Activated titanium anodes

Seawater or brine is electrolyzed in diaphragmless cells. Activated titanium anodes and titanium cathodes are used. The yield based on current consumed is relatively poor, 40 to 60%, due to the hydrogen produced reducing part of the hypochlorite formed. The electrolysis cells are technically uncomplicated and small. The hypochlorite solutions obtained contain several grams of hypochlorite per L. 

Panic V, Dekanski A, Milonjic 8K, Atanasoski R, Nikolic B (2000) The influence of the aging time of Ru02 sol on the electrochemical properties of the activated titanium anodes obtained by sol-gel procedure. fit Uskokovic DP, Battiston GA, Nedeljkovic JM, Milonjic 8K, Rakovic DI (eds) Trends in advanced materials and processes, vol 352, Materials science forum. Trans Tech, Zurich... 

In 1800, Cruickshank was the first to prepare chlorine electrochemically [38] however, the process was of little significance until the development of a suitable generator by Siemens and of synthetic graphite for anodes by Acheson and Castner in 1892. These two developments made possible the electrolytic production of chlorine, the chlor-alkali process, on an industrial scale. About the same time, both the diaphragm cell process (1885) and the mercury cell process (1892) were introduced. The membrane cell process was developed much mpre recently (1970). Currently, more than 95 % of world chlorine production is obtained by the chlor-alkali process. Since 1970 graphite anodes have been superseded by activated titanium anodes in the diaphragm and mercury cell processes. The newer membrane cell process uses only activated titanium anodes. 

Current Efficiency. The theoretical electrochemical equivalents representing the materials produced or consumed in the electrolysis of sodium chloride or potassium chloride brines are given in Table 4. In practice, the yield is ca. 95 - 97% of the theoretical value, owing to side reactions at the electrodes and in the electrolyte. With activated titanium anodes, the yield is largely independent of the distance between the electrodes. 

Computer-controlled cells with activated titanium anodes are run with il-feictors from 0.085 to 0.11. The corresponding cell voltages at 10 kA/m are 4.00-4.25 V (Fig. 19). 

The initial attemps to replace the graphite anodes with activated titanium anodes began as early as 1957 with platinized titanium and Pt/lr-coated anodes. However because of the short lifetimes of the anodes, they were not economic. The use of mixed metal oxides was first patented by Beer in 1965 and 1967 [150]. The initial patent described a coated metal electrode in which the active material was a mbced metal oxide coating containing one or more of the platinum metal group oxides. The second patent described coatings in which mfred metal oxide crystals contained a non-platinum metal oxide in addition to the platinum metal oxide (including Ti, Ta, and Zr oxides). 

Further improvements in the coating and the anode structures followed rapidly along with the commercialization of anodes by De Nora [151] under the trade name Dimensionally Stable Anode (DSA). Because of the dimensional stability and the lifetime of the coating and the ability to increase the current densities, rapid introduction of the activated titanium anodes was possible. At present only a few plants still use graphite anodes, largely due to the initial investment costs for titanium anodes. 

In the chlor-alkali industry titanium brings its properties to application as a material in activated metal anodes. In fact this is the major use of titanium in the chlor-alkali industry. 

Scientists of ICI (28) found that in solutions containing approximately 20 g H2S04/liter at temperatures up to 40°C, titanium anodizes rapidly forming a Ti02 layer of time-independent thickness of fractions of micrometers, Between 50 and 70°C, continuous film growth is observed, whereas the titanium begins to dissolve actively above 70°C. 

Vukovic, M., Marijan, D. and Cukman, D. (1999) Electrocatalytic activity and anodic stability of electrodeposited ruthenium-rhodium coatings on titanium. J. Mater. Sci. 34, 869-874. 

Operation of membrane cells The same processes take place on the anodes and cathodes as in diaphragm cells. Activated titanium is used for the anodes and stainle.ss steel or nickel is preferred for the cathodes. No water transport takes place in the absence of current, but upon application of current solvation-water is transported by the current-carrying Na" ions as they travel from the anode chamber to the cathode chamber. 

In non-carbonated concrete without chlorides, steel is passive and a typical anodic polarization curve is shown in Figure 7.3. The potential is measured versus the saturated calomel reference electrode (SCE), whose potential is +244 mV versus the standard hydrogen electrode (SHE). Other reference electrodes used to measure the potential of steel in concrete are Ag/AgCl, CU/CUSO4, Mn02, and activated titanium types. From this point on in the text, unless explicitly stated otherwise, potentials are given versus the SCE electrode. 

Other macrocell effects. A special case of macrocell effects has been observed on structures contaminated by chlorides where an activated titanium mesh anode was installed in order to apply cathodic protection when the cathodic protection system is installed but is not in operation, locahzed corrosion on steel can be slightly enhanced by the presence of the distributed anode [4]. 

Anodic acidification. At the anode surface, the anodic process of oxygen evolution takes place 2H2O —>62 + 4H + 4e . In the presence of chlorides, even chlorine develops 2Cr —> CI2 + 2e . Such processes may directly or indirectly produce acidity and may thus lead to destruction of the cement paste in contact with the anode [34]. Experience shows that such deterioration is negligible for activated titanium mesh anodes if the anodic current density does not exceed 100 rtiA/m (or values 3-4 times greater for brief periods). Design of the anodes for cathodic prevention and cathodic protection must respect these hmits. 

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