Effects of compositional and structural features on corrosion behavior of nickel-tungsten alloys

Ni-W alloys were electrodeposited onto copper foil from citrate solution. Coatings containing from 11 to 21 at.% W and having 7-52 mu m in thickness were obtained. The structure of these alloys was analyzed by X-ray diffraction and by using electron and light microscopy techniques. Alloys with 11 and 15% W are composed of two phases: solid solution of W in fcc Ni and solid solution of Ni in bcc W. An increase in W content in the Ni-W alloys to ca. 18-19% of W resulted in the grain refinement and the transition to amorphous structure. The corrosion behavior of obtained Ni-W and unalloyed Ni coatings was studied in 0.5 M NaCl solution by means of electrochemical impedance spectroscopy, potentiodynamic polarization and light microscopy. Comparing to pure Ni, the obtained Ni-W coatings exhibited a clearly decreased corrosion resistance (in terms of corrosion current density and polarization or charge transfer resistance at the open circuit potential). Despite of the quite wide range of composition of the alloys under test, the related grain refinement, and the transition to the amorphous structure, no clear relation between the corrosion rate and W content was detected. This behavior can be a result of the interplay of the activating effect of grain refinement or preferential dissolution of W from one side and diffusion barrier action or inhibition provided by the surface film of W oxidation products from the other side. The differences observed in the corrosion resistance of Ni-W coatings are more related to their morphological imperfections arising from various deposition conditions than to the W content. Some samples showed a rather non-uniform nature of corrosion (pronounced attack along cracks). An inversion in the dissolution behavior of Ni-W and unalloyed Ni was observed with increasing anodic potential. Contrary to pure Ni, Ni-W coatings were resistant to pitting corrosion in NaCl solution.