19. Cathodic and Anodic Protection

The current flowing during measurement is given by the ratio of the potential difference and the circuit resistance which practically coincides with the internal impedance of the voltmeter. If, for example, the reference electrode has a surface area of 10 cm² and the potential difference is 1 V with a voltmeter impedance of 1 MΩ, current density exchanged on the reference electrode surface is 1 mA/m² which is less than the exchange current density of active metals like copper, zinc and iron, but is greater by at least one order of magnitude than the passivity current density of Ti. Therefore, when using a small size MMO activated Ti as reference electrode, to avoid its polarisation the voltmeter impedance must be greater than 10 MΩ for short measurements and higher than 1 GΩ for continuous monitoring.

Piontelli wrote: «Volta’s attitude against the ‘chemical theories’ must be considered remembering that the chemical phenomena considered at this epoch very often were ‘local processes’ at the electrodes». [3] According to Piontelli many of the theories of the pile given after Volta where affected by the confusion between local chemical phenomena at the metal surface/solution and the chemical reaction connected with the current.

Fabbroni wrote: «J’avois observé aussi que l’alliage employé a la soudure des plaques de cuivre qui couvrent le toit mobile de l’Observatoire de Florence, s’étoit promtement altéré, chargé manifestement en oxide blanc à son conctact extreme avec ce métal. J’avoix appris [..], en Angleterre, que le clous de fer dont on se servoit autrefois pour assujettir le feuilles de cuivre employées au doublage des vaisseaux, les rongeoient tellement par leur conctact que bientôt le trou étoit dilaté, jusqu’à surpasser la tête de clou qui le retenoit. Il me parut qui il n’en falloit pas davantage pour reconnoître que les métaux exerçoient dans set cas un action réciproque, et que c’etoit elle qu’on devoit attribuer le cause des phénomènes qui s’opéroient par leur réunion ou contact». With observations like these Fabbroni laid the foundations both of the chemical theory of galvanism and of the electrochemical theory of corrosion.