Polarization resistance current-potential relationships

Indirect methods of corrosion rate measurement involve aspects of the electrochemical process other than metal dissolution. These measurements involve cathodic reactions, such as the evolution of hydrogen, or consider current-potential relationships, such as polarization curves or polarization resistance values. 

Polarization probes rely on the relationship of the applied potential to the output current per unit area (current density). The slope of applied potential versus current density extrapolated through the origin, yields the polarization resistance Rp, which can be related to the corrosion rate. 

Comparison between noise and impedance measurements has shown a reasonable agreement between the spectral noise plots and the impedance plots. Mansfeld and Lee [132] also concluded that although for very protective polymer coatings, Rn did not have a relationship to a particular coating property, for degraded coatings the noise resistance was identical to the polarization resistance. This, however, was limited to the cases in which the PSD plots of potential and current had the same slopes. 

Linear polarization measurements are executed rapidly. The currents in linear polarization measurements are measured in the potential range between 10 and 20 mV from the equilibrium potential. The E-I dependence in this potential range follows a linear relationship. The slope of the plot, dE/ di, represents the polarization resistance. The corrosion current is calculated using the Stem-Geary equation for known values of the anodic and cathodic Tafel slopes. The ratio of the overpotential to the current represents the resistance in Ohm s law and is often termed the charge transfer resistance or the polarization resistance, Rp. 

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... 

This is called applying a polarization overpotential. When it is small, the polarization overpotential has a linear relationship with polarization current, similar to Ohm s Law. Hence, the relationship (potential divided by current) is called polarization resistance. The corrosion rate is inversely proportional to the polarization resistance. 

This is perhaps the singly most important technique it arises from the Stern-Geary equation which describes a linear overpotential - current relationship in the vicinity (typically + 20 mV) of the corrosion potential where the linear polarization resistance is ... 

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. 

Zviagin and Liutovich (11) found similar minimum values for p-type Si as we did for the Ge samples. The theoretical curve of the Russian authors is calculated on the assumption that the minority carriers are depleted. This is possible for a p-type semiconductor only in the case of cathodic polarization. Since the Russian authors did not take into account the possibility of enrichment of the minority carriers, they did not get a distinct minimum of the theoretical capacity-potential curve. We found the minimum for n-type Ge under reverse bias, i. e., under anodic current. This result is to be expected (in contrast to a common rectifier) as long as the resistance across the phase boundary (R ) is high compared to the recombination rate or the rate orformation of free carriers. It is to be expected, in other words, as long as the electrochemical potential of the free carriers remains nearly constant across the space charge up to the surface. The Russian authors point out that the measured capacity is not equal to the space charge capacity, but should be related to it. This relationship is indicated by the measured frequency dependence of the measured impedances. It is in agreement with our assumption that the... 

A further barrier to corrosion reactions is provided by electrical resistance. When the anodic and cathodic reactions at the metal surface take place with locally different current densities, resistance in the current circuits can cause a measurable drop in potential (resistance polarization). This resistance polarization is a linear function of the current. Resistance polarization frequently arises through the formation of passive films. The resulting relationship between the change in potential and the current usually no longer follows Ohm s law, but instead is subject to a logarithmic relationship. 

At an intermediate resistance in the circuit, some current begins to flow and the potentials of both half-cells move slightly toward each other. This change in potential is called polarization. The resistance in the circuit is dependent on various factors, including the resistivity of the media, surface films, and the metal itself. Figure 1.2 shows the relationship between the polarization reactions at each half-cell. The intersection of the two polarization curves closely approximate the corrosion current and the combined cell potentials of the freely corroding situation. 

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