Modeling of MRP Protection Considering Chemical Inhibition, Electrochemistry, and Mass Transport
Authors: Vicente Moraes Carolina , Kelly Robert , Scully John R. , Santucci Raymond 
Published: December 2020

Aluminum alloy AA2024-T3 is widely used in aerospace applications due to its high strength-to-weight ratio. However, because of its heterogeneous microstructure, it is susceptible to microgalvanic corrosion and pitting in chloride containing environments, requiring additional corrosion protection methods. Organic coatings are the primary corrosion protection method used in aluminum alloys and they are usually composed of pigments embedded in a polymeric matrix. The polymeric matrix promotes a high resistance to ionic movement and it provides barrier against the ingress of aggressive species onto the substrate. The pigments can provide corrosion protection by inhibiting corrosion reactions, providing cathodic protection to the substrate, and/or depositing a protective layer on the surface of the substrate. Recently, a coating system containing a Mg-rich primer (MgRP) has emerged as an effective alternative to the chromate-based coatings commonly used for this purpose. Due to the lower potential of Mg, MgRP can protect aluminum alloys via galvanic coupling, serving as a sacrificial anode and preventing the corrosion of the Al substrate. Additionally, recent studies showed that the electrolyte chemistry resulting from Mg dissolution is beneficial to localized corrosion inhibition of AA2024, providing corrosion protection even after pigment depletion.

Localized corrosion of coated Al alloys can be affected by a large number of variables, such as water layer thickness, salt loading, pigment volume concentration of primer, chemistry of the primer, galvanic effects of pigment. and component geometry. The efficacy of the organic coating also depends on many variables. Exposure of the coating to UV radiation and aggressive solutions causes degradation of the polymeric matrix, which reduces the resistance to ionic movement and protection of the substrate against aggressive species. 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 to increase efficiency of experimental work. The objective of this work was to model the protection mechanisms offered by a MgRP coating system in AA2024, and investigate the influence of different variables on the effectiveness of the protection mechanism. The mechanisms of importance include protection via (a) the Mg pigment particles acting as sacrificial anodes, (b) the barrier to ion transport effected by the organic coating, and (c) changes in the electrolyte chemistry from dissolution of the Mg pigment particles. The modeling considers 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 system and serve as a framework to study new coating systems.

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.