Corrosion phenomenon Polarization resistance

An electrochemical reaction is said to be polarized or retarded when it is limited by various physical and chemical factors. In other words, the reduction in potential difference in volts due to net current flow between the two electrodes of the corrosion cell is termed polarization. Thus, the corrosion cell is in a state of nonequilibrium due to this polarization. Figure 4-415 is a schematic illustration of a Daniel cell. The potential difference (emf) between zinc and copper electrodes is about one volt. Upon allowing current to flow through the external resistance, the potential difference falls below one volt. As the current is increased, the voltage continues to drop and upon completely short circuiting (R = 0, therefore maximum flow of current) the potential difference falls toward about zero. This phenomenon can be plotted as a polarization diagram shown in Figure 4-416. 

Resistance to current flow also occurs as a consequence of solid corrosion product buildup on the metal surface. This phenomenon is most pronounced in environments containing H2S. Iron sulfide is a semiconductor whose conducting properties depend on the nature of the environment. It had been observed [39] that the anodic and cathodic polarization curves on iron sulfide covered electrodes are linear rather than exponential. In this case, the current flow is entirely controlled by the charge transfer across the interphase (not interface) consisting of FeS. The polarization admittance (1/Rp) becomes... 

The most commonly-used steady state techniques are potentiodynamic tests to determine the corrosion rate in systems that experience a uniform corrosion process. This type of attack can also be studied by measuring resistance to polarization. Cyclical polarization ciurves are also used to study localized corrosion and potentiokinetic reactivation is the most suitable study technique for evaluating intergranular corrosion produced by a sensitization phenomenon following ASTM G108 standard test. 

Approaching a potential from more active potentials at a certain scan rate will create a surface structure different from that created when approaching the potential from more noble potentials. The positive hysteresis shown in Fig. 7.20 is caused by the polarization to more noble potentials making the surface more passive. The negative hysteresis in Fig. 7.19 is caused by a decrease in passivity, often produced by the initiation of localized corrosion. This latter phenomenon is usually a reflection of a propensity for localized corrosion in the form of either pitting or crevice corrosion. From a practical standpoint, a positive hysteresis usually signifies that the alloy will be more resistant to localized corrosion than does a negative hysteresis. ... 

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