Effect of NO₃ ⁻ intercalation on corrosion resistance of conversion coated Zn-Al-CO₃ LDHs on electrogalvanized steel

To modify the corrosion protection provided by conversion coated Zn-Al-CO₃ layered double hydroxides (LDHs) on electrogalvanized steel sheets (EG steels), methods to intercalate NO₃ ⁻ during post treatment were explored and the effect of intercalation on corrosion resistance was evaluated. EG steels were immersed in a Na₂Al₂O₄ based solution to form a Zn-Al-CO₃ LDH conversion coating layer, and were then immersed in a NaNO₃ methanol solvent to intercalate NO₃ ⁻. After 2 h of immersion, it was found that NO₃ ⁻ had completely exchanged with the originally intercalated CO₃ ²− anions in the conversion coated Zn-Al-CO₃ LDHs. Corrosion resistance measurements of the EG steel surfaces protected by the NO₃ ⁻ intercalated in the LDH layer were performed by a simple immersion test and a droplet test. When the samples were immersed in a large amount of 0.1 M NaCl, the corrosion resistance of the NO₃ ⁻ intercalated LDH layer was inferior to that of the control sample. In contrast, the samples that were exposed to a 100 μl droplet of 0.1 M NaCl displayed improved corrosion resistance. The difference in these results was attributed to the solubility of the LDH layer and the resulting effect on the release process of NO₃ ⁻ from the LDH layer.