Our common experience tells us that iron left out in the elements rusts more quickly than iron that has been kept clean and dry. Water provides a medium for ion transport.
The observations from the
experiment also suggest that dissolved ions in the water have
considerable effect on the corrosion process. In fact some ions appear to cause corrosion to
occur very rapidly while others stop it altogether.
Although a drop of solution on a piece of iron
seems to be a pretty simple system it sets up a series of
interconnected processes which eventually lead to the formation
of rust. The key is how the drop changes the amount of oxygen
which can get to the metal.
At the surface of the drop the concentration of dissolved oxygen is higher than it is at the bottom of the drop, in contact with the metal. The appearance of each drop in which corrosion was detected can tell us something about what is happening where.
