm out to a plating shop rather than do yourself because of the fumes, the waste products and the hazardous materials that are involved. Good luck.

Regards,

A. Brett-
I think maybe your idea is not really a shortcut, compared with obtaining an all-new cylinder or saw.
I think it's pretty hard to wet-etch chromium without etching aluminum. I'm proud to note that up to a micrometer of Chrome can evaporate at 200-300 °C in an oxygen plasma vacuum machine, and aluminum wouldn't, but I suspect that won't solve your problem. Other than that I have no ideas.
Chrome is kind of special. Mechanically hard, high melting temperature, corrosion resistant. That's why they used it in that bore. The only thing that might be a poor substitute is nickel plating. Maybe you want to smooth out the exhaust port and nickel-plate the whole thing, not stripping the chrome. Nickel can be a lot softer than chrome, but some plating processes make it a little harder. Don't know, maybe that would be good. Electroless nickel comes to mind. Probably much thinner than the original chrome is, and might have to be re-done periodically. Amateurs can do electroless nickel.
I hope it covers the aluminum well. Some platings find it difficult to get started on aluminum, because the very surface is an oxide not a metal.

A. Around the valve opening is a very tough place to plate and nearly impossible for most to re-plate as combustion products and oils will have literally soaked into the aluminum or formed organo-metallic compounds.
A new motor or a new saw would be a lot safer and probably less expensive in the long run.

A. Hi, hard chrome stripping from aluminium is probably the most difficult -- depending on the thickness of chromium deposit. If it is only thin dense chrome, then only sulfuric etch (electrolytic) will remove it safely. The cheapest and safest option would be to have it machined off.
As for any suitable re-plating options, electroless nickel is the best bet ... but it is in no way as wear resistant as hard chrome.

A. Immersion in hydrochloric acid or treatment in alkali on reverse voltage will strip chrome. Care must be taken as both treatments attack aluminum. Also, both produce regulated waste.

A. Stripping aluminum in caustic has a high probability of destroying the part. Attack by 50% hydrochloric acid is much slower.
Stripping the part in 15% sulfuric acid at 15 volts will strip the chrome and anodize the aluminum as soon as the chrome in that area is stripped. Then use a weak caustic to strip the anodize. You should not get pits and will lose about 0.001 inch per side maximum if you are careful.
You could try stripping electrolytically in a phosphoric acid solution for a slower attack on the aluminum.