Abstract
A finite element model of the protection mechanisms offered by Mg-based organic coatings was developed. The model predicted the change in the corrosion potential of AA2024-T351 as a function of pH, water layer thickness, and the inhibition of oxygen reduction reaction. The pH in the solution was calculated taking into account Mg dissolution, precipitation of Mg(OH)2, Al dissolution, and hydrolysis of Al3+ ions. The predicted critical pH value at which the corrosion potential of AA2024-T351 sharply decreases to values below pitting and pit repassivation potentials under full immersion conditions was in accordance with experimental observations. A limiting water layer thickness below which the pH-induced pit repassivation mechanism is not predicted to occur was calculated. If the inhibition of oxygen reduction reaction by Mg(OH)2 is considered, the pH-induced repassivation mechanism becomes feasible at thinner water layers. Cathodic protection offered by Mg-rich primers was modeled as a function of coating resistance, water layer thickness, and electrolyte chemistry. The magnitude of the resistance of the film in which Mg pigments are embedded mitigates the extent of the cathodic protection. The change in local pH due to corrosion reactions affected the galvanic potentials obtained. The framework developed can be used to help identify chemical inhibitors that can operate by the chemical protection mode described in this work.
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