Activated titanium electrode

Figure 17.9 Average value in time and standard deviation of potential of activated titanium electrodes embedded in alkaline and carbonated concrete (Portland cement, iv/c 0.65, outside exposure, unsheltered) [23]...

After five months, in the carbonated specimens protected at 10 mA/m, the potential difference between the two activated titanium electrodes increased significantly and reached a value up to 200 mV, suggesting that the atkahnity produced by the cathodic reaction at the steel surface induced locaHzed reaUsaUzation of the concrete in the vicinity of the rebar. For the other specimens (i. e. in carbonated concrete at 2 and 5 mA/m ) realkalisation of concrete and repassivation of steel did not occur even after five years of testing [45]. In the same study, it was found that the application of a start-up current density of 70 mA/m for 1 month proved to be an effective way for achieving repassivation of steel in carbonated concrete [46]. The tests showed that, once repassivation is induced, even a current density of 5 mA/m is sufficient to guarantee protection. 

P. Castro, A. A. Sagues, E. I. Moreno, L. Maldonado and J. Genesca, Characterization of activated titanium solid reference electrodes for corrosion testing of steel in concrete , Corrosion, 52(8), 1996, pp 609-617. 

Disinfection of municipal water contaminated with coliforms and fecal streptococci was the subject of a study by Patermarkis and Fountoukidis [31]. Disinfection was achieved using titanium electrodes and direct current. The polarity was alternated every minute to eliminate titanium oxide buildup. No additives or supporting electrolytes were used in this room-temperatures process. At a current density of 2.5 mA/cm and an applied voltage of 45 V, no microbial activity was detected after 30 min of operation. Noncontaminated, electrochemically treated water possessed a residual disinfection capacitiy addition of treated water to a contaminated sample destroyed the microbial life in the sample. 

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. 

Linear polarization resistance (LPR). These measurements allow the actual corrosion rate of embedded probes or of the reinforcing bars to be monitored over time. The measurement principle is described in Section 16.2.3. In addition to the reference electrode a counter-electrode of a corrosion resistant material (e. g. stainless steel or activated titanium) has to be embedded. Several compact LPR sensor systems were developed and installed in structures such as precast deck elements in a road tunnel [6,20]. When existing structures have to be monitored for corrosion rate, a corroding piece of rebar can be isolated (by cutting) to get... 

The variations in the pH of concrete around the cathode were monitored by means of two pH sensitive probes (activated titanium wires) placed respectively 1 mm and 3 mm far from the steel surface. During the first 4 months, the potential difference between the two electrodes for all specimens was negligible, showing that the pH was low (carbonated) throughout the concrete. 

For electrochemical chloride extraction (abbreviated CE, also called chloride removal, or desalination), a direct current is applied between the reinforcement (cathode) and an anode that is placed temporarily on the outer surface of the concrete. The anode is an activated titanium wire mesh or a reinforcing steel mesh. The anode is surrounded by tap water or saturated calcium hydroxide solution in ponds (upper, horizontal surfaces) or tanks (vertical or overhead surfaces) or as a paste that can be sprayed onto all types of surface. Chloride ions migrate from the reinforcement to the anode. Due to a relatively high current density of 1 to 2 A/m, relatively large amounts of chloride can be removed from the concrete within a relatively short time, usually 6 to 10 weeks. After that, the anode, the electrolyte and the incorporated chloride ions are removed from the stracture. The principle layout and electrode reactions involved are indicated in Figure 20.8. 

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. 

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). 

These batteries are new systems which use a lithium-manganese composite oxide for the active material of the positive electrode a lithium-titanium oxide with a spinel... 

Appreciable interest was stirred by the sucessful use of nonmetallic catalysts such as oxides and organic metal complexes in electrochemical reactions. From 1968 on, work on the development of electrocatalysts on the basis of the mixed oxides of titanium and ruthenium led to the fabrication of active, low-wear electrodes for anodic chlorine evolution which under the designation dimensionally stable anodes (DSA) became a workhorse of the chlorine industry. 

Since the discovery of photoelectrochemical splitting of water on titanium dioxide (TiOj) electrodes (Fujishima and Honda, 1972), semiconductor-based photocatalysis has received much attention. Although TiO is superior to other semiconductors for many practical uses, two types of defects limit its photoeatalytic activity. Firstly, TiO has a high band-gap (E =3.2 eV), and it can be excited only by UV light (k < 387 nm), which is about 4-5% of the overall solar spectmm. Thus, this restricts the use of sunlight or visible light (Kormann et al., 1988). Secondly, the... 

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