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Zinc plating of Cu
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Dieter Britz
science forum beginner


Joined: 04 May 2005
Posts: 45

PostPosted: Fri Jun 30, 2006 2:09 pm    Post subject: Zinc plating of Cu Reply with quote
There is a recipe for plating Cu with Zn. You dissolve some
ZZZZZn metal in hot KOH or NaOH, leaving some undissolved.
Then touch the Zn metal with the Cu, and Zn will deposit on it.

Does anybody here know what the reaction is? Clearly, the
zincate solution gets reduced to Zn on the Cu, but how, why?
I tried to Google this and got the recipe, but the explanations
don't seem right to me, e.g. that hydrogen on the Cu reduces
zincate to Zn. The standard pootntial of Zn(oH)_4^{2-} is
-1.36 V, and presumably the Cu, on which no reaction is going
on, is moved to that potential when it touches the Zn. Might
this have something to do with different overpotentials, that
is, different rates of electrochemical reactions, on different
metals?
--
Dieter Britz, Kemisk Institut, Aarhus Universitet
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beav
science forum addict


Joined: 18 Oct 2005
Posts: 63

PostPosted: Mon Jul 10, 2006 1:58 pm    Post subject: Re: Zinc plating of Cu Reply with quote
On Fri, 30 Jun 2006 16:09:54 +0200, Dieter Britz <britz@chem.au.dk>
wrote:

Quote:
There is a recipe for plating Cu with Zn. You dissolve some
ZZZZZn metal in hot KOH or NaOH, leaving some undissolved.
Then touch the Zn metal with the Cu, and Zn will deposit on it.

Does anybody here know what the reaction is? Clearly, the
zincate solution gets reduced to Zn on the Cu, but how, why?
I tried to Google this and got the recipe, but the explanations
don't seem right to me, e.g. that hydrogen on the Cu reduces
zincate to Zn. The standard pootntial of Zn(oH)_4^{2-} is
-1.36 V, and presumably the Cu, on which no reaction is going
on, is moved to that potential when it touches the Zn. Might
this have something to do with different overpotentials, that
is, different rates of electrochemical reactions, on different
metals?


to make zincate, usually, you'd dissove ZnO in hot caustic. the
coating you'd get (zincate) is not Zn..

Zn on Cu? electrolessly, at least by this method? i'm skeptical. Cu
on Fe is obvious, but Zn on Cu seems counterintuitive. Can you post
the google resource?.
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Dieter Britz
science forum beginner


Joined: 04 May 2005
Posts: 45

PostPosted: Tue Jul 11, 2006 7:15 am    Post subject: Re: Zinc plating of Cu Reply with quote
beav wrote:

Quote:
On Fri, 30 Jun 2006 16:09:54 +0200, Dieter Britz <britz@chem.au.dk
wrote:

There is a recipe for plating Cu with Zn. You dissolve some
ZZZZZn metal in hot KOH or NaOH, leaving some undissolved.
Then touch the Zn metal with the Cu, and Zn will deposit on it.

Does anybody here know what the reaction is? Clearly, the
zincate solution gets reduced to Zn on the Cu, but how, why?
I tried to Google this and got the recipe, but the explanations
don't seem right to me, e.g. that hydrogen on the Cu reduces
zincate to Zn. The standard pootntial of Zn(oH)_4^{2-} is
-1.36 V, and presumably the Cu, on which no reaction is going
on, is moved to that potential when it touches the Zn. Might
this have something to do with different overpotentials, that
is, different rates of electrochemical reactions, on different
metals?


to make zincate, usually, you'd dissove ZnO in hot caustic. the
coating you'd get (zincate) is not Zn..

Zn on Cu? electrolessly, at least by this method? i'm skeptical. Cu
on Fe is obvious, but Zn on Cu seems counterintuitive. Can you post
the google resource?.

I was skeptical too, but this seems to be a well known
technique:

http://www.chymist.com/copper%20silver%20gold.pdf

http://www.geocities.com/CapeCanaveral/Hall/1410/lab-C-16.html

The first one seems to have the chemistry a bit wrong. My
guess is that as you connect the metals, you have the Cu at
a potential where there should be Zn on it, in equilibrium with
zincate in solution, but there isn't any. So some deposits
immediately. Feasible?
--
Dieter Britz, Kemisk Institut, Aarhus Universitet
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beav
science forum addict


Joined: 18 Oct 2005
Posts: 63

PostPosted: Tue Jul 11, 2006 6:14 pm    Post subject: Re: Zinc plating of Cu Reply with quote
On Tue, 11 Jul 2006 09:15:09 +0200, Dieter Britz <britz@chem.au.dk>
wrote:

Quote:
beav wrote:

On Fri, 30 Jun 2006 16:09:54 +0200, Dieter Britz <britz@chem.au.dk
wrote:

There is a recipe for plating Cu with Zn. You dissolve some
ZZZZZn metal in hot KOH or NaOH, leaving some undissolved.
Then touch the Zn metal with the Cu, and Zn will deposit on it.

Does anybody here know what the reaction is? Clearly, the
zincate solution gets reduced to Zn on the Cu, but how, why?
I tried to Google this and got the recipe, but the explanations
don't seem right to me, e.g. that hydrogen on the Cu reduces
zincate to Zn. The standard pootntial of Zn(oH)_4^{2-} is
-1.36 V, and presumably the Cu, on which no reaction is going
on, is moved to that potential when it touches the Zn. Might
this have something to do with different overpotentials, that
is, different rates of electrochemical reactions, on different
metals?


to make zincate, usually, you'd dissove ZnO in hot caustic. the
coating you'd get (zincate) is not Zn..

Zn on Cu? electrolessly, at least by this method? i'm skeptical. Cu
on Fe is obvious, but Zn on Cu seems counterintuitive. Can you post
the google resource?.

I was skeptical too, but this seems to be a well known
technique:

http://www.chymist.com/copper%20silver%20gold.pdf

http://www.geocities.com/CapeCanaveral/Hall/1410/lab-C-16.html

The first one seems to have the chemistry a bit wrong. My
guess is that as you connect the metals, you have the Cu at
a potential where there should be Zn on it, in equilibrium with
zincate in solution, but there isn't any. So some deposits
immediately. Feasible?

IMMERSION ZINC!!!!

well. hmmm. whatya know?

actually, this clears up something that has confused me for some
time...

I'd plated brass on aluminum by first immersing the Al in NaZnO2 to
get a zincate coating, then putting the work into a low concentration,
at low power, brass cyanide bath.

i could never reconcile WHY brass would plate on the zincate...
clearly, its an immersion Zn layer that adheres to the Al, and the
brass plates on that. i had assumed (heh!) tha the brass was adhering
to a crystalline layer of zincate that makes some kind of bizarre
intermetallic structure that causes the brass to adhere very well.
immersion Zn is the obvious answer.

caustic non cyanide Zn plating baths (basically a zincate bath) are
well known, but that's using a rectifier to plate the Zn on steel.
the hurdle to actual Zn deposition from zincate must be very low.

further, the follow-on experiment discusses using Sn in the same
fashion, and stannating Al as a preplate step is also well known. the
comparative expense of Sn over Zn means that zincating is far more
common.

i may not look it, but i certainly feel smarter... thanks for posing
the question.
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