Investigating the effect of current density in ultra-fast electrolytic zinc phosphate deposition

Abstract

The influence of the current density utilized during electrolytic zinc phosphate deposition on the properties of the final coatings has been investigated. The adoption of a concentrated electrolyte allows satisfactory coating properties with current densities as high as 100 mA cm−2, resulting in exceptional deposition rates. The size of the Zn3PO4 crystals was shown to decrease with the current density, which in turn affected the coatings final properties. An explanation to this finding was provided following classical nucleation theory. Samples obtained at 25 and 50 mA cm−2 featured more compact layers while samples obtained at 100 mA cm−2 were thicker but porous. Scratch test was used to assess wear resistance, which was observed to be increasing with the applied current density. The more compact layers obtained at 25 and 50 mA cm−2 provided an enhanced corrosion resistance as highlighted by potentiodynamic polarization and electrochemical impedance spectroscopy. The adoption of an electrolytic setup allowed for co-deposition of metallic Zn which can further protect the steel substrate by galvanic coupling.