Plasma electrolytic oxidation of magnesium by sawtooth pulse current

Abstract

Mg 2 SiO 4 coatings were formed on cp-Mg by Plasma Electrolytic Oxidation (PEO) using electrical regimes based on sawtooth anodic current pulses with extended ascending or descending segments (ramps). Voltage, current and light emission signals were simultaneously acquired to derive and analyse the evolution of system current and discharge population density during PEO process, while optical emission spectroscopy was employed for identification of plasma species. Coating morphological characterisation was carried out using Scanning Electron Microscopy. Phase composition was identified by X-ray diffraction. The corrosion performance was evaluated with Electrochemical Impedance Spectroscopy at various periods of exposure to corrosive media (0.5 wt% NaCl). It was shown that the application of sawtooth pulses allowed the controlling of microdischarge characteristics and coating morphology in both pulsed unipolar and reversed current regimes. Results further demonstrate that the application of anodic sawtooth pulses with the negative ramp facilitated defect healing during coating formation leading to production of more uniform PEO coatings with lower porosity and a slower degradation rate of magnesium substrate. A steady decay in the coating resistance with immersion time provides a possibility for controlling magnesium degradation and thereby for optimisation of component lifetime in applications where predictable Mg dissolution rate is required, such as sacrificial anodic protection of metallic structures and resorbable biomedical implants. • The quality of PEO coatings is modified by sawtooth pulse current regimes with controlled positive or negative anodic ramp. • Applying sawtooth waveforms with positive ramps refined the coating morphology. • Negative ramps introduced a defect healing effect reducing coating porosity and roughness. • Corrosion resistance of refined coating was proved to be better by EIS measurement.