Experimental potentiodynamic scans

Figure 6.7 Experimental potentiodynamic polarization curves for (50%CR) RSA 304 as fimctions scan rate in ION sulfuric acid solution at pH = -1.2 [4]...

The pitting potential is that potential at which the forward or ascending portion of the scan shows a rapid rise in current, followed by a negative hysteresis between the forward and reverse portions of the scan, as in Fig. 7.19. Often, the electrode surface exhibits small pits after the experiment. Controversy still surroimds the meaning of these potentials. The values measured are not intrinsic properties of the alloy and are influenced by a variety of experimental variables. The pitting potential as determined by the potentiodynamic scan has been shown to be related qualitatively to the resistance of a material to a loss of passivity by pit initiation. If a crevice develops in a portion of the specimen— between the electrode and its holder, for example— the pitting potential will probably reflect the breakdown of passivity in that crevice.22... 

The experimental techniques used are optical and scanning electron microscopes, electron microprobe, potentiodynamic polarization, X-ray diffraction, Fourier transform infrared spectroscopy and transmission Mossbauer spectroscopy. 

The experimental arrangement for potentiodynamic polarization experiment is shown in Figure 1.26. The experiment is done using the software, and polarization curves (both anodic and cathodic branches of polarization) are recorded at a suitable scan rate. The software performs the calculations and gives the data for corrosion potential and corrosion current density for the system on hand. 

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... 

ASTM G5 describes a procedure for a potentiodynamic polarization test on type 430 stainless steel in sulfuric acid that is intended to be a means to check experimental technique and instrumentation [2]. It indicates that the multinecked flask shown in Fig. 2(a) and the electrode configuration shown in Fig. 1(f) are suitable for the cell and working electrode configuration, respectively. A scan rate of 0.167 mV s is specified. 

Figure 3.12 (a) OCP transients and (b) potentiodynamic polarization plots recorded at a scan rate of 5 mV/s using Ru and Cu disc samples in alkaline (pH = 10) test solutions containing different surface modifying additives. The line plots in (b) represent experimental data, and the symbols placed on these lines are used to label the different systems used. 

Potentiodynamic tests for anodic polarizarion curves were carried out at 80 1°C at a scanning rate of 1200 mV/h, starting from—250 mV versus rest potential. The inhibitor concentrations used were 1% for both inhibitors. The experimental apparatus was as reported in ASTM G5-82 standard practice. 

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