Local Volta potential differences (Δψ) between a platinum coated AFM tip and various pure metal specimens with a thin humidity induced surface electrolyte layer have been determined via scanning Kelvin probe force microscopy (SKPFM). SKPFM derived Δψ display a linear correlation with the same quantities obtained under nominally identical environmental conditions using a scanning Kelvin probe (SKP); the slope is within experimental error one. By exploiting this correlation and using a SKP previously calibrated versus a series of metal/aqueous metal ion redox couples it is possible to indirectly calibrate atmospheric SKPFM derived Δψ with electrochemical potential. Good correlation is also obtained between immersion corrosion potential (Ecorr) in NaCl electrolyte and SKP determined Δψ in the presence of a NaCl dosed humidity layer. This implies that SKPFM may be calibrated directly against immersion Ecorr provided SKPFM measurements are performed under suitably controlled conditions of atmospheric relative humidity and surface electrolyte dosing.