Accelerated Oxidation Of Iron When Coating Breaks

Question

I was presented with the following information:

If iron is painted, then oxygen and water are prevented from contacting the metal, and corrosion is avoided. Sometimes the iron is coated with a thin layer of another metal, such as Tin (the layer of Tin keeps oxygen and water away from the iron) If the layer of Tin is worn away or scratched, the presence of Tin will actually accelerate the corrosion of the underlying iron. (http://www.mikeblaber.org/oldwine/chm1046/notes/Electro/Corrode/Corrode.htm)

I understand why Iron would be oxidized when the coating breaks, as an observation of the reduction (oxidation) potential would show that it is more favorable to oxidize iron, over tin.

However, the portion I am confused on is:

Why would corrosion be accelerated?

The passage I was reading suggested that it was because Tin acts as a cathode and thus would accelerate the corrosion, but that does not make a lot of sense to me. As I would see it, there would be little difference whether the Tin is there or not, and thus the reaction (corrosion) should not be accelerated, but simply as quick as it would have been without the coating.

Any insight would be greatly appreciated! Thanks!

Answer 1

The main factor for this phenomena is the the values of the standard electrode potentials for of the metal ions.

The UC Davis ChemWiki page Corrosion, uses this scenario as an example:

the presence of the second metal can actually increase the rate of corrosion. The values of the standard electrode potentials for $\ce{Sn^2+}$ (E° = −0.14 V) and $\ce{Fe^2+}$ (E° = −0.45 V) show that $\ce{Fe}$ is more easily oxidized than $\ce{Sn}$. As a result, the more corrosion-resistant metal (in this case, tin) accelerates the corrosion of iron by acting as the cathode and providing a large surface area for the reduction of oxygen

Tin is therefore more of a noble metal than iron - a galvanic corrosion chart is shown below, showing that tin is more 'noble' than iron (Image source):

The significance of tin being more noble than iron is that, according to the UC Davis ChemWiki page Electrochemical Corrosion, it can act as a depolariser, removing more electrons than the water/air alone - hence, accelerate the corrosion process.

Instead of the copper used in the example, it would be tin in your example.

A somewhat simplied illustration of the process is shown in the diagram below: