A Quantitative Framework Connecting the Critical Factors Determining the Stability of Localized Corrosion

Localized corrosion processes like pitting require the presence of a concentrated solution of cationic chlorides at the highly spatially discrete corroding surface. This chemistry is the consequence of the anodic dissolution of the metal and consequent cation hydrolysis and chloride migration from the bulk, leading to the development of a locally acidic environment that promotes continued corrosion. The minimum aggressive chemistry required to sustain pitting is therefore described in terms of the critical concentration of metal cations and pH. If these minimum conditions are not met, the chemistry in the pit is unfavorable towards continued dissolution and repassivation commences. The electrochemical nature of corrosion processes implies that there are also associated critical values for the dissolution flux and the potential characterizing this transition between stable dissolution and repassivation. These critical values represent threshold conditions for pit propagation and, if known, could be used as inputs for life prediction models based on the maximum damage size that can result from the exposure of a material to a localized corrosion environment. There has been much debate in the literature as to the mechanistic origin and measurement of these critical values. Although it is generally acknowledged that critical dissolution flux, critical solution chemistry, and critical potential all represent aspects of the same phenomenon, studies to date have investigated only one or a few of these factors without developing an integrated, quantitative relationship to connect them.
This dissertation addresses this knowledge gap in the literature by proposing a comprehensive, quantitative, and mechanistic framework relating the critical conditions for localized corrosion with a focus on stainless steels. The framework was structured upon experiments and modeling which employed the artificial pit technique, using 316L in chloride media as a candidate system. First, the steady state dissolution flux was modeled as a function of one-dimensional (1–D) pit geometry in order to quantify the phenomenology of pit stability with respect to pit depth and bulk electrolyte concentration. These results were utilized in designing 1-D artificial pit experiments to extract estimates of the critical dissolution current density and the critical potential – in terms of the Galvele pit stability product (i·x) and the repassivation potential Erp – at various pit depths. Mass transport modeling was then employed to determine the surface concentration at which repassivation commenced in the pit. The critical surface concentration of 50% saturation so obtained was observed to be in agreement with independent kinetics measurements which displayed a distinct transition from dissolution to repassivation at this value. The contribution of the local cathodic reaction towards pit stability was also studied using cation hydrolysis calculations which indicated that the critical pH was a key factor in inducing repassivation via oxide nucleation. The critical pH value for 316L was estimated in this manner to be 2.65, which was consistent with the effects of the individual anodic and cathodic kinetics at the estimated critical surface concentration. Mixed potential theory was utilized to rationalize these results in terms of the electrochemical processes accompanying repassivation. This study therefore demonstrated the development of a general, unified framework to quantitatively relate the critical factors controlling localized corrosion. The utility of such a framework lies in its extension to any system susceptible to localized corrosion, with particular value in its application towards predictive structural integrity analyses and corrosion inhibition strategies.