EFFECT OF GRAIN SIZE ON ATMOSPHERIC CORROSION RESISTANCE OF ULTRA--LOW CARBON IF STEEL

Abstract

Three kinds of ultra-low carbon IF steel with different grain sizes,and same chemical composition were prepared by different rolling and heat treat process.The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test,cyclic immersion corrosion test,AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test,the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15μm to 220μm.The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15μm to 46μm.As grain size increase from 15μm to 220μm,the whole compactness of rust are increased,the rust resistance and the atmospheric corrosion resistance are increased.The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed.The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased.Meanwhile,the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased.The atmospheric corrosion resistance is influenced by the two factors simultaneously.