ELECTROCHEMICAL CHARACTERIZATION OF PHOSP HATE LAYERS ON ZINC COATED STEEL

The electrochemical test for determination of the coverage of phosphat ed steel [1] is transferred to zinc coated steel. Potentiodynamic curr ent and capacity measurements as well as impedance spectroscopy were u sed to evaluate the coverage. For electrogalvanized steel the free met al area decreases to 20% within 60 s phosphating time. The layer growt h kinetics are determined for different systems by gravimetric measure ments. Time dependent measurement of the potential shows a nickel ceme ntation during trication phosphating. The combination of electrochemic al methods and XPS-/EDX-measurements shows the incorporation of mangan ese and nickel from the trication bath into the phosphate layer. Inhib iting passive layers of aluminium and lead were also detected. The inf luence of iron originating from the alloy layer on galvannealed steel on the phosphating process is examined.