Abstract: The effect of phosphorus on the pitting propagation of low carbon steels was studied by specific occluded corrosion cell tests and by analyzing the corrosion products with an energy dispersive spectrometer.The results showed that there were phosphate ions in sediments on the surface of the steels in the occluded hole,and the phosphate ions were the production of phosphorus in the steels.During the activation of pits in corrosion,phosphorus converted into phosphate ions on the surface of the metal,then phosphates formed and deposited,and these phosphates inhibited the propagation of pits.Different contents of phosphorus in the steels could lead to a great difference of phosphate content on the surface of the steels,and the effects of pit propagation inhibition had a large difference too.In the same steel sample,because of phosphorus segregation,the corrosion rate was lower in the region of high phosphorus content than that of low phosphorus content.The formation of convexo-concave corrosion grooves was due to phosphorus segregation.