Potentiodynamic Polarisation and Electrochemical Impedance Spectroscopy EIS

The polarisation resistance can be related to the rate of general corrosion for metals at or near their corrosion potential, corr. Polarisation resistance measurements are an accurate and rapid way to measure the general corrosion rate (CR) of bioinert Ti alloys, biotolerant austenitic surgical steels or biodegradable magnesium alloys. 

According to ASTM G59-97 (2009), a small potential scan, AE(t), defined with respect to the corrosion potential (AE = E — corr), is applied to a metal sample and the resultant currents are recorded. The polarisation resistance, Rp, of a corroding electrode is defined from Eq. (7.33) as the slope of a plot of the potential 

The current density i can be expressed as a function of the overpotential tj (tj = applied - - open circuit) by the Tafel equation (Burstein, 2005) as follows  

The corrosion current density iC0II is related to the polarisation resistance Rv by the Stern-Geary coefficient B  

The units of the Tafel slopes are V. The CR in mm/year can be determined from Eq. (7.37) in which EW is the equivalent weight of the corroding species in grams and p is the density of the corroding material in gem 3. 

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