The Nature of Oxide Films on Tungsten in Acidic and Alkaline Solutions

Surface enhanced Raman spectroscopy (SERS) data of W in acidic solution were characterized by modes associated with hydration (WO3(H2O)x, 942 cm-1) as well as modes characteristic of WO3 (816 cm-1). At applied anodic potentials the WO3(H2O)x content in the oxide was greater than the WO3 content, while at applied cathodic potentials the WO3 cont nt was larger. These results are consistent with a bi-layer film on W in acidic solution which consists of a compact inner layer of WO3 and an outer layer of WO3(H2)x. Rotating disk electrode experiments (RDE) demonstrated that the passive dissolution (ipass) of W in acidic solution increased with the angular velocity of the electrode. An increase in ipass nd a corresponding decrease in the thickness of the surface layer with increasing angular velocity is consistent with a rev rsible dissolution mechanism where dissolution is limited by the mass transport of a loosely bound WO3(H2O)x layer from the surface. Electrochemical impedance spectroscopy measurements found that W dissolution in acidic solution was associated with an adsorption pseudocapacitance presumably due to the outer WO3(H2O)x layer. The value of this psuedocapacitance increased with increasing angular velocity indicating thinning of the layer consistent with RDE and SERS results. Results for W in alkaline solution are also presented.