AbstractThe pitting potential Upis an important characteristic of every pitting system. It separates the region of passivity where the metal is practically immune from the region of heavy localized corrosion attack. On the basis of our own experimental results on pitting corrosion in the aluminum/halide‐system, the present paper rediscusses fundamental questions concerning the pitting potential and the pitting process in general.At or above Up, pitting of pure aluminum in chloride solutions is the result of the combined action of single, short‐life tunnelling events, which themselves propagate discontinuously. The same type of attack, to a minor extent, also occurs down to potential values 300 mV below Up. Therefore, the pitting potential is neither a lower limit for pit nucleation nor for pit growth, but appears to be the threshold value where the inactivation of tunnels is just balanced by tunnel nucleation. Taking the direct halogenation of the aluminum surface as the decisive step involved in tunnel nucleation, the pitting potential is given by the sum of the equilibrium potential of the aluminum/aluminum halide‐electrode and the overpotential necessary for an appropriate nucleation rate to gurantee stable pitting. This pitting overvoltage has been evaluated, using the nucleation concept by Volmer, to calculate tunnel nucleation rate and experimental results on the life time of single tunnels. Values of the pitting potential calculated on this basis agree well with experimental results in chloride‐, bromide‐and iodide solutions. Only the case of fluoride pitting is
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