Modeling Potential Thresholds of Sacrificial Metallic Coatings for Protection of Al-Mg Alloys in Marine Environments

Abstract

Repair and replacement of ship structures due to material degradation are high cost drivers in naval platform maintenance. Future ship designs and modernization of existing platforms rely on lightweight aluminum-magnesium (Al-Mg) 5XXX series alloys. A technological gap exists in the predictive capability for lifetime predictions of corrosion preventative sacrificial metallic coatings and identification of the underlying electrochemical, chemical and metallurgical mechanisms that control the potential thresholds for intergranular cracking (IGC) and intergranular stress corrosion cracking (IGSCC). Future ships (e.g. Littoral Combat Ship) which plan to use 5XXX alloys extensively will also be at risk. IGC and IGSCC initiation and growth are both strongly controlled by applied potential (regardless of sensitization level). Potential has been shown to significantly determine whether IGC or IGSCC occurs, onset incubation time and subsequent rates of penetration. Metal-rich primer(s) (MRP(s)) can be used as sacrificial metallic coatings that can control the applied potential across a macro defect site could provide protection against IGC of sensitized Al-Mg alloys. It is currently unknown whether a coating can exert sufficient potential control to mitigate IGC. MRPs (as well as pigmented organic coatings) offer the potential to serve as sacrificial anodes capable of performing this function. A multiphysics finite element package was used to computationally model galvanic protection conditions provided by MRPs in electrical contact with Al-Mg alloy AA5456. Primers included Zn-, Al- and Mg-based MRPs. In addition, pure metals including Mg, Al and Zn were evaluated as galvanic protection systems to validate the model against known mixed potential diagrams. Parametric variables included electrolyte thickness (1 micron to 1 meter) and composition (0.6M and 5.45M (sat’d)) NaCl. Primer evaluation was based on its ability to produce a global galvanic couple potential (Ecouple) below the threshold breakdown/pitting potential of the Mg-rich AA5456 beta-phase (beta-Epit). beta-Epit has been shown to be a critical parameter in the susceptibility of AA5456 to IGC. Sacrificial metallic primers with corrosion potentials (Ecorr) greater than Ecorr of AA5456 were predicted to result in the preferential corrosion of AA5456 based on initial boundary conditions. Zn-based primers exhibit the characteristics for providing galvanic protection to sensitized AA5456 without cathodic polarization at high hydrogen evolution overpotentials.