Effect of Cerium Salt in Electrolyte on Corrosion-resistance of LDH / AAO Composite Coating

It is difficult for a common anodic oxidation(AAO) coating to protect an aluminum alloy; sealing treatment or adding a corrosion-inhibitor to the AAO electrolyte is required to improve the poor corrosion-resistance of the coating. Rare-earth cerium salt is a green and efficient cathodic corrosion-inhibitor for aluminum alloys. It can increase the thickness of the anodic oxide coating, extend corrosion pathways, and improve the pitting-resistance of the coating. A layered double-hydroxide(LDH) is a type of host-guest two-dimensional nano-layered hydroxide material. As a nano-container, it can load a large number of corrosion inhibitors, with intelligent release on the surface of aluminum alloy. In this paper, an AAO coating containing Ce3+ was obtained by adding cerium salt to the electrolyte. On this basis, an LDH coating was prepared using an in situ growth method to seal the AAO, resulting in an LDH / AAO composite coating. The effect of trivalent cerium salt concentration in the anodic oxidation electrolyte on the corrosion-resistance of AAO film and LDH / AAO composite film was studied. The corrosion-resistance of the LDH / AAO composite coating was further improved by loading a vanadate inhibitor on the outer LDH layer for active anti-corrosion of the coating. The microscopic morphology and elemental composition of the films were characterized by scanning electron microscopy (SEM), energy spectrometry(EDS), and glow discharge emission spectrometry(GDOES). The corrosion-resistance of the films was examined by electrochemical alternating current impedance(EIS), kinetic potential polarization curve analysis, and an acid salt spray test. The results show that the corrosion-resistance of the coating was improved by adding cerium salt as a corrosion inhibitor in the anodic oxidation electrolyte. The porous anodic oxide layer was sealed by LDH and improved the growth of the LDH coating on the AAO coating surface. LDH can grow in AAO holes and defects, and completely cover the AAO layer. The corrosion-resistance was significantly better than that of cerium-salt sealing. When the concentration of cerium salt was 0.03 mol / L, the corrosion-resistance of the LDH / AAO composite coating had the highest impedance and better resistance to acid salt spray. It was demonstrated that 0.03 mol / L cerium salt can effectively improve the corrosion-resistance of the coatings. After loading vanadate in the LDH layer, the self-corrosion current density decreased by more than two orders of magnitude and the corrosion rate decreased significantly compared with that of AAO samples. The corrosion-resistance of the composite coating was further optimized by adding vanadate corrosion-inhibitor to the LDH layer. The corrosion-resistance of the composite coating is mainly attributed to addition of cerium salt to the electrolyte to improve the physical barrier properties of the AAO inner layer. Thus, addition of cerium salt to the anodic oxidation electrolyte can improve the density and corrosion-resistance of the AAO coating and improve the growth of the LDH coating on the AAO coating surface. The dual effects of LDH loaded with vanadate to trap corrosive ions in the outer layer and release the interlayer corrosion inhibitor provide long-lasting protection of the substrate aluminum alloy. A layered bimetallic hydroxide anodic oxidation composite coating was proposed and a "nano-container" was successfully used to load the vanadate corrosion-inhibitor.