Fig. 1 illustrates the microstructure and surface morphology of samples which include the bare substrate， the fluorinated with and without CeO2 thin film. Fig. 1a shows the initial surface state of the polished bare substrate. Some fine ste... Fig. 1 illustrates the microstructure and surface morphology of samples which include the bare substrate， the fluorinated with and without CeO2 thin film. Fig. 1a shows the initial surface state of the polished bare substrate. Some fine steaks resulting from the polishing progress are visible in the image. The surface is considerably rough and the activity of surface for magnesium alloy is various from sample to sample and within the same sample due to the presence of different phases [24，25]. In Fig. 1b， the micrograph was taken when the sample was fluorinated in the 20% HF for 20 h. There are a large number of pores and cracks on the surface of this sample; however， we cannot find the presence of remarked grinding scratches originated from the sample preparation on the surface in contrast with the bare substrate. It is well known that the magnesium alloy reacts with HF to form the fluoride coating via displacement reaction and the fluoride is insoluble， thus form a barrier on the surface of magnesium alloy. The pores and cracks distributed uneven also suggest that the activities on the surface of magnesium alloy are not very identical. The SEM image of the fluorinated sample with CeO2 thin film is shown in Fig. 1c. As can be seen from this image， most of the pores and cracks existed on the surface of the fluorinated sample have disappeared which is ascribed to the presence of CeO2 thin film. Moreover， the surface of CeO2 thin film is more uniform and more compact than that of the fluorinated sample， and this may explain why this kind of samples is the best corrosion resistant.
XPS analysis is performed aiming at identifying the chemical composition of the film and the typical survey scan spectra are shown in Fig. 2. The main element peaks of Mg， C， O， F and Ce are shown in the spectra. A significant amount of carbon is present on the surface of the sample due to some contaminant that originates from the polishing， cleaning and heat-treatment procedure; moreover， there may be some remnant since the deficiently burning for celloidin during the heat-treatment. There are three peaks in the survey scan spectra as follows: Mg 2p3/2 at about 50 eV， Mg 2s at about 88 eV and Mg Auger peak at about 301 eV. The peak of Mg 2p around 50 eV is shown in Fig. 2 which reveals only one peak at 50.2 eV belonging to Mg 2p being the fingerprint for Mg2+ in the transitional region. Hence， it is most probable that the presence of Mg in the coating is a result of the formation of MgF2 in the course of the fluorinated process and the formation of MgO during the heat-treatment. Both the main components of O 1s peak located at 530.6 eV and the F 1s peak located at 684.7 eV give the evidence that the major states are MgO and MgF2. It is found that almost a pure Ce(IV) state in the spectrum of CeO2 and this is similar to these reported by ?koda et al. [26]. 展開
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