Oxide films continued passive

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

Passivation under a wide range of operating conditions and in all boiler designs is a goal of all BW treatment programs. It is achieved through the formation of a stable, continuous, uniform, self-limited, and nonporous magnetite film. (This objective also applies to the maintenance of cuprous oxide, the copper equivalent of a film of passive iron.)... 

Continuous (barrier, passivation) films have a high resistivity (106Q cm or more), with a maximum thickness of 10 4cm. During their formation, the metal cation does not enter the solution, but rather oxidation occurs at the metal-film interface. Oxide films at tantalum, zirconium, aluminium and niobium are examples of these films. 

It was mentioned above that a chromium anode continues to dissolve in the passive state forming chromate ions in this case the invisible oxide film may be suflSciently porous to allow ions to penetrate it alternatively, the oxide film may become oxidized to C1O3 which dissolves to form chromate, but is immediately regenerated by oxidation of the anode. 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

Moreover, gradients of stoichiometry limit the accuracy of thickness determinations that often only refer to parts of the passive film. Many oxide films show an increase of oxidation state from metal to the electrolyte, for example, on Cu [13]. In case of passive iron, many traditional techniques only evaluate the thickness of the outer Fe203-film, but not that of the inner Fc3 04-layer. While Vetter [28] described the film by a duplex model, Wagner [14] presented a model with continuous change of stoichiometry. 

As a final note it must be stressed that laboratory static tests should only be used to eliminate dangerous metal-metal combinations, and not as an acceptance criterion. Fretting may substantially alter the properties of the interface, by continuously removing the passive oxide film, thereby inducing severe attack of a metal which, otherwise, would remain unaffected. 

The oxide film is often the first line of defense a material has against the corrosive influences of its environment. A passive oxide film forms upon oxygen adsorption from the environment. These films can be beneficial, serving to slow or block metal dissolution and further corrosion. This effect is called oxide passivation. On the other hand, metals may continue to oxidize/corrode, even after an oxide film has formed or this film may break down leading to active corrosion. Because of these drastically different outcomes, an understanding of how these films are formed under different environmental conditions is needed to understand how to mitigate corrosion. The focus of this chapter is the use of quantum mechanics simulations to understand the thermodynamics of passive film formation. In particular, we look at calculations of first-principles phase diagrams as a function of environmental conditions and how these studies fit with experimental data. We focus on Pd, Mg, and Pt metal systems as they are well studied, represent different types of oxide film formation, and fall within the authors areas of expertise. 

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