Halide Nuclei Theory of Pit Initiation in Passive Metals

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 surfac.e 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 filml 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. ABSTRACTPolarization behavior of the Ni-200 electrode in 75% Na2S-25% FeS at 1000~ under H2-0.32% H2S at 1 atm is linear over an overpotential range of -600 -< W -< 600 mV and is consistent with resistance polarization in the electrolyte. Similar polarization behavior of the 304 stainless steel electrode and of the Ni-200 electrode in 85% NasS-15% NiS suggests that the charge transfer mechanism is a sulfide/polysulfide exchange reaction. Nonelectrodic dissolution of nickel in the 75% Na2S-25%

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