Electrochemically triggered nucleation and growth of zinc phosphate on aluminium-silicon-coated steel

Steels with an aluminium silicon (AS) metallic coating behave passive in standard phosphating solutions. An electrochemical process was used to increase pH near the surface by polarising to potentials in which hydrogen is evolved, subsequently triggering the formation of phosphate coatings. These conversion coatings have similar morphology and composition than those produced on reactive metal surfaces. Size histograms after different active phosphating times obtained from image analysis show instantaneous nucleation of phosphate crystals with subsequent growth. The use of a commercial titanium phosphate based activation bath leads to homogeneous crystal growth, but appears not to affect significantly the crystals observed shortly after nucleation. Phosphating with a sequence of short potential pulses leads to nucleation of new crystals near the interface of previously nucleated crystals and is hence not an alternative to obtain morphologies with small crystals. Comparing layers prepared here with layers on the same AS coating prepared under open circuit conditions using fluoride additives shows that electrochemically grown layers are more homogeneous.