Predicting the Effectiveness of Mg-Rich Primers in Mitigating Galvanic Corrosion of Al Alloys Coupled with SS316 Fasteners Using Finite Element Modeling

Abstract

Aerospace aluminum alloys, such as AA2024-T3 and AA7075-T6, are widely used in aerospace applications due to their high strength-to-weight ratio. However, because of their heterogeneous microstructure, they are susceptible to localized corrosion in chloride-containing environments. Corrosion attack is aggravated at fastener/sub-structure joining locations, where the Al alloy is in contact with more noble alloys, such as stainless steels. To protect these structures against corrosion attack, organic coatings are commonly used. Typical coating systems are usually composed of three layers: a conversion coating, a primer, and a topcoat. Historically, Al alloys have been protected by chromate-based coating systems. However, Cr(VI) has carcinogenic effects; thus, environmental regulations are restricting its use, stimulating the research in alternative corrosion protection technologies. Metal-rich primers, such as Mg-rich primers, have emerged as an effective and environmentally-friendly alternative to chromate-based primers. Depending on the type of the metallic pigment, these primers can provide chemical and electrochemical protection mechanisms to Al alloys.Localized corrosion of coated Al alloys can be affected by a vast number of variables, such as water layer thickness, electrolyte chemistry, salt loading density, pigment volume concentration of primer, chemistry of the primer, galvanic effects of pigment and noble fasteners, and component geometry. Thus, it is difficult to predict and quantify damage by localized corrosion, in addition to the efficacy and lifetime of a coating system. In this context, modeling can be used as a valuable assessment of parameter space, increasing the efficiency of experimental work. The objective of this work was to model the protection mechanisms offered by MgRP and investigate the influence of different environmental and geometric parameters on the effectiveness of the MgRP protection mechanisms in mitigating galvanic corrosion around fastener/substructure joints. The mechanisms of importance include protection via (a) the Mg pigment particles acting as sacrificial anodes, (b) the barrier to ion transport affected by the organic coating, and (c) changes in the electrolyte chemistry from the dissolution of the Mg pigment particles. The models consider each of these protection mechanisms in isolation as well as in combination. Ultimately, this model will provide a better understanding and prediction of the protection mechanisms of the MgRP, and it will elucidate critical conditions that should be further investigated with experimental work.Acknowledgements:Financial support from the SERDP program through the Office of Naval Research (C. Sanders) via Contract N00173-19-1-G011 as well as the DOD Corrosion Program Office through the US Air Force Academy via Contract FA7000-18-2-0006 is gratefully acknowledged. Technical discussions with Prof. John Scully (University of Virginia) and Dr. R.J. Santucci (Naval Research Laboratory) are also gratefully acknowledged.