Electroplating

We have covered galvanic and electrolytic corrosion and decomposition, but we have ignored up until now a very important concept of electrochemistry: deposition.

In the galvanic cell formed by zinc and copper, the zinc electrode corroded away while the copper electrode swelled with copper. We never did examine that growth on the copper electrode.

When the copper electrode received electrons, the copper ions surrounding that electrode accepted the electrons to become solid copper, and they added to the mass of the solid copper electrode as they gathered towards it in search of electrons.

This is the reason that the copper solution is required in the zinc-copper cell. Otherwise, there is nowhere for the electrons to go once zinc gives them up, leading to no reaction occurring. What would happen if you had the copper electrode, but instead of copper ions, you had some other solution that could be reduced by the incoming electrons?

$AuCl_{4(aq)}^-+3e^-\rightarrow Au_{(s)}+4Cl^-_{(aq)}$ $E^\circ = 0.93V$

As this reaction has an $E^\circ$ higher than zinc, when it is connected to a zinc half-cell, the two react to make a functional galvanic cell.

The operation of this cell creates a solid gold deposition on the cathode. With a copper cathode, that translates to a copper rod getting covered in gold. This is one form of gilding objects. If you had a different electrode from a simple copper rod, perhaps an ornament of some sort, you could use this process to coat it in gold.

This process is similar to electroplating, but it requires an oft-replenished source of gold tetrachloride ions, which can be difficult to provide. Another option is to try and use the anode to coat the cathode. That wouldn’t work, though, as the anode in a galvanic cell would just degrade and develop ions in solution.

This is where the process of electroplating comes in. Electroplating takes a galvanic cell and turns it into an electrolytic cell, by applying a current across the cell opposing the current normally generated by the cell.

In this sort of setup, the supply of metal to be deposited comes directly from the copper itself. By inducing a current between the two electrons, you can force the solid copper to become oxidized, causing copper ions to appear in the solution.

These copper ions then get attracted to the cathode, as that is being pumped full of electrons. The copper ions accept the electrons and become reduced, forming a solid sheet of copper on the zinc cathode.

Through the addition of an external current source, such as a battery, you can force many metals to be the anode, supplying metal ions to deposit on the cathode. This technique is used to cover many metallic objects with various metals.

Technically, the cathode of the electroplating (the item that gets plated in metal) doesn’t have to be metal itself, metal is just the typical example due to its conductivity. Any conductive surface can be hooked into an electroplating set-up, meaning that if you take any object and make it conductive somehow, perhaps by a conductive paint or some other technique.

One of the most common examples of electroplating is the penny, which is a zinc disk that is electroplated with copper. This type of penny has been in use since 1982, and has made the penny more economical to produce.