Alloys, anodic behavior current-potential curves

Botella, R, Frayret, J., Jaszay, T, et al. (2003). Experimental study, via current-potential curves, of the anodic behavior of Alloy C-276 and T60 titanium in chlorinated and oxygenated aqueous media under sub- to supercritical conditions, J. Supercrit. Fluid, 25, pp. 269-278. 

The electrochemical behavior of metals and alloys which are passivated can be conveniently described by the schematic current-potential curve shown in Figure 3-1. Such a curve is also referred to as a poten-tiokinetic or anodic polarization curve. Three regions can be distinguished on this curve. The first region corresponds to the active state of the metal, where anodic dissolution takes place according to the following global reaction... 

Electrochemical tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

Another example of a polarization curve is the anodic polarization behavior of metals and alloys in aqueous solutions when they are passivated. Figure 4.3.14 shows the polarization curve of a carbon steel in dilute NaOH solution, obtained by using the potential step method. The anodic current density increased with increasing polarization... 

The behavior is illustrated by the anodic polarization curves of two gold-copper alloys and the pure metals shown in Figure 7.29, which were measured in a concentrated chloride electrolyte capable of dissolving copper as well as gold. The value of the critical potential increases with the noble-metal content in the alloy. The current in the subcritical potential region is lower at higher gold concentration. 

The potentiodynamic technique is used to examine the passivation behavior of a metal or alloy in an electrochemical system. During the potentiodynamic scan, the metal surface may undergo several different electrochemical reactions, wherein the anodic current may vary over several orders of magnitude [34]. Generally, analysis of the anodic curve can provide potentials for active, passive, transpassive, and repassive zones a rough estimation of corrosion current and corrosion potential and a measure of the stability of passivity. Moreover, one can determine whether the passivation is spontaneous or needs to be polarized to induce passivation. In addition, one can determine whether the electrochemical system can induce a spontaneous transition from passive... 

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