Cathodic treatment of galvanic zinc coatings in solutions of molybdates

Cathodic behavior of zinc coatings is studied in solutions of ammonium moly bdate at various pH. It is found that a solid-phase film of molybdenum oxid es forms in a weakly acidic solution (pH 5.4) in no less than two stages. T he first involves the transition of Mo(VI) to Mo(V) under diffusion control and the second is a further electroreduction under kinetic control. The pr otective properties of molybdate films are affected by both the electrolyte acidity and the treatment mode (cathodic current density, duration). The f ilm hinders the Zn(OH)(2) formation on the zinc surface.