The effects of manganese on the corrosion behaviour of welded and un-weld as–cast Al-Fe-Si alloy has been studied. Alloys of varying percentage of manganese from 0.019-0.24 (0.019, 0.15, 0.16, 0.17, 0.24 wt. %) were sand cast into rectangular test sheets of dimension 300 ´ 20 ´ 5 mm . Each of the alloys were cut into two equal parts and welded together. The corrosion characteristics of the five categories of alloys (welded and un-weld) in 0.5M NaCl at room temperature over a period of 1464 hours were investigated by weight loss method. The results obtained showed that Al-Fe-Si samples (un-weld and welded) in 0.5 M NaCl medium displayed a very good resistant to corrosion according to the percentage of manganese added. It was observed that sample A1 and A2 with manganese percentage of 0.17Mn in NaCl medium displayed a very good resistance to corrosion while sample D1 and D2 with manganese percentage of 0.24Mn in the same medium has the lowest corrosion resistance. The result actually confirmed that the addition of manganese aided corrosion resistance but to a certain level, since the addition of 0.150-0.170Mn leads to progressive inhibiting but after this range the corrosion rate increases.