Cathodic Protection to Control External Corrosion of Concrete Pressure Pipelines and Mortar-Coated Steel Pipelines for Water or Wastewater Service
Introduction
Concrete and steel are considered compatible materials because they have similar coefficients of thermal expansion and because concrete usually provides steel with excellent corrosion protection. Because of the high alkalinity of portland cement, a stable, corrosion-mitigating, passive oxide film forms on the surface of the encased steel.
The protective oxide film formed on steel encased in concrete does not form or will be destroyed if the concrete does not fully encase the steel, the alkalinity of the concrete is lost by reaction with aggressive gases or liquids, or excessive amounts of chloride or other aggressive ions are present. If one or more of these conditions exist and moisture and oxygen are in contact with the steel, corrosion can occur.
Corrosion occurs because of the formation of an electrochemical cell. An electrochemical cell consists of four components: an anode, at which oxidation occurs; a cathode, at which reduction occurs; a metallic path through which electrical current passes as a flow of electrons; and an electrolyte (concrete pore solution) through which electrical current passes as a flow of ions in an aqueous medium. If any one of the four elements of the electrochemical cell is eliminated, corrosion is prevented.
Within the electrochemical cell, the location of relative anodic and cathodic areas can be determined through potential (voltage) measurements. This is accomplished by measuring the potential between a metal immersed or embedded in an electrolyte and a stable reference electrode. This technique may also be used to assess the effectiveness of CP.
Cathodic Protection (CP)
The basic principles of corrosion can be used to understand the theory of CP. CP is defined as a technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell (see Figure 1 and NACE/ASTM G193).
Polarization Diagram
If corrosion of steel is found in a concrete pressure pipeline or a mortar-coated steel pipeline, CP may be used to control further corrosion. However, CP does not replace lost steel or return corroded steel to its original cross-section.
Electrical continuity of the steel elements within the pipe sections and between individual pipe sections is required for adequate CP; see Paragraph 6.8.
This standard serves as a guideline for establishing minimum requirements for CP of the following:
New pipelines: CP is usually not required on new pipelines because of the passive film that forms on steel embedded in portland cement concrete or portland cement mortar. Cracks or damage caused by construction activities may be a consideration for application of CP. Environmental conditions such as those described in Paragraph 1.1.2 or the presence of nearby sources of interference also warrant consideration of CP; otherwise, the pipeline should be monitored periodically to determine whether corrosion might be occurring.
Existing pipelines: Studies should be made to determine the extent of active corrosion on the pipeline. If these studies indicate that corrosion is affecting the safe or economic operation of the pipeline, adequate corrosion control measures, which may include CP, should be taken.
CP should be provided and maintained if investigations indicate that corrosion poses a risk or might be occurring and adequate electrical continuity exists or can be established.
Special conditions sometimes exist in which CP is ineffective or only partially effective, such as shielding by nearby structures. Deviation from this standard may be warranted in specific situations provided that the corrosion control personnel in charge demonstrate that the objectives expressed in this standard are achieved.
For accurate and correct application, this standard must be used in its entirety. Using or citing only specific paragraphs or sections can lead to misinterpretation and misapplication of the recommendations and practices contained in this standard. This standard does not designate practices for every specific situation because of the complexity of conditions to which buried or submerged pipelines are exposed.
This AMPP standard provides general corrosion control guidelines for embedded steel in buried concrete pressure pipelines and mortar-coated steel pipelines for water or wastewater service. In accordance with AMPP’s standards review policy, this document is to be updated every 5 years to reflect current industry trends and regulations. Revisions include clarifications, added definitions, consistency with related AMPP standards, general organization, and adherence to the latest AMPP formatting for standards.
