Carbide anode

Electrolytic recovery of iron from pickling solutions using tungsten carbide gas diffusion anodes was studied by Streng et al. [23]. Replacement of Tainton anodes (99% lead and 1% silver) with a gas diffusion tungsten carbide anode led to a 1.5-1.7 V decrease in the cell voltage and a 50% reduction in energy consumption. Unwanted oxidation of ferrous ions to ferric ions did not occur on these gas diffusion anodes, since the cell was operated at a lower potential. 

Considering the influence of applied conditions on stoichiometry deviation in SiC under electrochemical treatment, let us also present some data related to silicon carbide anodization in the potentiostatic regime. The treatment was performed using HF-based electrolyte under conditions where anodic current density values are 4-10 mA cm-2. In spite of the value of the current being comparable with that used for the formation of nanoporous PSC structures, SiC anodization under potentiostatic conditions results in built-in adherent film ( anodic film in the... 

Kretzschmar C, Wiesiner K. Study of the operation of tungsten carbide anode and iron phthalocyanine cathodes in fuel cells with a sulphuric acid electrol3rtc. Elektrokhimiya 1978 14 1330-4. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

Compared with XPS and AES, the higher surface specificity of SSIMS (1-2 mono-layers compared with 2-8 monolayers) can be useful for more precise determination of the chemistry of an outer surface. Although from details of the 01s spectrum, XPS could give the information that OH and oxide were present on a surface, and from the Cls spectrum that hydrocarbons and carbides were present, only SSIMS could be used to identify the particular hydroxide or hydrocarbons. In the growth of oxide films for different purposes (e.g. passivation or anodization), such information is valuable, because it provides a guide to the quality of the film and the nature of the growth process. 

For Sm, Eu, and Yb, on the other hand, nanocapsules containing carbides were not found in the cathode deposit by either TEM or XRD. To see where these elements went, the soot particles deposited on the walls of the reaction chamber was investigated for Sm. XRD of the soot produced from Sm203/C composite anodes showed the presence of oxide (Sm203) and a small amount of carbide (SmC2). TEM, on the other hand, revealed that Sm oxides were naked, while Sm carbides were embedded in flocks of amorphous carbon[12J. The size of these compound particles was in a range from 10 to 50 nm. However, no polyhedral nanocapsules encaging Sm carbides were found so far. 

Another indication of the influence of precipitated phases on anodic behaviour may be seen in the curve for Alloy C in Fig. 4.28, where the small peak in the middle of the passive range is probably attributable to anodic dissolution of an intermetallic phase (n) and MjC carbide . ... 

Fig. 19.15 Schematic representation of range of corrosion potentials expected from various chemical tests for sensitisation in relation to the anodic dissolution kinetics of the matrix (Fe-l8Cr-IONi stainless steel) and grain boundary alloy (assumed to be Fe-lOCr-lONi) owing to depletion of Cr by precipitation of Cr carbides of a sensitised steel in a hot reducing acid (after Cowan and Tedmon )...

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

The electroextraction process for molybdenum involves the use of its oxides, carbides or sulfides as soluble anodes in a potassium chloride-potassium hexachloromolybdate (K3MoCl6) molten electrolyte. An inert atmosphere electrolytic cell, with a provision for semicontinuous electrolysis, is used for this purpose. The process operation consists of the following steps. 

It has been found that when molybdenum carbide (Mo2C) is used as the soluble anode, a loose carbon crust forms on the surface of the pellets as the dissolution of molybdenum progresses. X-ray diffraction analysis of the spent anode has indicated a predominance of the Mo2C phase. This suggests that the anodic reaction proceeds as... 

The passive layer is subsequently formed from Ti02+ and was described as Ti02 x H20. During corrosion CO or C02 is formed from the carbon of the carbide. The electrochemical behaviour of TiC in acid electrolytes was reinvestigated with respect to the depassivation of titanium substrates for anodic PbOzdeposition by M. Cappa-donia et al. [122] using XPS. 

After many attempts with several anode materials we found a stable anode. Silicon carbide and iron silicide, etc. in a conducting form are stable towards chlorine. The chlorine formed on the anode then reacts with the solvent (THF) forming chlorinated organic compounds. 

Therefore, we tried silicon carbide as the anode, which turned out to be completely stable against oxidation. Iron silicide and other silicides also seem to be stable. In our case, the reaction at the anode is the chlorination of the solvent. So, for instance, chlorinated products of THF can be detected by... 

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