Pulsed unipolar plasma electrolytic oxidation (PEO) processes for magnesium and resultant Ca- and P-containing coatings were investigated. Electrical waveforms of the PEO process were quantitatively studied, and correlated with the coating characteristics. It was found that the system relaxation during the voltage pulse-off period included capacitor discharging and dielectric relaxation of the coating material. The extent of the latter depended on the pulse frequency, and a critical frequency of 2000 Hz was identified above which only capacitor discharging could be observed. Correspondingly, the treatments at the critical frequency resulted in the highest internal stress and the lowest corrosion resistance of produced PEO coatings in a simulated body fluid, as evaluated by electrochemical impedance spectroscopy and potentiodynamic polarisation scans. At the same time, the corrosion resistance of Mg was improved, by a factor of >10, by the PEO treatments at a pulse frequency of 3000 Hz, which allowed simultaneous increases in coating thickness, porosity refinement and internal stress relaxation without significant cracking.