Film Growth and chloride penetration on austenitic stainless steels in NaCl solution

For austenitic stainless steels potentiodynamically polarized in 0.6 M NaCl solution, a film growth equation was derived using the generalized Fick's first law. This law consists of two terms of ionic transport by diffusion and migration. During potentiodynamic polarization, the ionic transport by diffusion can be neglected when the potential across surface film is relatively large. Then, the ionic fluxes for film growth depend only on the cation concentration at the metal/film interface, the oxygen ion concentration at the film/solution interface and the electric field across the film. It was shown that the film thickness was linearly proportional to the potential drop across the film and consequently the electric field across the film would be constant during potentiodynamic polarization. When the cation and oxygen ion concentrations are assumed to be constant, the fluxes of ions are also constant without regard to the film thickness or potential drop across the film. Finally, an equation for accumulated chloride ion concentration at the metal/film interface was derived. Under the assumptions adopted in this study, the accumulated chloride concentration at metal/film interface is linearly increasing with the applied potential during potentiodynamic polarization, which is consistent with the surface analysis results on 316L stainless steel by ToF SIMS (time of flight secondary ion mass spectrometry).