Linear Polarization Resistance (LPR) and the Stern-Geary Equation

With this widely used technique in corrosion monitoring, the polarization resistance of a material is defined as the slope of the potential-current density (DE/Di) curve at the free corrosion potential, yielding the polarization resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation:

where: Rp is the polarization resistance icorr the corrosion current The proportionality constant , B, for a particular system can he determined empirically (calibrated from separate weight loss measurements) or, as shown by Stern and Geary, can be calculated from ba and bc, the slopes of the anodic and cathodic Tafel slopes, i.e.

Corrosion MonitoringLinear Polarization Resistance (LPR)Polarization resistance is particularly useful as a method to rapidly identify corrosion upsets and initiate remedial action, thereby prolonging plant life and minimizing unscheduled downtime. The technique is utilized to maximum effect, when installed as a continuous monitoring system. This technique has been used successfully for over thirty years, in almost all types of water-based, corrosive environments. Some of the more common applications are:Cooling water systems Secondary recovery system Potable water treatment and distribution systems Amine sweetening Waste water treatment systems Pickling and mineral extraction processes Pulp and paper manufacturing Hydrocarbon production with free water The measurement of polarization resistance has very similar requirements to the measurement of full polarization curves. There are essentially four different methods of making the measurement according to whether the current or the potential is controlled and whether the current (or potential) is swept smoothly from one value to another, or simply switched between two values. In addition the measurement may be made between two nominally identical electrodes (a two-electrode system), or a conventional three-electrode system (working, reference and counter) may be used

Electrochemical impedance spectroscopy (EIS)EIS has been successfully applied to the study of corrosion systems for thirty years and been proven to be a powerful and accurate method for measuring corrosion rates. But in order to access the charge transfer resistance or polarization resistance that is proportional to the corrosion rate at the monitored interface, EIS results have to be interpreted with the help of a model of the interface. An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal, usually a voltage between 5 to 50 mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000 Hz. See how our Sponsor can help you with these test methodsThe EIS instrument records the real (resistance) and imaginary (capacitance) components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and input by the operator. The program then fits the best frequency response of the given EIS spectrum, to obtain in fitting parameters. The quality of the fitting is judged by how well the fitting curve overlaps the original spectrum. By fitting the EIS data it is possible obtain a set of parameters which can be correlated with the coating condition and the corrosion of the steel substrate. Amongst the numerous equivalent circuits that have been proposed to describe electrochemical interfaces only the following apply in the context of a freely corroding interface at or close to kinetic equilibrium: Simplest RC representation of an electrochemical interface One relaxation time constant with extended diffusion Two relaxation time constants Impedance of pitting processes of Al based materials.

Model for a corroding paint on steel substrate

Electrochemical Noise Electrochemical Noise, commonly abbreviated ECN, is an electrochemical technique in which the potential and/or current signals that arise directly from the electrochemical reactions taking place on an electrode surface are measured and interpreted. ECN is of intense ongoing interest because of the totally non-invasive nature of the measurement when performed with a Zero Resistance Ammeter (ZRA). The electrochemical instrument is not applying any signal to the sample that might improperly influence the result. This perturbation may be of concern in every electrochemical experiment other than noise.

ECN can also be measured with the electrochemical instrument configured as a potentiostat or galvanostat. The number of actual applications of this sort is quite small. Most ECN experiments are carried out with ZRA's, so that no signal at all is applied to the sample by the instrument.ECN has been used to monitor localized corrosion (pitting), uniform corrosion through measurement of the Noise Resistance, and the deterioration of paints on metal substrates. ECN does not yet enjoy the level of acceptance of DC techniques and Electrochemical Impedance Spectroscopy.