The structure and composition of zirconium based chromium free conversion layers on magnetron sputtered pure aluminium were characterised as a function of immersion time in the aqueous conversion bath to understand the mechanism of film formation. The influence of the conversion time on the thickness of the formed layer on pure aluminium was investigated using scanning electron microscopy, Rutherford backscattering spectroscopy and transmission electron microscopy. The conversion time was varied between 30 and 600 seconds. The SEM micrograph reveals zirconium, aluminium and fluorine are the main constituents of the film, with zirconium accounting for about 24% of the total film composition. EDX analysis shows zirconium based conversion coating covered the sputtered superfine aluminium uniformly, with cracks evenly distributed throughout the film layer.