Perhaps the simplest method is to use two nominally identical electrodes. This is then proportional to the inverse of the polarisation resistance and hence is directly proportional to the corrosion rate. A more sophisticated approach is to use a potentiostat and a three electrode arrangement. Again, this is directly proportional to the corrosion rate. Problems arise when the response is non-linear or is linear over only a very small range of potential.
Electrochemical noise can make reliable measurements difficult.
And, as with all these perturbative techniques, the method effectively averages over the whole of the test electrode - localised corrosion attack can be hard to detect. Some of these problems are overcome by the a.c. impedance measurement technique.