What Acid to Strip Chrome Plating from Plastic?

[GrahamHR]

Try electrochemistry; I wanted to remove a chromium plating from a zinc alloy diecast steering wheel centre. Your plastic is not conductive, but the metal it is coated with is.

My attempts with a 12V battery were not entirely successful but very colourful indeed with the various oxidation states of the chromium. nickel and copper compounds. Green, orange, red, yellow and black when I switched to a stainless steel electrode when the copper one disappeared Strangely, the zinc diecasting was unaffected during the whole process

Parallel to this I made a satin black sprayed polycarbonate steering wheel centre which in reality is a lot nicer than the original "chrome" one !

 

[cliveshelton]

I'm pretty sure we used this stuff at school for cleaning the glassware, although it could have been Chromic Acid!

That was almost certainly chromic. Not used much professionally nowadays because it can form explosive mixtures in some rare circumstances. Also Cr(VI) now classified a carcinogen. The elf and safety boys have poopood it for most purposes.

It it won't dissolve your electroplating. Can't think of anything that will easily although I'm no electro-chemist.

 

[nigelmercier]

I found this video, which looks promising: http://youtu.be/GMm8iWW2FK4

MSDS says: Sodium Metasilicate, Sodium Hydroxide (which I have already used), Surfactant: glycol blend

Sodium Metasilicate is the stuff used to grow crystal "gardens", but WikiPedia says this regarding its use as a flocculant: "... microscopic negatively charged particles suspended in water interact with sodium silicate. Their electrical double layer collapses due to the increase of ionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate."

Any comments from the chemists?

 

[cliveshelton]

I found this video, which looks promising: http://youtu.be/GMm8iWW2FK4

MSDS says: Sodium Metasilicate, Sodium Hydroxide (which I have already used), Surfactant: glycol blend

Sodium Metasilicate is the stuff used to grow crystal "gardens", but WikiPedia says this regarding its use as a flocculant: "... microscopic negatively charged particles suspended in water interact with sodium silicate. Their electrical double layer collapses due to the increase of ionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate."

Any comments from the chemists?

I think what you might be looking at there is a blackening of the "silvering" layer rather than removal of the layer. They normally use a hot stamping foil to apply it and it is bonded into the underlying polymer. It is not likely that you will remove it chemically to a clean and smooth finish. But if blackening is OK for what you need then try it. I assume you want to recover with another silver/chrome effect. Good luck with that! I would replace them with new rather than sweat over the DIY.

 

[VicS]

I found this video, "

Any comments from the chemists?

I am inclined to agree with cliveshelton.

I am sure if the chrome had been removed you would see some change in appearance of the Superclean

Have you tried the idea with acid and a piece of zinc. Although there were no details given it came from a respected chemistry text book ( Partington)

 

[nigelmercier]

Have you tried the idea with acid and a piece of zinc...

Yes, I scrubbed a small area with zinc (which didn't like it the acid one bit!) but no difference.

Mechanical methods are out, the substrate is too thin. As for replacing them, I've got 12, and they are almost £40 each!

 

[nigelmercier]

Try electrochemistry... when I switched to a stainless steel electrode when the copper one ...

Right. I put in a lump of stainless steel and connected up a 12V battery, this is still in the "1 litre of 18% HCl plus 200g Oxalic Acid crystals".

There is massive fizzing from either the chrome or the stainless, depending which way round I connect it. I left if for a while with the chrome fizzing, it has either been removed, or has got a copper coating, not sure which.

So my new questions are:

Which way round to connect the battery?
What electrolyte?
How to remove the nickel, or copper base? If copper, I guess Ferric Chloride, I've got some somewhere but can't find it.

 

[GrahamHR]

I just used a saturated salt solution; negative to the item you are trying to strip.

 

[VicS]

I just used a saturated salt solution; negative to the item you are trying to strip.

Wouldn't you connect it the other way round ???

care need anyway if salt or HCl is used as the electrolyte as chlorine gas will be produced.

