2017 W.R. Whitney Award: Perspectives on Chloride Interactions with Passive Oxides and Oxide Film Breakdown

The nature of passivity and its breakdown have garnered great interest before and since Schonbein used the term “passivity” in 1836 to describe the “altered state” of iron. There has been a large body of experimental work, and a number of theories describing passivity and its breakdown leading to pitting corrosion have been proposed. However, there continues to be debate on this topic, which includes the discussion as to whether pit initiation is controlled by oxide film breakdown or by the pit growth kinetics. This communication will focus on oxide film breakdown without drawing any conclusions on the rate controlling step. As all currently proposed mechanisms require Cl− interactions for oxide film breakdown in Cl-containing environments, the question becomes what is the nature of the interaction of Cl− with the passive film, adsorption and/or incorporation, or neither? The interaction of Cl− with the passive film on pure aluminum and Type 316 stainless steel will be reviewed and summarized using available experimental data concerning Cl− interactions both from prior work at the Naval Research Laboratory and work reported in the literature. A point will be made that choosing the appropriate experimental procedure and data analysis is of great importance for getting high-fidelity data.