Role of Electrolyte Evolution on Damage Distributions and Rates during Atmospheric Corrosion Under Sodium Chloride Droplets

Abstract

Corrosion under sodium chloride droplets on copper is being studied via electrochemical modeling and experimentally characterized under rigorously controlled laboratory conditions. The objective of this work is to understand the governance of thin alkaline films that emanate from saline droplets upon corrosion initiation on corrosion rates and damage distributions. The radial spreading of thin alkaline films has been reported for both macroscopic and microscopic saline droplets on a variety of alloys that exhibit active corrosion under humid conditions [1-6]. This process is termed secondary spreading. It is resultant from oxygen reduction at cathode sites near the drop edge which produce concentrated hydroxide solution (e.g., NaOH) that subsequently spreads as a film over the metal surface [1,2,7,8]. These films are characteristically comprised of fields of microdroplets on the order of 100 μm diameter or less and can spread several times the original droplet radius from which they emanated. A number of investigators have characterized secondary spreading, demonstrating the potential for these films to serve as sites for oxygen reduction [7]. Unknown is the extent to which these secondary spreading films provide cathodic current to the corroding system and, in turn, their influence on corrosion rates and damage distributions. In this work, we explore the degree to which secondary spreading films influence corrosion that occurs under NaCl droplets on copper for a range of droplet diameters (50 µm to 1 mm). Our approach is to (1) develop a continuum-based model of the system (with non-continuum informed model extensions) to explore the influence of the film under various conditions and (2) define the physicochemical properties of the droplet-film system and experimentally realize the influence of the film on corrosion behavior to inform the model. Specifically, a transient, multiphysics finite element model is being constructed to predict current and potential distributions during the initial stage of corrosion over a range of droplet-film geometries and electrochemical parameters. Initial model assumptions include Nernst-Planck transport kinetics in non-dilute solution and the absence of heterogeneous reactions in the bulk electrolyte; both assumptions will be improved upon in subsequent iterations. Additionally, Butler-Volmer electrochemical kinetics is assumed, with oxygen reduction as the only cathodic reaction and metal oxidation as the only anodic reaction. The behavior of the droplet-film system is being experimentally studied via electrochemical capillary techniques and the physicochemical evolution of the droplet and film characterized to inform and benchmark the model. Key to the experimental efforts is the ability to rigorously control electrolyte geometry. To do so, a novel testing platform has been constructed that will allow production of varying size microdroplets and allow electrochemical probing within them. Experiments are currently under way to characterize the effect of restricted size secondary spreading regions on corrosion kinetics and damage distributions using this system. Together, this integrated computational and experimental approach seeks to deliver new mechanistic insight into atmospheric aerosol-driven corrosion.