Standard: NACE 01101
ELECTROCHEMICAL CHLORIDE EXTRACTION FROM STEEL REINFORCED CONCRETE - A STATE-OF-THE-ART REPORT
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Reinforced concrete is a versatile and widely used construction material. Its excellent performance and durability rely on the compatibility of the steel with the concrete surrounding it and the ability of the concrete to protect the steel from corrosion in most circumstances. Unfortunately, corrosion protection is not guaranteed, and can fail if sufficient chlorides (usually in the form of sea salt, deicing salt, or chloride contamination of the original mix) or atmospheric carbon dioxide (CO2) penetrate the concrete and break down the passive layer that protects the steel. This breakdown of the passive oxide layer leads to corrosion of the reinforcing steel if sufficient oxygen and water are available.
Regardless of the cause of depassivation (chlorides or carbonation), corrosion occurs by the movement of electrical charge from an anode (a positively charged area of steel where steel is dissolving) to the cathode (a negatively charged area of steel where a charge-balancing reaction occurs, turning oxygen and water into hydroxyl ions). This means that the process is both electrical and chemical, i.e., electrochemical. In the case of chloride attack, patch repairs are only a local solution to corrosion, and repairing an anode can accelerate corrosion in adjoining areas.
One solution to this problem involves applying an electrochemical treatment that suppresses corrosion. Figure 1 shows the basic components of an electrochemical treatment system. The components are a direct-current (DC) power source and an anode (temporary or permanent) usually distributed across the surface of the concrete. Electrochemical methods work by applying an external anode and passing current from it to the reinforcing steel so that all of the steel becomes a cathode.
Three electrochemical techniques are used to counter corrosion of steel in concrete. The first of these techniques is cathodic protection. A newer alternative for chloride-contaminated structures is electrochemical chloride extraction (ECE), also known as electrochemical chloride removal (ECR), or desalination, as the process is called in Europe. A method for treating carbonated concrete has been developed and is gaining rapid acceptance as a rehabilitation method for carbonation in buildings and other structures. This is known as realkalization.
Chloride removal was the subject of two major studies conducted under Federal Highway Administration(1) (FHWA) contracts in the 1970s.1,2 Both of these studies, as well as follow-up reports, concluded that chloride removed by electrochemical migration is a promising technique for use on salt-contaminated concrete.
Further research was undertaken in Norway by a private company, and under the Strategic Highway Research Program(2) (SHRP). As a result of that research a number of patents were published. A list of some of the principal U.S. patents directly relating to ECE is given in the bibliography. The list is not comprehensive and does not include patents from other countries.
The chloride ion acts as though it is a catalyst to corrosion, and is not consumed in the corrosion reaction. Chlorides enable corrosion to develop and expand once they are present beyond a threshold level at the steel surface. Because chlorides are negatively charged, the electrochemical process can be used to repel the chloride ion from the steel surface and move it toward an external anode. The ECE process uses an external anode that is installed for the duration of the treatment process. A higher electrical current density is applied than that used for cathodic protection (see NACE Standard RP02903 on cathodic protection and NACE Standard TM02944 for testing embeddable anodes for cathodic protection of atmospherically exposed steel-reinforced concrete). Normally, the ECE system runs for a limited time (typically four to eight weeks), and is then dismantled and removed from the structure. No permanent system is installed.
(!) Federal Highway Administration (FHWA), 400 7th St. SW, Washington, DC 20590.
(2) Strategic Highway Research Program (SHRP), National Research Council, National Academy of Sciences, Box 289, Washington, DC 20055.
|Document Number:||nace 01101|
|Most Recent Revision:||YES|
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