NACE SP0408-2008

1.1 Background

1.1.1 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 passive oxide film on the surface of the reinforcing steel by highly alkaline portland cement contained in the concrete. This passive oxide film can be compromised by (1) excessive amounts of chloride or other corrosive ions and gases, or (2) the steel not being sufficiently encased by the concrete.

1.1.2 Corrosion 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 electrons flow; and an electrolyte (concrete), where ions flow. The anodic and cathodic areas occur as a result of coupling dissimilar metals, exposure to different environmental conditions, or both. If any one of the four elements of the electrochemical cell is eliminated, corrosion can be prevented.

1.2 Cathodic Protection

1.2.1 The basic principles of corrosion can be used to understand the theory of CP. CP reduces the corrosion of a metal surface by making the protected surface the cathode of an electrochemical cell.

1.2.2 CP is a proven technique for controlling corrosion of steel in chloride-contaminated concrete structures. However, CP neither replaces lost steel nor returns corroded reinforcing steel to its original crosssection.

1.2.3 CP of reinforcing steel in atmospherically exposed concrete is described in NACE SP0290. Many of the practices described in that document are relevant to buried and submerged elements. Other anode types are also applicable to buried and submerged elements, as the soil or water provides a somewhat homogenous medium for the anode system, which need not be fixed directly to the concrete. The application of CP to prestressed concrete cylinder pipelines is described in NACE SP0100.

1.3 Scope and Limitations

1.3.1 The provisions of this standard should be applied under the direction of a registered Professional Engineer or a person certified by NACE International as a Corrosion Specialist or a CP Specialist. The person's professional experience should include suitable experience in CP of reinforced concrete structures. Under certain circumstances, a CP system may either become a structural component or significantly affect the serviceability and structural performance of a reinforced concrete structure; therefore, such impact by the CP system should be reviewed by a qualified registered Structural Engineer or the equivalent.

1.3.2 The guidelines presented here are limited to CP systems for new or existing buried or submerged reinforced concrete elements.

1.3.3 When the reinforcing steel is bonded to facility grounding, as commonly required by the National Electrical Code,6 the resulting galvanic corrosion cell and the possible adverse affects on the quantity and distribution of CP current to the reinforcing steel shall be considered.