R13 Combating corrosion: surface treatment
Aim: To illustrate the advantages and disadvantages of surface treatment in combating corrosion |
There are
various ways in which corrosion can be combated. These involve changing:
1.
the anode surface or the anodic half-reaction.
2.
the cathode surface
3.
the electrolyte
4.
the nature of the oxidising agent
5.
the ability to transport electrons
The anodic surface can be changed by protecting the metal with a thin
layer of paint (lacquering), zinc (galvanising) or tin (tinning).
When considering illustration R13, it is necessary to bear in mind the
table of standard reduction potentials for the half-reactions involved
in the process.
Standard reduction potentials for the half-reactions considered in
illustrations R12 and R13.
the more negative
value of the Zn2+/Zn redox couple ensures that corrosion of zinc
to zinc oxide will be favoured over oxidation of iron. Furthermore, even when microscopic defects are present in the zinc layer an electrochemical cell is set up with the more strongly reducing Zn2+/Zn redox couple as an anode and the Fe2+/Fe redox couple as the cathode resulting in the zinc anode being preferentially oxidised, so protecting the iron underneath and preventing it from rusting.
Although the layer of zinc is very thin the protection is good for a long time.
In the food industry iron is protected by a very thin layer of tin in tin cans. The
of Fe2+/Fe
redox couple is above that of Sn2+/Sn therefore if the tin layer
in the can is scratched it will rust since iron oxidises in preference to
tin as shown on the right-hand side of illustration R13. Considerable localised
rusting can occur and food in dented tin cans should be avoided as these
can be rusty on the inside. The tin ions which may then enter the food are,
however, not very toxic. Despite these disadvantages tin is used extensively
in the food industry as a good way of protecting the iron and the chances
of it being scratched on the inside are minimal. 
