Effect of pH on the Anodic Behavior of Tungsten

Abstract

Potentiodynamic and potentiostatic polarizations, and a rotating disk electrode technique were used to study the influence of diffusion on the anodic reactions of tungsten (W) in a broad pH range (∼0.3-13.5) in H3PO4 buffered solutions. The anodic current was controlled totally by the kinetics of H + -assisted dissolution below pH 1 (Region A). At around pH 2.5 (Region B), in addition to the kinetics of H2Oassisted dissolution, specimen rotation had an effect on the current. This observation was attributed mainly to the physical damage to an oxide phase by hydrodynamic effects and partly to the diffusion of surface H2WO4(aq) species. The anodic current was under the mixed control of the kinetics of OH −assisted dissolution and slow diffusion of surface WO 2− 4 ions between pH 4 and 7 (Region C). The kinetics of the H2WO4(s) solubilization and slow diffusion of surface WO 2− 4 ions were observed to influence the anodic current between pH 7.5 and 11.5 (Region D). In highly alkaline solutions (pH > 12; Region E), however, W anodic current was controlled totally by the slow diffusion of bulk OH − ions to the electrode surface.

Dissolution kinetics of WO3 in acidic solutions

“…In our previous work [20,21] the effect of pH on the anodic behavior of tungsten was investigated over a wide pH range (pH 0.5-13):…”

“…Above the anodic potentials at which WO 3 coats W surface completely ( [20]; above 300 mV; pseudo-plateau region), the anodic reaction rate of W is controlled by the chemical reactions in Equations 2, 3 and 4 ( [20]; the rate determining steps). Therefore, at these potentials the steady-state W anodic current, which reflects the total rate, can be expressed in acidic solutions as:…”

“…In the hydrometallurgical extraction literature there are some reports on the chemical dissolution behavior of WO 3 (s) in alkaline solution [18,19]. In contrast, no comparable information is available for acidic solution, although the stability of this oxide in low pH environments is important for CMP as well as other technologically important processes [1][2][3][4][5][6][7][8][9]20].…”

Corrosion resistance of Cr–C–W alloys produced by electrodeposition

“…2d). It should be noted that the anodic polarization curves plotted in semilogarithmic coordinates resemble the anodic polarization curves of pure tungsten in these corrosive environments [25][26][27]. This also, indirectly, proves that the layer characterized by a high content of tungsten The comparison of polarization diagrams showed that the rates of corrosion for Cr-C-W coatings are less than for Cr-C deposits in all corrosive solutions.…”

Dynamic electrochemical-etching technique for tungsten tips suitable for multi-tip scanning tunneling microscopes

“…The shut-off time, which is important for controlling the sharpness of tip apex, was set to be less than 100 ns. The simplified process of the anodic oxidation of metal tungsten in a KOH or NaOH solution can be expressed by the following two steps [104][105][106]:…”