CATHODIC PROTECTION CURRENT DISTRIBUTION METHOD AND APPARATUS FOR CORROSION CONTROL OF REINFORCING STEEL IN CONCRETE STRUCTURES

Abstract

Mixed-metal-oxide (MMO) coated precious-metal tape is installed directly on concrete surfaces using an electrically non-conductive adhesive with semi-conductive coating or overlay, thereby obviating the need for slots or holes to the concrete structures which are not subject to direct moisture. The tape anodes may be installed on the concrete surfaces exposing metals without developing an electrical short circuit between the anode and the metals due to the non-conductive adhesive. The semi-conductive layer over the metal tape can distribute the CP current uniformly to the entire concrete surface without developing electrical short circuit. Overall the invention provides for quick and low cost installation on many concrete structures. Interconnections between the tape anodes and bare metal distribution elements may be made with conductive adhesive or spot welding.

Description

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates tape anode installation on a concrete surface and covered with a semi-conductive coating. The tape anode is fixed on the concrete surface by non-conductive adhesive and the top of the tape discharge current;

FIG. 2 illustrates a mesh anode installation within a semi-conductive coating;

FIG. 3 is an example of a possible installation method of tape anodes to a bare-metal element; and

FIG. 4 illustrates chlorine gas generated on an anode diffusing away to atmosphere.

DETAILED DESCRIPTION OF THE INVENTION

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.

FIG. 1 is a simplified cross-sectional diagram showing a metal tape anode 1 attached to concrete 4 through non-conductive adhesive 3. To distribute cathodic protection current to the entire concrete surface before traveling into the concrete and reinforcing steel bars, thin coated semi-conductive media 2 is coated over the metal tapes.

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 FIG. 2, the anode tapes are typically spaced on concrete surfaces according to the cathodic protection current requirement for the reinforcing steel in concrete. The spacing is also based on the current requirement of the reinforcing steel. As shown in FIG. 3, the tape anodes 1 or 6 may be electrically interconnected at points 8 to bare metal tapes 7 by means of spot welding or conductive adhesive. The “bare” metal tape may be the same metal as the tape anode or different materials may be used.

The semi-conductive coating is resistant to the acid which may develop on the anode surface. FIG. 4 illustrates how chlorine gas evolved on the metal tape surface and within the semi-conductive coating may diffuse away from the anode system though the porous coating 2. Because the invention allows the anodes to operate at anodic current densities higher than 110 mA/m² without generating acid, this also allows using as chloride removal system using larger current densities. The negatively charged chlorides which are attractive to the positive charged anode, they turn to chlorine gas. The gas diffuses away to the surrounding atmosphere through the semi-conductive layer.

Claims

1. A system for controlling the corrosion of reinforcing steel in a concrete body having a surface, comprising:

a mixed-metal-oxide (MMO) coated precious-metal anode indirectly bonded to the surface of the concrete body through an electrically non-conductive material; and

a semi-conductive coating covering the anode and at least portions of the surface of the concrete body to distribute cathodic protection current from the concrete surface to the anode without the anode making a direct electrical connection to the concrete body.

2. The system of claim 1, wherein the electrically non-conductive material is a non-conductive adhesive.

3. The anode of claim 1, wherein the anode is composed of carbon, titanium, tantalum, zirconium, niobium, or alloys thereof, or MMO coated titanium, tantalum, zirconium, niobium, or alloys thereof.

4. The anode of claim 1, wherein the coating is composed of oxides of titanium, tantalum, iridium, ruthenium, palladium, or cobalt.

5. The system of claim 1, wherein the anode is in the form of an elongate tape.

6. The system of claim 1, wherein the anode is in the form of an elongate tape having a width of 5 mm or greater and the thickness in the range of 0.001 mm to 1 mm.

7. The system of claim 1, including a plurality of anodes interconnected with a bare metal tape.

8. The system of claim 1, including a plurality of interconnected anodes spaced-apart on the surface of the concrete.

9. The system of claim 1, including a plurality of anodes interconnected with a bare metal tape using an electrically conductive adhesive.

10. The system of claim 1, including a plurality of anodes interconnected to a bare metal tape using spot-welding.

11. The system of claim 1, wherein the semi-conductive coating includes a layer of carbon fibers or passive metal fibers.

12. The system of claim 1, wherein the electrical resistivity of the semi-conductive coating is in the range of 100 ohm-cm to 20,000 ohm-cm.

13. The system of claim 1, wherein the semi-conductive coating is sufficient porous for chlorine gas escape therethrough to atmosphere.

14. The system of claim 1, wherein the anode is in the form of a mesh.