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If an alloy is placed in an acid where selective leaching is occurring, will the alloy act as both anode and cathode?

If the alloy does act as both anode and cathode then will the potential difference be zero as the cathode and anode are shorted together?

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  • $\begingroup$ Hello! Welcome to Chemistry.SE. Can you expand a bit, give us your line of thought? Do you have any theories or ideas behind what could be the answer? $\endgroup$
    – Melanie Shebel
    Nov 7 '15 at 3:53
  • $\begingroup$ Well lets say I have a piece of of iron and aluminum alloy immersed in acid. Then shouldn't it act as both cathode and anode since aluminum is oxidized and the hydrogen is reduced at the surface of the alloy? $\endgroup$
    – Mike
    Nov 7 '15 at 4:37
  • $\begingroup$ Thinking about something like leaching zinc from brass? Zn and Cu both dissolve, but the Cu plates back. $\endgroup$
    – MaxW
    Nov 7 '15 at 4:41
  • $\begingroup$ Yes like leaching, but the piece of alloy will act as both cathode and anode? $\endgroup$
    – Mike
    Nov 7 '15 at 4:44
  • $\begingroup$ @Mike - Let's try again. Leaching brass. There are two reactions happening in te solution. (1) Both Cu+2 and Zinc+2 are leached form brass. So the brass is acting like an anode. (2) Zinc+2 stays in solution, but the Cu+2 plates back. So in second reaction the brass is acting like a cathode. // The net result is that the brass has less zinc. $\endgroup$
    – MaxW
    Nov 7 '15 at 5:56
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You can setup a situation where one metal is acting as a cathode and the other metal acting as an anode. There will be a small potential difference due to their differences in Fermi Levels much like a thermocouple but the alloy will be at an electrochemical potential that is constant.

This is not that different than a single metal(like a piece of iron) in solution. In the corrosion reaction, the counter reaction may happen somewhere else on the iron. So the stick of iron may be the anode and the cathode at the same time.

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In an alloy, the atoms of the different metals are usually so homogenized that there is no automatic anode-cathode separation. A separation can develop, as in the brass example mentioned by MaxW.

And as Burak Ulgut notes, even a single metal can have anodic and cathodic sites. In the old days, the zinc-carbon battery https://en.wikipedia.org/wiki/Zinc–carbon_battery had a problem in that the zinc might develop cathodic sites that ran the battery down very quickly. This was solved by coating the zinc with a very tiny amount of mercury that had a high overvoltage for hydrogen evolution, yet allowed the zinc to move freely and be at the surface so as not to decrease the cell voltage appreciably. Purer zinc and perhaps the different medium of alkaline batteries has reduced the need for mercury, hence the frequent claim "Mercury-free!"

Electrode visualization can be achieved with a single metal or a combination:

enter image description here

https://tgreen230493.wordpress.com/2014/10/30/galvanic-corrosion-of-an-iron-nail/

enter image description here

http://icaseonline.net/parsel/www.parsel.uni-kiel.de/cms/fileadmin/parsel/Material/Hatfield/pdf/Sculpture-_teacher_notes.pdf

In the bottom picture, the iron bar on the left is wrapped with zinc, and becomes the cathode while the zinc becomes a sacrificial anode. The cathodic reaction is

1/2 O$_2$ + H$_2$O + 2 e$^-$ --> 2 OH$^-$

The buildup of OH$^-$ turns the originally neutral solution (perhaps activated with chloride ions) pink/red from phenolphthalein.

The iron bar on the right is wrapped with copper sheet and corrodes away. In some work I did a few years ago, I added K$_2$Fe(CN)$_6$, which also indicated the presence of the Fe$^{2+}$ ions coming off the bar. In that case, I thickened/gelled the solution with agar and added NaCl to accelerate corrosion. The intent was to show that addition of Ca(NO$_2$)$_2$ would stifle corrosion by oxidizing some Fe$^{2+}$ to Fe$^{3+}$ and precipitating Fe$_3$O$_4$ at the potential anode. It did and so became a useful corrosion inhibitor.

Not all metal corrosion reactions can be analyzed as if they were electrochemical. A piece of sodium does not usefully develop an electrochemical potential in an aqueous system: too rapid. The iron exhaust manifold on cars develops an oxide coating: no solvent, just metal and air.

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