Effect of Copper Ions on Chromate Conversion Coating of Pure Aluminum

Chromate conversion coatings (CCC) are formed an aluminium alloys by natural immersion in a bath containing fluoride ions and chromate species. The present studies employed high purity aluminium substrates, with addition of copper to the bath as copper fluoride, in order to investigate the influence of additions of Cu++ ions to the chromate coating bath and to assist understanding of the mechanism of detachment of conversion coating from aluminium-copper alloys. The coatings were formed in solutions of various compositions and the electrochemical behaviour of the aluminium was evaluated by open circuit potential time measurement. The structure, composition and morphology of the conversion coatings were examined by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis and Rutherford backscattering spectroscopy (RBS). Thus, the findings suggest that the influence of copper addition to the coating bath, as Cu++ ions, is small, and limited to a slight reduction in coating thickness.