The influence of surface roughness on the pitting potential using the electrochemical microcell technique is reported. Two Al-Mg-Si model alloys of varying manganese content were investigated. With potentiodynamic anodic polarization in 0.1 M NaCl solutions and with different sizes of exposed area it is shown, that a critical defect density is present which controls the pitting potential at small polarization potentials. This defect density is related to the crevice distance. With smoother polished surfaces a larger crevice distance is measured. The pitting potential measured on an exposed area smaller than the crevice distance reveals that small effects of the alloying element Mn become visible. For the fine polished surface it is shown that the addition of 0.5 wt% Mn to the model alloy lowers the pitting potential.