Effect of Plasma Nitriding Temperature on Corrosion Resistance of Nitriding Layer on X80 Pipeline Steel in 3.5%NaCl Solution

Abstract

X80 pipeline steel was nitridated by plasma nitriding and then its electrochemical corrosion behavior in 3.5% NaCl solution was studied by means of polarization curves and impedance spectroscopy. As compared to the bare X80 steel, the nitriding steels exhibit a remarkable increment in hardness, which increases with the rising nitriding temperature; the nitriding steels possess higher corrosion resistance with a lower corrosion current density by about one order of magnitude.The formation of e phase(Fe3N) with a small amount of γ ' phase(Fe4N) dominates on the surface of the steel nitridated at 450 ℃, resulting in a maximium in corrosion potential about-214 m V and a minimum in corrosion current density about 0.56 μA/cm2, respectively, togeter with the hardness increase to about 810 HV. Only the γ 'phase forms on the surface of the steel nitridated at 570 ℃,which contributes to the hardness increase to 930 HV and a obvious decrease of corrosion resistance. The plasma nitriding temperature determines the phase constitutes of the surface nitriding layer and affects greatly its hardness and corrosion resistance.