镀液中锌镍离子浓度比对锌镍合金镀层耐蚀性的影响

Influence of concentration ratio of zinc ion to nickel ion in plating bath on corrosion resistance of zinc–nickel alloy coating

采用氯化铵体系在45钢基体上电镀制备Zn–Ni合金镀层,镀液组成和工艺条件为:NH_4Cl 200 g/L,H_3BO_3 20 g/L,Zn^(2+)、Ni^(2+)总浓度1.1 mol/L,pH 4.3,电流密度20 m A/cm^2,温度30°C,时间15 min。采用扫描电镜、能谱仪和X射线衍射仪表征了不同锌镍离子浓度比时所得锌镍合金镀层的微观形貌、镍含量和相结构。通过塔菲尔曲线测量、电化学阻抗谱和中性盐雾试验对比研究了从不同锌镍离子浓度比的镀液中所得合金镀层及纯锌镀层的耐蚀性。结果表明,随镀液锌镍离子浓度比的增大,合金镀层表面颗粒由棱锥状逐渐转变为球状,镍含量减小,厚度稳定在2.45μm左右,晶面(330)的织构系数逐渐增大,晶面(600)与(552)的织构系数逐渐减小,耐蚀性先改善后变差,但Zn–Ni合金镀层的耐蚀性均优于纯锌镀层。镀液中锌镍离子浓度比为1.2时,所得Zn–Ni合金镀层的Ni质量分数为13%,微观形貌和耐蚀性最优。

Zn-Ni alloy coatings were prepared on 45 steel substrate from an ammonium chloride bath consisting of Zn2＋ and Ni2＋ ions 1.1 mol/L totally, NH4Cl200 g/L, H3BO3 20 g/L, at temperature 30℃, pH 4.3 and current density 20 mA/cm2 for 15 min. The microscopic morphology, nickel content and phase structure of the Zn-Ni alloy coatings obtained from the bath with different Zn2＋-to-Ni2＋ concentration ratio were characterized by scanning electron microscope, energy-dispersive spectroscope and X-ray diffractometer, respectively. The corrosion resistance of the alloy coatings obtained with different concentration ratios of Zn2＋ to Ni2＋ as well as pure Zn coating were comparatively studied by Tafel curve measurement, electrochemical impendence spectroscopy and neutral salt spray test. With the increasing of the concentration ratio of Zn2＋ to Ni2＋ in bath, the surface particles of alloy coating changes from pyramid shape to spherical shape, the content of nickel in alloy coating is decreased, the thickness remains at 245 μm or so, the texture coefficient of crystal plane （330） is increased gradually while those of the crystal planes （600） and （552） are decreased, and the corrosion resistance gets better initially and then worse. However, the corrosion resistance of Zn-Ni alloy coating is better than that of pure zinc coating. The Zn-Ni alloy coating obtained from the bath with a Zn2＋-to-Ni2＋ concentration ratio of 1.2 contains 13wt% Ni and has the best microscopic morphology and corrosion resistance.