(Corrosion Division Morris Cohen Graduate Student Award Address) A Systematic Study of Critical Parameters Affecting the Galvanic Corrosion between Al Alloy and Stainless Steel in Atmospheric Condition
Authors: Liu Chao (Gilbert) 
Published: December 2020

Connections between dissimilar materials are frequently encountered in complex structures involved in the aerospace industry. In a typical aircraft structure, one of the most common couples encountered is that of an aluminum-alloy-based airframe component and noble fasteners such as stainless steel. When this type of galvanic couple is exposed to a atmospheric marine environment, it can develop cracks that pose increased risk and require costly maintenance and repair. In this work, a complementary modeling and experimental approach is applied to systematically investigate the effect of electrolyte layer thickness, solution chemistry and materials surface properties on the electrochemical and corrosion distributions in the galvanic coupling between AA7050 and SS316L, and capture the underlying mechanism of the dependence of corrosion distributions on each external variable.
To validate the robustness of our modeling approach, a Laplace-Equation based modeling coupled using mathematically-fitted electrochemical kinetics from experiment as boundary conditions was applied to simulate galvanic corrosion between Zn plate and SS 316 rods under thin layer electrolyte conditions. The modeling results were compared with experimental results from a four-day modified B117 test. It shows that: electrolyte layer thickness (WL)=3,500~4, 000 𝜇m would be the appropriate WL range formed on the sample surface during modified B117 salt fog testing; the robustness of Laplace-Equation based modeling is highly dependent on the boundary conditions, experimentally-determined electrochemical kinetics as boundary conditions can truly reflect the electrochemical response of anode/cathode to the surrounding solution environment, which in turn yields the simulated results mostly close to experimental measurement.
In the study of the effect of electrolyte layer thickness, the total cathode current capacity of a surface on the electrolyte film thickness and cathode size in a galvanic couple to support the corrosion of AA7050 was accessed. The total cathode current capacity was studied over the range of WL thickness from full immersion conditions (where the total cathode current scales directly with cathode size) to the thin film regime in which the WL is the diffusion boundary layer thickness. In order to fully assess the transition from thin film to thick film condition, an understanding of the natural convection layer thickness is required. For the conditions studied here, the natural convection boundary layer was found to be close to 800 um. The WL thickness, cathode length and electrochemical kinetics interact to create four regimes of total cathode current as a function of WL thickness.
In the study of the solution chemistry, the effect of Al3+ on the electrochemical and galvanic corrosion distributions between SS316L and AA7050was emphasized as it represents the major dissolved metal species from AA7050. It is found that Al3+ accelerates the hydrogen evolution reaction (HER) kinetics due to enhanced proton diffusivity, with the effect being observed not only on Al alloys, but also on Pt and stainless steel. The cation study for the galvanic couple indicates that Al3+ is also galvanic corrosion accelerator.
The effect of oxide film properties on the cathodic kinetics of stainless steel were investigated because cathodic kinetics can significantly affect the degree of galvanic corrosion based on mixed potential theory. The study focused on the oxygen reduction reaction (ORR) and was studied by electrochemical (potentiodynamic polarization, cyclic voltammetry, potentiostatic test, and electrochemical impedance) techniques and x-ray photoelectron spectroscopy surface analysis. This study shows that the kinetics of ORR on stainless steels is determined by a combined effect of oxide film composition, film thickness, and resistivity.
This work has improved the fundamental understanding of the effect of several pertinent variables on the galvanic-coupling induced localized corrosion of Al alloy, provided an approachable modeling means to predict corrosion distribution at given conditions in a timely fashion as well as develop corresponding corrosion mitigation strategies, and complemented continuum/meso-scale atmospheric corrosion modeling studies in this field.

Acknowledgements

The financial support from the Office of Naval Research (ONR) via Grants No. N00014-14-1-0012 and No. N00014-17-1-2033, the Sea-Based Aviation Program, and William Nickerson, Program Manager is gratefully acknowledged. The assistance of my advisor, Prof. R. G. Kelly is gratefully acknowledged. Helpful technical discussions with Profs. S. J. McDonnell, J. R. Scully, and J. T. Burns (University of Virginia), Dr. Jayendran Srinivasan (the Ohio State University), Dr. Piyush Khullar (Medtronic), Dr. Veronica Rafla (SpaceX) and Mr. Victor Yang (University of Virginia) are also acknowledged.