Electrochemical precipitation of Mg-Al Layered Double Hydroxide on AA6061 Aluminum Alloy: Effect of Water Content and Electrolyte Concentration on Coating Morphology and Corrosion Resistance

Mg-Al layered double hydroxide intercalated with NO3−（Mg-Al LDHNO3）coatings were formed on AA6061 alloy by electrochemical precipitation at applied voltages of 5 and 10 V to enhance its corrosion resistance. The effects of water content and total concentration of Mg（NO3）2 and Al（NO3）3（TMAIC）in the electrolyte on the coating morphology and the corrosion behavior were investigated. The water content in 2-propanol-based electrolytes was varied to 2.4%, 20%, and 100%, and the TMAIC was set to 2.4 or 24 mmol/L. In electrolytes containing 2.4% or 20% water and 24 mmol/L TMAIC, low electrical conductivity localized water reduction reactions on intermetallic particles（IMPs） and preserved the passive film on the Al matrix, leading to the preferential formation of island-like LDHNO3 on IMPs. In contrast, in 100% water with 24 mmol/L TMAIC, higher conductivity enabled water reduction across the surface, resulting in a continuous LDHNO3 layer. Polarization tests in 0.1 mol/L NaCl solution revealed that crack-free, island-like LDHNO3 effectively suppressed the oxygen reduction reaction on IMPs, resulting in a corrosion current density ten times lower than that of the as-polished specimen. Continuous LDHNO3 layers, in contrast, showed higher passive current density, indicating passive film degradation due to increased pH during electrochemical precipitation.