Patent Application
20140251793
This invention relates generally to corrosion control in reinforced-concrete structures and, in particular, to mixed-metal-oxide (MMO) coated precious-metal tape and mesh that may be installed directly on concrete surfaces without the need for slots, holes, cementitious grout or concrete.
Cathodic protection (CP) is a method for controlling corrosion of reinforcing steel, including steel structures in chloride contaminated concrete. Various types of impressed current cathodic protection anodes for reinforced concrete structures have been developed in the past. The anode is one of the most critical components for a cathodic protection system, as it is used to distribute cathodic protection current to the reinforcing steel.
One of the most effective and durable anodes is made of a material which is resistance to corrosion, for example a mixed-metal-oxide (MMO) coated titanium substrate. MMO coated anodes are manufactured by coating a mixture of precious metal oxides on a specially treated precious metal. The coated substrate undergoes multiple thermal treatments at elevated temperatures to achieve good bonding properties between the substrate and the coating. Although titanium is widely used as substrate material due to its resistance to corrosion, resistance to chemical attacks and high mechanical strength, other anodes such as tantalum, niobium and zirconium anodes are also used for different applications.
Since the first MMO-coated titanium anode was developed in 1984, many concrete structures have been protected using this material. To install the anodes, however, they must be embedded in concrete or cementitious grout. For example, titanium mesh with a concrete overlay, titanium ribbon or ribbon mesh embedded in cemetitious grout in saw-cut slots, or discrete anodes embedded in grout in drilled holes. However, these types of installations add burden to the structure and lead to some durability concerns. A useful review of MMO-coated anodes and installation techniques may be found in “Cathodic Protection of Steel in Concrete” By Paul Chess, Taylor & Francis (1998), ISBN 0419230106, the entire content of which is incorporated herein by reference.
For the slotted or discrete types of installations, the existing concrete must be cut or drilled to install the anodes. However, when the concrete covers over the reinforcing steel are shallow or congested, such installation procedures are not feasible. Even if the anodes are somehow installed in the slots or drilled holes, the vicinity of the reinforcing steel near the anodes may cause an electrical short circuit, resulting in malfunction of the cathodic protection system.
When a MMO-coated titanium anode is operated greater than 110 mA/m2 of anode current density in chloride contaminated concrete, acid is generated at the anode-concrete interface due to the chlorine gas evolution by the anodic reaction. As a result, cement paste of the concrete as the electrolyte which contact to the anode is dissolved by the acid. This leaves the non-conductive aggregates at the anode-concrete interface and causes the increases of the circuit resistance, diminishing the cathodic protection current.
When MMO coated precious metal tape is installed with electrically conductive adhesive, or when any form of MMO coated precious metal anodes are embedded in the dry concrete structure which is not subject to direct moisture, rain or seawater splashes, the circuit resistance increases with time due to the electrochemical osmosis at the anode-concrete interface. Once the circuit resistance exceeds the maximum DC power supply, the current from the anodes decrease with time. The electrochemical osmosis condition increases with increasing the anode voltage. Eventually, the anodes cannot discharge any current at the maximum voltage of the power supply.
When high conductive media is used as an anode to cover the concrete surface, an electrical short circuit is often developed by the exposed metals, such as rebar chairs, steel wire ties. When highly conductive media is used as an anode to cover the concrete surface, the concentration of cathodic protection current from the local portion of the anode system to shallow rebars develop acid generation, resulting in poor current distribution to a concrete structure.
This invention overcomes the shortcomings of prior art by allowing mixed-metal-oxide (MMO) coated precious-metal tape to be installed on dry concrete structures without the need for slots or holes. In the preferred embodiments, a semi-electrically conductive and gas-permeable coating or overlay is used in combination with a MMO-coated precious-metal tape anode to improve current distribution at low voltages. The electrical resistivity of the semi-conductive coating is lower than that of typical concrete. However, when the semi-conductive coating is moist in a local area, the resistivity is still high enough to prevent the concentration of cathodic protection current discharging to the concrete substrate.
According to the invention, MMO-coated tape anodes may be installed on the concrete surfaces using non-conductive adhesives without developing an electrical short circuit between the anode and the reinforcing steel. Interconnections between the tape anodes and bare metal distribution elements may be made with conductive adhesive or spot welding. Alternatively, MMO-coated mesh anodes may be embedded in the semi-conductive coating without contacting to the concrete substrate. Interconnections between the mesh anodes and bare metal distribution elements may be made with conductive adhesives or spot welding.
Since MMO coated anodes are not embedded or contact to concrete, the anodic can be operated at anode current densities higher than 110 mA/m2. The chlorine gas evolved on the anode diffuses away though the porous semi-conductive coating before it tunes to hydro-chloric acid. By utilizing this high current discharge capability of the anode system with the semi-conductive coating, the system facilitates electro-chemical chloride extraction from the chloride contaminated concrete.
Through the use of a semi-conductive layer covering over the anodes, electrical short circuits can be prevented even though the semi-conductive coating contacts with exposed metals from the concrete surface. The semi-conductive media may include carbon, MMO coated metal(s) or any passive bare metal powder or fibers mixed with any electrolytic cementitious or plastic media.
When carbon powder or fibers are used as the electrical media to produce semi-conductive layer, the carbon is consumed by passing through the cathodic protection current with time. However, when a large current is required for a long time of period, MMO coated metal powder or fibers may be used to extend the life of the semi-conductive layer. When passive metal powder or fibers are used under their break-down potential, they can be used without consumption of the metals.
This invention relates to protection and corrosion prevention of reinforced concrete structures using mixed-metal-oxide (MMO) coated precious-metal tape anodes. In the preferred embodiments, the anodes are attached to a concrete surface using a non-conductive adhesive and covered with a semi-conductive layer to provide cathodic protection or chloride removal. As such, the bare metal surface of the tape is fixed on the concrete surface using a non-conductive adhesive, such that the MMO-coated metal surface of the tape is exposed.
The substrate metal tape anode may be composed of titanium, tantalum, zirconium, or niobium. However, the most preferred metals are titanium or titanium alloys because of the corrosion resistance and availability. The tape anode width is preferably over 5 mm, and the thickness is in the range of 0.001 mm to 1 mm, preferably between 0.1 mm to 0.3 mm.
The semi-conductive coating preferably comprises a cementitious or plastic material with suspended conductive or semi-conductive particles to adjust the electrical resistivity. In accordance with one preferred embodiment, the semi-conductive coating comprises a flowable, hardening cementitious or plastic medium, which may include a layer of carbon fibers or passive metal fibers, further including a distribution of oxides of titanium, tantalum, iridium, ruthenium, palladium, or cobalt. By adjusting the composition of the semi-conductive media 2, the electrical resistivity may range from 100 ohm-cm to 20,000 ohm-cm. These moderately high resistances prevent electrical short circuits if any metal exposed from the concrete. However, the electrical resistivity is low enough to distribute the current to the entire covering concrete surface. The thickness of the semi-conductive media 2 is in the range of 1 mm to 25 mm, preferably between 3 mm to 7 mm.
Furthermore, as shown in
The semi-conductive coating is resistant to the acid which may develop on the anode surface.
