Steady State Modeling of the Influence of Galvanic Coupling on the Structural Degradation of Airframe Components

A thin, chloride-containing electrolyte film can be established on surfaces in many marine environments, due to splash/spray, rain, or salt-enabled deliquescence [1]. Defects in the corrosion protection system are common and inevitable due to the intense operating/maintenance conditions of ships and aircraft. Such defects are likely intensified at complex component joining/fastener locations. Furthermore, the geometry of faying surfaces and fastener holes inherently aid in the trapping/wicking of the electrolyte into tight crevices typical of these component joining locations. This geometry leads to a severe localized corrosion environment where a galvanic cell can be established, for example, between aluminum alloy components and stainless steel fasteners.

The overall goals of this work are to develop a quantitative scientific understanding of the effects of important external variables on the electrochemical potential and current distributions that develop in fastener hole configuration involving a realistic thin film/galvanic couple configuration between a 7050- T7451 component and a CRES 316 fastener. The earlier modeling of atmospheric localized corrosion [2-4] will serve as a framework, but needs to be extended to more complicated geometries.

A combination of experimental and modeling approaches are being used to characterize key processes and important external factors in atmospheric localized corrosion. Regarding the modeling framework, two aspects require definition: the geometry to be studied, and the differential equation to be solved. The generic thin film/galvanic couple configuration of interest is shown in Figure 1. There are a substantial number of external variables describing the geometry. Determination of the significance of each of these geometric variables has been accomplished using a Design of Experiment (DOE) method. From DOE analysis, geometric parameters W₁, G₁, W₂, L₁ and L₄ are confirmed to be the most important. The Laplace's Equation is used to model steady-state current and potential distributions under thin-electrolyte conditions, ignoring ion species diffusion and changes in solution chemistry. The use of the Laplace's Equation relies on a knowledge or estimation of the electrolyte characteristics (primary conductivity) and its dependence on position and other experimental variables. FEM modeling was applied to numerically determine the potential and current distributions in the thin film at steady state for the range of geometries and environments. To accomplish this objective, the appropriate electrochemical kinetics for the CRES 316 and 7050-T7451 alloys will be determined experimentally. The electrochemical kinetics so determined will serve as the boundary conditions for solving the Laplace's Equation. In the current work, electrochemical kinetics from a related study [4] were used for the boundary conditions. The results of the DOE matrix and its implications for localized corrosion and the mitigation of such attack will be discussed.

Acknowledgement

This work has been supported by the Office of Naval Research (ONR) Grant N00014-14-1-0012. Mr. William Nickerson, Technical Officer at Office of Naval Research is gratefully acknowledged.

References

[1] G.A. Shoales, S.A. Fawaz, M.R. Walters, in: M. Bos (Ed.) ICAF 2009 - Bridging the Gap Between Theory and Operational Practice, Springer, Rotterdam, The Netherlands, 2009, pp. 187-207.

[2] F. Cui, F.J. Presuel-Moreno, R.G. Kelly, Corrosion, 62 (2006) 251-263.

[3] F.J. Presuel-Moreno, H. Wang, M.A. Jakab, R.G. Kelly, J.R. Scully, J Electrochem Soc, 153 (2006) B486-B498.

[4] D. Mizuno, R.G. Kelly, Corrosion, 69 (2013) 681-692.