Electrochemical metal dissolution kinetics

Almost aU metals corrode, but many metals corrode very slowly under normal environmental conditions, due in part to kinetic limitations of the metal dissolution reaction. Thus, the rate of metal corrosion can be anticipated and controlled by developing kinetic rate expressions for metal oxidation reactions. There is a major difference, however, between classical electrochemical metal dissolution kinetics and metal dissolution in a corrosion system, that difference being the occurrence of one or more oxidation and reduction reactions on the same metal. 

The rate of electrochemical metal dissolution kinetics is characterised by the corrosion ciurent, j, which is usually measured in terms of mA/em and converted... 

Kiss, L. (1988), Kinetics of Electrochemical Metal Dissolution. Amsterdam Elsevier. 

Metallic corrosion occurs because of the coupling of two different electrochemical reactions on the material surface. If, as assumed in the discussion of iron dissolution kinetics above, only iron oxidation and reduction were possible, the conservation of charge would require that in the absence of external polarization, the iron be in thermodynamic equilibrium. Under those conditions, no net dissolution would occur. In real systems, that assumption is invalid, and metallic dissolution occurs with regularity, keeping corrosionists employed and off the street. 

In this expression, i is current density, p is density, n is the number of electron equivalents per mole of dissolved metal, M is the atomic weight of the metal, F is Faraday s constant, r is pit radius, and t is time. The advantage of this technique is that a direct determination of the dissolution kinetics is obtained. A direct determination of this type is not possible by electrochemical methods, in which the current recorded is a net current representing the difference between the anodic and the cathodic reaction rates. In fact, a comparison of this nonelectrochemical growth rate determination with a comparable electrochemical growth rate determination shows that the partial cathodic current due to proton reduction in a growing pit in A1 is about 15% of the total anodic current (26). 

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. 

Although the author believes that the generalized concept was originally responsible for the electrochemical treatment of corrosion processes by the early workers, it appears that Hammett and Lorch (23) and Frumkin (24) were among the first to specifically describe metallic dissolution according to this concept. Wagner and Traud (16) showed that the electrode kinetics for hydrogen evolution are not affected by the simultaneous dissolution of the metallic ions. 

The potential at which the current for multiple electrochemical reactions is equal to zero is termed the mixed potential or, in the case of metal dissolution, the corrosion potential. Concepts of thermodynamics, kinetics, and transport must be applied to calculate values for the mixed or corrosion potential. 

The best-known examples of electrochemical oscillators are reactions involving the anodic dissolution of a metal in acidic solution. With the exception of the complex bifurcation scenarios observed during Cu dissolution, they have not yet been discussed in this chapter. This is because their kinetics are much more complicated than those of the examples reviewed. Thus, despite the fact that oscillatory metal dissolution reactions have been an intense subject of research over decades, there does not seem to be a single example where the reaction mechanism is identified unambiguously and understood in depth. This is for the most part due to complicated passivation and reactivation kinetics which involve the for-... 

Chapters 18-21 discuss core-shell and advanced Pt alloy catalysts (which also can be considered to have a core-shell structure). Chapter 18 studies the fundamentals of Pt core-shell catalysts synthesized by selective removal of transition metals from transition metal-rich Pt alloys. Chapter 19 outlines the advances of core-shell catalysts synthesized by both electrochemical and chemical methods. The performance, durability, and challenges of core-shell catalyst in fuel cell applications are also discussed. Chapter 20 reviews the recent analyses of the various aspects intrinsic to the core-shell structure including surface segregation, metal dissolution, and catalytic activity, using DFT, molecular dynamics, and kinetic Monte Carlo. Chapter 21 presents the recent understanding of activity dependences on specific sites and local strains in the surface of bulk and core-shell nanoparticle based on DFT calculation results. 

In conclusion, the present discussion of proposed film breakdown and pit initiation mechanisms suggests that several phenomena are responsible for the loss of passivity and the onset of pitting when a metal is polarized to a high potential in presence of aggressive anions. Structural defects in the passive film reflecting those of the metal, anion adsorption on the film and the metal surfaces and the effect of anions on the kinetics of the electrochemical reactions governing oxide formation and metal dissolution are most critical. Practical consequences of these phenomena for pitting corrosion will be discussed in Section 7.3. 

The corrosion rate depends on the electrode kinetics of both partial reactions. If all of the electrochemical parameters of the anodic and cathodic partial reactions are known, in principle the rate may be predicted. According to Faraday s law, a linear relationship exists between the metal dissolution rate at any potential Vm the partial anodic current density for metal dissolution... 

Grygar T (1998) Phonomenological kinetics of irreversible electrochemical dissolution of metal-oxide microparticles. J Solid State Electrochem 2 127-136. 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Under -> open-circuit conditions a possible passivation depends seriously on the environment, i.e., the pH of the solution and the potential of the redox system which is present within the electrolyte and its kinetics. For electrochemical studies redox systems are replaced by a -> potentiostat. Thus one may study the passivating properties of the metal independently of the thermodynamic or kinetic properties of the redox system. However, if a metal is passivated in a solution at open-circuit conditions the cathodic current density of the redox system has to exceed the maximum anodic dissolution current density of the metal to shift the electrode potential into the passive range (Fig. 1 of the next entry (- passivation potential)). In the case of iron, concentrated nitric acid will passivate the metal surface whereas diluted nitric acid does not passivate. However, diluted nitric acid may sustain passivity if the metal has been passivated before by other means. Thus redox systems may induce or only maintain passivity depending on their electrode potential and the kinetics of their reduction. In consequence, it depends on the characteristics of metal disso-... 

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