The influence of chloride ion concentration on the corrosion behavior of thixomolded AZ91D alloy under anodic polarization and natural immersion into sodium chloride solution was investigated. A small passivation following its breakdown at critical potential was observed on anodic polarization curves. The critical potential showed a linear relationship with square root of scanning rate and logarithm of sodium chloride concentration. In natural immersion condition, electrochemical noise at corrosion potential and an inductive loop on the impedance diagram appeared after certain period. Those behaviors indicated the start of depassivation which would bring about filiform corrosion and pitting in later stage. The critical concentration of depassivation was estimated to around 10(-2) kmol/m(3). If it was lower than this value, the corrosion rate showed a constant value around 7 mg/m(2)/h (0.03 mm/y).