NACE 01104

Abstract

<strong>Introduction</strong> <br>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. Corrosion of the steel reinforcement does not occur, despite the presence of moisture and oxygen in the concrete pores, because of the alkalinity of the concrete pore water creating a passive oxide film on the reinforcing steel. 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. This leads to the breakdown of 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. <br>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). <br>One solution to carbonation-induced reinforcement corrosion involves applying an electrochemical treatment that suppresses corrosion. Figure 1 shows the basic components of an electrochemical treatment system for realkalyzing concrete. The components are a direct-current (DC) power source and a temporary anode distributed across the surface of the concrete encased in a conductive medium or electrolyte. <br>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. <br>Three electrochemical techniques are used to counter corrosion of steel in concrete. Cathodic protection can be applied by impressed current or galvanic anodes. Electrochemical chloride extraction (ECE) uses a temporary anode and high current over a period of 4 to 6 weeks (see NACE Publication 01101¹). Realkalization is a method for treating carbonated concrete. It is similar to ECE but takes approximately one week and is gaining rapid acceptance as a rehabilitation method for carbonation in buildings and other structures. Both ECE and realkalization use currents up to about 1 A/m² (0.1 A/ft²) of steel surface area.