Chyrkin A. Formation of oxide nanofilms on the surface of MoSi2, WSi2, and TiSi2 under anodic polarization

The peculiarities of anodic oxidation of molybdenum, tungsten and titanium disilicides in a 3% NaCl solution, the kinetics of a silica nanofilm growth on the MoSi2 surface under anodic polarization as well as the effect of WSi2 additives on the corrosion behavior of the TiB2-WSi2 ceramics have been studied. The polarization voltamperometry along with the surface analysis techniques applied to the oxidized samples showed that the anodic oxidation of MoSi2, WSi2, and TiSi2 proved to be a complex multi-stage process, leading to formation of oxide nanofilms (5-15 nm of thickness) on the surface of the disilicides under investigation. The oxide nanofilm structure was characterized as follows: the nanofilms consist of a silica matrix with transition metal nanocrystallites dispersed in it, its chemical composition being defined by the nature and the sequence of electrochemical anodic reactions. It was shown that silica formation during anodic oxidation proved to be critical for the sample surface passivation. It has been established that the anodic dissolution of titanium disilicide occurs in three stages with alternate formation of Ti3+ and Ti4+ ions. The AES and XPS data allowed to identify the presence of silica and TiO2 in the outer scale in relation 4 : 1. The rutile grains, immersed in silica, serve as fast diffusion paths for oxygen diffusion towards the TiSi2 / oxide interface. The silica nanofilm growth kinetics on the MoSi2 surface under anodic polarization in a 3% NaCl solution was studied using stationary chronamperometry. It was shown that the oxide film growth proved to be a two-stage process; i.e. the reaction rate decreases rapidly during first few minutes due to formation of a diffusion barrier followed by the stage of a stationary diffusion controlled oxide growth. The kinetic data obtained can be explained in terms of Mott-Cabrera theory for thin oxide films. The investigation of the effect of WSi2 additives on the corrosion resistance of the TiB2-WSi2 ceramics in 3% NaCl solution was carried out as well.