Pitting potentiostatic measurements

As mentioned earlier, the pitting potential is sensitive to several experimental parameters. The influence of some of these parameters on the potentiostatically measured potential-independent CPT was examined by Arnvig and Bisgaard. ... 

D.M. Hewette, Pitting of Austenitic Stainless Steel, the Electrochemical Basis and Validity of Potentiostatic Measurements, master s thesis, University of Tennessee Library, Knoxville, TN, 1978... 

Because pitting exhibits probabilistic behavior, the induction time T (the time for the occurrence of the first pit) in potentiostatic conditions is a probabilistic value and here the model described above does not hold. In the same way, the pitting potential measured in potentiokinetic conditions is probabilistic. 

Potentiostatic measurements on a sufficient number of samples, or simultaneous measurements using a multicharmel device [2,3ab], given the survival probability P and then the elementary pitting probability (F, t) as a function of the polarization time t (Fig. 2a). The probability density for the random function x is -dP/Pdt = Sg(y, t). Figure 2a and b show the pit generation rate time dependence... 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

Gupta, R.K., Sukiman, N.L., Cavanaugh, M.K. et al. (2012) Metastable pitting characteristics of aluminium alloys measured using current transients during potentiostatic polarisation. Electrochimica Acta, 66, 245-254. 

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