Thermal sprayed zinc

Several promising sacrificial anodes such as thermal-sprayed zinc, thermal-sprayed Al-Zn-In, zinc hydrogel, and zinc mesh pile jacket have been developed for use in CP of substructure members especiaUy in marine environments. Industries in some states in cooperation with FHWA carried out some developments and identification of some anodes suitable for impressed current CP of inland concrete structures. 

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. 

Figure 7.5 Thermal sprayed zinc being applied to a bridge substructure in the Florida Keys. Courtesy Florida DOT.

The next major development was of humectants which are sprayed onto thermal sprayed zinc anode surfaces. A humectant penetrates into the concrete and increases the humidity to maintain a low electrical resistance and maintain current flow between the anode and the steel (Bennett et al., 2000). 

Thermal sprayed zinc - Zinc is flame or electric arc sprayed onto the concrete surface and a direct connection made to the reinforcement. It can be used as sprayed on marine splash or tidal applications. In drier locations a humectant solution of hygroscopic salts can be applied. Over 50,000 m has been applied, mainly in Florida. 

Thermal sprayed Aluminium/Zinc/Indium - A proprietary variation on thermal sprayed zinc that uses an alloy to enhance the current rather than a humectant. A 300 micrometre thick coating is applied by arc spray. Approrimately 35,000 m had been applied up to May 2003. Adhesive zinc sheet - A proprietary system consisting of rolls of zinc 0.25 mm thick, 25 mm wide with a conductive gel adhesive on one side. The installed system can be painted. The edges must be sealed against water ingress as the gel can swell and leak. 

Figure 7.10 Thermal sprayed zinc applied to the leaf piers of Golden Fleece Interchange on the M6, UK. Probe anodes are applied to the cantilevered ends, the bearing shelf and the diaphragms between the longitudinal steel.

American Welding Society. Specification for thermal spraying zinc anodes on reinforced concrete. AWS/ANSI Standard. 2002 AWS C2.20/C2.20M 2002. 

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). 

Disadvantages no control to ensure that protection is maintained, limited anode life (say 15 5 years), no published standards other than impressed current criteria which require installation of monitoring equipment and circuitry. Anodes intrusive either thermal sprayed zinc, anodes in repairs or cored holes. 

Table 2.30 Relative Performance of Thermally-Sprayed Zinc-Aluminum Coatings 34 Years"... 

Work in Spain (Guillen and Feliu, 1970) on painted, thermally sprayed zinc immersed in seawater showed that epoxy resin and polyurethane paints performed especially well. 

Butler, J. T., and Morrow, H. (1988 and 1990). Corrosion performance of thermal sprayed zinc-aluminum alloys (typescript). ZMTG, P.O. Box 2664, Greenwich, CT 06836, 16 pp. 

German, G. (1978). Corrosion prevention with thermal-sprayed zinc and aluminum coatings. Proc. Anniversary Symposium, Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646. ASTM, Philadelphia, pp. 74-82. 

In thermal-sprayed zinc coatings, layer thicknesses of 0.050-0.150 mm are the maximum reached. These layers consist mainly of pure zinc, with a certain amount of zinc oxide and pores depending on the layer thickness and spraying technique. The much lower protective effect of spray galvanisation at the same layer thickness is probably due to the lack of the iron-zinc alloy phase. 

Thermal sprayed zinc, a few hundred microns in thickness applied directly to the concrete surface (Fig. 13.21). A humectant may be added for increasing moisture, reducing concrete resistance, and increasing the current level. 

Thermally sprayed zinc or aluminum for moderate protection... 

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