Potentiodynamic scan rate, determining

Measurement Techniques. DC polarisation curves on freshly abraded mild steel in bulk paints were determined using a traditional 3-electrode potentiodynamic technique. A 50 ml cell employed a disc mild steel electrode (area 0.33 cm ), saturated calomel reference and platinum counter electrode. Polarisation curves were made at a scan rate of 2V/Hr between -950 to -450 mV vs see. 

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. 

The polarisation behaviour was determined using a GillAC potentiostat (ACM instruments) in a three-electrode cell containing 3.5 % NaCl solution. A saturated calomel electrode and a platinum electrode were used as reference and counter electrodes, respectively. Potentiodynamic scans were acquired in the region from -400 to +1200 mV at a scan rate of 1 mV/s. 

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]. 

Parkins has devised a slow and rapid potentiodynamic scanning method to determine both the relative susceptibility to slip-film rupture-repassivation SCC susceptibility, as well as possible ranges of potential where SCC might occur [60], The method applies to metals and alloys whose oxide films can initially be cathodicaUy reduced. The validity of the method relies on the notion that SCC only occurs when crack walls are readily passivated while the crack tip dissolves at a high rate due to continual destabilization of the passive film. In this method, the rapid scan anodic polarization curve provides a measure of the bare dissolution kinetics over a range of anodic potentials. A slow anodic potentiodynamic scan provides a measure of the passive crack flank dissolution kinetics over the same range of potential. Alloy-electrol5 combinations that produce potential... 

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. 

The advantage of the Stem diagram over the Evans diagram is that it can easily be obtained using the potentiodynamic polarization technique at a constant potential sweep (scan rate) and no prior knowledge of the above kinetics parameter is necessaiy for determining the Ecorr Wr point. The resultant curve is known as a potentiodynamic polarization curve. 

The authors also noted that following potentiodynamic polarization from the corrosion potential to 0 mV at a scan rate of 1 mV/s, XPS analysis was still able to detect significant quantities of surface nitride. This is illustrated in Figure 11. The most active of the alloys studied, type 304, was determined to have dissolved approximately 20 monolayers. This suggested that the nitride may form a kinetic barrier that is protected by the oxide passive film from rapid protonation to ammonia and ammonium in the active range of potential. In the same study the nitride phase formed on Ni had little effect on anodic behavior in 0.1 M HCl,... 

H2SO4 in which the potential was scanned from —20 to -1-20 mV relative to the OCP. A relatively linear response is observed in this potential range. From Fig. 4, the polarization resistance can be found to be 80 cm. Using the Tafel slopes for this system determined from the potentiodynamic polarization curve given in Fig. 3 and the polarization resistance taken from Fig. 4, the corrosion rate is found to he 1.4 x 10 A cm , which is close to the value determined hy the Tafel extrapolation technique. 

It is possible to use the data from a standard potentiodynamic polarization curve to determine polarization resistance. So, even though the scan was performed over a wide potential range, the data near the zeroTafel slopes are determined from the potentials further away from the zerocorrosion rate is determined from... 

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