Halide Nuclei Theory of Pit Initiation in Passive Metals

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Abstract
A kinetic approach is used to test the theory of pit initiation in passive metals in contact with a solution containing aggressive ions. A new process, hemispherical halide nuclei formation at the surface of the passive film and inward growth under restricted conditions, is introduced. The possibility of nuclei growth is discussed, taking into account the irreversible thermodynamic stability of the system. The direction of the irreversible change of a system consisting of the metal substrate, the passive film, the halide nuclei, and the solution is determined using the general evolution criterion of irreversible thermodynamics proposed by Glansdorff and Priogogine. A critical radius of the halide nucleus r* is deduced. This radius must be surpassed for growth and pitting to take place. The present theory explains the nature of the pit initiation processes, especially some characteristic values of the pitting phenomenon, such as the critical pitting potential and the induction period of pit nucleation.

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