The Effect of the Methanol–Water Interaction on the Surface Layer on Titanium in CH3OH-H2O-LiClO4 Solutions

Abstract

The purpose of this study was to explain the mechanism of formation and to examine the composition of the anodic film formed on the surface of titanium in an anhydrous neutral methanol solution of electrolytes. In an environment deprived of water molecules, the growth of a 3D-phase titanium oxide layer is not possible. Electrochemical investigations demonstrated that the Ti surface in CH3OH-LiClO4 solutions experienced a pseudo-passivation with the formation of a methoxy layer, which resulted from the reaction of the metal surface with alcohol molecules. The presence of this methoxy surface film was confirmed through XPS and in situ FTIR measurements. The layer blocked the Ti anodic dissolution at the potential range corresponding to the stability of methanol and methoxy ions (i.e., <0.55 V). At potentials over 0.55 V, the methoxy layer was oxidised, which caused the “depassivation” of the metal surface and the etching of titanium. The addition of water changed the properties of Ti in CH3OH-LiClO4 solutions, but only with a water content above 0.2 mole fraction. Below this concentration of water, titanium behaved like it would in an anhydrous solution of methanol. In the range of water concentration of 0.2 to 0.7 mole fraction, the structure of the solution is strengthened because both components of the solvent formed separate percolating networks. The strengthening of the solution structure resulted in a strengthening of the surface layer of Ti(OH)m(OCH3)n. Such a layer had strong barrier properties similar to the properties of an organic polymer film. The formation and growth of a stable layer of TiO2 were possible only in a solvent when the water concentration was higher than ≈0.7 mole fraction.