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The main purpose of this research is to obtain a hard anodic film on aluminium and to study the effects of molybdic oxyhydroxide films on the formation and properties of anodic films. The experiments begin by immersing the A1100 aluminium specimens in ammonium molybdate solution and then anodising them in a 10 wt-% sulphuric acid solution by dc with 20 V at 20uC. Finally, the specimens are sealed in hot water at y95uC. The study proves that a longer immersing time, a higher immersing temperature, a higher concentration of ammonium molybdate and a pH value of 3 result in a harder anodic film. The molybdic oxyhydroxide film (the preimmersing process) retards the formation of the anodic film, producing a harder anodic film. The SEM images of the anodic film after the preimmersing process show that the pores of the film are closely and regularly distributed. From the EDS analysis, molybdenum is detected in the anodic films. The preimmersing process decreases the anodic film thickness. However, the anodic film with preimmersing treatment has a maximum microhardness of 601¡10 HV(50 g), which is 62% higher than that without preimmersing. SE/S286