Limitations in the use of the potentiodynamic polarisation curves to investigate the effect of Zn on the corrosion behaviour of as-extruded Mg–Zn binary alloy

•Alloyed Zn accelerates the H2 evolution rate of the Mg-(1-4)Zn binary alloys.•Alloyed Zn enhances protectiveness of the passive film of the Mg–Zn binary alloys.•Acceleration of the H2 evolution by Zn leads to increase in net corrosion rate of the alloys.•Polarisation test has some limitations to investigate the Mg–Zn alloys showing passivity.•Polarisation test should be understood in association with long-term corrosion tests. The effects of Zn on corrosion behaviour of as-extruded Mg-(1-4)Zn alloys were investigated using an immersion test, a zero resistance ammeter technique, and a potentiodynamic polarisation test. As a result, it was revealed that the solutionised Zn enhanced protectiveness of the passive film, and accelerated the H2 evolution rate of the Mg–Zn binary alloys. The acceleration of the H2 evolution rate by addition of Zn leads to an increase in the net corrosion rate of the Mg–Zn alloy. In this research, the polarisation test was found to have some limitations for evaluating the true corrosion behaviour of passive Mg–Zn alloy.