Corrosion rates determination

Stern and Weisert " by taking arbitrary values of the Tafel constants showed that corrosion rates determined by the polarisation resistance techniques are in good agreement with corrosion rates obtained by mass loss methods. 

Neufeld, P. and Queenan, E. D., Frequency Dependence of Polarisation Resistance Measured with Square Wave Alternating Potential , Br. Corros. J., 5, 72-75, March (1970) Fontana, M. G., Corrosion Engineering, 3rd edn., McGraw-Hill, pp 194-8 (1986) Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement , Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

The cathodic reactions normally are slower than the anodic reactions and are therefore the corrosion rate-determining steps. 

Electrochemical impedance, weight loss, and potentiodyne techniques can be used to determine the corrosion rates of carbon steel and the activities of both sulfate-reducing bacteria and acid-producing bacteria in a water injection field test. A study revealed that the corrosion rates determined by the potentiodyne technique did not correlate with the bacterial activity, but those obtained by electrochemical impedance spectroscopy (EIS) were comparable with the rates obtained by weight loss measurements [545]. 

Furet, N.R. Haces, C. Corvo, F. Diaz, C. Gomez, J. (1990) Corrosion rate determination using Fe Mossbauer spectra of corrosion products of steel. Hyperfine Interactions 57 833-1838... 

Thus the determination of Rp is attainable in media of high resistivity because Rp can be mathematically separated from Rs by taking the difference between Z(co) obtained at low and high co (Rp = Zm >0 - Z , >, ). In other words, determination of Rp can be achieved by subtracting the results of Eq. (10) from the results of Eq. (9). This is a particularly useful approach if Rs is nearly the same value as Rp, as shown in Fig. 6. This situation may result from either low conductivity environments or placement of the reference electrode far away from the working electrode. Note that corrosion rate determination in Eq. (3) requires knowledge of pa and pc, which are not obtained in the impedance experiment. 

In this expression, bd and bc refer to the appropriate anodic and cathodic Tafel constants. Comparison of weight loss data collected as a function of exposure time determined from R , Rf from EIS, and gravimetric measurements of mild steel exposure to 0.5 M H2S04 are often within a factor of two. This suggests that use of Rn in the Stern-Geary equation may be appropriate for the estimation of corrosion rate (147-150). However, Rn measurements may underestimate corrosion rates. / p is often measured at effective frequencies of 1(T2 Hz or less in linear polarization or EIS measurements, while Rn is measured at 1 Hz or greater. An example of this is provided in Fig. 57, which shows the corrosion rate of carbon steel in 3% NaCl solution as a function of exposure time determined by EIS, linear polarization, noise resistance, and direct current measurement with a ZRA. Among these data, the corrosion rates determined by noise resistance are consistently the lowest. 

Many field methods of corrosion rate determination use some form of electrical measurement and these instruments are commercially available. Figure 15 shows some of the probe types used in these measurements. 

Multiple concentration fields are used here in an attempt to capture the dominant effects brought about by equations 1 and 2 on the spatially discrete anodic and cathodic areas formed during exposure to 5% NaCl solution. Multiion electrolyte simulation has also been documented [12] using a nested radial basis function (RBF) approach to predict concentration profiles around a rotating disk. Here, the evolution of Mr", [02], [OH ] and [H+] fields around a planar interface is predicted, governed by corrosion rates determined using data obtained from the rotating disc technique. 

Figure 4. Comparison of different corrosion rate determinations of the two carbon steels SA 106 grade B and SA 213 grade Til in an HCl 5% inhibited solution at 75 °C.

Figure 4, which compares three corrosion rate determinations related to the electrochemical measurement (CM), the mass losses of the 5 electrodes of the probe (MWL) and the mass loss of the working electrode (WWL) as a function of the flow rate of the solution, testifies to the goodness of the new calibration of the corrosion meter. After this operation, the instrument can be used for monitoring the corrosion of carbon steels in inhibited HCl solutions quantitatively. 

