Active metals passivation phenomena

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. 

The phenomenon of metal passivation, or sudden electrochemical potential-induced transition from an active dissolution state to a passive state, is responsible for the low corrosion rates observed in many metals and alloys. This condition has usually been attributed to the electrochemical formation of metal-oxide protection films or coverage of the surface by corrosion films. In either case, the metal becomes partially protected from the environment, and the corrosion current drops sharply. This condition is of enormous practical importance, as the integrity of metallic structures of great structural significance can be maintained by keeping their electrochemical potenticds at some predetermined ("passivation") value. 

Figure 14.11 demonstrates the relationship between mass change of CVD SiC and Po at 1873 K in Ar-02. P02 increased step by step with time from 16.7 to 160 Pa. Mass loss (active oxidation) occurred up to Po. = 146 Pa, and at Po = 160 Pa it drastically changed to slight gain of mass (passive oxidation). The Po for the transition from active to passive oxidation ( 02) is 160 Pa at 1873 K. This transition phenomenon can be commonly observed in the oxidation of Si-based ceramics and many kinds of metals. 

The sudden transition of a metal-solution interface from a state of active dissolution to the passive state is a phenomenon of great scientific and technological interest. This transition has been attributed to the formation of either a monolayer (or less) of adsorbed oxygen on the surface or to the coverage of the surface by a three-dimensional corrosion product film, hi either case, the reactive metal is shielded from the aqueous environment, and the current drops sharply to a low value that is determined by the movement of ions or vacancies across the film. 

According to equation (6.2), the anodic formation of oxides is the result of a reaction between the metal and water. Thus, the smaller the activity of water, the more difficult it becomes to passivate a metal. Figure 6.17 highlights this phenomenon in an acetic acid/acetate/water mixture, the passivation current density decreases sharply when the proportion of water increases from 10 to 50%. If there is no water, no passive film is formed. Non-aqueous corrosive environments are encountered mostly in the chemical industry. 

In addition, the volumetric expansion of the lithiated particle is such that a phenomenon of electrochemical prrlverization occurs, whereby large particles crumble into smaller particles. Unpassivized active surfaces are thus created during the cotrrse of cycling, leading to the continuous formation of a passivation film, which consiunes electrolyte and electrons (irreversible capacity). This is particularly true for metals accepting lithium insertion below 0.8 V versus aHA such as Sn, Al, Si, etc. 

Imagine what would happen if the structure is now polarized in the opposite direction. It would amount to polarizing the potential of the anode to that of cathode in the positive direction. Theoretically, such a practice should result in creating corrosion rather than protection. But for some metals, positive polarization forms a protective oxide/hydroxide surface film and this phenomenon of passivation for a limited number of metals results in retardation of corrosion. By this method called anodic protection, it is possible to passivate active-passive metals. Metals, such as iron, chromium and nickel are passivated by anodic polarization, which leads to retardation of corrosion. The potential of this must, however, be maintained in the region of passivity by a potentiostat. Anodic protection is widely applied in transport of acids and corrosives in containers and other applications. 

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