Effect of NO₃⁻Intercalation on Corrosion Resistance of Conversion Coated Zn-Al-CO₃LDHs on Electrogalvanized Steel

Abstract

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 CO32− 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.