Determination of the Polarization Resistance

Determination of the polarization resistance. The polarization resistance method is based on the observation that the polarization curve close to the corrosion potential... 

S. Feliu, J. A. Gonzales, C. Andrade, V. Feliu, On site determination of the polarization resistance in a reinforced concrete beam , Corrosionf... 

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 electrochemical impedance spectroscopy (EIS) method is very useful in characterizing an electrode corrosion behavior. The electrode characterization includes the determination of the polarization resistance (/J ), corrosion rate (Cfl), and electrochemical mechanism [1,4,6,19-28]. The usefulness of this method permits the analysis of the alternating current (AC) impedance data, which is based on modeling a corrosion process by an electrical circuit. Several review papers address the electrochemical impedance technique based on the AC circuit theory [22-24,29-30]. 

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... 

Figure 7-5 shows a simulation of the impedance of this circuit in Nyquist and Bode plot presentations. In the complex plane (Nyquist plot), an ideal capacitive semicircle with Rp as the diameter is displayed. Adopting this simplified model, analysis of corrosion systems is often reduced to the determination of the polarization resistance Rp available from the low frequency limit... 

As can be seen, the method does not require knowledge of the Tafel coefficients. The authors proposed the determination of Rp values by means of finite differences without switching off cathodic polarization, but only changing the polarization of the electrode by a small value AE and measuring the resulting increase of current A/. The ratio of these quantities approximately determines the required resistance Rp. As a more elegant solution they recommend measurement by the impedance method, which allows more accurate determination of the polarization resistance with elimination of the ohmic and diffusion components. 

Among the other electrochemieal methods are the measurement of the corrosion potential and Ihe determination of the polarization resistance (i p). Both methods are relatively simple but may give relevant information. 

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 coulostatic method was applied to in situ measurement of the polarization resistance i p (Eq. (25)). Thus, the rate of electroless plating was determined by Suzuki et al. [21-23]. Two advantages of this technique have been cited by the inventors (1) measured values are not influenced by the solution resistance, and (2) measurements can be finished very rapidly, within a few tens of milliseconds. The principle is briefly explained below. 

As mentioned in Chapter 1.3, the slope, d /di, at the zero-current potential is a measure of the polarization resistance, Rp. The corrosion can be determined from the polarization resistance using the Stern-Geary equation, if the Tafel slopes are known ... 

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. 

The current density of various materials was determined as a function of the potential difference between the anodic and cathodic branches of the current potential curves in 0.9% NaCl with a stable redox system Fe (CN)6" 7Fe (CN) [1]. The saline solution containing this redox system had a resting potential closely resembling that of a tissue culture fluid which has a redox potential of 400 mV. Ti and its alloys Ta and Nb exhibit a better resistance than the stainless steel AISI 316L and a wrought CoNiCr alloy. The same ranking can be observed during the measurement of the polarization resistance of the different materials [1]. Breakdown potential measurements of various implant materials in... 

Thus, if the Tafel coefficients are known, the measurement of the polarization resistance at the corrosion potential allows us to determine /cor. 

At the corrosion potential the absolute values of the two partial current densities are equal. The reciprocal of the slope of the curve / =f( is called polarization resistance. A measurement of the polarization resistance at the corrosion potential, allows one to determine the corrosion current density provided the anodic and... 

Fig. 9.4 Potential difference between the working and reference electrodes (a) and relative error in the determination of WE polarization resistance (b) as functions of misalignment between the WE and CE normalized by the solid-electrolyte thickness (s/d), as calculated by the finite-element analysis assuming linear electrode kinetics [8, 25]. At high s/d ratios, the experimentally measured value stabilizes at a small Nemst potential due to gas-phase polarization of the working electrode,...

The disadvantage of the polarization-resistance technique is that, while the values of and cannot be determined, these values must be known for the calculation of i orr- Therefore, the Tafel slopes must be determined with some other technique, or their values must be estimated. 

Protection current density and coating resistance are important for the current distribution and for the range of the electrochemical protection. The coating resistance determines, as does the polarization resistance, the polarization parameter (see Sections 2.2.5 and 24.5). For pipelines the protection current density determines the length of the protection range (see Section 24.4.3). 

It should be clearly pointed out that with anodic interference according to the data in Fig. 2-6 in Section 2.2.4.1, the corrosivity of the electrolyte for the particular material has no influence on the current exit corrosion. On the other hand, the conductivity of the electrolyte has an effect according to Eqs. (24-102) and (20-4). Chemical parameters have a further influence that determines the formation of surface films and the polarization resistance. 

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