Plug-in specimens enable measurement of reliable Mg polarization curves. Cathodic polarization curves were measured for high purity Mg in 3.5% NaCl saturated with Mg(OH)2 using (i) mounted specimens and (ii) plug-in specimens. Polarization curves yielded the corrosion current density icorr and the corresponding corrosion rate Pi, which was compared with corrosion rates evaluated from hydrogen evolution, PH, and weight loss, PW. Mounted specimens produce Pi values three times larger than plug-in specimens, due to crevice corrosion in the mounted specimens. Plug-in specimens had no crevice and allow simultaneous measurement of PH and Pi. Pi was less than PH and indicated an apparent valence of 1.45 in support of the existence of the uni-positive Mg+ ion.