Enhanced Corrosion Resistance of AA6061 Aluminum Alloy via Electrodeposition of Mg-Al Layered Double Hydroxide Coatings

Mg-Al layered double hydroxide (MgAl-LDH) containing NO₃^- ions in the interlayer (MgAl-LDH-NO₃) was electrodeposited onto AA6061 aluminum alloy to improve its corrosion resistance. Electrodeposition was performed in 2-propanol-water mixed electrolytes containing Mg(NO₃)₂ and Al(NO₃)₃, with Mg²⁺ and Al³⁺ ion concentration set at 73, 24, and 2.4 mmol/L (mM), maintaining an Mg:Al ratio of 4:1. Potentiodynamic polarization, electrochemical impedance, and wet-dry corrosion tests were conducted. MgAl-LDH-NO₃ was deposited preferentially onto the intermetallic precipitates (IMPs) of AA6061, forming island-like deposits (LDH islands), while a nanometer-thick layer formed on the matrix. The coverage of IMPs by LDH islands was higher for the sample electrodeposited in the 24 mM electrolyte compared to those in the 73 mM and 2.4 mM electrolytes. IMP covereage by LDH islands suppressed the oxygen reduction reaction on the IMPs, resulting in reduced corrosion current density (I_corr) and increased polarization resistance (R_p). Therefore, the 24 mM-electrolyte sample exhibited the lowest I_corr and the largest R_p values. In the wet-dry tests, coverage of IMPs by LDH-islands effectively inhibited the corrosion initiation of AA6061. Raman spectroscopy revealed a decrease in NO₃^- and an increase in CO₃^2- ions in the interlayer of LDH, suggesting exchange of NO₃^- with Cl^- and CO₃^2- ions during the wet-dry cycles. These results indicate that the optimal Mg²⁺ and Al³⁺ ion concentration for MgAl-LDH-NO₃ coating is approximately 24 mM or less, providing nearly twice the corrosion protection ability compared to coatings formed from the 73 mM- and 2.4 mM electrolytes.