NACE SP0107

Abstract

General Background Following this General section, this standard is divided into two standalone sections, the first on electrochemical chloride extraction and the second on electrochemical realkalization. This will help the user by ensuring that all the relevant provisions are in one place. Reinforcing steel is compatible with concrete because of similar coefficients of thermal expansion and because concrete normally provides the steel with excellent corrosion protection. The corrosion protection is the result of the formation of a highly alkaline passive oxide film on the surface of the reinforcement by Portland cement contained in the concrete. This passive oxide film may be compromised by (1) excessive amounts of chloride or other aggressive ions and gases such as carbon dioxide, or (2) the concrete not fully encasing the steel.  Corrosion of the steel occurs as a result of the formation of an electrochemical cell. An electrochemical cell consists of four components: an anode, where oxidation occurs; a cathode, where reduction occurs; a metallic path, where the electrons flow; and an electrolyte (concrete), where the ions flow. The anodic and cathodic areas occur as a result of coupling of dissimilar metals (e.g., reinforcing steel to copper, brass or galvanized fittings or aluminum window frames), exposure to differential environmental conditions, or both. If any one of the four elements of the electrochemical cell is eliminated, corrosion can be prevented. Corrosion of reinforcing steel in concrete is a serious problem in certain environments throughout the world. This corrosion is directly attributable to the presence of significant amounts of aggressive substances and/or conditions at the steel surface. Parking structures, bridges and roadways, buildings, sanitary and water facilities, marine structures, concrete pipe, storage facilities, and other reinforced concrete structures are being damaged by corrosion. Industry published information indicates that refurbishment and replacement of concrete structures with corroded reinforcement and/or cracking and spalling damage of the concrete costs billions of dollars each year. These losses can be reduced if proper corrosion control factors are considered and addressed during rehabilitation and maintenance repair of reinforced concrete structures. Carbonation of concrete is a major factor that leads to reinforcing steel corrosion. Carbonation is a process by which atmospheric carbon dioxide reacts with the alkalis in the pore water of the concrete, reducing the pH to near neutral. A carbonation front proceeds through the cover concrete to the reinforcement, where it leads to the breakdown of the passive oxide layer, allowing corrosion to proceed. Electrochemical realkalization can be used to reverse this process and restore the alkaline environment to the reinforcement, preventing further corrosion. Chloride contamination of the concrete is another major factor that leads to reinforcing steel corrosion. Depending on the environment, it has been shown that chloride ion content as low as approximately 0.2 percent by weight of cement (or approximately 0.6 kg/m3 [1 lb/yd3] of concrete, depending on the cement content of the mix) at the steel depth can initiate the corrosion process.6 Electrochemical chloride extraction (ECE) can be used to move chloride ions away from the steel surface and reestablish the protective passive oxide layer. Electrochemical Treatments Electrochemical treatments for reinforced concrete include cathodic protection (CP), ECE, and electrochemical realkalization (EcR). ECE and EcR are short-term treatments with a temporary installation that is removed after treatment. Treatment is intended to remove the cause of corrosion. On the other hand, CP is a permanent installation. NOTE: CP of atmospherically exposed steel in concrete is described in NACE SP0290.1 Many of the practices described in SP0290 are relevant to ECE and EcR in terms of preparation of the structure, testing, and wiring. Scope and Limitations The provisions of this standard shall be applied under the direction of competent persons who, by reason of their knowledge of the physical science and the principles of engineering and mathematics acquired by education and related practical experience, are qualified to engage in the practice of corrosion control on reinforced concrete. NOTE: The competent person shall have a minimum of ten (10) years of experience in the field of corrosion of steel in concrete and at least three (3) projects similar to the one to be undertaken. They should also be a certified Cathodic Protection Specialist or Corrosion Specialist of NACE International or have an international equivalent qualification or documented equivalent level of education and experience. The requirements presented here are limited to impressed current ECE and EcR systems for new or existing atmospherically exposed reinforced concrete elements. The provisions of this standard practice are not applicable to prestressed concrete. Normal reinforcing steel in post-tensioned elements with the post-tensioning strands fully protected in ducts can be treated as long as adequate precautions are taken to ensure that the prestressed steel is not susceptible to hydrogen embrittlement and that it is protected such that the instant off (IR free) potential of the steel does not rise above the hydrogen evolution potential