"Mechanisms Governing the Passive Film Breakdown and Pit Stabilization in Austenitic Stainless Steel" 

Thang Duc Nguyen, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

Chloride-induced stress corrosion cracking (CISCC) is a degradation mode in austenitic stainless steel exposed to chloride-containing environments under tensile stress. In this process, localized pitting corrosion typically acts as a stress concentrator, inducing crack initiation and propagation. Accordingly, this paper focuses on the initial stage of pitting corrosion, emphasizing the breakdown of the passive film and the dynamic cycle of film rupture and repassivation, ultimately leading to pit formation. To understand the mechanism governing passive film breakdown, the structural, chemical, and electronic characteristics of the oxide scale on stainless steel must be considered. The passive film typically exhibits a duplex structure, with a chromium-rich inner layer and an iron-rich outer layer, both of which exhibit semiconducting behavior. Several mechanisms have been proposed to explain passive film breakdown, including (i) chloride ion penetration, (ii) mechanical rupture driven by applied or residual stress or electrical failure due to the semiconductor behavior of the oxide scale, and (iii) chloride ion adsorption at the film/electrolyte interface. Furthermore, the transition from metastable to stable pit growth has been described using diffusion-controlled models, highlighting the critical role of the corrosion product concentration within the pit. The corrosion product, primarily metal cations, is considered a factor in establishing a locally aggressive electrolyte and in attracting chloride ions from the bulk solution to maintain electrochemical neutrality within the pit. This mechanism is further supported by the presence of a lacy metal cover at the pit mouth and chloride precipitation at the pit bottom, both of which act as diffusion barriers that maintain a high cation concentration, thereby preventing repassivation until the pit reaches its critical depth for propagation. Finally, studying the evolution of the oxide layer during pitting corrosion will not only reveal the breakdown mechanism but also the pit-to-crack transition and crack propagation driven by the coupled electrochemical and mechanical effects of the oxide at the crack tip.