Molybdate conversion coatings on zinc surfaces

The morphology and electrochemical features of zinc surfaces converted in acid baths of sodium molybdate (Na2MoO4 · 2H2O) by simple immersion were characterized. Various bath compositions were employed in order to evaluate the layer properties as a function of the acid used to adjust the pH (H3PO4, H2SO4 and HNO3), the pH itself (3 and 5), dipping time (1, 5 and 10 min) and additives (F−, NO 2 - and NO 3 - ). Open circuit potential and electrochemical impedance measurements were used to monitor the layer performance during immersion in sulfate solution, according to procedure established elsewhere for chromating conversion coatings. Scanning electron microscopy revealed that the morphology of the molybdate layers was very sensitive to the acid used to adjust the bath pH. Layers obtained with sulfuric and nitric acids were darker, thicker and cracked when compared with layers obtained with phosphoric acid. Among the conditions tested, the conversion with 0.3 M molybdate baths at pH 3 acidified with phosphoric acid for 10 min provided the best performance in the corrosion test. In comparison with chromating, electrochemical impedance revealed that such a layer has a very similar protection mechanism in spite of a quite inferior performance.