KAA 504 ELECTROCHEMICAL METHODS
Lecturer: Dr. Mohd. Hazwan Hussin
Electrochemical Corrosion Laboratory
Practical
Aims
To familiarise students with fundamental concepts of corrosion and introduce them to using a computer controlled potentiostat for electrochemical corrosion parameters measurements.
Introduction
Electrochemical techniques provide a powerful tools that corrosion practitioner can used to access corrosion and make field prediction from that assessment. The speed with which such information becomes available means that the assessment can be made in real time or shortly after the problem arises. Numerous of electrochemical techniques are available, for this practical class only two techniques are discussed, Potentiodynamic Polarization, Electrochemical Impedance Spectrocopy (EIS).
Potentidynamic Polarisation
In corrosion, metal is able to dissolve because oxidation of the metal is accompanied by reduction of another species, often oxygen, or in acids protons, which are reduced to hydrogen. The anodic reaction (oxidation – with release of electrons) takes place more rapidly at more positive potentials, the cathodic reaction (reduction) more rapidly at more negative potentials. Thus there is only one potential (the corrosion potential, Ecorr ) where the rates of the anodic and cathodic reactions are in balance without an external source of current (i.e. Ia = Ic = Icorr).
In electrochemical measurements we usually control the potential of a metal sample with a potentiostat and measure changes in current. The measured E/I characteristic is called a polarisation curve. At high potentials we see only the anodic reaction (the cathodic current is very small) and the current is positive (electrons out of the sample). Under activation control (rate controlled by charge transfer) both anodic and cathodic currents increase exponentially with overpotential (η) - so that if log I is plotted against E (a Tafel plot) linear regions should be seen on both anodic and cathodic curves. The linear regions correspond to potentials where either anodic or cathodic reaction dominates and the reaction is under activation control. Each Tafel line describes the rate of an electrochemical process. Extrapolation of the straight Tafel lines to the corrosion potential should indicate the corrosion current, Icorr, (when anodic and cathodic current are equal). Near the corrosion potential the measured current is the difference between the anodic and cathodic currents, so this is non-linear on a Tafel plot.
[In an Evans diagram the separate anodic and cathodic lines are plotted and the crossover point gives both corrosion current and corrosion potential – but we don’t show the net (measured) current that would be seen in an expt. - given by the difference between the two currents at each potential]
The slopes of the lines can provide mechanistic information, but we use them here as a simple method of finding corrosion current by extrapolating kinetic information gleaned from the curve. Note that for large currents the presence of concentration polarisation (arising from diffusion effects) or resistance polarisation (from potential drops in solution between the sample and the reference electrode) can lead to non-linear regions in the Tafel plot. If there is a diffusion-limited current for the cathode, we may be able to extrapolate this line to estimate corrosion rate, or if the anode is passive (see below) then the passive current gives the corrosion rate. Both estimates should coincide with values obtained from extrapolation of the second reaction to Ecorr.
Electrochemical Impedance Spectroscopy (EIS)
EIS has becomes a routine tool for pratical corrosion prediction. Typically, the impedance spectra are modelled by assuming a circuit made up of resistors, capacitors, and inductors and then fitting to the spectra to extract values of the circuit elements. The values may then be related to physical phenomena to try to verify that the circuit model is a reasonable representation of the corrosion process. The area that have been demonstrated a appropriate for using EIS for corrosion measurement are:
• Rapid estimation of corrosion rates [within 30 min to 24 hours (coatings)]
• Estimation of extremely low corrosion rates and metal contamination rates (˂0.01 mpy)
• Estimation of corrosion rates in low conducting media.
• Rapid assessment of corrosion inhibitor performance in aqueous and nonaqueous media.
• Rapid evaluation of coatings.
