Decavanadate-Intercalated Zn--Al Layered Double Hydroxide Epoxy Coatings for Chloride Corrosion Protection of Mild Steel
Abstract
Corrosion of mild steel below organic films occurs as a result of electrolyte penetration, interface water accumulation, filler-polymer contact, and the presence of inhibitor molecules adjacent to damaged zones. This paper investigates epoxy coatings based on diglycidyl ether of bisphenol-A and crosslinked using polyamide as a curing agent and filled with silane-functionalized Zn-Al layered double hydroxides. The performance of three types of films was assessed: bare epoxy, epoxy with nitrate-intercalated APTES-functionalized Zn-Al LDH (LDH-NF), and epoxy with decavanadate-intercalated APTES-functionalized Zn-Al LDH (LDH-DF). Both filled films had the same content of LDH ($3 \, \text{wt}.\%$) which allows to study the effect of gallery chemistry and functionalization of particles on film characteristics without varying the amount of LDH filler. LDH-DF has the decomposition temperature of $340^\circ\mathrm{C}$, compared to $275^\circ\mathrm{C}$ of LDH-NF, whereas the total thermal weight loss decreased from $37.6 \, \%$ to $25.4\,\%$. X-ray photoelectron spectroscopy proved the existence of Si-O and C-N environments associated with APTES molecule in both kinds of fillers and showed the presence of vanadium oxoanion in LDH-DF by detecting its characteristic V 2p peaks at $516.2$ and $523.6 \,\mathrm{eV}$. In addition, LDH-DF surface turned from hydrophilic state (contact angle close to $29^\circ$) to hydrophobic one with contact angle $\approx90^\circ$ after silane functionalization. Results of potentiodynamic polarization tests have indicated that LDH-DF epoxy decreases the corrosion current density from $3.4\times10^{-5}$ to $1.2\times10^{-6}\,\mathrm{A\,cm^{-2}}$ while increasing polarization resistance from $1.3\times10^{4}$ to $3.8\times10^{5}\,\Omega\,\mathrm{cm^{2}}$. Coating behavior is accounted for due to a synergistic effect of LDH platelet tortuosity, silane-assisted interaction between filler and resin, low affinity of particles to moisture at the boundary, chloride exchange in the LDH gallery and decavanadate-induced electrochemical activity inhibition at the interface of exposed steel.