Passivating potential polarization curves related

Though processes occurring under photopassivation have not so far been understood in detail, they may be related with certainty (Izidinov, 1979) to the acceleration, under illumination, of one of the two conjugated reactions, which constitute the overall process of electrochemical corrosion. Depending on the initial state of corroding silicon, either the anodic (at the active surface) or the cathodic (at the passive surface) partial reaction is accelerated. This leads to the shift of the potential, and the system jumps over the maximum of the polarization curve from one stable state to the other. 

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].

Reference has been made to the observation that both anionic and cationic species in the environment can influence the anodic polarization of active-passive types of metals and alloys. Specific examples have related to the effect of pH as it influences the stability and potential range of formation of oxide and related corrosion product films. The effect of pH, however, cannot be treated, even with single chemical species, independent of the accompanying anions. For example, chloride, sulfate, phosphate, and nitrate ions accompanying acids based on these ionic species will influence both the kinetics and thermodynamics of metal dissolution in addition to the effect of pH. Major effects may result if the anion either enhances or prevents formation of protective corrosion product films, or if an anion, both thermodynamically and kinetically, is an effective oxidizing species (easily reduced), then large changes in the measured anodic polarization curve will be observed. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

Figure 1-25. Thicknesses of the inner Fe(II), of the outer Fe(III), and d of the total layer of passive iron as a function of the electrode potential E (SHE) (StrehblowandSpeckmann, l984 Hanptetal., 1986). and the related potentiodynanuc polarization curve pi is the passivation potential, f2 the Flade potential insert phase model.

Fe(III). The polarization curve (insert) shows the clear relation of its features to the XPS results and the calculated values for the passivation potentials. For E > 0.2 V, the Fe(II) photoelectrons may be fully attenuated by the Fe(IlI) outer part and the presence of Fe(II) cannot be proved. The evaluation of the Ols signal indicates the presence of 45% of hydroxide for E = -0.9 V, decreasing continuously with increasing potential to less than 10% at E = 0.8 V. 

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