Electrode potential, effect dissolution mechanism

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

The local change of pH is obtained only if the convection is stopped and can have either beneficial (dissolution of films) or detrimental effect (precipitation of hydroxides precipitation of reaction intermediates) depending on the composition of the solution. Polarization to very positive potentials causes corrosion of the electrode itself (formation of oxide films, disintegration of carbon) and should be avoided. It seems, that the best results are obtained with automatically applied potentlostatic pulses and a measuring period taking place at a defined time after the pulse application. Electrochemical activation is usually combined with a periodical mechanical cleaning. 

Many individual controlled-potential experiments were performed using different electrode rotation rates and in electrolytes of different composition, pH and temperature. In this way, this set of data allowed the determination of the effects of these parameters and potential upon the kinetics and mechanism of magnetite dissolution. 

---