Localized Corrosion: An Empirical Approach to the Study of Passive Film Breakdown Rates

Electrochemical tests lasting 600 h to 6,000 h, at different potentials, were carried out on Type 316L (UNS S31603), Type 904L (UNS N08904), and nickel alloy 825 (UNS N08895) in sodium chloride (NaCl) solutions. A statistical approach was adopted using samples of 10 to 60 specimens. The pitting or crevice corrosion behavior was reported using cumulative frequency of passive film breakdown vs. time. In this work, we considered the formation of stable occluded cells as the breakdown of passive film. Each corrosion system “passive alloy-environment-geometry-time” was characterized by straight lines (i.e., breakdown rate of passive film, ρ_b), with a higher slope in the initial stage and a lower slope after some tens/hundreds of hours. The ρ_b can be used in the design for material selection and industrial maintenance schedules.