Anodic and Cathodic Limitations on Localized Corrosion and Stress Corrosion Cracking Propagation of Stainless Steel 304L in Atmospheric Environments

The formation of localized corrosion, characterized by the highly focused metallic dissolution of an otherwise passive alloy, is likely in marine environments. Localized dissolution is characterized as an autocatalytic process and requires a minimum concentration of metal chlorides, that undergo hydrolysis, destroying localized passivity. If these conditions are not met, repassivation will commence and corrosion will stifle. Anodic processes must be supported by cathodic reduction reactions on the external environment. In atmospheric scenarios, a finite water layer thickness combined with solution properties can provide significant ohmic drop in solution creating a finite cathode. At some point, localized corrosion features grow to a size that requires a cathode current that cannot be obtained by the surrounding material, creating a maximum pit size. Both anodic and cathodic processes are influenced by solution concentration, solution composition, and temperature, therefore, influencing the extent of corrosion on the surface of an alloy. Finally, solution properties are dictated by the exposure relative humidity. As the relative humidity changes both diurnally and seasonally, a wide range of solutions are possible in marine environments. This dissertation addresses environmental changes (relative humidity and temperature) on anodic and cathodic properties. These properties are combined in order to inform upon localized corrosion through analytical and Finite Element Modeling.

The main goals of this dissertation are to (i) develop a robust understanding of reaction mechanisms as a function of environmental parameters, (ii) utilize and enhance analytical predictive capabilities to understand governing factors and more accurately predict pitting corrosion damage, and (iii) utilize FEM modeling approach, electrochemical techniques, and pertinent instrumental characterization tools to systematically investigate the effect of electrolyte layer thickness, solution chemistry, materials surface properties, and geometry on localized corrosion and SCC damage distributions.

The goal of this dissertation was fulfilled by combining experimental and modeling techniques to inform upon the underlying corrosion mechanisms, and governing factors (including environmental factors) for localized corrosion of stainless steel 304L as a candidate system. First, cathodic reduction reaction mechanisms were determined in simulated atmospheric environments utilizing a rotating disk electrode and identified that the relative humidity dictates the reaction mechanism. It was also noted that corrosion can cause for a pH increase in the cathodic region which can further change the reaction mechanism and stifle cathodic currents. Additionally, a novel technique was introduced in order to visually inspect the surface of an alloy with in-situ Raman spectroscopy under cathodic control in thin film conditions. Second, anodic properties were assessed in similar environments in which critical parameters, namely the pit stability product and repassivation potential, were determined. Third, the water layer thickness was determined in common exposure environments and the boundary between thin film and bulk corrosion scenarios was identified as a function of relative humidity. These water layer thicknesses were compared to the boundary layer present in accelerated corrosion testing which was determine through the creation of a resistance-based sensor. Fourth, cathodic and anodic properties were combined with water layer thicknesses to predict localized corrosion. Various trends in cathodic kinetics were identified as a function of environment. Additionally, governing parameters for maximum pit sizes were identified for multiple scenarios. Localized corrosion predictions were validated based on long term exposure data in controlled environments. Finally, environmental influences on the electrochemical conditions were assessed for stress corrosion cracking in a Laplace-Equation based FEM modeling approach and highlighted the importance of considering the external environment on crack tip conditions.

Overall, this dissertation developed a fundamental understanding of the effect of environmental parameters on localized, pitting corrosion and stress corrosion cracking. The work provides for a complete means of predicting localized pitting corrosion of SS304L in marine environments as a function of relative humidity which can be used for diagnostic measures. Electrochemical characterization in this dissertation provides meaningful information for better understanding of corrosion processes and identifies governing factors (both experimental and modeling techniques) of corrosion processes.