Patent Publication Number: US-7905993-B2

Title: Corrosion control method and apparatus for reinforcing steel in concrete structures

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 that may be installed directly on concrete surfaces without the need for slots, holes, cementitious grout or concrete. 
     BACKGROUND OF THE INVENTION 
     Cathodic protection is a method for controlling corrosion of reinforcing steel 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 and 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 gain 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 globally 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 the installation add some burden to the structure and 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 &amp; Francis (1998), ISBN 0419230106, the entire content of which is incorporated herein by reference. 
     The overlay concrete cathodic protection system causes additional dead load to the structure. Frequent disbonding between the overlay and existing concrete is also a serious problem. For the slotted or discrete types of system, 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, installation of these types of the systems is 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. 
     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 directly on concrete surfaces without the need for slots, holes, cementitious grout or concrete. In the preferred embodiment, an electrically conductive adhesive is used to bond the tape to the surface of the concrete. The electrically conductive adhesive is preferably formed by disposing mixed-metal-oxide (MMO) coated precious-metal particles in a rubberized adhesive. 
     According to the invention, MMO-coated tape anodes may be installed on the concrete surfaces including a shallow concrete cover or congested reinforcing steel without developing an electrical short circuit between the anode and the reinforcing steel. 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a tape anode installation and protection of the tape anode using an optional over-coating; 
         FIG. 2  illustrates the tape anode installation and protection of the tape anode using an optional coating and non-conductive overlay; and 
         FIG. 3  is an example of a possible installation method of tape anodes to a bare-metal element. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention disclosed concerns the protection and the prevention of corrosion of reinforced concrete structures using mixed-metal-oxide (MMO) coated precious-metal tape anodes. In the preferred embodiment, the tape anode is attached to a concrete surface using a conductive adhesive. More particularly, MMO-coated precious-metal powders are introduced into epoxy or other adhesive, including a rubberized adhesive, capable of bonding the anode tape to concrete. 
     An appropriate rubberized adhesive may be similar to that used for clear packing tape. Because this is electrically non-conductive, the current from the MMO tape does not transfer to the concrete underneath. However, if the MMO-coated particles have diameters larger than the thickness of the dried adhesive film, some portions of the particles are exposed through the adhesive layer. As such, when the tape is compressed on the concrete surface, the particles make acceptable contact to the concrete. By including enough particles in the adhesive, the contact resistance between the MMO tape and the concrete is sufficiently low enough to provide adequate cathodic protection. 
     The substrate metal tape anode may be composed of titanium, tantalum, zirconium, niobium. However, the most preferred metal is 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 conductive adhesive uses precious-metal powders coated with mixed metal oxides of titanium, tantalum, iridium, ruthenium, palladium, cobalt or mixtures of the same. As with the body of the anode itself, the metal substrate of the powder may be titanium, tantalum, zirconium, niobium, or alloys thereof. 
     The size of the powder may ranges from 10 to 1000 mesh. By mixing the MMO-coated powders in the adhesive, the contact electrical resistance between the tape anode and the existing concrete is low enough to flow the cathodic protection current into the reinforcing steel through the concrete electrolyte. 
       FIG. 1  is a simplified cross-sectional diagram showing a metal tape anode  1  attached concrete  4  through conductive adhesive  2 . To reinforce the bonding durability of the tape anode to the concrete for a longer period of time, an optional concrete coating, water proof membrane, liner, or cap  3  may be used to cover the tape anode. Furthermore, as shown in  FIG. 2 , solid or mesh tape  5  may be placed over the tape anode with or without coating  3  to further reinforce the durability of the anode tape. The tape  5  may be FRP (fiberglass-reinforced plastic) or other non-conductive material. 
     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 distribution to the reinforcing steel. As shown in  FIG. 3 , the tape anodes may be electrically interconnected at points  7  to bare metal tapes  6  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.