Hydrotalcite coating formation on Al-Cu-Mg alloys from oxidizing bath chemistries

The effects of bath oxidizing power on hydrotalcite coating formation and breakdown were examined in this study. Overall, results indicate that increasing the oxidizing power of the coating bath shortens the immersion time required to form highly corrosion-resistant coatings. Hydrotalcite conversion coatings were formed on Al 2024-T3 (lUNS A92024) Al-4.4%Cu-2.5% Mg-0.6%Mn) panels by immersion in alkaline lithium salt solutions containing either carbonate, nitrate, or nitrate/persulfate additions. The resulting coating structure was characterized by scanning electron microscopy. Corrosion resistance was characterized by electrochemical impedance spectroscopy (EIS) and salt spray exposure testing. When nitrate and persulfate oxidizers were present in the bath, coating formation time and hydrogen evolution were sharply reduced while coating weight was increased, indicating an overall increase in coating efficiency. A decrease in pinhole-type coating defects was attributed to early cessation of hydrogen evolution. Coatings with high corrosion resistance were formed in 6 min or less of simple immersion. Coatings formed in the presence of oxidizers were much more corrosion resistant than those formed in carbonate-only solutions. Coatings formed in nitrate-containing baths withstood 168 h of ASTM B117 salt spray exposure with little or no pitting. In EIS tests, coatings exhibited total impedances of 2 MΩ-cm² to 5 MΩ-cm² after 24 h exposure to aerated 0.5 M sodium chloride (NaCl) solution. It was also found that, in EIS tests in chloride solution, the total impedance of coatings formed in oxidizing baths increased with increasing exposure time. This "healing" process is so far unique to hydrotalcite coatings formed in the presence of oxidizers and is thought to be associated with low-temperature hydrothermal sealing of the coating.