Anode noble-metal-coated titanium

J.3.2.8 Noble-Metal-Coated Titanium Anodes (NMCT)... 

The Krebscosmo Bipolar Cell Electrolyzer BMZ 7.5 which is basically made from steel and titanium. The partition wall of the bipolar electrode is a PTFE foil. Anodes are expanded titanium sheet with noble metal coatings and the cathode structure is a perforated steel sheet. The cell units can be arranged in parallel groups of bipolar elements. Each cell element has 2.5 m of membrane area that operate at a nominal amperage of 7.5 KA. The cell block of the BMZ 7.5/64 electrolyzer consists of 64 elements with 16 series elements with 4 current paths with a nominal block amperage of 30 KA. Electrolytic performance is not disclosed (74). 

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

Figure 8.42 shows the basic configuration of electrofiltration, where an electric field is applied across micro or ultrafiltration membranes in flat sheet, tubular, and SWMs. The electrode is installed on either side of the membrane with the cathode on the permeate side and the anode on the feed side. Usually, the membrane support is made of stainless steel or the membrane itself is made of conductive materials to form the cathode. Titanium coated with a thin layer of a noble metal such as platinum could, according to Bowen [93], be one of the best anode materials. Wakeman and Tarleton [94] analyzed the particle trajectory in a combined fluid flow and electric field and suggested that a tubular configuration should be more effective in use of electric power than flat and multitubular module. 

The DSA-type anodes are inert , coated anodes made of a valve metal (titanium, niobium, or tantalum) base coated with an electrochemically active coating. The active coating is made either of noble metals or of mixed metal oxides. Noble metals in active coatings are usually platinum or platinum alloys. Mixed metal-oxide coatings contain active oxides and inert oxides the active components are usually ruthenium dioxide (R.UO2) and iridium dioxide (IrC>2) and the inert components are mostly titanium dioxide (TiC>2) and other oxides such as tantalum... 

The advantages of using chloride electrolytes compared with sulfate electrolytes are higher electrical conductivity, lower electrolyte viscosity, lower overpotential for nickel reduction, and higher solubility and activity of nickel. An important factor is the lower anode potential of chlorine evolution compared with oxygen evolution in sulfate electrolytes using the common lead anodes. Chloride electrolytes require insoluble or dimensionally stable anodes, usually titanium coated with an electroactive noble metal or oxide, and a diaphragm system to collect the CI2 gas from the anode. The chlorine liberated at the anode is recycled for use in the leach circuits. In practice, some decomposition of water... 

Cathodic protection by impressed current involves the use of a rectifier connected to a power line. Contrary to sacrificial anodes, which operate at a fixed potential, the use of a rectifier permits to adjust the voltage (or the current) to the particular requirements of a protection scheme. This not only allows one to optimize the electrochemical conditions for protection, but the method is also well suited to protect large surfaces. On the other hand, protection by impressed current needs more maintenance than the use of sacrificial anodes. In order to protect buried structures by impressed currents one uses consumable anodes such as scrap iron or, more often, non-consumable anodes made of iron-silicon alloy, graphite or of titanium coated with noble-metal oxides. 

Ruthenium is not as industrially important as other noble metals, but it is used as Ru02 to coat the dimensionally stable titanium anodes used in the production of chlorine, chlorate, and caustic, as well as oxygen in water electrolysis. Ruthenium has also found some use as a binder for high-temperature cemented carbides. 

Electrochemical oxidation of organic conpounds requires an anode that is stable under anodic polarization, and that hs a low catalytic efficiency for oxygen evolution. Noble metals, such as platinum, are resistant to oxidation, but they have hi catalytic efficiencies for oxygen evolution and are prone to fouling (d, 7). Dimensionally stable anodes, such as titanium coated widi active or inactive catalysts, are less prone to oxygen evolution and fouling, but ftiey do suffer from leaching of the catalyst from e electrode surface. 

Mixed metal oxide coated anodes, also called dimensionally stable anodes (DSA), are based on electrode technology developed in the early 1960s for the production of chlorine and caustic soda. The mixed metal oxide films are thermally applied to a noble metal such as titanium, niobium, and tantalum as substrate materials and are available in a variety of sizes and shapes. These oxide coatings have excellent conductivity, are resistant to acidic environments, are chemically stable, and have relatively low consumption rates. Groundbed installation in soils usually specifies that the anode be prepackaged in a canister with carbonaceous backfill material. 

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