E. Perre, S. Albayrak, C. Becker-Willinger
INM-Leibniz Institute for New Materials,
Keywords: zinc phosphate, corrosion protection, coating, steel, nanoparticles
Summary:Corrosion protection is a constant concern to steel manufacturers and users. Various processes were developed in the past to provide corrosion protection for steel such as metallic layers like e.g. zinc, coating of the metal surface with films with barrier or active corrosion protection properties, or delivery of a current to prevent corrosion. Zinc phosphates were often used as protective films directly grown on a metal surface by conversion reaction or as particulate additives in a coating film. The corrosion inhibition mechanism of zinc phosphate is still not clearly understood and was attributed to different phenomena such as passivation function, pore clogging, shielding ability or cathodic disbonding. The use of spherical zinc phosphate pigments limits protective action because of their low surface to volume ratio in combination with their intrinsic low solubility by hindering the active components to be liberated fast enough in order to prevent corrosion. Modifying the particles morphology by increasing their aspect ratio and in this way increasing their surface to volume ratio would be a strategy to improve the protection ability. In the present investigation flake type zinc phosphate particles with an aspect ratio between 10 and 30 and a thickness in the nanometer range were synthesized by kinetically controlled precipitation reaction starting from aqueous solutions of zinc acetate in the presence of a complexing agent such as e.g. citric acid. Particle size and aspect ratio of the particles could be controlled by systematic variation of the concentration of the complexing agent in relation to the amount of zinc ions present in the solution. XRD-analysis revealed that the resulting particles are fully crystalline and possess an orthorhombic lattice structure (Zn3(PO4)2 • 4 H2O, Hopeite). SEM measurements proved their anisotropic platelet type shape. Potentiostatic measurements on mild steel samples immersed in aqueous salt solutions containing 0.75 wt. % Hopeite particles revealed a corrosion inhibition effect. The corrosion current Icorr decreased by a factor of 10 from aqueous salt solutions containing no particles to aqueous salt solutions containing the flake type Hopeite particles. Composite coatings were prepared by dispersing the flake type Hopeite particles in concentrations up to 10 wt.-% in organic polymeric matrix after appropriate surface modification and were been applied on mild steel. The morphological analysis on polished cross-sections showed that it was possible to disperse the particles homogeneously in the matrix. Corrosion protection ability was monitored by means of neutral salt spray test and electrochemical impedance spectroscopy (EIS).