From that point of view sulphuric acid might be preferable

 

[121]

Dont mess with this stuff unless you have the necessary experience, gloves, goggles, fume extraction etc. it eats everything , especially organic matter (including flesh and lung tissues). If you do, dispose of carefully. Gets hot when diluted like many conc. acids). Also it might form nasty things with some chromium compounds. But you won't care 'cos you flesh will be dropping off anyway.

I doubt if it will work though to your satisfaction. I would use mechanical means to get back to the substrate you need. Quicker in the long run than experimentation.

Agreed - was an Analytical Chemist in previous life and never had any real cause to use it.

Arguably nitric acid (conc.) is worse regarding flesh...

 

[GrahamHR]

Wouldn't you connect it the other way round ???

From that point of view sulphuric acid might be preferable

Oops,

Sorry, Yes, it is the positive that I had on the item being stripped.

What is strange is that despite copious gassing, I didn't smell any chlorine, it's usually easy to detect ( it was being done ouside though

If the chlorine evolved is an issue, something like sodium carbonate would do instead of the salt.

 

[Sans Bateau]

Definitive advice, sure not to fail........



......Buy a new one.

 

[VicS]

Oops,

Sorry, Yes, it is the positive that I had on the item being stripped.

What is strange is that despite copious gassing, I didn't smell any chlorine, it's usually easy to detect ( it was being done ouside though

If the chlorine evolved is an issue, something like sodium carbonate would do instead of the salt.

carbonate is not going to do it surely?

Definitive advice, sure not to fail........



......Buy a new one.

12 of them at £40 each!

 

[nigelmercier]

12 of them at £40 each!

Indeed, saving that would pay for an awful lot of experiments.

 

[VicS]

Indeed, saving that would pay for an awful lot of experiments.

Come to think of it it would pay for Frost's plate stripping kit!

 

[GrahamHR]

carbonate is not going to do it surely?

It's only another ionic compound, to increase the conductivity of the water ( which is rather poor in it's pure form)

 

[VicS]

It's only another ionic compound, to increase the conductivity of the water ( which is rather poor in it's pure form)

So what reaction is going to remove the chromium if you use sodium carbonate as the electrolyte ?

 

[GrahamHR]

Wish I knew; it's gone though!

My inorganic chemistry/ electrochemisty lectures were a very long time ago.

The process seems to rely on surface defects; where the original plating has failed, the electrochemical effects seem to creep below the interface.

In the early 1980s I worked on electrochemical generation of nitronium ions. (Not for nitration of aromatics for explosivs , but it would no doubt work). Instead for the production of expandable graphite where the nitronium ion is the effective expansion agent. It's done normally with concentred nitric and sulphuric acids ( those Chinese convicts working on open vats supplying cheap treated graphite to Europe did not have pleasant life then !)

 

[mrplastic]

Wish I knew; it's gone though!

My inorganic chemistry/ electrochemisty lectures were a very long time ago.

The process seems to rely on surface defects; where the original plating has failed, the electrochemical effects seem to creep below the interface.

In the early 1980s I worked on electrochemical generation of nitronium ions. (Not for nitration of aromatics for explosivs , but it would no doubt work). Instead for the production of expandable graphite where the nitronium ion is the effective expansion agent. It's done normally with concentred nitric and sulphuric acids ( those Chinese convicts working on open vats supplying cheap treated graphite to Europe did not have pleasant life then !)

Why don't you try calling Borough Plating in Southend? They are hard chrome plastic platers and may be able to help. I used to use them a lot as a supplier and they definitely know how to do what you need. What I can tell you is that the plastic is almost certainly ABS, when plating they extract the butadiene and then replace it again but use it to carry the chrome, well that's how they explained it to me!

 

[GrahamHR]

In the early 1980s I worked on electrochemical generation of nitronium ions. Instead for the production of expandable graphite where the nitronium ion is the effective expansion agent. )

Edit, due to brain getting rusty; water is the expansion agent, the nitronium ion is the means of getting something into the graphite plates, subsequently removed by flushing with water.