Transition metals passive properties

More effort has probably been devoted to study of the corrosion and passivation properties of Fe-Cr-Ni alloys, e.g. stainless steel and other transition-metal alloys, than to any other metallic system [2.42, 2.44, 2.55, 2.56]. The type of spectral information obtainable from an Fe-Cr alloy of technical origin, carrying an oxide and contaminant film after corrosion, is shown schematically in Fig. 2.13 [2.57]. 

Although the transition metal ehaleogenides usually are quite refractory, direct reaction is feasible in many cases. In some cases (e.g., W and Mo), the oxide is volatile, making the surface at least accessible to reaction. In addition, the metals often have high rates of diffusion in the compounds, thus reducing the surface passivation effect of compound formation. This is probably because diffusion jumps are more probable in the presence of elements that can change their charge states. This property can be helpful in conversion of an oxide to a sulfide via H2S or CS2. 

Transition metal carbides and nitrides find broad interest in chemistry and technology. In the form of nanopowders they can be used in electronics and for catalysis. As catalysts they have similar properties to those of the expensive noble metals. They are extremely hard and therefore are often used in cutting tools or in harsh abrasive conditions. However, nanoparticulate nitrides and carbides are more reactive due to heir higher specific surface area. Some of them oxidize rapidly in air and are typically passivated with a thin layer of oxide before exposure to ambient conditions. 

Metals have long been used in magnetic applications because they exhibit the best magnetic saturation and remanent magnetization (1). As a result, the magnetic properties of single transition metals (Fe, Co, Ni) and their alloys (Fe-Co, Fe-Ni) have been studied extensively. These properties can be varied as the composition of the particles is varied. However, metals suffer from corrosion problems, and their use requires further processing to passivate the surface of the particles (20). 

Because of the presence of an oxide film, the dissolution rate of a passive metal at a given potential is much lower than that of an active metal. It depends mostly on the properties of the passive film and its solubility in the electrolyte. During passivation, which is a term used to describe the transition from the active to the passive state, the rate of dissolution therefore decreases abruptly. The polarization curve of a stainless steel in sulfuric acid, given in Figure 6.2, illustrates this phenomenon. In this electrolyte, the corrosion potential of the alloy is close to -0.3 V. Anodic polarization leads to active dissolution up to about -0.15 V, where the current density reaches a maximum. Beyond this point, the current density, and hence the dissolution rate, drops sharply. It then shows little further variation with potential up to about 1.1 V. Above that value the current density increases again because transpassive dissolution and oxidation of water to oxygen becomes possible. 

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