Polarization resistance method

The corrosion current density icoa is evaluated by the electrochemical polarization resistance method assuming that both the anodic and the cathodic partial currents obey the Tafel relation ... 

The Polarization Resistance Method for Determination of Instantaneous Corrosion Rates... 

The complications and sources of error associated with the polarization resistance method are more readily explained and understood after introducing electrical equivalent circuit parameters to represent and simulate the corroding electrochemical interface (1,16-20). The impedance method is a straightforward approach for analyzing such a circuit. The electrochemical impedance method is conducted in the frequency domain. However, insight is provided into complications with time domain methods given the duality of frequency and time domain phenomena. The simplest form of such a model is shown in Fig. 3a. The three parameters (Rp, Rs, and C d,) that approximate a corroding electrochemical inter-... 

Error-producing complications related to the polarization resistance method and possible remedies are reported in the literature (14,15,22-27). The most common errors involve (1) invalidation of the results through oxidation of some other electroactive species besides the corroding metal in question, (2) a change in the open-circuit or corrosion potential during the time taken to perform the measurement, (3) use of AE that is too large, invalidating the assumption of a linear relationship between im and E required by Eq. (2) (i.e., AE/p < 0.1), (4) too... 

The polarization resistance method, when performed properly, enables reliable determinations of instantaneous corrosion rates (45). Possible sources of error include violating linearity, high solution resistance, fast scan rates or inadequate... 

In contrast to the EIS method, the Tafel-extrapolation, Tafel-curve-modeling and polarization-resistance methods are conducted under essentially dc conditions. In these cases, in generating the appropriate Eexp versus log iex or iex curve, the potentiodynamic potential scan rate is very slow, or the time between potentiostatic potential steps is very long. The common practice is a potential scan rate of 600 mV/h or an equivalent step rate of 50 mV every 5 min. Underthese conditions, it is assumed that a steady-state, extemal-current-density results at every discrete potential. Consequently, every element in the electrical circuit is purely resistive in nature, and therefore, the applied potential and resultant extemal-current-density are exactly in phase. Since the impedance (normalized with respect to specimen area) is dEexp/diex, under these conditions, the impedance, Z, at Ecorr is given by (see Eq 6.29) ... 

Details of the EIS Method (Ref 14-18). In the EIS method, potentials are applied over the frequency range of approximately 10 3 to 104 Hz in order to provide full information on the corrosion process and cell performance. Since the amplitude of the potential wave is small, on the order of 10 mV, the assumption is made (as with the polarization-resistance method) that the surface of the material is not disturbed. 

Once Rp is determined by the EIS method, icorr is evaluated in the same way as with the polarization-resistance method (i.e., with Eq 6.28). Therefore, the Tafel constants still must be experimentally determined. The intrinsic value of the EIS method lies in the fact that extensive information is extracted (i.e., Rp, Rs, and C are all determined) and, ideally, interpreted to not only determine the corrosion rate but also the rate-controlling mechanisms at the material surface and within the electrolyte. 

In the polarization-resistance method, why is it generally assumed that repeat measurements may be made without removing the sample from the electrolyte and re-preparing the surface ... 

In most cases, the corrosion rate of a metal in contact with an aggressive environment is monitored using direct current and specific equipments [26, 27, 28] whose operation is based on the polarization resistance method [5]. 

Our experience has shown that the application of the polarization resistance method for a reliable evaluation of the corrosion rate requires that the Tafel slopes should refer exclusively to the polarization curve under examination. 

The presented data is a confirmation that it is expedient to use oil-soluble Cl as components of polymer coatings. If their concentration is optimal, the strength of adhesive joints coating metal ware can improve by 10-20%. The inhibited coatings are more resistant than those made of initial polymers. The crevice corrosion rate of the metal substrate recorded by the polarization resistance method presented in [80] lowers on transfer from the initial polyethylene coating to an inhibited one by three to ten times [81]. 

Quantitative information on the corrosion rate of steel in concrete is of great importance for the evaluation of repair methods in the laboratory, for service-life prediction and structural assessment of corroding structures as well as control of repair work on site. The only technique available today is the polarization resistance method a RILEM recommendation covering this subject has been published recently [31]. 

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

The other main electrochemical method for determination of corrosion rates is the (linear) polarization resistance method (the LPR method). In a limited potential range around the corrosion potential (up to 20 mV) a linear relationship exists betw een the potential increment AE and the increment in external current Ale, as shown in Figure 9.2. It can be shown mathematically that the slope of the curve in this potential range is given by Stem-Geary s equation... 

Rp is called the polarization resistance and can be determined from the slope of the linear portion of the i-E curve in the vicinity of the corrosion potential by using a polarization of only a few mV from Ecorr (this method is known as the polarization resistance method). From the measured Rp value and the known (or estimated) j8 values, the corrosion current can be calculated from Equation 15.9. The corrosion rate is then determined from Faraday s law [17] ... 

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