Pulse reverse electrodeposition and characterization of nanocrystalline zinc coatings

Nanocrystalline zinc coatings are produced through pulse reverse electrodeposition in an acid sulfate electrolyte with polyacrylamide (PAM) as the only additive and are characterized by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The influence of the additive and electrodeposition parameters on the surface morphology, grain size and crystallographic preferred orientation of the coatings are investigated. The mechanical, wear and corrosion resistance properties of the nanocrystalline and coarse-grained zinc coatings are also evaluated by nanoindentation, a ball-on-disc tribometer and an immersion test, respectively. The results show that changing the electrodeposition parameters cannot produce the nanocrystalline zinc coating in the absence of the additive, and the PAM plays a crucial role in reducing the grain size of the coating from the micro to nano scale. The grain size of the coating decreases asymptotically with an increased forward pulse current density in the presence of PAM, and the pulse reverse current makes the coating smoother. The hardness of the nanocrystalline zinc coating (31 nm) is almost three times than that of the coarse-grained counterpart. The wear and corrosion resistance properties of the nanocrystalline zinc coating are superior to its coarse-grained counterpart.