Kinetics and Characterization of Phosphate Coatings Prepared on Aluminum Surfaces

The electrochemical formation of phosphate thin coatings on aluminum alloy АА1050 has been studied. The mass/thicknesses of the films were determined by gravimetrical measurements at different conditions: concentration of the working solutions 2.0- 11.0 vol.%; duration of the processes 1.0-10.0 min; temperature in the range of 20.0-70.0°С; cathodic current densities 0.1- 0.5 Adm–2. The chemical elements containing in the phosphate coatings were determined by EDX-analysis. The morphology and topography of the coatings were studied by SEM. The corrosion behavior and the protective ability of the phosphate coatings are investigated by potentiodynamic polarization method (PDPM) and by electrochemical impedance spectroscopy (EIS). It has been found that as the concentration, temperature and density of the polarizing cathode current density rises, the mass of the phosphate films increases – the thinner ones being dense and uniform, while the thicker ones are cracked. Mainly phosphorus, oxygen, molybdenum and nickel exist in the coatings and their amounts increase with temperature of the phosphating bath is raised while the Al content in the coatings reduces at the same conditions. The corrosion of the phosphated specimens increase slightly with a rise in the temperature of the solutions during their phosphating, when the cracks in the coatings and resp. the bare metal of the substrate increases. The uncoated specimen has lower barrier ability, while the phosphated has higher barrier ability in the corrosion medium. Keywords: Aluminum alloys; conversion coatings; phosphating; tin films; corrosion