Influence of LaCl3 addition on microstructure and properties of nickel-electroplating coating

The influence of rare earth chloride LaCl3·7H2O addition on the microstructural features, phase structure, corrosion resistance and microhardness of nickel-electroplating was investigated. The Watts-type with different additive amounts of LaCl3·7H2O (0–1.2 g/L) were used in the experiment. Surface morphologies of coatings were examined by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM) was used to measure the coatings' grain size and the microstructure of coatings was detected by X-ray diffraction (XRD). Corrosive investigation was carried out in 3.5 wt.% NaCl solution. The microhardness values of the coatings with different amounts of LaCl3·7H2O were measured, and the mechanism of the variation in microhardness was studied. Results showed that the addition of rare earth lanthanum refined the grain size and improved the surface consistency of the coatings, meanwhile the microhardness and corrosion property of coatings were improved and achieved a maximum with arround 1.0 g/L LaCl3·7H2O addition in electrolyte. The preferred growth orientation of lanthanum doped coating was crystal face (200), meanwhile the La2Ni7 phase was detected in the nickel coating by XRD and this was due to the induced co-deposition of elements La and Ni. The reason maybe was that the special out-layer electronic structure of element La raised the polarization of Ni cathode deposition, accelerated the nucleation of Ni and reduced hydrogen evolution from cathode surface.