Improving the Surface Corrosion Resistance of AMX601 Magnesium Alloy by Acid-Alkaline Treatment

A drawback of acid cleaning as surface finishing of magnesium (Mg) surface is the absence of a protective oxide film on its surface. Acid-alkaline treatment is proposed to enhance the surface corrosion resistance of AMX601 Mg alloy. Acid-alkaline treatment was conducted by first dipping the alloy in HNO₃-H₃PO₄ solution and then immersing the alloy in NaOH solution. The potentiodynamic polarization test in 0.9% NaCl solution at 37 °C revealed a nobler corrosion potential of −1.36 V_Ag/AgCl and a lower corrosion current density of 36.0 µA·cm⁻² of the acid-alkaline-treated specimen than the acid-treated (−1.44 V_Ag/AgCl, 89.7 µA·cm⁻²) and untreated (−1.52 V_Ag/AgCl, 40.0 µA·cm⁻²) specimens. Acid treatment induced a significantly higher surface roughness (20 µm) than acid-alkaline (10 µm) and grinding (0.5 µm) treatments because of the selective dissolution of the Mg matrix and the accumulation of intermetallic precipitates. The film formed on the acid-alkaline-treated specimen was thick and free of cracks, whereas that formed on the acid-treated specimen was thin and cleaved. The formation of a protective oxide film and the enrichment of cathodic intermetallic particles on the acid-alkaline-treated specimen enhanced the corrosion resistance of the surface.