Abstract <p>Gd–O–N coatings were deposited by the method of reactive anodic evaporation of Gd in Ar/O₂/N₂ low-pressure arc plasma (~0.4 Pa) under conditions of high (~90%) degree of metal vapor ionization. The coatings were analyzed using X-ray diffraction and Raman spectroscopy. It was shown that the addition of N₂ to the gas mixture during the coating synthesis promotes an increase in the crystallite size of the cubic phase of Gd₂O₃ and the formation of a structure, close in its parameters to defect-free samples. The effect is manifested in the range of N₂ pressures limited from above by values (~3.4 × 10^–2 Pa under the experimental conditions), exceeding which leads to the formation of the monoclinic phase of Gd₂O₃ and the dominance of GdN in the coatings. Gd–O–N coatings provide effective protection of the biodegradable magnesium alloy AZ31B from corrosion in a physiological solution. Based on potentiodynamic polarization and impedance spectroscopy of magnesium alloy samples, it was found that Gd–O–N reduces the corrosion rate in Earl’s solution by two orders of magnitude (down to 0.07 mm/year). A correlation was found between the corrosion characteristics and the structural-phase state of the coatings obtained at different N₂ flows. It was shown that the coatings synthesized at an N₂ flow providing the largest crystallite sizes (~21 nm) of cubic Gd₂O₃ are characterized by the highest polarization resistance (686 kΩ cm²). In contrast, the appearance of the monoclinic Gd₂O₃ phase and nitride in the coatings and a decrease in the crystallite size at high N₂ flows reduce the polarization resistance by half.</p>