Polarization resistance techniques corrosion-rate measurements

Polarization resistance (Rp) techniques can be used to continuously monitor the instantaneous corrosion rate of a metal. Mansfeld provided a thorough review of the use of the polarization resistance technique for the measurement of corrosion currents. R is defined as ... 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

The linear polarization technique estimates instantaneous corrosion rates under various process conditions. The corrosion current, according to the Stem-Geary equation, is inversely proportional to polarization resistance, which allows the measured polarization resistance to be normalized directly into corrosion rates. Because the current follows the appHed overvoltage, the polarization resistance curve is plotted automatically. Because this technique accurately measures corrosion rates <0.1 mpy, it is of a great importance in water distribution systems and food industries that face problems with traces of impurities and contamination. It can be used to measure the corrosion rates in civil engineering structures that cannot be subjected to weight loss measurements. Usually, Hnear polarization measurements are executed in 10 min. As shown in Fig. 5.3, the current as a... 

The ohmic resistance, Ra, is the same at all firequencies. The polarization resistance can also be measured by a DC technique and can be used to calculate the corrosion rates of any metal in a given corrosion environment. 

With the advent of advanced electronics and computerization, electrochemical techniques have evolved rapidly. The most common technologies today are the polarization resistance technique, electrochemical impedance, and Tafel extrapolation. Regardless of the technique used, each relies on the same basic principles in each test, a metallic coupon in an electrolyte is subject to an electrical perturbation. This perturbation is the appUcation of a current from an external source (power supply). This current stimulates the surface corrosion reactions. The voltage (potential) response of the coupon is measured and correlated with the current appUed—a galvanodynamic test. Conversely, the coupon potential is controlled and correlated with the requisite current—a potentiodynamic test. In either case, the resultant current is representative of the rate determining mass transfer or charge transfer rate. This may be related to the corrosion rate. 

There are many other techniques for measuring corrosion. Some of these may rely on chemical solution analysis or physical measurement of metal loss. One widely used additional technique is provided by ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods). This guide covers two basic procedures. In one, a sample of the material, usually in the form of a continuous wire, is immersed into the environment of interest (liquid, air, solid, or multiphase). The electrical resistance through the wire is determined. As corrosion consumes the cross section of the wire, the resistance increases proportionately. The second procedure uses the polarization resistance technique described in ASTM G 59 to determine the corrosion rate in the environment. 

Electrochemical tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

To the uninitiated engineer, the plethora of available corrosion monitoring techniques can be overwhelming in the absence of a categorization scheme. The first classification can be to separate direct from indirect techniques. Direct techniques measure parameters that are directly associated with corrosion processes. Indirect techniques measure parameters that are only indirectly related to corrosion damage. For example, measurements of potentials and current flow directly associated with corrosion reactions in the linear polarization resistance technique represent a direct corrosion rate measurement. The measurement of the corrosion potential only is an indirect method, as there is at best an indirect relationship between this potential and the severity of corrosion damage. 

The practical polarization-resistance technique involves one more assumption than those listed in Section 11.1. It is assumed that the polarization equation can be approximated by a linear relation between the measuring current density and the polarization for small polarization values. Equation (7) can be obtained by expanding the exponentials in Eq. (1) into Taylor series and ignoring all terms higher than first power. There has been a certain amount of controversy about the criteria of validity for this approach, both for the corrosion case and for the electrode kinetics case (see Ref. 116 and references therein). By now, this controversy seems to have been cleared up with the following conclusions. While the polarization curve is never absolutely linear at finite polarization values, the assumption of linearity often introduces only a small error. Furthermore, this error approaches zero, by mathematical definition, as A approaches zero, but the rate of approach depends strongly on the values of and b. The error... 

A corrosion monitoring technique rarely gives wrong information, unless the equipment used is faulty. Nonsense results arise because the information is correct, but irrelevant in the corrosion sense. The polarization resistance method, for example, measures the combined rate of any electrochemical reactions at the surface of the test sample. If the main reactions are corrosion, the rate measured is the corrosion rate. If however, other reactions are possible at rates that are comparable or greater, the measured rate includes the other reactions. 

This set of experiments has focused on the use of two nondestructive electrochemical techniques to measure polarization resistance and thereby estimate the corrosion rate. In addition, the effects of scan rate and uncompensated ohmic resistance were studied. Three main points should have been made by this lab (1) Uncompensated ohmic resistance is always present and must be measured and taken into account before Rp values can be converted into corrosion rates, otherwise an overestimation of Rv will result. This overestimate of Rp leads to an underestimate of corrosion rate, with the severity of this effect dependent upon the ratio Rp/Ra. (2) Finite scan rates result in current shunted through the interfacial capacitance, thereby decreasing the observed impedance and overestimating the corrosion rate. (3) Both of these errors can be taken into account by measuring Ra via EIS or current interruption and by using a low enough scan rate as indicated by an EIS measurement in order to force the interfacial capacitance to take on very large impedance values in comparison to Rp. 

The methods of measuring corrosion rates in the course of testing corrosion inhibitors are conventional weight loss, electrochemical techniques such as linear polarization resistance, potentiodynamic polarization, AC impedance, and electrochemical potential or current noise. 

This set of experiments has focused on the use of two nondestructive electrochemical techniques to measure polarization resistance and thereby estimate the corrosion rate. In addition, the effects of scan rate and uncompensated ohmic resistance were studied. Three main points should have been made by this lab ... 

Calculation of corrosion rate from the corrosion current Electrochemical Techniques to Measure Polarization Resistance... 

Electrochemical corrosion techniques are essential in predicting the service hfe ofmetal-hc components used in chemical and construction industries. They measure the corrosion rates, the oxidizing power of the environment, and evaluate the effectiveness of corrosion protection strategies. The following direct current dc electrochemical methods are discussed in this chapter linear polarization technique, Tafel extrapolation, and open circuit potential (OCP) vs. time measurements. Electrochemical impedance spectroscopy (EIS) is introduced as an alternating current technique ac. This technique uses alternating current to measure frequency-dependent processes in corrosion and estimates the change of polarization resistance. 

Macdonald summarized the hmitations of EIS technique when used to measure the corrosion current (corrosion rates) of metals [79]. A high level of mathematics is required to analyze data and interpret properties of the corrosion system. Analysis of impedance data results in determination of the polarization resistance. However, it requires obtaining a large number of low-frequency data for an accurate estimate. It is necessary to extract the noise from the data obtained at low frequency ranges to obtain meaningful mechanistic information. To calculate the corrosion rate using the Stem-Geary equation, the Tafel method should be used to estimate the Tafel slopes as a function of time. Due to the variation of porosity of corrosion products on metals, the corrosion products (oxides and hydroxides) contributions to the overall impedance spectra are difficult to evaluate. 

The corrosion rate is probably the nearest the engineer will get with currently available technology to measuring the rate of deterioration. There are various ways of measuring the rate of corrosion, including AC Impedance and electrochemical noise (Dawson, 1983). However, these techniques are not suitable for use in the field for application to the corrosion of steel in concrete so this section will concentrate on linear polarization, also known as polarization resistance or LPR, and will discuss various macrocell or galvanic current measurement techniques. 

Standard test procedures are defined within ASTM standards ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements G 5, "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements G 106, Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements and G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. Each of these methods describes a standard procedure or practice for the test method. A complete discussion of the technologies is beyond the scope of the current text. For the current text, the focus is on the application of the most simple and most widely used of these techniques, the polarization resistance measurement, ASTM G 59. The parameters discussed are, however, applicable concerns for all electrochemical tests. 

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