Un-electroplating?

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kfz
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Un-electroplating?

Post by kfz »

Anyone had any luck at this? Trying to remove what looks like chrome plate from a bronze rudder pintle.

Roger kindly gave me a rudder pintle for the GP, but its chrome or maybe nickel plated. Tried every household acid cleaning solution I could find to no success.

Had a great idea to un-plate it, so using some old battery acid, diluted down and an old 6 volt battey charger procceed to unplate the chrome from the pintle to some old honda Cb125 rear footpeg pins that I happen to need re-plating.

all went well and the chrome flew off until a small proportion of the underlying bronze was exposed, at which point the copper in the bronze decidedi t was a lot more interested in the pins than the chrome. The soltion went horrible gunky and stopped.

Anyone had more success at this than me?

Kev
chris
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Re: Un-electroplating?

Post by chris »

try emery cloth, going down the grades, steel wool, then repolish.
Michael Brigg
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Re: Un-electroplating?

Post by Michael Brigg »

Carl Douglas, of Sculling boat manufacture fame, holds himself out to be something of an expert in the field of electro chemical reactions, it having been his area of university expertise before he became a high quality wooden boat builder.

I recall him telling me all about the process whereby Stainless steel corrodes when used (for example) as a Keel bolt below the waterline, (or in Oak (acidic) below the waterline and without Oxygen) with description of "tiny little pits of corrosion" in which, with a magnifying glass it is possible to find tiny little residual beads of copper.

The thought I would have is that your pintle may not be pure bronze, (a Copper/Tin alloy,) but more likely Brass, or some other cheap "Copper alloy" which contains Zinc.

If this is the case, the Zinc is a little more reactive than Chrome, and in your acid bath, the brass will undergo the reaction seen in "nail sickness" or de-zincification.

The murky solution will then presumably contain Zinc, and copper deposits will appear on the surface of the Brass item in much the same manner as the corrosion pits on the submerged Stainless steel Keel bolts.

As another thought, if you have Zinc in there it is also possible you are straying into more complex alchemy and are producing Zinc Chromate, (a nasty carcinogenic paint compound previously used as a metal primer and still used to prevent your rivets corroding in an aluminium mast.) or some other Chromate.

This is where my chemistry runs out of steam.

Another thought is that Battery acid may not be the best as Anhydrous Chromium Sulphate is insoluoble.

My suggestion then is to start polishing. It will eventually rub off!! :?
Michael Brigg
alan williams
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Re: Un-electroplating?

Post by alan williams »

Hi
Defeated by the electrochemical series. I would suggesst that you now stop trying to deplate as you will be removing the copper from the bronze not the Nickel or chrom due to it being move reactive. Only solution I would have thought is to rub it down with emery cloth.
Cheers Al
kfz
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Re: Un-electroplating?

Post by kfz »

alan williams wrote:Hi
Defeated by the electrochemical series. I would suggesst that you now stop trying to deplate as you will be removing the copper from the bronze not the Nickel or chrom due to it being move reactive. Only solution I would have thought is to rub it down with emery cloth.
Cheers Al

Yea, Thats what i guessed happened but, that go me thinking how it got plated in the first place if the copper is higher than the chromium????

Because it was the cathode not the anode.

???

Kev
Michael Brigg
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Re: Un-electroplating?

Post by Michael Brigg »

kfz wrote:
alan williams wrote:Hi
Defeated by the electrochemical series. I would suggesst that you now stop trying to deplate as you will be removing the copper from the bronze not the Nickel or chrom due to it being move reactive. Only solution I would have thought is to rub it down with emery cloth.
Cheers Al

Yea, Thats what i guessed happened but, that go me thinking how it got plated in the first place if the copper is higher than the chromium????

Because it was the cathode not the anode.

???

Kev
Copper is not higher than Chromium, it is near the bottom...It is the Zinc that is higher.

In electroplating, the metal "gives" it's electron to the Cathode thereby becoming a positive (and soluoble) ion.
At the Anode, and electron is "given" by the anode to the Positive Ion, thereby converting it to an uncharged elemental Metal Atom that will be deposited on the Anode.
The flow of elecricity (ie a stream of electrons) is from the anode to the positively charged metal Ion in solution, and from the elemental metal Atom to the cathode.

For each Ion formed at the Cathode, an elemental metal atom is deposited on the Anode resultin in a net flow of electrons. Metal is consumed from the Cathode, and deposited on the anode.

Provided that the solution only contains one metal element (ie a salt of the acid) and provided the metal salt is a soluoble one, and the Cathode is a pure ingot of that metal, then a single metal element will be deposited. Acid solutions are used because that way the only positive ions are the Metal ion and Hydrogen ions.

If the Pintle was a pure Bronze, this would work as Chromium is more reactive than both Copper and tin, but assuming it is actually Brass, then it contains Zinc, which is one step above Chromium in the electrochemical series, and will therefore become the main vehicle in the electroplating sequence. This will cause "Dezincification." Pure Copper and any othe lower Metals in the base aloy, (Lead, nickel etc) should be left behind on the Cathode.

Simples?

The Solution will become a witches brew of Hydrogen ions, and Sulphate, with Zinc and Chromium and perhaps a bit of Copper, tin and even some Chromate and Stanneate!!
Michael Brigg
alan williams
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Re: Un-electroplating?

Post by alan williams »

Shows how much chemistry I've forgotten in 43 years.
Cheers Al
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