Combined Numerical and Experimental Approach to Study the Effect of External SS316 Cathode on the Localized Corrosion in AA7050-T7451 Galvanically Coupled with SS316 Under Atmospheric Conditions
Authors: Liu Chao , Kelly Robert G. 
Published: February 2020

Connections between dissimilar metal alloys (e.g., aluminum (Al) alloy components and stainless steel (SS) fasteners) are frequently encountered in airframes exposed to an atmospheric marine environment. This exposure results in the formation of thin layer of electrolyte containing chloride species on the materials surface. The thin film electrolyte can wick into the tight crevice formed between the Al alloy component and the SS fastener when breakdown of corrosion protection coating happens. As a result, an electrochemical cell is established due to this galvanic coupling, and localized corrosion is likely aggravated in the crevice that forms. This corrosion damage can serve as a preferential site for fatigue crack formation, leading to failure of Al alloy components during operation. Developing mechanistic understanding of metal-environment interactions can provide guidelines for current protection system evaluation, as well as develop corrosion mitigation strategy during materials design stage.
There are several influential environmental factors affecting atmospheric corrosion, including relative humidity (RH), water layer thickness (WL), concentration of dissolved species in the solution, temperature, etc. [2]. For localized corrosion due to galvanic coupling under atmospheric condition, it has been shown that the effect of external cathode outside the crevice formed in the galvanic coupling controls the extent of localized corrosion inside the crevice [3-4]. In the literature there is a limited amount of corrosion modeling work which focuses on atmospheric corrosion, and in those few cases that do exist, the electrochemical kinetics used in the modeling framework are mainly based on full immersion conditions, which might lose key features under atmospheric conditions. Hence atmospheric corrosion modeling work with accurate electrochemical kinetics is greatly needed to better understand the effects of the key external factors on electrochemical distributions that developed in the corrosion system involving a realistic thin film/galvanic couple configuration.
In this work, a combined numerical and experimental approach is being applied to study potential and current density distributions as a function of two important variables for external cathode of SS316: water layer thickness on the external cathode as well as the size of external cathode. For the experimental work, a sintered Ag/AgCl electrode serving as a combined reference electrode (RE) and counter electrode (CE) [5] is implemented to characterize thin film electrochemical kinetics of AA7050-T7451 and SS316. A series of thin film electrolyte with thicknesses from 10m to 2000 m were used to study the effect of WL on electrochemical kinetics of testing materials as a function of scan rate and electrolyte concentration. An example of cathodic kinetics for SS316 in terms of WL is shown in Figure 1. In addition, the evolution of th ecrevice solution chemistry of AA7050 is mimicked by dissolving the alloy galvanostatically in the thin film cell before determining the anodic kinetics of AA7050 in that solution. The electrochemical kinetics of tested materials serve as boundary conditions in the modeling framework. For the computational work, the finite element method (FEM) [6] is applied to study both the effect of external cathode size and water layer thickness on potential and current density distributions along the AA7050 inside the crevice. Comparisons of potential/current density distributions with different external cathode sizes as well as water layer thicknesses under corresponding boundary conditions are used to illustrate the effects of atmospheric variables on localized corrosion damage along the crevice.

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.

Reference

[1] N. E. Co, and J. T. Burns, In Corrosion Research in Progress Symposium , pp. 47–51, NACE, Vancouver, Canada (2016).
[2] H. Simillion, O. Dolgikh, H. Terryn, J. Deconinck, Corros. Revs., 32, 73-100 (2014).
[3] Z. Y. Chen, F. Cui, and R. G. Kelly, J. Electrochem. Soc., 155, C360- C368 (2008).
[4] Z. Y. Chen, and R. G. Kelly, J. Electrochem. Soc.,, 157, C69-C78 (2010).
[5] P. Khullar, J. V. Badilla, and R. G. Kelly, ECS Electrochem. Lett., 4, C31-C33 (2015).
[6] C. Liu, and R. G. Kelly, Electrochem. Soc. Interface, 23, 47-51 (2014).

Figure 1