Abstract: The corrosion of pipeline delivering the reverse osmosis permeate of seawater in one power plant was investigated.Erosion-corrosion test and electrochemical test were made and scanning electron microscopy(SEM) and three-dimensional video microscope were adopted to study the corrosion behavior rules of 20 steel and ductile iron in reverse osmosis permeate of seawater.The change of water quality parameters was also checked and recorded during the tests.The results indicated that after 45 days of erosion-corrosion,the corrosion rate of 20 steel was higher than 0.1 mm·a-1(the sixth level of corrosion resistance)while the corrosion rate of ductile iron was about 0.1 mm·a-1.The corrosion rates of two metals were decreased with the increase of erosion time.From a macro perspective,the corrosion morphologies of two metals were relatively uniform without obvious pitting.The results of electrochemical tests showed that with the erosion going on,the corrosion current density of the two metals decreased while the change of charge-transfer resistance was not obvious.The medium values of p H and dissolved oxygen contents changed irregularly while the medium values of salinity and conductivity increased with dissolution of corrosion products in the medium.