Recently, superior corrosion properties of novel zinc coatings alloyed with magnesium have been reported.The protection mechanism of magnesium in the coatings was studied on model zinc-magnesium alloys containing from1 to 32 wt. % Mg prepared by casting. Their chemical and phase composition was determined. The samples werecontaminated with NaCl and exposed to wet air. Weight loss was evaluated after 28 days of exposure. The compositionof corrosion products was analyzed using different techniques. The exposure tests were completed with scanning Kelvinprobe measurements. Alloying of zinc with small quantities of magnesium significantly decreased the weight lossof zinc-magnesium alloys. The effect was strongest at 4–8 wt. % Mg in the structure. Weight loss of these alloys was upto 10 times lower than that of zinc. This was found to be connected dominantly to the efficiency of the oxygenreduction, which was significantly limited on the surface of ZnMg phases covered with magnesium-based oxide layers.