Kulim Kht ACM Cyclic Voltammetry UTM

 Cyclic Voltammetry 

Cyclic sweeps are used to measure corrosion that proceeds at about the same rate all over the metals surface (uniform) and corrosion at discrete sites on the surface e.g. pitting crevice and stress corrosion cracking (localised). A typical experiment comprises of an electrochemical cell containing the electrolyte a reference electrode a platinum auxiliary and a working electrode of the metal under test. The instrument is connected RE to reference electrode AE to the platinum and WE1 to the working electrode. This applies to the Gill AC the Gill 8 and 12 the Field Machine and the manual potentiostat. A sweep is programmed into the sequencer say -1000 mV to +1000 mV @ 20 mV/min and the test started. The results can be interpreted in the Analysis package. To measure uniform corrosion the method of Tafel extrapolation is used the Tafel slope being the slope of the straight line portion of the semi-logarithmic polarisation curve. To determine the degree of localised corrosion the amount of hysteresis between the positive going sweep and the negative going sweep is calculated.

 At large perturbations away from Ecorr the reaction measured becomes almost totally oxidised (when going positive) or almost totally reduced (when going negative). The equations that describe the reactions at large overpotentials can be simplified to a linear relationship i.e. Anodic overpotential = ba.log(iapp/icorr) Cathodic overpotential = (-bc.log(iapp/icorr)). This allows an extrapolation of iapp from either the anodic or cathodic Tafel region to the open circuit potential and hence to obtain the corrosion current

Kulim Kht ACM AC Impedance/EIS UTM

AC Impedance
The Gill AC; Gill 8; Gill 12 and the Field Machine are all used for this test. A typical experiment sweeps from 10 kHz to 0.01 Hz with a 10 mV perturbation around the rest potential. The usual result is an nyquist impedance plot of half a semi-circle: the high frequency part giving the solution resistance and the width of the semi-circle giving the corrosion rate in the same manner as LPR. The analysis of this data is performed by circle fitting in the analysis software. One useful benefit of AC is the ability to measure the solution resistance at high frequency. This allows any instrument that incorporates AC to perform automatic IR compensation during DC tests.
At each frequency a sine wave is generated and fed into the potentiostat. This wave is then imposed on the cell and its potential and current flow measured. The measured values of current and voltage are compared for amplitude and phase and an impedance calculated. This is repeated for the rest of the frequencies and a plot generated. The standard starting point with AC impedance is the basic Randles circuit.

An alternate name for AC impedance is Electrochemical Impedance Spectroscopy (EIS).