TWIP钢表面Ni-P化学镀层组织结构及性能研究OA北大核心CSTPCD

As advanced high-strength steel,twinning-induced plasticity (TWIP) steel has the high product of strength and plasticity,excellent impact resistance and good weldability.However,TWIP steel has poor corrosion resistance due to the presence of active Mn elements with high content,especially in the non-oxidizing acid solutions or environments containing chloride.In this paper,in order to improve the corrosion resistance of TWIP steel,high-phosphorus Ni-P binary coatings were prepared on the Fe-Mn-Al-C TWIP steel.Prior to the electroless plating test,samples with dimensions of 10 mm×10 mm×3 mm were mechanically ground with sandpaper,then polished with a 1 μm diamond paste,followed by ultrasonic cleaning using ethanol and acetone.The pre-treatment process before plating mainly included alkali washing,acid pickling and activation.The Ni-P coatings on the TWIP steel samples were electrolessly deposited in an acidic hypophosphite-reduced nickel bath at 90 ℃,and a pH of 4.7 for 2 h plating duration.In the electroless plating bath,nickel sulphate (NiSO4·6H2O) was used as the source of nickel,sodium hypophosphite (NaH2PO2·H2O) as the reducing agent and P source,lactic acid (CH3CHOHCOOH) and acetic acid (CH3COOH) as the complexing agents,and a little amount of thiourea as the stabilizing agent.The depositions were annealed in a tube furnace under high purity argon inert atmosphere at 300,400,500,600 ℃ for 1,2,4 h,respectively.The effect of heat treatment temperature and time on the microstructure evolution and morphologies of electroless Ni-P coatings was analyzed by scanning electron microscopy (SEM),energy dispersive spectrometer (EDS),electron probe microanalyzer (EPMA) and X-ray diffractometer (XRD).The surface roughness of the coatings after annealing treatment was measured by means of atomic force microscope (AFM).The corrosion resistance of Ni-P coatings on the surface of TWIP steel was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The results showed that compared with the annealing time,the annealing temperature had a greater influence on the surface morphology of the Ni-P coating.The boundary of cell structure in the coating became blurred,and the roughness decreased with the increase of the annealing temperature.The evolution process of the coating microstructure was as follows:amorphous state (25 ℃)→partial crystallization and precipitation of metastable nickel-phosphorus compounds such as Ni12P5 and Ni5P2 (300 ℃)→complete crystallization of the amorphous coating and formation and growth of the stable Ni3P phase (400 ℃)→grain growth (500 ℃,600 ℃).In the NaCl aqueous solution,the corrosion resistance of the electroless coating under different heat treatment conditions was better than that of the TWIP steel.As the annealing temperature increased,the corrosion resistance of the coating decreased at first,and then increased.The corrosion current density of the coating after being annealed at 600 ℃ for 1 h was 0.25 μA/cm2,which was reduced by 80.9% compared with ordinary Ni-P coatings and 99.6% compared with TWIP steel substrates.The smooth,dense and defect-free surface of the coating with 600 ℃ annealing treatment promoted the formation of a protective oxide film.The corrosion resistance of the coating was improved.The appropriate heat treatment process can improve the compactness and protective ability of Ni-P coating.A compact and defect-free coating can provide excellent protection for TWIP steel substrates.