Improved Adhesion and Corrosion Resistant Performance of Polyurethane Coatings on Anodized Mg Alloy for Aerospace Applications

In the current investigation, we targeted to improve the adhesion and corrosion resistant behavior of AZ31 Mg alloy by executing the two-stage treatment; initially by anodization then followed by the deposition of polyurethane (PU) coatings. The impact of the different anodized layers on the adhesion and corrosion protective performance of PU-coated AZ31 Mg substrates was systematically estimated by various characterization techniques. Surface characterization results revealed the influence of silicate content in the anodization bath on the surface microstructure of anodized layers. Surface wettability of Mg substrates after anodization was significantly improved (nearly 25°), confirming the enhanced surface hydrophilicity, possibly due to the higher surface roughness. From the scratch test results, the PU/A75Si coating exhibited the highest adhesion strength with a critical load of 4.8 ± 0.3, which corresponds to about ten-fold of improvement in comparison to PU/Bare. Corrosion test results confirmed the enhanced corrosion protection behavior of PU/A75Si coatings with higher values of R_ct (8.72 × 10⁷ Ω cm²), and R_f (1.881 × 10⁷ Ω cm²) with the lowest values of i_corr (2.0120 × 10⁻¹⁰ Acm⁻²) compared to that of PU/Bare samples. Based on the attained outcomes, it was concluded that the anodized layer could increase the adhesion between the polyurethane coatings and base substrate, which is the necessary factor in enhancing the corrosion resistant behavior of polyurethane coatings on AZ31 Mg substrates.