Corrosion kinetics polarization behavior

This chapter outlines the basic aspects of interfacial electrochemical polarization and their relevance to corrosion. A discussion of the theoretical aspects of electrode kinetics lays a foundation for the understanding of the electrochemical nature of corrosion. Topics include mixed potential theory, reversible electrode potential, exchange current density, corrosion potential, corrosion current, and Tafel slopes. The theoretical treatment of electrochemistry in this chapter is focused on electrode kinetics, polarization behavior, mass transfer effects, and their relevance to corrosion. Analysis and solved corrosion problems are designed to understand the mechanisms of corrosion processes, learn how to control corrosion rates, and evaluate the protection strategies at the metal-solution interface [1-7]. 

Figure 11 Schematic Evans diagram illustrating the effect of a change in the cathodic reaction kinetics on the corrosion conditions. Case 1 would be representative of Fig. 5. Case 3 would lead to the polarization behavior described in Fig. 6. Case 2 would lead to the polarization behavior shown in Fig. 8. (After Ref. 71.)...

Polarization behavior relates to the kinetics of electrochemical processes. Study of the phenomenon requires techniques for simultaneously measuring electrode potentials and current densities and developing empirical and theoretical relationships between the two. Before examining some of the simple theories, experimental techniques, and interpretations of the observed relationships, it is useful to characterize the polarization behavior of several of the important electrochemical reactions involved in corrosion processes. 

Chapters 1 to 3 describe the theory of corrosion engineering and offer analyzed case studies and solved problems in the thermodynamics of corrosion processes, the relevance of electrochemical kinetics to corrosion, low field approximation theory, concentration polarization, the effects of polarization behavior on corrosion rate, the effect of mass transfer on electrode kinetics, and diffusion-limited corrosion rates. 

I , and Ij., respectively these are both assumed to obey Tafel kinetics. The rates of these two reactions are equal to /q at the corrosion potential, Ecorr- When the potential is changed to a more positive value appb the rate of the normal anodic partial reaction would be expected to increase along the curve marked to the value /Mg,e and simultaneously the normal cathodic reaction would be expected to decrease along curve Ij. to the value /n.e- This is true of the normal electrochemical polarization behavior of most metals, e.g. iron and steels. 

In Chapter 4, analysis of the kinetics of coupled half-cell reactions shows how the corrosion potential and corrosion current density depend on the positions of the anodic and cathodic polarization curves. The anodic polarization curves are generally represented as showing linear or Tafel behavior, and the cathodic curves are shown with both Tafel and... 

Evaluation of corrosion behavior is normally done through a function that depends on kinetic parameters depicted in Figure 3.2. Hence, the current density function for polarizing an electrode irreversibly from the corrosion potential is similar to eq. (3.8). Hence,... 

This chapter is coniined to analyze the complex aqueous corrosion phenomaion using the principles of mixed-potential, which in turn is related to the mixed electrode electrochemical corrosion process. This theory has been introduced in Chapter 3 and 4 as oxidation and reduction electrochemical reactions. Basically, this Chapter is an extension of the principles of electrochemistry, in which partial reactions were introduced as half-cell reactions, and their related kinetics were related to activation and concentration polarization processes. The principles and concepts introduced in this chapter represent a unique and yet, simplified approach for understanding the electrochemical behavior of corrosion (oxidation) and reduction reactions in simple electrochemical systems. 

Damborenea (1995) also studied the behavior of coated steels in aqueous media (NaCl 0.6 M and HCl 2 M) through voltametry experiments. It is observed that the values for the corrosion potential measured as a function of time range between 220 and 140 mV in the case of coated steels, and between -200 and -50 mV on uncoated steel. However, the kinetic measurements using complex impedance and polarization resistance showed that the resistance to corrosion diminished notably over time, with the R.p values reaching the same levels as those of naked steel after 24 h (Fig. 19-11). This downturn revealed the presence of defects or pores in the coating, which permitted the movement of ions and, consequently, the contact of the environment with the metal substrate. 

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,... 

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