Excellent plasma electrolytic oxidation coating on AZ61 magnesium alloy under ordinal discharge mode

In the preparation of plasma electrolytic oxidation (PEO) coating, the rapid heating of freely-happened electron avalanche under traditional discharge (TD) mode inevitably results in a strong eruption of electric breakdown melt. The PEO coating is loose and invariably composed of a very thin inner dense layer and an outer loose layer, as a result of which its properties and application have been limited greatly. In this work, for purpose of weakening the eruption of breakdown melt, thickening the inner dense layer, densifying the outer loose layer and improving the performance of PEO coating, ordinal discharge (OD) mode of PEO coating is developed by regulating the mass ratio of MgF₂ to MgO (α) and voltage in the PEO investigation on AZ61 magnesium alloy in KF-KOH electrolyte. The formation mechanism under different discharge mode, electrochemical corrosion and wear of PEO coatings are investigated. The results show that the suitable α and voltage for effective OD are 1.3 and 130V under which the freely-happened electron avalanche in MgF₂ under TD mode can be restricted by the adequate adjacent MgO. Compared with TD mode, the inner dense layer, in which the (101) plane of MgF₂ is parallel to the (111) plane of MgO at their well-knit semi-coherent interface, is thickened to 2.4~7.2 times, the corrosion potential (E_corr) improvement is enlarged to 3.6~13.2 times and the corrosion current intensity (I_corr) is reduced from 10.8~9.499 to 0.433 (10⁻⁶ A/cm²). The outer loose layer is densified and the wear rate is lessened 65.5%~89.8% by the evident melioration in surface porosity, impedance and hardness. This work deepens the understanding about the discharge of PEO coating and provides an available OD mode for preparing excellent PEO coating.