Corrosion rate measurements are made using the Linear Polarization Resistance  technique. The instrument measures the current required to polarize the  electrodes of a probe to a known potential. From polarization potential and the  measured current, polarization resistance can be calculated. Then, using  Faraday’s law, instantaneous corrosion rate is calculated from polarization  resistance. 

For purposes of drinking water applications, the method is simplified to a very  basic concept- a small voltage (or polarization potential) is applied to an electrode  (corrosion probe tips) in water. The current needed to maintain a specific voltage  shift (typically 10 millivolts) is directly related to the corrosion on the surface of  the corrosion tips in water, from which the corrosion rate can be calculated. 

The InstantMPY probe is made with two or three metal tips. A small D.C. voltage  of about 10 mV is applied across the two metal tips. The current flow between  the metal tips is measured after 60 seconds. The ratio of voltage to current is the  linear polarization resistance which, according to the Stern-Geary relationship, is  inversely proportional to the corrosion current. The corrosion rate can be  calculated from the corrosion current reading using Faraday’s Law since the  constant introduced current is known. The corrosion rate is then calculated by the  corrosion analyzer and displayed as a corrosion rate in Mils Per Year which is the  standard unit of measure for reporting corrosion rates in drinking water.