Potentiodynamic scan rate, determining pitting potentials

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

Potentiodynamic polarization determines Eap for positive scans, whereas negative scans yield E, . If they are different, the polarization curve shows a hysteresis. In many cases, this difference gets smaller with decreasing scan rates, indicating that the critical potentials are influenced by the composition of the pit electrolyte as well as kinetic factors like pit nucleation and pit growth. The ASTM standard G61 applies 10mVmin [11]. Potentiostatic tests depend less on the experimental conditions and thus are more reliable but time consuming. Usually a potential is applied and the current density is followed for some time. If the current decreases continuously, E < np will hold, whereas it increases when E exceeds np-If pits are formed at > np and then the potential is stepped to less positive values, the current density will drop continuously when E < rp is reached. For some systems, both critical potentials are... 

The potentials that indicate the susceptibility to SCC can be determined by the scanning of potential-current curves at different scan rates. An example for carbon steel is shown in Figure 1.20. Potentiodynamic polarization curves involve the recording of the values of current with changing potentials (scan rate 1 V/min). This simulates the state of crack tip where there is very thin film or no film at all. To simulate the state of the walls of the crack, a slow sweep rate of lOmV/min is needed such that the slow scan rate permits the formation of the passive oxide film. The intermediate anodic region between the two curves is the region where SCC is likely to occur. This electrochemical technique anticipates correctly the SCC of carbon steel in many different media. The polarization curves also show the active zone of pitting and the stable passive zone before and after the expected zone of SCC susceptibility, respectively. 

As stated above, Ejj and Eprot often dejiend strongly on the method by which they are determined and, therefore, do not uniquely define intrinsic material properties. The Eprof values determined from the scanning method can be complicated by scan rate, pit size or depth, vertex potential/current, polarization curve shape, and specimen geometry [86,87]. Investigators have found more consistent Eprof values after a critical charge has passed, while others report a single critical potential [85]. Often this potential is difficult to choose from E-I data and has been taken at various points on the reverse scan of a cyclic potentiodynamic polarization curve [89]. 

The basic electrochemical method for evaluating the susceptibihty of metals to pitting and crevice corrosion is covered by ASTM G 61 [30]. An anodic polarization curve is measured by a cyclic potentiodynamic polarization method using a specific scanning rate, from which breakdown potential and protection potential can be determined. See Fig. 11. 

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