Growth mechanism of 2024 aluminum alloy micro-arc oxide layer in cobalt-containing electrolyte

The coatings were prepared on the surface of 2024 aluminum alloy by micro-arc oxidation. The effect of the addition of cobalt salt on the growth mechanism of the coating was investigated. The addition of cobalt salt prolonged the spark discharge phase of microarc oxidation and achieved the co-film formation of the solution component and the substrate. XRD, SEM, EDS, and XPS were used to investigate changes in the physical composition, surface morphology, and chemical composition of coatings prepared at different times. The hardness and corrosion resistance of the coatings were investigated as oxidation time increased. The surface of the coatings can be divided into two regions: the loose "cobalt-rich zone" and the dense "aluminum-rich zone", and with the increase of oxidation time, the surface of the coating becomes more dense and flat, and the pores are mostly closed. The time-voltage curve shows that the growth of the micro-arc oxide coating in cobalt-containing electrolyte can be divided into three stages. The coating grows rapidly in the first 15 min, and a black coating is obtained, with the color-revealing substance being CoO; the coating turns from black to blue after 15 min, as CoO transforms to blue CoAl2O4. XPS and XRD analyses showed that the addition of cobalt salt significantly promoted the formation of alumina in the coating, and the conversion of CoO to CoAl2O4 and γ-Al2O3 to α-Al2O3 occurred at the later stage of micro-arc oxidation, when the hardness of the coating would obtain a large enhancement. The corrosion resistance test results showed that the coating prepared by treatment in the cobalt-containing electrolyte for 60 min showed the best corrosion resistance with a corrosion potential 100.3 mV higher than that of the substrate and a corrosion current reduced by 4 orders of magnitude to 6.20 × 10−9 A·cm−2.