John P Broomfield
5 Cathodic protection using thermal sprayed metals
John P. Broomfield
Chapter 4 gave an overview of impressed current and galvanic cathodic protection. One of the most important choices in the design of a cathodic protection system is the anode system. Thermal sprayed metals are increasingly popular as anodes in both impressed current and galvanic anode systems.
5.1 Thermal sprayed zinc as an impressed current anode
Thermal spayed zinc was first used as an impressed current anode by the California Department of Transportation (Apostolos et al. 1987). The anode is applied using either flame spraying or electric arc spraying. The latter is now more common as deposition rates are far higher. There is a standard for thermal spraying anodes, see American Welding Society (2002). After initial surface preparation by a light grit blast, the zinc is sprayed onto the surface of the concrete using flame or electric arc equipment. Although the material is non-proprietary, it requires specialist applicators and bulky spray equipment.
As the coating is highly conductive it is important to avoid short circuits to the steel caused by tramp steel or tie wires. A simple electrical circuit with an audible alarm can be set up to warn of shorts between the reinforcement cage and the anode as it is applied; however, it can still be difficult to precisely locate short circuits. These must be eliminated for impressed current cathodic protection to work.
The anode is ideal for bridge substructures where it has been widely used in the USA. The coating is more tolerant of water after application than conductive paint anode systems. However, bulky specialist equipment is required for installation and it cannot be applied in any other way. The anode system has a typical life-expectancy of up to 25 years and has a medium to high cost of installation compared with other impressed current cathodic protection anodes.
Like a conductive paint there is little visual or other impact. The grey colour means it has limited acceptability on buildings. It can be overcoated, usually with silicate-based materials.
Due to its limited use in the UK and elsewhere in Europe there is limited home-grown information on its performance. However, based on the extent of US usage, there do not appear to be any major issues with its use. Hundreds of thousands of square metres have been applied to bridges in the USA.
The largest installations have been in Oregon where a number of historic landmark bridges were protected with this system (Covino et al. 2002). There have been other large installations on inland bridges elsewhere in the USA and Canada. Several hundreds of thousands of square metres of impressed current thermal sprayed zinc anodes have been applied to bridges in the USA. Figure 5.1 shows an impressed current cathodic protection installation on a bridge substructure in the UK.
5.2 Thermal sprayed zinc as a galvanic anode
The development of thermal spayed zinc as an impressed current anode was followed by its use as a galvanic anode by Florida Department of Transportation (Kessler and Powers, 1990) where thousands of marine bridge piles and columns have been protected. For marine applications it is
Figure 5.1 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 where moisture run-down might degrade the life of the zinc anode.
recommended to use a bulk zinc anode at low tide level to protect the lower steel and to reduce the consumption of the zinc coating anode.
Many of the piles it has been applied to are prestressed. The use of galvanic zinc anodes means that the risk of hydrogen embrittlement of the prestressing is minimised.
The main advantage of thermal sprayed metal anodes are that they do not change the profile or dead load of the structure or require excavation of concrete. Most other galvanic anodes have at least one of these drawbacks.
While the thermal sprayed galvanic zinc anode was developed for application in a marine splash and tidal zone environment, it has been used inland in de- icing salt exposure conditions. In this case a humectant of hygroscopic salts was developed (Bennett, 1998). This was designed to reduce the electrical resistance of the cover concrete and the anode/concrete interface to boost the current flow of the anode in dryer inland conditions. A proprietary system with protective/cosmetic top coat is also offered in Europe.
In monitored galvanic anode bridge systems in Florida, sections of rebar have been isolated from the network and connected via an ammeter to the rest of the steel. This allows monitoring of protective current flows into a particular area. Similarly, areas of anode have been isolated and current flow measured to determine how much current the anode delivers. Occasional coring can be used to assess the amount of anode consumed and the anode replaced when necessary. Large-scale galvanic systems are in place protecting
many piles in Florida where over 50,000 m2 has been applied.
Figure 5.2 Thermal sprayed zinc being applied to a bridge substructure in the Florida Keys (courtesy Florida DoT).
5.3 Aluminium–zinc–indium galvanic anodes
At about the same time as the humectant was under development, a proprietary aluminium–zinc–indium alloy thermal spray was developed (Funahashi and Young, 1999). This was aimed at boosting the galvanic current throw without the use of a humectant for inland de-icing salt applications. The original material was electric arc sprayed using a wire with a core of powdered metal. Later developments of other proprietary aluminium–zinc– indium anodes use a solid wire and are now widely offered in Europe and the Middle East.
5.4 Applying thermal sprayed metals
The anode is applied using either flame spraying or electric arc spraying. The latter is now more common as deposition rates are far higher than flame spraying. There is a standard for thermal spraying anodes on concrete, see American Welding Society (2002). After initial surface preparation by a light grit blast, the zinc or Al–Zn–In is sprayed onto the surface of the concrete using flame or electric arc equipment. The coating application rate and thickness must be carefully controlled as thick coatings can debond due to thermal shock of the hot ‘splatters’ of zinc (or Al alloy) hitting the cold surface. Coatings are usually a few tenths of a mm thick (typically 0.3 mm).
Zinc vapour can lead to ‘zinc flu’ if breathed in. Therefore all operatives must be suitably protected and the area shrouded to contain the zinc overspray.
This anode system has a life-expectancy of up to 25 years and has a medium to high cost of installation compared with other anode systems. Like a conductive paint there is little visual or other impact. The grey colour means it has limited acceptability on buildings.
References
American Welding Society. ‘Specification for thermal spraying zinc anodes on reinforced concrete’. AWS/ANSI Standard. 2002; AWS C2.20/C2.20M:2002. Apostolos, J. A. Parks, D. M., and Carello, R. A. ‘Cathodic protection using metallized
zinc’. Materials Performance. pp. 22–28. 1987.
Bennett, J. E. ‘Chemical enhancement of metallized zinc anodes performance’. Corrosion 98. Mar 1998. Paper No. 640, San Diego CA..
Covino, B. S. Cramer, S. D. Bullards, S. J. Holcomb, G. R. Russell, J. H. Collins, W. K. Laylor, H. M., and Cryer, C. B. Performance of Zinc Anodes for Cathodic Protection of Reinforced Concrete Bridges. FHWA/Oregon DOT Report. FHWA- OR-RD-02-5. Mar 2002.
Funahashi, M. and Young, W. ‘Three-year performance of aluminium alloy galvanic cathodic protection systems’. NACE Corrosion 99 (Paper No 550) Apr 1999. Kessler, R. J. and Powers, R. G. ‘Zinc metalizing for galvanic cathodic protection
of steel reinforced concrete in a marine environment’. Corrosion 90. Apr 23–27 1990; Paper 324: Las Vegas Nevada. NACE International, Houston, Texas.