Potentiodynamic polarization curve

Scan Rates Sweeping a range of potentials in the anodic (more electropositive) direction of a potentiodynamic polarization curve at a high scan rate of about 60 V/h (high from the perspective of the corrosion engineer, slow from the perspective of a physical chemist) is to indicate regions where intense anodic activity is likely. Second, for otherwise identical conditions, sweeping at a relatively slow rate of... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Fig. 14. Potentiodynamic polarization curves obtained on FeTb and FeTb/Si02 samples in pH 3.1 CF-containing solution [159], (Reprinted by permission of The IEEE).

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

Fig. 8. Potentiodynamic polarization curve for a rotating Cu disc in 0.1 M NaOH with two analytical Pt half rings detecting soluble Cu+ (i jji) and Cu2+ ion formation (ijq) [62],...

Fig. 20. Composition (Fe(II) and Fe(III)) of the passive layer formed for 300 s on Fe in 1 M NaOH calculated from XPS measurements on the basis of a bilayer model including the potentiodynamic polarization curve with indication of formation of soluble Fe2+ and Fe3+ species. Hp and Epi are the passivation potentials in alkaline solution and acidic electrolytes (Flade potential) extrapolated to pH 12.9 [12],...

Fig. 22. Potentiodynamic polarization curve of Co in borate buffer pH 9.3 with potential ranges of active behavior, primary and secondary passivity and their relation to the oxidation peaks A1 and A2 [57].

Fig. 24. Potentiodynamic polarization curve of Cu in 0.1 M KOH with anodic and cathodic current peaks and the related reactions of oxide formation or reduction dissolution of cations and the indication of the stability ranges of the CU2O and duplex oxide layer, z ph at CII indicates oscillating photocurrent due to a chopped light beam [86],...

Fig. 27a. Potentiodynamic polarization Curve of Fe5Cr in 0.5 M H2SO4 with potential ranges of hydrogen evolution, active dissolution (Cr2+), passivity (Cr3+), transpassivity (C Cb2-), and oxygen evolution [69].

Fig. 32a. Potentiodynamic polarization curve of rotating disc electrodes of Fe/Al alloys in phthalate buffer pH 5.0 with dE/dt = 20 mV s-1 [76],...

Figure 3 Potentiodynamic polarization curves from a 0.003% S austentic stainless steel measured in 1 M NaCl with capillaries of differing size. (From T. Suter, T. Peter, H. Bohni. Mater. Sci. Forum, 192-194, 25 (1995).)...

Figure 6.58 Potentiodynamic polarization curve and electrode potential values at which stress-corrosion cracking appears (Jones)5...

Figure 6.59 Potentiodynamic polarization curves showing especially the domain of suscep-tiblity to SCC1...

Potentiodynamic polarization curves for steel and A1 in solutions of interest ... 

Potentiodynamic polarization curves and the retrieved corrosion data for 316L stainless steel in 1% and 3% NaCl are shown in Figures 5-9. The polarization curves shown in Figures 5 and 6 indicated that the tendency of the stainless steel... 

In addition, the cathodic PCT technique offers an exceptionally powerful tool for understanding the kinetics of the cathode reaction, in the case where electrochemical reactions are self-enhanced over long periods of time ( 4 h) under the cathodic polarization. In contrast to the cathodic potentiodynamic polarization curves with a short measuring time ( 10 min), the cathodic PCTs allow observation of variations in the steady-state current with polarization time, which may provide valuable information when analyzing the reaction rate under cathodic polarization [120]. 

The anodic potentiodynamic polarization curve for zinc in 1 N NaOH is shown in Fig. 5.1(c). In this case, the curve again starts to rise due to diffusion polarization but rather suddenly decreases near -800 mV... 

Fig. 7.74 Schematic representation of the effect of scan rate on potentiodynamic polarization curve of an active-passivetype alloy and the range of potentials of predicted stress corrosion cracking...

Fig. 7.75 Potentiodynamic polarization curves at two scan rates for carbon steel in boiling 35% NaOH and potential range of cracking. Redrawn from Ref 112...

Measurements showed that, even in the chloride-free Na2S04 solution, MnS inclusions are dissolved [7]. Since the inclusions are dissolved rather slowly and no stable pitting occurs, the dissolution processes can be assigned to single inclusions quite well. Measurements at sites with inclusions are shown in Fig. 10. Two local potentiodynamic polarization curves of the steel DIN 1.4301 (0.017% S) were measured. In the case of an active 10 pm x 5 pm inclusion, the electrochemical current shows an abrupt increase over a limited potential range (shaded areas). To avoid the dissolution of an inactive 3 pm x 3 pm inclusion in the transpassive range, the measurement was stopped at 1000 mV. Subsequent optical microscopy studies of the same area revealed that the inclusion had been dissolved during the experiment. The SEM pictures indicate the nearly complete dissolution of the oval, active inclusion (Fig. 10, top left), whereas the inactive inclusion of a rounded shape (Fig. 10, bottom) did not dissolve at all. 

Local potentiodynamic polarization curves of the steel DIN 1.4301 (0.003% S) in a I M NaCl solution showed that the pitting potential does not have a constant value. With the usual large-area technique a value of about 300 mV is obtained. Diminishing the exposed surface to an area of 50 pm in diameter leads to an increase of the pitting potential to about 1200 mV (Fig. 11(a)). The pitting potential is usually considered to be independent of the area. There exist specific values for a particular combination of material and electrolyte. However, this work shows that the pitting potential also is an area-dependent value. 

Fig. 11. Local potentiodynamic polarization curves in 1 M NACI, dF/d/ diameter of the microcell (a) and the sulfur eontent (b) are varied.

Many different electrochemical and non-electrochemical techniques exist for the study of corrosion and many factors should be considered when selecting a technique. Corrosion rate can be determined by Tafel extrapolation from a potentiodynamic polarization curve. Corrosion rate can also be determined using the Stem-Geary equation from the polarization resistance derived from a linear polarization or an electrochemical impedance spectroscopy (EIS) experiment. Techniques have recently been developed to use electrochemical noise for the determination ofcorrosion rate. Suscephbility to localized corrosion is often assessed by the determination of a breakdown potenhal. Other techniques exist for the determinahon of localized corrosion propagahon rates. The various electrochemical techniques will be addressed in the next section, followed by a discussion of some nonelectrochemical techniques. 

---