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43 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 ground and polished; then the samples were immersed in a beaker of acetone and cleaned in an ultrasonic vibration appara- tus. Finally, the samples were washed and dried. The alkaline electrolyte of a silicate system was selected, with pH from 11 to 13 at room temperature. The electrolyte was mainly composed of 15 g/L sodium silicate (Na 2 SiO 3 ) and 30 g/L sodium hydroxide (NaOH). The cathode voltage varied from 4 to 24 V. The treatment temperature was maintained at 10±2 °C by a recyclable water cooling system during the MAO process. The morphologies of MAO coatings were characterized by Sirion Field Emis- sion † scanning electron microscopy (SEM). Phase constitution was identified by an XD-3A † x-ray diffractometer (XRD). 10 The corrosion resistance of MAO coatings was evaluated using a salt spray test. The 2A12 aluminum alloy samples were subjected to a 5% sodium chloride (NaCl) solution to simulate an extremely corrosive environ- ment. The temperature of the salt spray chamber was maintained at 37 °C for 96 h. During the process, the pH value ranged from 6.5 to 7.2. 11 Results and Discussion Surface Morphologies of Microarc Oxidation Coatings The surface morphologies of MAO coat- ings with cathode voltages of 4 and 24 V are shown in Figure 1. These SEM micrographs show that different surface morphologies result when applying dif ferent cathode voltages. The coating surface became uni- form, dense, and smooth with increasing cathode voltage, and many micropores dis- tributed on the surfaces of these MAO coat- ings functioned as conductive paths. When the cathode voltage was 4 V, a stratification phenomenon was observed and the electrically conductive path was minimal. The coating surface was uneven and coarse and a small amount of white ceramic particles was found. Furthermore, the scratches generated in the polishing process were still visible with the naked † Trade name. FIGURE 1 Surface morphologies of MAO coatings with different cathode voltages: (a) 4 V and (b) 24 V. FIGURE 2 XRD patterns of MAO coatings with different cathode voltages: (a) 4 V and (b) 24 V. eye. The results showed that the MAO coat- ing formed under this condition was very th i n . W h e n th e c a th o d e v o lt a ge w a s increased to 24 V, the distribution of micro- pores was uniform, the size of micropores became smaller, and the surface of the coating becam e smooth and compact. Thus, it can be concluded that increasing the cathode voltage facilitated the forma- tion of dense and smooth MAO coatings. 12 Phase Components of Microarc Oxidation Coatings Figure 2 displays XRD patterns of the MAO coatings formed on the 2A12 alumi- num alloy using different cathode voltages. The MAO coating is mainly composed of aluminum and aluminum oxide (Al 2 O 3 ) per XRD analysis. The intensity of Al 2 O 3 diffrac- tion peaks increased with increasing cath- ode voltage. When the cathode voltage was 4 V, the diffraction peaks of the Al 2 O 3 phases in the XRD pattern (Figure 2[a]) were very weak, which indicates the ceramic coating formed on the 2A12 aluminum alloy sur- face is very thin. In Figure 2(b), the diffrac- tion peaks of the Al 2 O 3 phases strengthened significantly with the increased cathode voltage. It can be concluded that the con- tent of the Al 2 O 3 phase in the MAO coatings and the thickness of the ceramic coatings both increased with the increase of cathode voltage. 13 Corrosion Resistance of Microarc Oxidation Coatings The corrosion rate of MAO coatings produced with different cathode voltages was determined by measuring the weight loss of the coated samples after salt spray treatm ent. W h en th e catho d e v olt a ge increased from 4 to 24 V, the weight loss of