Temporal Instabilities in Corrosion Processes
In this chapter, temporal dynamic instabilities in the electrode processes associated with anodic electrodissolution of metals and semiconductors are described. Oscillations in these processes are generally associated with the periodic buildup and destruc
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Temporal Instabilities in Corrosion Processes
6.1 6.1.1
Oscillations in Anodic Dissolution of Metal Electrodes General Characteristics of Passivation/Dissolution Processes
Oscillatory dissolution of metal electrodes belongs to the earliest known examples of the nonlinear dynamic phenomena, reported already in the first half of nineteenth century, as it follows from work of Fechner, dated 1828 [1]. In this section, representative examples of such corrosion systems are briefly described. Currently known examples of the corrosion processes that can exhibit oscillatory dynamics include, among others, the following experimental systems: Fe/H2SO4; Fe/NaClO3; Fe/HNO3, Fe/NaCl; Cu/H3PO4; Cu/HCl; Cu/NaCl, Cu/NaClO3, Ni/H2SO4; Ag/KCN þ K2CO3; Ag/HClO4, Ag/HCl; Al (including alloys)/KOH; Au/H2SO4 þ Cl, Co/HCl þ CrO3, Co/H3PO4, Co/H2SO4 þ HNO3; Pb/LiAlCl4; Pb/H2SO4, Pb/NaOH; Sn/NaOH; Zn/NaOH; Ti/H2SO4; Bi/NaCl; Cd/KOH; Nb/HBF4; and W/NaOH, V/H3PO4. Reviews of these phenomena published up to 1994 include, among others, papers by Wojtowicz [2], Hudson and Bassett [3], and Tsotsis and Hudson [4]. Figure 6.1, taken from [2], collects some representative experimentally recorded current or potential oscillations for such systems, as well as, for comparison, examples of oscillations disscussed in Chapter 5. Early discoveries of oscillatory corrosion processes were made during the simple measurements of the I–U polarization curves, measured in a two-electrode arrangement, so the ohmic potential drops in the electric circuit were not compensated. Typical polarization curves for such systems are characterized with the abrupt decrease of the anodic current, caused by the passivation effect, at the potentials close to the Flade potential (EF). In the simplest description of those processes, a dissolution of metal electrode to ions in the solution is represented by reaction: M ! Mnþ ðaqÞ þ ne
M. Orlik, Self-Organization in Electrochemical Systems I, Monographs in Electrochemistry, DOI 10.1007/978-3-642-27673-6_6, # Springer-Verlag Berlin Heidelberg 2012
(6.1)
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6 Temporal Instabilities in Corrosion Processes
Fig. 6.1 Examples of electrochemical oscillations: (a) Fe/1 N H2SO4, EH ¼ þ0.49 V, (b) Au/4 N HCl, EH ¼ þ1.8 V, (c) Zn/4 N NaOH, EH ¼ 1.1 V, (d) Fe/14 N HNO3, I ¼ 40 mA/cm2, (e) Cu/ 0.4 N CrO3 þ 1 N HCl (rest potential), (f) Pt–Pt/1 M CH2O þ 3.75 M H2SO4, I ¼ 30 mA/cm2, (g) Pt/0.1 M N2H4 þ 1 M KOH (rest potential), (h) Pd/1 M HCOOK þ 1 M KOH, I ¼ þ0.2 mA/cm2 (after [2] and references cited therein). Reprinted from [2] with kind permission from Springer Science +Business Media B.V.
characterized with the formal potential E10, while the process of the formation of the passive oxide layer can be described with the reaction scheme: M þ ðn=2ÞH2 O ! MOn=2 þ nHþ þ ne
(6.2)
which is characterized with the respective value of E20, more positive than E10. In terms of this notation, the thermodynamically determined Flade potential can be considered as equal to equilibrium potential of the reaction (6.2), and at 298.15 K: EF
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