The use of this technique enables the electrochemist to determine information relating to the corrosion rate, passivity and pitting susceptibility. Using the 1287A Electrochemical Interface or the 1285A Potentiostat plots of log[I] vs. E (termed potentiodynamic scans) may be obtained. The instrument is set to a series of pre-defined scan limits.
The starting point will be set depending on the nature of the information required. It is normal to start the scan at a potential close to that of the open circuit potential which the instrument software provides. The limits are set to scan to provide either anodic or cathodic information.
Experimental limits determine that once a certain current density has been passed progressing further is self defeating (i.e. anodic scan where pitting has been initiated, or a cathodic scan where the potential is sufficiently negative for hydrogen evolution to occur). The scan rate is typically expressed in mV/sec.
At higher scan rates the system under evaluation will not have time to stabilize at each potential. Scanning at higher potentials will have the effect of pushing the values obtained (icorr, Ecorr) to a more positive value. The ASTM (American Standard of Testing and Methods) stipulates 0.1667 mV/sec (10 V/min) for the analysis of corroding systems.
The technique can be used to determine icorr, which in turn can be used to calculate the corrosion rate. Scans are performed close to the open circuit potential (-200mV for an anodic scan and +200mV for a cathodic scan). The corresponding trace must have a point where the current measured is equal to zero.
The Tafel plot extrapolated to the zero current/potential gives a set of co-ordinates relating to Ecorr (x axis) and icorr (y axis). The icorr value may be calculated using the Tafel constants (βa + βc) and Rp. The value for βa can be determined by taking the slope for the anodic portion of the curve and βc for the corresponding cathodic part.
Using the Rp value and the Stern-Geary equation the value for icorr can be determined. The corrosion rate can then be calculated from this value in mm per year.
From the Ecorr value it is possible to obtain a value for icorr which in turn is related directly to the rate constant of the electrode reaction.
icorr = corrosion current (A cm-2)
EW = equivalent weight of the sample (g)
d = density of sample (g cm-3)
The CorrWare software package used with the 1287A Electrochemical Interface and 1285A Potentiostat allows autotafel fitting. Individual markers are moved to the appropriate positions on the plot and the Ecorr and icorr values calculated. If the material density and surface area are known it is also possible to compute the corrosion rate. The plotting of different comparable metal electrodes using the same physical/chemical environment provides information relating to general corrosion mechanisms and their rate. Techniques such as Tafel plots cannot be used to provide information relating to corrosion types such as pitting and crevice corrosion, although this information is more readily obtainable using electrochemical noise measurement, where the current and voltage are measured simultaneously in a static (unperturbed) system.