There are a number of electrochemical techniques of corrosion rate measurement. These pages giva a brief introduction to some of them. There are many other web sites which go into more detail.
Introduction - gives a basic introduction to the electrochemistry of corrosion
Tafel - deals with corrosion current density estimation from full polarisation sweeps.
LPR - explains the basic idea behind the linear polarisation resistance measurement technique.
EIS - is to do with electrochemical impedance spectroscopy (what we used to call a.c. impedance measurement)
Harmonics - covers corrosion rate measurement using harmonic analysis.
ZRA - stands for zero resistance ammetry.
Noise - is electrochemical noise.