From an experimental point of view, particularly notable is the possibility of determining the electrochemical parameters without exciting the system with signals of such amplitude that its free corrosion state will be irreversibly perturbed. The usefulness of this procedure is apparent if one considers the problem of comparing different corrosion rate determinations. 

Both galveinostatic and potentiostatlc measurements have been very useful in characterizing passivation processes, in spite of the earlier mentioned weaknesses in their application, especially since they have been supplemented by other techniques, such as potentiodynamic analysis, coulometry and ac-impedance(49-52) investigation on either stationary or rotating electrodes. They can indicate if a metal undergoes an active-passive transition under the given conditions, provide the rates and mechanisms of relevant reactions, give corrosion rates, determine the presence of reaction intermediates (impedance(49)), evaluate electrode capacity, speculate on oxide composition, (28,29,44,45,53) and detect its first appearance(21) eind thickness. (28,44)... 

Breakdown of the protective layer is the necessary prerequisite for the initiation of corrosion. Once this layer is destroyed, corrosion will occur only if water and oxygen are present on the surface of the reinforcement. The corrosion rate determines the time it takes to reach the minimally acceptable state of the structure (Figure 4.1) but it should be borne in mind that this rate can vary considerably depending on temperature and humidity. 

Fig. 4 Change of corrosion rate, determined by polarization measurements, of electrodes made from Siemens Martin steel in...

The most simple attempts to quantify noise and correlate it with corrosion rates were based on the standard deviation either of the potential - corrosion rate, determined hy some other method. 

The corrosion rate determined by the fit for the polarization curve shown in Fig. 3(b) is 1.4 X 10 A cm , and the fitted anodic and cathodic Tafel slopes are 47 and 98 mV decade" respectively. The corrosion rate is between the values determined by extrapolation of the anodic and cathodic Tafel regions, and the Tafel slopes are different than those determined manually. 

Corrosion Rate Determination by Electrochemical Noise Analysis (ENA)... 

The effect of uncompensated IR drop on corrosion rate determination using polarization resistance measurements was discussed in depth by Mansfeld [1-3]. He showed that in electrochemical measurements of the polarization resistance the experimental value Rp is the sum of the true value Rp and the uncompensated ohmic resistance R which is essentially the electrolyte resistance but can also contain the resistance of surface films. 

J. Jankowski, Electrochemical methods for corrosion rate determination under cathodic polarisation conditions—a review part I—DC methods, Corros. Rev. 20 (2002) 159—177. 

Distribution and intensity of corrosion is therefore usually determined by the access of oxygen to the various parts of the surface. Uneven oxygen access causes concentration cells (Section 7.5). Very often the corrosion is diffusion controlled, i.e. the diffusion-limiting current density is of special interest. Several examples of corrosion rate determined by the oxygen reduction rate are dealt with in Chapter 7. 

By the use of method a) with a noble metal like copper, the measurements are mainly expressing the wet time, while probes including a metal such as steel or zinc (also in method b) give results with a certain relation to corrosion rates in the actual environment. The ratio between corrosion rates determined with weight loss coupons and electrochemical probe, respectively, stays constant under varying conditions at a given site, but varies from one site to another [9.13],... 

Chemical corrosion tests focus primarily on resistance to surface and selective corrosion. In general, the effect of material and corrosive medium variables can be understood with these methods. The variable potential, on the other hand, is more or less undefined and can experience time changes depending on the properties of the various partial reactions involved in corrosion. The fluctuation range of the potentials found in practice cannot be taken into account in the immersion test. Often, therefore, the results obtained in chemical corrosion testing using electrochemical methods must be further differentiated to take account of the variable potential. If the corrosion rate determined in chemical corrosion testing depends heavily on the potential, and has little to offer, but if there is only a little potential dependence it is more reliable. 

Dawson, J. L., Callow, L. M., Hlady, K. and Richardson, J. A., Corrosion Rate Determination By Electrochemical Impedance Measurement, Conf. On-Line Surveillance and Monitoring of Process Plant, London, Society of Chemical Industry (1977)... 

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