Corrosion inhibitors for bare and PEO-coated Mg alloys
Magnesium (Mg) alloys offer excellent strength-to-weight ratio and biocompatibility, making them attractive for automotive, aerospace, and biomedical applications. However, their widespread use is limited by their high corrosion susceptibility. Plasma Electrolytic Oxidation (PEO) coatings have emerged as a promising solution, providing a protective barrier and enabling the incorporation of corrosion inhibitors for active protection. Despite this potential, little is known about which inhibitors are truly effective in PEO-coated Mg systems. This thesis systematically investigates a wide range of corrosion inhibitors to enhance the durability of PEO-coated AZ21 Mg alloy. Screening over twenty candidates revealed that inhibitors effective on bare Mg do not necessarily perform well in PEO systems. For instance, 2,5-pyridinedicarboxylate sodium salt (2,5PDC-Na) was found to degrade the coating rather than protect it. This unexpected behavior was studied in depth to understand the underlying mechanisms. In contrast, sodium salts of 8-hydroxyquinoline (8HQ) and decylphosphonic acid (DP) significantly improved the corrosion resistance of PEO-coated Mg. DP, in particular, extended the coating's lifetime by more than 50 days compared to a reference system without inhibitors. Its superior performance was linked to the formation of insoluble compounds with Mg²⁺ ions, effectively blocking corrosion pathways. Based on these findings, the thesis proposes a set of criteria for selecting inhibitors for PEO-coated Mg alloys, emphasizing the importance of forming stable, insoluble compounds. These guidelines were applied retrospectively to validate their broader applicability. The insights provided contribute to a better understanding of inhibitor-PEO interactions and support the development of more robust Mg-based materials for critical applications.
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