Anodic oxidation of tungsten in nonaqueous electrolyte. II: Electrochemical coloring and bleaching of the oxide film

• Published in: Journal of the Electrochemical Society 1992-03, Vol.139 (3), p.728-732
• Main Authors: ORD, J. L, DE SMET, D. J
• Format: Article
• Language: English
• Subjects: 30 DIRECT ENERGY CONVERSION; 300504 - Fuel Cells- Applications; 300505 - Fuel Cells- Electrochemistry, Mass Transfer & Thermodynamics; 400400 - Electrochemistry; ACETIC ACID; ANODES; CARBOXYLIC ACIDS; CHEMICAL REACTIONS; CHEMISTRY; ELECTRIC CONDUCTIVITY; ELECTRICAL PROPERTIES; ELECTROCHEMISTRY; ELECTRODES; ELECTROLYTES; ELEMENTS; ELLIPSOMETRY; Exact sciences and technology; General and physical chemistry; HYDROGEN PRODUCTION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONIC CONDUCTIVITY; Kinetics and mechanism of reactions; MEASURING METHODS; METALS; MONOCARBOXYLIC ACIDS; ORGANIC ACIDS; ORGANIC COMPOUNDS; OXIDATION; PHYSICAL PROPERTIES; REDOX REACTIONS; TRANSITION ELEMENTS; TUNGSTEN
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.2069292

Ellipsometric and electrochemical measurements are used to determine the mechanisms by which anodic oxide films on tungsten are reduced (colored) and reoxidized (bleached) in acetic acid electrolyte. In the reduction process, a layer of HWO sub 3 forms on the outer surface of the oxide, and, as reduction proceeds, a phase boundary between HWO sub 3 and WO sub 3 moves inward toward the substrate. Competition from hydrogen evolution limits the depth to which the boundary penetrates, but films up to 50 nm in thickness can be reduced completely to HWO sub 3 with an appropriate sequence of cycles. The highly absorbing HWO sub 3 layer (refractive index 1.642 and extinction coefficient 0.811) is readily distinguished from the anisotropic transparent oxide (refractive index 2.170 transverse and 2.078 parallel to the 5.02 MV/cm anodizing field). The reoxidation process begins with the formation of a layer of WO sub 3 on the outer surface of the HWO sub 3 , and again a phase boundary moves inward across the film. A field on the order of the anodizing field is required to move H to the electrolyte through the outer reoxidized layer, and this field also moves oxygen into the outer layer, where it combines with mobile H cations to form H sub 2 O groups within the WO sub 3 . The buildup of H sub 2 O enables H to penetrate more deeply into the film on subsequent cycles. When the phase boundary between WO sub 3 and HWO sub 3 reaches the substrate, W begins to enter the film, and an interface between oxide and hydrated oxide sweeps outward across the film as W replaces H. When this interface reaches the electrolyte, anodizing conditions are re-established. Open-circuit transients applied during the reoxidation process are sensitive to the nature of the mobile ionic species, and detect the changes in the ionic transport process during the reoxidation.