Influence of conversion time on polymeric speciation in zirconium conversion coatings deposited on aluminium alloy 5754: a ToF-SIMS study

The influence of conversion time on polymeric speciation and protective performance of zirconium conversion coatings (ZrCCs) on aluminium alloy AA5754 was investigated using samples treated in 150 ppm H2ZrF6 at pH 4.6 for 230 s and 480 s. Characterisation combined time-of-flight secondary ion mass spectrometry (ToF-SIMS) with region-of-interest analysis, electrochemical impedance spectroscopy in dilute Harrison’s solution after one hour, atomic force microscopy, and contact-angle measurements. ZrCC thickening is conversion-time-dependent, with a preference for growth above intermetallic particles. ToF-SIMS identified monomeric, dimeric, and trimeric zirconium hydroxide fragments, while tetrameric species were detected only at very low intensities, in contrast to previous observations on steel, possibly due to the lower overall pH increase during ZrCC formation. Despite the thicker layer obtained at 480 s, EIS indicated no improvement in corrosion protection compared with the 230 s. Morphological and wettability analyses further showed that the shorter treatment produced a rougher but more compact and uniform coating, which correlates with better short-term protective behaviour. These findings emphasise that compactness and uniformity, rather than thickness or wettability, govern the early protective function of ZrCCs and raise the question of whether tetrameric polymeric forms are required to establish a protective layer, as hypothesised for cold-rolled steel.