Anode titanium mesh

Figure 8-22. Electrode arrangement for electrochemical restoration of reinforced concrete structures the anode (titanium mesh) is placed on the concrete surface and embedded in mortar (for CP) or in fibres (for ECR and ER).

Since the 1960s titanium mesh anode baskets have been used (21), especially in nickel plating solutions. Nickel anodes in the form of small round buttons and pellets combined with the titanium anode basket allows a constant anode area to be maintained with a minimum of effort. 

These systems have not been installed to any significant extent and have now been superseded by conductive paints, conductive polymers or titanium mesh anode systems. 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

The cathodes themselves are made, for instance, of steel mesh or woven steel and the anodes are DSA in form of expanded titanium meshes. The brine (310-320 g NaCl/L) enters the anode compartment and completely covers the anodes and the cathodes (called fingers or tubes). The chlorine leaves the cell through an outlet in the cell head. The... 

Tokuyama Soda Co., Ltd. has developed a large bipolar electrolyzer that has an electrolysis area of 2.7 m per unit. The cell bodies are metallic. The anode chamber is lined with titanium plating and the cathode chamber and structural frame are carbon steel. The anode surface is titanium mesh with an... 

Solutions of acid copper sulfate (containing only chloride and carrier) were used as the copper electroplating bath. A piece of titanium mesh (diameter = 55 mm) coated with iridium oxide was used as an insoluble anode. The bath was pumped through the anode to the cathode under 1 l/min and controlled at 25 °C. The cathode rotating speed was maintained at 165 rpm. The copper electrodeposition tests were conducted under different electric field waveforms with an average cathodic current density of 25 to 32 ASF, which was controlled by the cell voltage. Samples were cross-sectioned with a focused ion beam scanning electron microscope (FIB-SEM) to inspect both the quality of the copper deposits in the trenches or via-holes. 

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]. 

In the 1980s, anodes based on titanium meshes activated with special oxides (especially ruthenium and iridium) or conductive paints were developed. The technique, by now utihsed beyond North America, is applied not only to bridge decks, but also to bridge substructures (girders, cross beams, piers), and marine structures, parking garages, industrial, office and residential buildings, etc. 

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. 

A second type of anode is based on a conductive coating with carbon powder in an organic matrix. It has the following advantages it does not require an overlay, it can easily be appHed to structures of any form, and it does not present problems of additional weight to the structure or limitations of dimension the cost of installation can be lower, down to about half that of activated titanium mesh systems. On the other hand, it cannot deliver current densities above 20 mA/m over long periods (and maximum levels of 35 mA/m ) and its service life can reach 10-15 y, but not in humid climates where there may be loss of adhesion and effects of premature deterioration. 

For electrochemical realkalisation (abbreviated RE), a direct current is applied between the reinforcement (cathode) and an anode that is placed temporarily on the outer surface of the concrete. The method is comparable to chloride extraction. The anode is an activated titanium mesh or a reinforcing steel mesh. The anode is surrounded by a sodium (or other alkali metal) carbonate solution of about 1 mole per litre in ponds (upper, horizontal surfaces) or tanks (vertical or overhead surfaces) or as a paste that can be sprayed onto all types of surface. Due to a relatively high current density of 1 to 2 A/m, a carbonated concrete cover of several centimetres can be realkaUsed within a relatively short time, usually a few days to a few weeks. After that, the anode and the electrolyte are removed from the structure. The layout and principle reactions involved are indicated in Figure 20.9. 

Rehabilitation is achieved by overlays such as latex-modified concrete, low-slump concrete, high-density concrete, and polymer concrete. They are commonly used for the rehabihtation of bridge decks. This procedure extends the life of a bridge deck by about 15 years. Impressed-current CP systems on bridge decks are now a routine rehabihtation teehnique because of the cooperative research with industry and states in the development of durable anodes, monitoring devices, and installation techniques. Titanium mesh anode, used in conjunction with a concrete overlay to distribute protective current, serves as a durable anode for use in impressed-current CP of reinforced concrete bridge deeks and widely accepted by state and other transportation agencies. 

