In the present work, the corrosion behavior of a twin-roll cast WZ73 magnesium alloy is investigated by potentiodynamic polarization and climate chamber tests in different sodium chloride solutions. The corrosion parameters E_corr and j_corr are extracted from the polarization curves, and the critical concentration for the initiation of pitting corrosion is thus identified. The critical concentration for the initiation of pitting is confirmed by means of climate chamber tests. As a consequence, the WZ73 magnesium alloy is found to be highly susceptible to pitting corrosion in sodium chloride solutions of concentrations higher than 10⁻⁴ mol/L. Moreover, when the NaCl concentration increases, the corrosion mechanism changes from pitting to filiform corrosion, which is attributed to micro-galvanic effects. At low NaCl concentrations, the corrosion attack remains localized. When the NaCl concentration increases, pitting occurs and, in addition to the pits that form, micro-galvanic corrosion leads to filiform corrosion attack. This is caused by the electrochemical potential of the long period stacking ordered phase, which is higher than that of the α-Mg matrix. Consequently, the corrosion attack is focused on the phase boundary between the α-matrix and LPSO phases.