Emerging Corrosion Control Technologies for Repair and Rehabilitation of Concrete Structures (page 3)


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Corrosion Special Topical Papers
Emerging Corrosion Control Technologies for Repair and Rehabilitation of Concrete Structures* Dr. Jianhai Qiu School of Materials Engineering Nanyang Technological University Nanyang Avenue, Singapore 639797 This zinc foil anode coated with hydrogel is provided in roll form for coverage of the concrete surface to be protected. The ionically conductive hydrogel performs two functions: (1) to enable the anode to be securely adhered to the exterior surface of the concrete structure, and (2) to serve as a continuous conductive electrolyte between the anode and the concrete structure. Fig. 1 Pressure-Sensitive Zinc-Hydrogel Anode (Illustration copyrighted by 3M) Fig. 2 Installation of Zinc-Hydrogel Anode (Illustration copyrighted by 3M) . 3.2.1. How It Work As illustrated in Fig. 2, the zinc foil is applied to the surface of structurally sound concrete structure. The pressure sensitive adhensive (conductive hydrogel) bridges the gap between the zinc anode (foil in this case) and the concrete surface – making possible the electrolyte continuity across the anode and cathode (rebars). The zinc foil must be electrically connected to the rebar network by wires so that electrons can flow through the wire from zinc (anode) to rebars (cathode). Conventional current flows in the opposite direction of electrons, i.e., from cathode to anode. Ionic current is carried by charged species such as Cl^-, Na⁺, OH^-, Ca²⁺ etc. across the electrolyte (concrete + hydrogel). The potential of rebar and the current flowing between the zinc foil and the rebar can give some indication about the degree of cathodic protection. 3.2.2 Why It Works Corrosion of rebar is the process involving the following oxidation reaction: Fe ==> Fe²⁺ + 2e A neutral iron atom lost 2 electrons and became a positively charged ion. The tendency of a metal to lose electrons can be considered as the tendency of metal to corrode. When zinc foil is applied to the concrete surface but not connected by wire to the rebar network, the steel rebar continues to corrode at a normal rate. When zinc foil is electrically connected to the rebar network, ELECTRONS flow from zinc foil into steel rebars, thus reducing the steel’s tendency to lose electrons and hence reducing the rate of corrosion (oxidation). This is because zinc is a much more reactive metal than steel, it has a much greater tendency to lose electrons than that of steel. Since electrons carry negative charges, the accumulation of electrons within steel rebar makes steel more negatively charged, leading to cathodic polarisation, - the shift of rebar potential in the negative direction. In general, the greater the tendency for the anode to donate electrons to the cathode, the greater the degree of protection rendered to the “cathode” (the steel rebar network). However, one must remember that when an anode donates electrons to the cathode, the anode material “sacrifices” itself. What this means is that the useful life of the anode is both limited by the chemical/electrochemical surface reactivity and the amount of physical material available. 3.2.3 Potential Applications The nature of the zinc hydrogel anode makes it a perfect rehabilitation option for structures or certain sections of structures exposed to atmospheres. The conductive hydrogel ensures the continuity of electrolyte across the rebar-concrete-hydrogel-anode, and hence cathodically protects the rebar network. Tips: It is a good idea to paint the zinc surface to preserve the anode material by reducing corrosion of zinc from external atmosphere.
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