Effect of additive Al₂ O₃ powders on micro-arc oxide coating of magnesium alloy

3.1 Influence of the additive amount of Al₂O₃ on film thickness and corrosion rate

The samples with added different additive amounts of Al₂O₃ powders were tested by thickness and a full immersion test. Respectively, we took five random test points on both sides of the relative surface, an averaged results as the thickness of oxide films. We put the sample in a concentration of 3.5% NaCl solution at room temperature for 72 h. The corrosion rate showed the results of testing.

Figure 1 shows that with increasing additive amount of Al₂O₃, the film thickness was on the rise. It arrived at 4 g/l, the average of film thickness achieved the highest. The corrosion rate appeared a turning point at this time. With the additive amount continuously increasing, the membrane thickness decreased, uniform film thickness was no longer uniform, and corrosion rate reduced slowly. Because of the increasing Al₂O₃ additive amount, there were a lot of white spots on the ceramic film surface, which could be observed by eye, and the roughness of the film increased. According to the above analysis, the additive amount of Al₂O₃ was 4g/l.

Figure 1

The influence of the additive amount of Al₂O₃ on film thickness and corrosion rate.

3.2 Al₂O₃ added before and after the MAO ceramic microstructure

As can be seen in Figure 2, the oxidation of the whole structure was not changed after adding Al₂O₃ powders. The coatings were still composed of a dense layer and a porous layer, which linked closely together. Before adding the powder, the porous layer had loose organization was of a great thickness and had holes, cracks, and other defects. The loose layer was more restrictive after adding Al₂O₃ powders.

Figure 2

MAO of the surface and cross-sectional morphology. (A) Added Al₂O₃ ceramic coating surface prior to SEM. (B) Added Al₂O₃ ceramic coating surface after the SEM. (C) Added Al₂O₃ before ceramic coating cross-section SEM. (D) Added Al₂O₃ after ceramic coating cross-section SEM.

The whole phenomenon of the MAO process was not changed after adding Al₂O₃ powders. Early discharge process happened at the surface of magnesium alloy with a small and dense spark. With the extension of oxidation time, the required breakdown voltage continued to rise, oxidation reaction became intense, the spark became large and sparse, and the sample was more and more difficult to oxidate. This may be because the surface of ceramic coating, after adding the Al₂O₃ powders was still composed at the top, of holes, uneven “volcano-like” melt, melting at the electrolyte with “cold quenching” effect of rapid solidification, so that a local area near the discharge channel of the film thickness thickened, while the discharge channel narrowed. With the oxidation time increasing, melting significantly increased before adding Al₂O₃ powders. The discharge channel became more reducible and smaller. Eventually it made the surface of the ceramic coating volcano-like with fewer and smaller holes. These phenomena are in line with Figure 2.

3.3 EDX energy spectrum analysis

Figure 3 illustrates the SEM images (Figure 3A and C) of the MAO film with Al₂O₃. When the Al₂O₃ powders were added into the electrolyte, it can be seen many bright particles were homogeneously distributed in the MAO coating. EDS result of the bright particle marked in Figure 3A (point 1) and dark region marked in Figure 3C (point 2) were shown in Figure 3B and 3D. Al and O content of the bright particle was higher than dark region’s obviously. From XRD and EDS analysis of the MAO coating, the bright particle was the Al₂O₃ powders. During MAO, when the molten MAO ceramic coating erupted from the discharge channels, Al₂O₃ powders would be mixed within the MAO coating.

Figure 3

Compound MAO ceramic coating energy spectrum analysis results. (A) EDX spectra of 1. (B) Spectra of one analysis result. (C) EDX spectra of 2. (D) Spectra of two analysis results.

3.4 Adding Al₂O₃ powders before and after the phase structure of the ceramic membrane contrast

Comparing (A) with (B) in Figure 4, the effect of the powder on phase composition of MAO coating can be seen after adding Al₂O₃ powders. The phase composition of ceramic membrane with Al₂O₃ powders was Al₂O₃, MgO, MgAl₂O₄, Mg₃ (PO₄)₂ and the ceramic membrane increased the phase of Al₂O₃ than before. Because of the corrosion resistance of Al₂O₃, the corrosion resistance of composite film is significantly improved. This is same with the result of Figure 1.

Figure 4

MAO coating XRD diagram. (A) Ceramic membrane pre-accession XRD diagram without Al₂O_3. (B) Ceramic membrane after added Al₂O₃ XRD diagram.

3.5 Ultrasonic waves on the phase structure of the ceramic membrane

Comparing (A) and (B) in Figure 5, nano-Al₂O₃ under ultrasonic waves’ peak intensity and peak area was significantly larger than that without ultrasonic irradiation, which means the content of Al₂O₃ in the films under the conditions of the ultrasonic waves is much higher than under the conditions of non-ultrasonic waves. The conditions of ultrasonic waves effectively promote the diffusion and the content of nano-Al₂O₃ powders on the surface coating, and then effectively improve the hardness and corrosion resistance of the film surface.

Figure 5

MAO coating XRD diagram with ultrasonic waves. (A) Ceramic coating XRD diagram without ultrasonic waves. (B) Ceramic coating XRD diagram with ultrasonic waves.

3.6 Electrochemical test

As can be seen in Figure 6, the corrosion potential of magnesium alloy is –1.630 V; Al₂O₃ not adding into the corrosion potential is –1.564 V, after adding Al₂O₃ into the corrosion potential is –1.291 V, after calculating the corrosion current is also significantly lower, indicating that corrosion resistance of AZ91D magnesium alloy MAO ceramic membrane after treatment in solution significantly improved. The corrosion resistance of ceramic coating with Al₂O₃ powder was improved. Analysis showed that the corrosion resistance significantly increased mainly due to a special composite ceramic membrane, which contains Al₂O₃ powders. The ceramic membrane layer becomes compact after adding Al₂O₃ powders, and the holes of the surface layer significantly reduce, which can be seen in the composite MAO ceramic membrane microcosmic structure.

Figure 6

Al₂O₃ MAO coating on the corrosion potential.