Deposition of zinc–zinc phosphate composite coatings on aluminium by cathodic electrochemical treatment

The role of cathodic electrochemical treatment as an energy efficient and eco-friendly method to deposit zinc–zinc phosphate composite coatings on Al is addressed. The deposition of zinc–zinc phosphate composite coatings was carried out under galvanostatic conditions using an electrolyte solution containing 5 g/l of ZnO and 12 ml/l of o-phosphoric acid (85%), maintained at 27 ± 1 °C and two graphite disc anodes, placed on the two sides of the Al cathode. The applied current density (5 to 50 mA/cm2), pH of the electrolyte (1.50, 2.30 and 3.00), treatment time (5 to 60 min) and concentration of ZnO (1 to 5 g/l) were used as the process variables. The methods of evaluation include, assessment of colour and uniformity by visual observations, adhesion by pull-off adhesion test, coating weight by a measure of gain in weight after deposition, morphological features by scanning electron microscopy, chemical composition by energy dispersive X-ray analysis and phase contents by X-ray diffraction studies. The corrosion resistance of the coatings, in 3.5% NaCl, was evaluated by potentiodynamic polarisation studies. The findings of this study reveal that the surface morphology, phase content and corrosion resistance of the composite coatings largely depend on the proportions of Zn and zinc phosphate in them. The cathodic electrochemical treatment method provides ample avenues to manipulate the volume fraction of Zn and zinc phosphate in the resultant composite coatings by a careful choice of the process parameters and hence, it is possible to deposit coatings on Al with varying degrees of corrosion protection.