A derivation of the polarization curve of alloys from those of the single metallic components is shown. At mixed metallic crystals of irregular lattice distribution the rate of dissolution of the alloy in the active state approaches that of the main metallic component of lowest rate of dissolution within certain limits of metal composition. This behavior is similar to the chemical attack on mixed crystals of a regular lattice distribution of the components. The exact validity of the present theory requires that the free energy of the metals involved is not essentially changed by alloying. The maximum of the polarization curve of a binary alloy is closely related to the crosspoint between the decreasing branch of the polarization curve of one metallic component and the increasing branch of the polarization curve of the other. In the passive state the oxide component of lowest solubility accumulates and decides on the rate of dissolution.
These conclusions have been corroborated by a comparison of polarization curves of ferritic and austenitic stainless steels with the curves of their components. In addition, some features of the mechanism involved in the growth of the passive film have been revealed. In both the active and passive states the metal of lowest rate of dissolution controls the dissolution of the alloy, if the alloy composition is properly selected.
3.8.3, 2.3.5, 6.2.5
