Totally Irreversible Dissolution of Metals

The systems described so far have involved metals in solutions of their corresponding ions. In such systems, in principle, true (thermodynamic) electrochemical equilibria can be established, and departure of the potential from the equilibrium value results in a more or less reversible cathodic deposition of anodic dissolution. Both cathodic and anodic partial currents are present at any instant of time and undergo a continuous change with potential, leading to a situation when one becomes negligible compared to the other. 

In practice, however, systems are often encountered in which the solution initially does not contain corresponding ions, and for that or some other reason, the cathodic partial current is negligible at any potential. In such cases, the system is not thermodynamically defined, and the concept of reversible potential cannot be used. Nevertheless, in many cases, well-defined steady states are established, giving stable potential values to the extent that some such systems are used as the most reproducible reference electrodes (e.g., Ag, AgCl, Cr Hg, Hg2Cl2, CF Hg, Hg2S04, S04 -. etc.). 

In others, well-defined steady states enable, e.g., the quantitative treatment of corrosion problems to be developed. Such situations are, therefore, important and will be given some attention here. 

---