Basic Passive Corrosion Kinetics

An important special case of complex kinetics is the simultaneous action of basic passive laws together with active corrosion. A typical case for this is when a scale grows but is consumed at the same time by evaporation. The resulting shape of curves is shown in Fig. 7 and has been described as para-linear [23] behavior. It may be analyzed with Eq. (12) omitting the logarithmic term. 

Resistance of ceramics to corrosion is due to one of three basic behaviors immunity, passivation, or kinetically limited corrosion. When a ceramic is thermodynamically incapable of spontaneous reaction with its environment it is referred to as having immunity. When the necessary thermodynamic data are available this type of corrosion resistance may be predicted by calculation. Metals, except for the precious metals such as gold, do not exhibit immunity. 

Chapter 1 provides an introduction to some of the basic terms and concepts of electrochemistry and corrosion and provides a detailed overview of the remainder of the book. Chapter 2 provides an overview of the important thermodynamic and kinetic parameters of relevance to corrosion electrochemistry. Chapter 3 focuses on what might be viewed as an aberration from normal dissolution kinetics passivity. This aberration—or peculiar condition, as Faraday referred to it— is critical to the use of stainless steels, aluminum alloys, and all the so-called corrosion resistant alloys (CRAs). 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Reaction (25) would suggest a passivating behavior but the solubility of silica is favored even at low temperatures in the alkahne water present due to the dissolution of NH3. Hence basic reaction kinetics are linear, it is a form of active corrosion. 

Although the basic organization of the book is unchanged from the previous edition, there is in this edition a separate chapter on Pourbaix diagrams, very useful tools that indicate the thermodynamic potential-pH domains of corrosion, passivity, and immunity to corrosion. A consideration of the relevant Pourbaix diagrams can be a useful starting point in many corrosion studies and investigations. As always in corrosion, as well as in this book, there is the dual importance of thermodynamics (In which direction does the reaction go Chapters 3 and 4) and kinetics (How fast does it go Chapter 5). 

This chapter examined gas-solid kinetic processes. We saw how to apply the basic tools we learned in calculating thermodynamic driving forces (Chapter 2), reaction rates (Chapter 3), and mass diffusion (Chapter 4) to understand and model a number of important gas-solid kinetic processes including adsorption/desorption, active gas corrosion, chemical vapor deposition, and passive oxidation. The main points introduced in this chapter include ... 

The pH of the electrolyte does not only have an effect on the passivation potential, but also on the passivation current density, because both the metal dissolution kinetics and the solubility of hydroxides depend on pH. Figure 6.16 shows that the passivation current density of iron becomes smaller at higher pH. This has been explained by a lowering of the solubility of ferrous hydroxide, which precipitates at the surface. Since both the passivation potential and the passivation current density decrease with increasing pH, spontaneous passivation of iron becomes possible in basic, aerated media. This explains why steel reinforcements in concrete (pH >13) resist corrosion well as long as chemical reactions with carbon dioxide from air (carbonation of concrete) do not modify the alkalinity. 

M. Prazak constructed an original electronic potentiostat which became the basic instrument for study of corrosion. It was used, for example, for following corrosion of corrosion-proof steels and alloys [122-124]. The effects of temperature on potentials of metals, on kinetics of their dissolution, and on activation energy of iron passivation in sulfuric acid were tested in the institute [125-130]. 

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

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