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 usually leads to the formation of thin film electrolyte which contains chloride species which functions as an ionic path. As a result a localized electrochemical cell is formed due to the galvanic coupling. This galvanic-coupling-induced localized corrosion could serve as a preferential site for fatigue crack formation which is detrimental to the structural integrity of the airframe.Water layer thickness (WL) is an important environmental variable which significantly impacts the degree of atmospheric corrosion. There have been many experimental studies regarding the influence of WL on an individual material surface[1–5] in the literature. Nevertheless there is a limited number of studies of the effect of WL on the galvanic coupling especially modeling study. A comprehensive study to develop a scientific understanding of the correlation between WL and electrochemical and corrosion damage distributions for the UNS A97050 and UNS S31600 galvanic couple is greatly needed .A combined numerical and experimental approach was applied to investigate the effect of water layer thickness on the potential current density as well as localized corrosion distributions for a ring-disk electrode. The ring-disk electrode consists of UNS A97050 as the outer ring and UNS S31600 as the inner disk. For experimental work the scanning Kelvin probe (SKP) technique was applied to measure the potential distributions above the ring disk electrode with varying WLs for different exposure periods. A series of 3D printed caps were installed on the top of the ring-disk electrode to control WL ranging from 2mm to 100um. The electrolyte solution was 0.6M NaCl and a chamber with RH=98% was used for sample exposures under different time periods. Optical Microscopy and Interferometry were used following exposure to characterize the corrosion damage at the UNS A97050/UNS S31600 interface. Meanwhile a thin film electrochemistry testing technique [6] was introduced to measure polarization curves of UNS S31600 and UNS A97050 for different WLs. These experimentally determined electrochemical kinetics served as boundary conditions in the modeling work. For modeling work a finite-element-method (FEM) based model with Laplace’s Equation as governing equation was applied to calculate potential and current density distributions under steady state for the entire galvanic couple. The simulated potential distributions were then compared to potential distributions derived from SKP measurements to demonstration the modeling compatibility to predict electrochemical distribution in a galvanic corrosion system.AcknowledgementThis 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. Tomashov N.D. Corrosion 20 (1964): p. 7t–14t.2. Stratmann M. and H. Streckel Corrosion Science 30 (1990): pp. 681–696.3. Stratmann M. and H. Streckel Corrosion Science 30 (1990): pp. 697–714.4. Stratmann M. H. Streckel K.T. Kim and S. Crockett Corrosion Science 30 (1990): pp. 715–734.5. Nishikata A. Y. Ichihara Y. Hayashi and T. Tsuru J. Electrochem. Soc. 144 (1997): pp. 1244–1252.6. Khullar P. J.V. Badilla and R.G. Kelly ECS Electrochem. Lett. 4 (2015): pp. C31–C33.

Key words: atmospheric corrosion, electrolyte layer thickness, finite element method (FEM), Laplace equation, electrochemical distributions