Two kinds of metal oxide coatings were fabricated on 7050 Al alloy by hard anodization (HA) and micro-arc oxidation (MAO) techniques. The microstructure, phase composition, and corrosion behavior of the two coatings were studied by scanning electron microscopy, X-ray diffraction, and corrosion tests, respectively. When compared with the HA coating, the MAO one was more effective to isolate the substrate from the corrosive environment. In addition, as confirmed by electrochemical tests, the MAO coating was of better corrosion resistance than the HA coating. Furthermore, it was revealed by neutral salt spray test that the MAO coating could protect substrate alloy over 1140 h, while the HA coating can only protect substrate alloy for 46 h due to the amorphous composition and through thickness defects (micro-pores and micro-cracks).

Micro-arc oxidation (MAO) was performed on AZ31 and AZ91 Mg alloys to explore the relationship between Mg substrates and corrosion behaviors of resultant coatings. The microstructure and long-term corrosion performance of the two coatings were investigated. Results showed that MAO coating on AZ91 alloy provided a longer-term corrosion protection to substrate than that on AZ31 alloy, despite having similar microstructure and phase composition. However, once corrosion occurred, the corrosion area of MAO coating on AZ91 alloy enlarged at a faster rate compared to that on AZ31 alloy during immersion tests, and their corrosion morphologies were different. Based on the corrosion features of Mg substrates and MAO coatings, roles of substrates on the corrosion processes of coatings were analyzed.

In this study, white thermal control coatings were produced on a 6061 Al alloy using plasma electrolytic oxidation (PEO). The coatings were mainly formed by incorporating K₂ZrF₆. The phase composition, microstructure, thickness, and roughness of the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), a surface roughness tester, and an eddy current thickness meter, respectively. The solar absorbance and infrared emissivity of the PEO coatings were measured using a UV–Vis–NIR spectrophotometer and FTIR spectrometer, respectively. The addition of K₂ZrF₆ to the trisodium phosphate electrolyte was found to significantly enhance the thickness of the white PEO coating on the Al alloy, with the coating thickness increasing in proportion to the concentration of K₂ZrF₆. Meanwhile, the surface roughness was observed to stabilize at a certain level as the K₂ZrF₆ concentration increased. At the same time, the addition of K₂ZrF₆ altered the growth mechanism of the coating. In the absence of K₂ZrF₆ in the electrolyte, the PEO coating on the Al alloy surface predominantly developed outwards. However, with the introduction of K₂ZrF₆, the coating’s growth mode transitioned to a combination of outward and inward growth, with the proportion of inward growth progressively increasing in proportion to the concentration of K₂ZrF₆. The addition of K₂ZrF₆ substantially enhanced the adhesion of the coating to the substrate and endowed it with exceptional thermal shock resistance, as the inward growth of the coating was facilitated by the presence of K₂ZrF₆. In addition, the phase composition of the aluminum alloy PEO coating in the electrolyte containing K₂ZrF₆ was dominated by tetragonal zirconia (t-ZrO₂) and monoclinic zirconia (m-ZrO₂). With the increase in K₂ZrF₆ concentration, the L* value of the coating increased from 71.69 to 90.53. Moreover, the coating absorbance α decreased, while the emissivity ε increased. Notably, at a K₂ZrF₆ concentration of 15 g/L, the coating exhibited the lowest absorbance (0.16) and the highest emissivity (0.72), which are attributed to the enhanced roughness resulting from the substantial increase in coating thickness caused by the addition of K₂ZrF₆, as well as the presence of ZrO₂ with higher emissivity within the coating.

S: Micro-arc oxidation (MAO) coatings were prepared on the AZ31 Mg alloy under the constant/gradient current control modes. The effects of current output modes on the growth rate, energy consumption, microstructure, and corrosion resistance of MAO coatings were studied. The results shows that the current step-down mode (G852) exhibited a lower unit energy consumption than the constant current mode (C5) and the current step-up mode (G258), even though the same average current density current was used during MAO process. Meanwhile, the G852 is conducive to a thick and dense coating, resulting in a better corrosion resistance. The method of energy saving preparation of corrosion resistant MAO coating by adjusting the current mode is of great significance for practical production.