Civil Engineering Reference
In-Depth Information
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.
