Modern inorganic chemistry

THE TRANSITION ELEMENTS 399

electrolytic. Defects in the iron lattice caused by strain or the
presence of impurities produce areas with differing electrode
potentials, i.e. the metal is no longer under standard conditions, and
a cell is produced. In the presence of an electrolyte the cells become
active and a current flows through the iron. The cell is shown
diagrammatically below (Figure 13.6).

Water drop N. Oxygen (air)

4e + O 2 +2H 2 O~4OH"(aqn

Iron

Anodic area Cathodic area

Figure J3.6. Rusting of iron in contact with a drop oj water

In the anodic areas iron goes into solution:

Fe-+Fe2+(aq) + 2e~

whilst oxygen is reduced in cathodic areas:

O 2 + 2H 2 O + 4e~ -> 4OH~(aq)

Clearly then, if either water or oxygen are absent, corrosion cannot
occur. The presence of an electrolyte, which imparts conductivity to
the solution, increases the rate of corrosion.
The existence of anode and cathode areas can be seen by the
following experiment. A few drops of phenolphthalein are added to
a solution of potassium hexacyanoferrate(III) and hydrochloric acid
added, drop by drop, until the solution is colourless. (The phenolph-
thalein turns pink due to hydrolysis of the potassium hexacyano-
ferrate(III).) Drops of this solution, about 1 cm in diameter, are now
placed on a sheet of freshly abraded steel when pink cathode areas
and blue anode areas appear.
Corrosion problems are particularly important when two metals
are in contact. The more reactive metal becomes the cathode of the
cell and goes into solution when the cell is activated by an electro-
lyte. A typical cell is shown in Figure 13.7. When the metal in
contact with iron is more reactive than iron itself, the iron is pro-
tected from corrosion. This is important when mechanical strength