Tafel slopes corrosion-rate measurements

The main concept that most of the corrosion data interpretation is based on was first introduced by Wagner and Traud (1938), according to which galvanic corrosion is an electrochemical process with anodic and cathodic reactions taking place as statistically distributed events at the corroding surface. The corresponding partial anodic and cathodic currents are balanced so that the overall current density is zero. This concept has proven to be very useful, since it allowed all aspects of corrosion to be included into the framework of electrochemical kinetics. Directly deduced from this were the methods of corrosion rate measurement by Tafel line extrapolation, or the determination of the polarization resistance Rp from the slope of the polarization curve at the open circuit corrosion potential... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

A qualitative measure of the corrosion rate can be obtained from the slope of the curves in Fig. 2. Z INT is given in units of s ohm" . Owing to the presence of the uncompensated ohmic resistance and lack of values for Tafel slopes [Eq. (2)], data in Fig. 2 should be viewed as indicative of significant changes in corrosion rates. Corrosion loss remained low during the first 2 months, followed by a large increase for both flushed samples and controls. The corrosion rate increased when the surface pH reached values of 1 or less. Total corrosion loss as determined from integrated Rp data was less for the control than for the flushed sample. 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

It can be seen that it was again difficult to obtain results from specimens where no stable rest potential could be measured. The harmonic currents in all cases were low and for certain specimens were of the same order as the distortion resulting from the input sine wave. The Tafel slopes obtained were in general anomalously high and the corrosion rates varied over several orders of magnitude. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

From this last equation, icon can be obtained if the Tafel slopes and the overall charge-transfer resistance are measured. This relation was first developed by Stem and Geary and is applied by some industrial instrumentation to determine the corrosion rate. 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

For the data in Fig. 6, the ohmic and polarization resistances can be determined to be about 0.3 and just under 100 cm, respectively. The value of Rp is slightly higher than that determined by linear polarization (Fig. 4) in a measurement that just preceded the EIS experiment on the same electrode. The double layer capacitance is seen to be 1/3000 SI cm = 333 pF cm . The polarization resistance determined by EIS can be used to determine the corrosion rate with the Stern-Geary equation, just as was described above for polarization resistance determined by linear polarization. EIS data provide no estimation of the Tafel slopes, which are required in the Stem-Geary equation. 

Linear polarization is simple, easy, and fast. The sample is not polarized far from the corrosion potential, so it is suitable for in situ monitoring. The polarization resistance is measured, but the Tafel slopes are needed to determine corrosion rate. Under certain conditions, Tafel slopes can be extracted from the data. 

EIS requires more sophisticated equipment and analysis, but provides more information on the behavior of the interface than DC techniques. Tafel slopes are needed to determine corrosion rate. Using a scanning probe to map the local current, local EIS measurements can be made. 

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 anode and cathode corrosion currents, fcorr.A and fcorr,B. respectively, are estimated at the intersection of the cathode and anode polarization of uncoupled metals A and B. Conventional electrochemical cells as well as the polarization systems described in Chapter 5 are used to measure electrochemical kinetic parameters in galvanic couples. Galvanic corrosion rates are determined from galvanic currents at the anode. The rates are controlled by electrochemical kinetic parameters like hydrogen evolution exchange current density on the noble and active metal, exchange current density of the corroding metal, Tafel slopes, relative electroactive area, electrolyte composition, and temperature. 

HER measurements showed the onset overpotential for acidic hydrogen evolution was found to be small (0.078 V) and material was also found to be resistant to corrosion in the acidic media. These two results suggest that both the synergistic effects ofthe alloys and the nitride component make this type of material a promising candidate for the HER reaction. The resulting Tafel slope was found to be 35 mV decade". This suggests the Tafel reaction recombination step to be the rate-limiting step in the HER. 

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