ZnO Thin Films as Promising Corrosion Protection on Mg-Based Alloys

The present study examined the microstructure and corrosion characteristics of MgCa4Zn1Gd1 and MgCa2Zn1Gd3 alloys that were coated with ZnO thin films, which were deposited by atomic layer deposition (ALD). Coatings of different thicknesses (42.5, 95.4 and 133.7 nm for 500, 1000, and 1500 cycles, respectively) were characterized using X-ray diffraction (XRD), Raman spectroscopy, SEM/EDS, AFM (atomic force microscope), and FTIR (Fourier transform infrared spectroscopy). XRD and Raman analyses were conducted to verify the formation of crystalline zinc oxide (ZnO) with a homogeneous granular morphology. Surface roughness decreased with increasing coating thickness, reaching the lowest values for the 1500-cycle ZnO layer on MgCa2Zn1Gd3 (R_a = 7.65 nm, R_s = 9.8 nm). Potentiodynamic and immersion tests in Ringer solution at 37 °C revealed improved corrosion resistance for thicker coatings, with the lowest hydrogen evolution (20.89 mL·cm^-2) observed for MgCa2Zn1Gd3 coated after 1500 cycles. Analysis of corrosion products by FTIR identified Mg(OH)₂ and MgCO₃ as dominant and then MgO and ZnO. Phase analysis also indicated the presence of ZnO coating after 100 h of immersion. The ZnO film deposited after 1500 ALD cycles on MgCa2Zn1Gd3 provides the most effective corrosion protection and is a promising solution for biodegradable magnesium implants.