TWIP钢表面Ni-P化学镀层组织结构及性能研究

Investigation on Microstructure and Corrosion Resistance of Electroless Ni-P Coating on TWIP Steel

目的在孪生诱发塑性钢(TWIP)表面制备Ni-P镀层,提高TWIP钢的耐腐蚀性能。方法通过化学镀工艺在TWIP钢表面制备了高磷Ni-P镀层,并进行不同温度的热处理,利用扫描电镜及能谱仪、X射线衍射仪及原子力显微镜等探究了热处理温度和时间对Ni-P镀层形貌和组织结构的影响。通过电化学方法研究了TWIP钢表面镀层的耐蚀性能。结果随着热处理温度的升高,镍磷原子发生迁移,胞状组织边界模糊,粗糙度降低,其中600℃热处理镀层表面最为致密平整。随着热处理温度的升高,镀层组织结构演变过程如下:非晶态(普通镀层)→非晶态部分晶化,Ni12P5、Ni5P2等亚稳态镍磷化合物析出(300℃)→非晶态完全晶化、稳态Ni3P相长大(400℃)→晶粒长大(500℃、600℃)。Ni-P镀层能够明显提升TWIP钢的耐腐蚀性能。随着热处理温度的升高,镀层的耐蚀性能先降低后升高。600℃热处理1h镀层的腐蚀电流密度为0.25μA/cm^(2),与普通Ni-P镀层相比降低了80.9%,与TWIP钢基体相比降低了99.6%。600℃热处理镀层光滑致密无缺陷的表面促进了保护性氧化膜的产生,使镀层的耐蚀性能提高。结论合适的热处理工艺提高了Ni-P镀层的致密性和保护能力,光滑致密无缺陷的镀层能够为TWIP钢提供良好的防护。

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 1μm 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(NiSO_(4)·6H_(2)O)was used as the source of nickel,sodium hypophosphite(NaH_(2)PO_(2)·H_(2)O)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 0.25μA/cm^(2),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.