Of all the systems cited above, only the titanium mesh anode and metallized zinc are in extensive use at present. The titanium mesh on bridge decks is durable over long time. The thermal-sprayed zinc is free from debonding problems but suffers from an increase in resistance over time. However, the Oregon DOT has had significant success with thermal-sprayed zinc anode on substructure components. The thermal-sprayed titanium has shown promise as a new anode. 

One of the most successful commercial anodes is the expanded titanium mesh with an activated precious or mixed metal oxide coating. This also comes in the form of an expanded titanium mesh, strips and other configurations. It is fixed onto the surface, usually with plastic fixings and a cementitious overlay applied. 

This anode consists of an expanded titanium mesh with a catalytic mixed metal oxide coating. The anode is fixed to the concrete surface and then overlaid with concrete. This is usually dry spray shotcrete on vertical and soffit surfaces or cast on top surfaces and decks. It is also available in a number of mesh sizes and as a ribbon (described separately). 

Figure 7. II (a) A mixed metal oxide coated titanium mesh anode being overlaid on a bridge in Baltimore MD. Courtesy Jack Bennett, (b) Extreme example of an overlay failure on a titanium mesh anode. Oregon Inlet N. Carolina USA.The whole bridge was swept away in a storm a few years after the author took this photograph. 

The anode is used where increase in dead load, change in profile or application of an overlay is undesirable but the durability of the titanium mesh anode is required. It requires good cover to the steel and can be more expensive to install than the shotcreted mesh anode. It is not as widely used but there are several thousand square meters of application around the world. It has been widely used in cathodic prevention systems installed on new or nearly new structures. 

This is a wet sprayed mortar containing nickel plated carbon fibres to achieve conductivity (Figure 7.14). It requires a primary anode, usually the titanium mesh ribbon. This is a proprietary anode and there are no variations. 

As stated in the anode descriptions earlier, there are also two NACE test methods for cathodic protection anodes. These are TM 0294 on embeddable anodes (mixed metal oxide coated titanium, mesh, ribbon, tnbes, rods and conductive ceramic tubes) and TMO1105-2005 on organic-based condnc-tive coating anodes. In addition there is a specification for applying thermal sprayed zinc anodes to concrete American Welding Society (2002). 

The anode system may be a single component, such as flame sprayed zinc, or multiple component such as a titanium mesh with a cementitious overlay. It may consist of a single continuous anode such as mesh or coatings, or multiple anodes connected together such as ribbon anodes or the discrete rod anodes. All anodes require electrical connections to the power supply. As for the rebar connections these should be duplicated for redundancy. 

The most popular anode is the same coated titanium mesh used for impressed current cathodic protection. Instead of embedding it permanently in a cementitious overlay a temporary anode system is used. A proprietary system developed in Norway consists of shredded paper and water sprayed onto the surface to form a wet papier mache . The mesh anode is then fixed to the surface on wooden batons and a final layer of papier mache applied. The system is kept wet for the operational period. Figure 7.20 shows an early installation underway using a mild steel mesh anode. This is rarely used now for ECE. 

Anode types are similar to those used for ECE. The sprayed cellulose is used by the owners of the patented system with a steel or coated titanium mesh. The steel is more likely to be used here as the treatment time is shorter and the steel is less likely to be completely consumed. 

Figure 4.5.19 depicts the schematics of the mercury cell anodes with an expanded titanium mesh [109]. Other types of mercury cell anodes use small diameter titanium rods or thin titanium blades. Electrical connection in the ceil is made through the boss using solid copper rods protected with a riser tube of titanium. A typical arrangement of anodes in a mercury cell is shown in Fig. 4.5.20. 

Membrane-cell anodes are coated expanded titanium mesh type or louvered type [110], and the anode is generally in contact with the membrane. The anode surface has to be absolutely smooth to ensure that no pinholes are created in the membranes [69]. It is claimed that punched steel metal is superior to a flattened expanded metal as the anode substrate [111]. 

Titanium mesh anode A type of impressed current anode consisting of an expanded titanium mesh coated by a corrosion re.sistant film of mixed metal oxides. After being fixed to the concrete surface the mesh is covered with concrete or mortar. 

Figure 6.9ta) and (b) The Elgard titanium mesh anode syiitem. 

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