Corrosion behaviour of amorphous and nanocrystalline Ni-W alloys

摘要

Nanocrystalline Ni-W alloy coatings show superior corrosion and wear resistance. Ni-W alloys have attracted attention in past years due to their outstanding catalytic properties that are supposed to be applied at hydrogen evolution. In general, nanocrystalline coatings produced by the means of electrodeposition are gaining attention due to tailoring properties reached by controlling electrodeposition parameters. Conceming corrosion resistance of nanocrystalline materials, an ambiguity appears. High volume fraction ofgrain boundaries contributes to the corrosion resistance showing different results among various metals and corrosion environment. In general the coatings are sensitive to the experimental conditions including used electrolyte, pH, temperature and current density. Nanocrystalline, amorphous, intermetallics and combination of these phases can be obtained while changing different parameters in Ni-W electrodeposition. Grain size decreases with increasing tungsten content in the alloy, which contributes to amorphous phase formation. The alloy properties are fundamentally affected by the composition and morphology. The alloy Ni-W was obtained in this work by the pulse current of rectangular shape using different process parameters including peak cathodic current, on and off time from a citrate type electrolyte. The microstructure and the electrochemical behaviour were investigated by the means of XRD, EDX, open circuit potential and polarization resistance. Corrosion behaviour of the alloys was analysed in the chloride medium under potentiodynamic polarization. Tafel equation was employed for determining corrosion current density and subsequently corrosion rate. It was found that the prepared alloys consist of two-phase system with prevailing either crystalline or amorphous phase depending on the parameters of the pulse current. The prevailing crystalline phase slows the corrosion rate down. On the other hand, the amorphous phase causes increasing of the corrosion rate. The reason is tungsten segregation at grain boundaries, which supports formation of corrosion active sites.