Question

Iron is coated with a layer of tin. What will be the effect on the rate of corrosion of iron if the tin layer is scratched ?

Answer 1

The iron will corrode more rapidly.

Consider the standard electrode potentials:

`Fe_((aq))^(2+)+2erightleftharpoonsFe_((s)) E^(0)=-0.44V`

`Sn_((aq))^(2+)+2erightleftharpoonsSn_((s)) E^(0)=-0.13V`

Effectively a galvanic cell is set up. The Sn/Sn2+ 1/2 cell is more positive so is driven left to right, taking in the electrons given out by the Fe/Fe2+ which is driven right to left:

`Fe_((s))rarrFe_((aq))^(2+)+2e`

This has the effect of accelerating the corrosion of the iron.

This is the opposite effect of galvanising where zinc is used to protect the iron. If the zinc is scratched the exposed iron continues to be protected as the Zn/Zn2+ 1/2 cell has a more negative `E^0` value of -0.76V so corrodes preferentially.