Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1994-04-14
1995-12-19
Tung, T.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204196, C23F 1300
Patent
active
054765765
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to cathodic protection of structures, or particularly it relates to a system of impressed current cathodic protection. It will be convenient to describe the invention with reference to particular application for cathodic protection of bridge and pier columns in marine environments however it should be noted that the invention has a wider application.
In the current economic environment a greater emphasis is being given to the general upkeep, preservation and overall extension of the physical life of existing bridge and pier structures in often excess of the original design life. As a significant proportion of such structures consist of steel reinforced/pre-stressed concrete, corrosion of the steel reinforcing members within the concrete presents a significant problem, particularly in marine and tidal environments where the kinetics of corrosion are greatly increased.
There are various known methods of protecting the steel reinforcement by way of cathodic protection. One method of such protection is achieved by the use of a sacrificial anode. A metal higher in the galvanic series than the metal sought to be protected is electrically connected to the latter. The sacrificial anode, such as magnesium supplies electrons to the steel reinforcement and gradually decays in the process. The sacrificial anode must be periodically replaced to provide continuous corrosion protection.
An alternative method of supplying electrons is by way of impressed current cathodic protection. This method involves supplying electrons from an external source, the steel reinforcing member being connected to the negative side of the electromotive force and the positive side is connected to an anode.
Over the last ten to fifteen years different types of impressed current cathodic protection systems have been developed for the protection of reinforced concrete structures. These systems have included titanium mesh/cementitious overlays, conductive coatings, spray-on metallised zinc, conductive rubber anodes and internally mounted graphite/titanium anodes. Problems associated with the above systems include complexity of design, specification and on-site works required including preparation of concrete surface and subsequent maintenance. Furthermore, it is necessary to enable installation at low costs as well as to minimize the level of expertise required to install these systems.
A further problem with impressed current cathodic protection systems is current dumping. In the case of marine pier or bridge structures three distinct exposure zones consisted with corresponding corrosion zones are identified: sun; influenced by the wetting and drying process which promotes ingress of water and difusion of oxygen and chloride ions; and saturated thus severely restricting the supply of oxygen which is vital for corrosion initiation and subsequent propagation. Some of the above impressed current cathodic protection systems require periodical wetting of an anode substrate in the tidal/splash zone to maintain conductivity for the protective current. In column tidal zones dumping of current to surrounding steel reinforcement occurs when the tide rises and conductivity is restored through the conductance of salt water which generally has a resistivity of between 25 and 1600 ohm.cm.
There is a need to provide a system for impressed current cathodic protection for reinforced concrete piling which reduces current dumping, is relatively easy to install and does not require exceptionally skilled labour for its installation and which is relatively inexpensive to maintain. Moreover there is a need for an impressed current cathodic protection system which provides an adjustable current density control to achieve an even polarization of the steel reinforcement. There is also a need for a protection system which does not significantly affect the volume occupied by the structure or add significantly to the mass of the structure to be protected.
It is an object of the present invention to provide a solution to one or more of the above
REFERENCES:
patent: 2423290 (1947-07-01), Bonwitt
patent: 4255241 (1981-03-01), Kroon et al.
patent: 4578215 (1986-03-01), Bradley
patent: 4900410 (1990-02-01), Bennett
patent: 5292411 (1994-03-01), Bartholomew et al.
patent: 5296120 (1994-03-01), Bennett et al.
Phang Bob
Solomon Isaac
Solomon Corrosion Consulting Services Pty. Ltd.
Tung T.
Winn and Coales International Limited
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