On the dissolution kinetics during acid pickling and Zr-based conversion coating of aluminum alloys using element-resolved electrochemistry

The acid pickling of Al-3at.%Mg, Al-3at.%Cu, and aluminum alloy (AA) 7449-T651 in nitro-sulfuro-ferric acid was investigated using element-resolved electrochemistry (AESEC) in terms of their elemental dissolution kinetics. The influence of this acid pickling on the subsequent Zr-based conversion coating process was also demonstrated on these alloys by monitoring the dissolution rates of the alloying elements during conversion and the final elemental depth profiles from calibrated glow discharge-optical emission spectroscopy (GD-OES). The separate influence of fluoride (F−) and nitrate (NO3−) as additives on the dissolution kinetics was also investigated when added to the conversion coating bath solution. F− increased the dissolution rate of Al but no significant effect was seen on Cu, while NO3− enhanced the dissolution rates of both elements. Fourier-transform infrared reflection absorption spectroscopy (FT-IRRAS) data suggested a greater Zr-fluoride presence if the conversion coating was performed on a non-acid-pickled surface.