Effect of morphology and manganese content of flake-type zinc-manganese-phosphate additives on the corrosion protection behavior of highly structured Nanomer® coatings

摘要

In the present investigation flake-type zinc-manganese- phosphate particles having an aspect ratio in the range of 10-30 and a thickness in the nanometer range were synthesized by kinetically controlled precipitation reaction and were incorporated in an organic epoxy-phenolic resin binder matrix. The resulting composite mixtures were coated on mild steel using wet chemical approach. Particle size, particle shape and aspect ratio of the flakes varied with the change of the molar ratio of zinc to manganese from 0/100 to 34/66 as could be proved by scanning electron microscopy (SEM). XRD-analysis showed that the resulting flake-type particles are fully crystalline. Composite coatings were prepared by dispersing the different zinc-manganese flake type particles in concentrations up to 5 wt.-% in the epoxy-phenolic resin matrix. Furthermore, the compositions were blended with additional platelet-shaped silica particles up to a final concentration of 20 wt.-%. The morphological analysis on polished cross-sections on cured coatings on mild steel showed, that it was possible to disperse the particles homogeneously in the matrix and that especially the compositions containing combinations of zinc-manganese-phosphate flakes and silica platelets show an improved alignment of the particles along the metal substrate surface. Neutral salt spray test revealed that after 336 h exposure the subsurface migration could be kept to a minimum for particle combinations indicating the excellent barrier functionality of the highly structured Nanomers®. Standard electrochemical measurements such as electrochemical impedance spectroscopy supported the results of the neutral salt spray test.