Corrosion Behavior and Anti-corrosion Mechanism of Lanthanum Nitrate Modified Chromium-Free Dacromet Coating

With zinc powder and aluminum powder as raw materials and lanthanum nitrate as additive, lanthanum nitrate modified chromium-free Dacromet coating was prepared. The phase composition and micromorphology of the corrosion products of the coating after immersion in 5wt% NaCl solution were studied, and the corrosion behavior and anti-corrosion mechanism of the coating were investigated. The results show that after immersion in NaCl solution for 10 d, some zinc-rich phases in the modified coating were preferentially corroded to form Zn₅(OH)₈Cl₂·H₂O, and the surface showed a honeycomb morphology; after immersion for 30 d, the corrosion products increased and Zn₅(OH)₆(CO₃)₂ was formed, and the surface showed a sponge-like structure; after immersion for 60 d, the corrosion products gradually dissolved and peeled off, and the surface showed a three-dimensional porous network structure. The anti-corrosion mechanism of the coating included the self-repairing effect of corrosion products and rare earth passivation film, the physical shielding effect of organic silane passivation film and lamellar zinc-aluminum powder overlapping, the sacrificial anodization effect of zinc-aluminum metal, the passivation effect of rare earth and the slow-releasing effect of corrosion inhibitor.