Linear polarisation resistance

Electrochemical Techniques Although the linear polarisation resistance technique has moved beyond the infancy status attributed to it in the original material, its inherent limitations remain, i.e. it is a perturbation technique, sensitive to environmental conductivity and insensitive to localised corrosion. Two developments have occurred ... 

Polarisation resistance This technique, sometimes referred to as linear polarisation resistance (LPR), has been applied widely in industrial monitoring because of its ability to react instantaneously to a corrosion situation or change in corrosion rate " " . The limitation of the technique arises from the necessity to have a defined electrolyte as the corrosive (the author has seen an LPR probe installed in a dry gas-line in an oil refinery). 

Impedance Some of the errors arising from the use of linear polarisation resistance led to interest and development in a.c. systems.An early development used a fixed a.c. frequency and a commercial instrument was produced in the UK. Inaccuracies still occurred, however, and were due to the electrode impedance which is fequency dependent. Electrode reactions have a capacitance component, in addition to resistance, resulting in a requirement to measure the impedance. However, the total impedance comprises values for the reaction, solution, diffusion and capacitance. Measurements at different frequency are more reliable, particularly where high solution resistances occur. Simplifications for industrial monitoring have been developed consisting of two measurements, i.e. at a high (10 kHz) and low frequency (0-1 Hz). The high-frequency measurement can identify the... 

Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long life when exposed to this environment. 

Berke, N.S., Shen, D.F. and Sundberg, K.M. (1990). Comparison of the Linear Polarisation Resistance Technique to the Macrocell Corrosion Technique. Corrosion rates of Steel in Concrete, ASTM STP 1065, N.S. Berke, V. Chaker and D. Whiting (eds), ASTM, Philadelphia, pp 38-51. 

Polarisation methods involve changing the potential of the WE and monitoring the current which is produced as a function of time or potential. One of the most relevant physical quantities measured by DC polarisation methods is linear polarisation resistance (LPR). Its definition is based on the mixed potential theory proposed by Wagner and Traud [4], that explains the corrosion reactions by assuming that cathodic and anodic partial reactions occur at the metal-electrolyte interface at a certain corrosion (or mixed ) potential,... 

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

The importance of the method in corrosion testing and research has stimulated other work, and since Stern s papers appeared there have been a number of publications many of which question the validity of the concept of linear polarisation. The derivation of linearity polarisation is based on an approximation involving the difference of two exponential terms, and a number of papers have appeared that have attempted to define the range of validity of polarisation resistance measurements. Barnartt" derived an analytical expression for the deviations from linearity and concluded that it varied widely between different systems. Leroy", using mathematical and graphical methods, concluded that linearity was sufficient for the technique to be valid in many practical corrosion systems. Most authors emphasise the importance of making polarisation resistance measurements at both positive and negative overpotentials. 

The advantage of the method is that measurements can be made in more resistive corrodents (crude oil and concrete) compared with linear polarisation measurements. However, corrosion expertise is required for both operation and interpretation, and most industrial applications, therefore, are provided as a specialised service. 

Polarisation Resistance slope of the linear plot of overpotential versus current density measured at potentials close to the corrosion potential, or the tangent of such a curve at the corrosion potential if the plot is not linear. If a small change in potential, A , gives rise to a change in current density. A/, then the polarisation resistance is / p(Q m ) = AE/Ai. 

Inhibitor control can be effected by conventional methods of chemical analysis, inspection of test specimens or by instrumentation. The application of instrumental methods is becoming of increasing importance particularly for large systems. The techniques are based on the linear (resistance) polarisation method and the use of electrical resistance probes. They have the advantage that readings from widely separated areas of the plant can be brought together at a central control point. (See Section 18.1.)... 

As a result of the transferred species, loss mechanisms occur. In terms of the first law of thermodynamics these losses are well known as polarisation losses. Polarisation losses are sensitively influenced by numerous mechanisms, which are strongly non-linear with respect to a change of the operational parameters like the current density, electrical potentials, temperature, pressure, gas compositions and material properties. These parameters are assumed to be constant in case of a differential cell area. Thus, the loss mechanisms are summarised in a constant area specific resistance ASR [ 2cm2]. A change of the local overpotential (EN(Uf) — Vceii) at constant ASR complies with a proportional change in the local current density. 

A schematic representation of this polarisation curve is given in Rg. 15.2. The first part of the curve corresponds to activation loss, mainly the kinetics of oxygen reduction (with jo = 10 -10 A cm ) involved the second part is linear and due to ohmic loss, mainly the electrolyte resistance the third part is due to mass transfer or diffusion loss when the value of becomes close to /umcat or E(j) tends to zero. The optimal operating point is located in the linear part of the curve. These current-density-potential curves are very important for any type of fuel cell, because they summarise the influence of all the important parameters on the performance of a cell. Even though the equation is more complex in the case of high-temperature fuel cells, the general features of the current density vs potential characteristics are similar. 

Electrochemists have shown that the polarisation curves E = f i) are linear in the vicinity of the corrosion potential, within a zone of about 10 mV in the anodic and cathodic directions, and that their slope is inversely proportional to the corrosion rate and bears the dimension of a resistance (Figure B.4.5) [17]. 

For MFC, ferricyanides and manganese oxides have been used as alternatives for oxygen. The main application of MFC is to treat the wastewater. The ohmic resistance plays dominant role in the polarisation curves. The polarisation and power curves for MFC are shown in Fig. 2.12 for starch substrate. The polarisation curves for MFC are usually linear in nature and hence the internal resistance value can be easily obtained from their slope. The performance of MFC depends upon the electrochemical reactions occurring between the electron acceptor with high potential and organic substrate with low potential. In MFC, it becomes uncertain to have an idea of the cell voltage because the electron transfer takes place via the organic substrate and complex chain which varies from microbe to microbe. 

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