The corrosion performance of two crystalline Co–Ni–Pr and Co–Ni–Er alloys (Pr 5.5Co 58Ni 36.5 and Er 6.5Co 57.5Ni 36) in an alkaline solution, like the one used in water electrolysis, was studied and compared with that of a commercially employed binary Co–Ni alloy (Co 57Ni 43) in order to assess the influence of rare earth elements on its corrosion behaviour. Light electron microscopy, scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples prior to the electrochemical tests. The experiments were carried out in 1 M KOH solution at 298 K. Potentiodynamic polarization curves and potentiostatic current decay transients revealed that Co–Ni–R alloys significantly improved resistance to corrosion compared with Co–Ni alloys, in spite of their two-phase microstructure, as a direct consequence of the presence of the praseodymium and erbium rare earth metals in the anodic layer formation.