Morphological, structural, microhardness and electrochemical characterisations of electrodeposited Cr and Ni-W coatings

This study examines the corrosion of electrodeposited Cr and of two electrodeposited Ni-W coatings in 0 1 mol L-1 NaCl solution, as well as the influence of heat treatment on the crystallographic structure and microhardness properties of these coatings. Physical characterisation is carried out using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. Electrochemical characterisation is carried out using both the potentiodynamic linear polarization technique and open circuit measurements during long-term immersion tests The corrosion products on the coating surfaces are characterised by ex situ Raman spectroscopy. As-electrodeposited Ni-W samples do not present defects, and the surface evolves from fine globular grains to rough polycrystalline morphology with decreasing electrodeposition current density All the studied coatings corrode in the chloride medium and the corrosion is non-uniform for the Ni-W coatings Raman analyses carried out after the immersion tests reveal Cr2O3 and Cr(OH)(2) corrosion products on the Cr coating surface, and Ni(OH)(2), NiO and WO3 corrosion products on the Ni-W coating surfaces. Ni, Ni4W and Ni-W phases are formed after heat treatment of the Ni86W14 coating at 600 degrees C Although all the annealed Ni-W layers are cracked, their microhardness increases as the annealing temperature increases, suggesting that Ni-W coatings are potential substitutes for chromium in industrial applications in which good microhardness properties and stability at temperatures higher than 100 degrees C are required. (C) 2009 Elsevier Ltd. All rights reserved.