Cell-to-Cell Surveys for Identifying Active Corrosion on Mortar-Coated Steel Pipes
Introduction
Corrosion in infrastructure systems, such as steel pipes, significantly threatens their structural integrity and longevity. Detecting and monitoring corrosion promptly is crucial to ensure the safe operation of these infrastructure assets. In the case of mortar-coated steel pipes, corrosion can occur underneath the protective mortar layer, making it challenging to identify without invasive investigative techniques. Cell-to-cell surveys assess the likelihood of active corrosion on mortar-coated pipe materials due to the loss of intimate contact between the steel cylinder and the mortar coating. This corrosion assessment method is commonly used as a preliminary screening tool with the following objectives:
Measure soil-to-soil potentials at ground level directly above the pipeline;
Identify locations where corrosion is likely occurring; and,
Prioritize locations for excavation where data indicates corrosion activity.
A cell-to-cell survey involves using electrical potential measurements to assess the corrosion activity between adjacent cells on the pipe surface, providing a reliable and non-intrusive means of corrosion detection. This method is well-suited for assessing mortar-coated pipe materials due to the large electrical potential difference between the pipe surfaces in contact with the mortar coating and the corroding pipe surfaces that have lost intimate contact with the mortar coating. A schematic of a cell-to-cell survey performed on a mortar-coated pipeline with an actively corroding area is presented in Figure 1.
Figure 1: Cell-to-Cell Survey Schematic
Approach
The cell-to-cell survey method effectively evaluates mortar-coated pipelines installed in shallow buried conditions. Corrosion is an electrochemical process accompanied by current flow from the pipeline surface through the soil; the cell-to-cell test method determines current flow in the soil and, thus, identifies potential corrosion cells on the pipeline. The current flow in the soil is determined by measuring soil-to-soil potential gradients at ground level above a buried pipeline. The measurements obtained during the cell-to-cell survey are used to indicate the location of holidays (coating defects) and provide a relative indication of the magnitude of the individual defects and the associated corrosion.
Figure 2: Cell-to-Cell Survey - Operator Setup
For a typical cell-to-cell survey, soil-to-soil voltages are recorded every 5 feet directly over the pipeline. Direct contact with the soil is required during a cell-to-cell survey. As the operator enters the “voltage gradient field” associated with an actively corroding pipeline area, the measured cell-to-cell voltages will increase and feature a positive polarity. As the operator gets closer to the anodic (corroding) location, the cell-to-cell voltages will increase more rapidly until the cell-to-cell voltage peaks. After the peak, the cell-to-cell voltage will decrease rapidly to zero, indicating the operator is straddling the defect. Just after the defect, the cell-to-cell voltage will continue to decrease rapidly to a minimum inverse peak voltage (most negative potential); after that, it will increase at a diminishing rate as the operating moves farther away from the defect.
A typical cell-to-cell survey setup with an operator is shown in Figure 2.
A schematic of the voltage readings during a cell-to-cell survey approaching a defect is presented in Figure 3.
Figure 3: Cell-to-Cell Survey Data when Approaching a Defect
Benefits of Cell-to-Cell Surveys for Mortar-Coated Pipe Materials
Used as a preliminary screening tool, the cell-to-cell survey is well-suited for mortar-coated pipe materials for the following reasons:
Non-invasive detection – Cell-to-cell surveys offer a non-destructive and non-intrusive approach to detecting active corrosion. This eliminates the need to physically damage the mortar coating or the pipe itself, minimizing the potential for further damage during the inspection. The pipeline also can remain in service during a cell-to-cell survey.
High sensitivity and accuracy – By measuring the electrical potential between adjacent cells on the pipe surface, cell-to-cell surveys provide a highly sensitive method for identifying active corrosion. This allows for the early detection of corrosion activity, enabling timely intervention and maintenance actions to prevent further deterioration.
Cell-to-cell surveys can be done relatively quickly compared with other evaluation methods, such as leak detection and in-pipe inspection tools. Generally, cell-to-cell surveys are cost-effective for maintenance planning and identifying locations for targeted condition assessment or corrosion evaluation.
Long-term monitoring capability – Cell-to-cell surveys can be conducted periodically, allowing for long-term monitoring of corrosion activity on mortar-coated steel pipes. This helps in assessing the effectiveness of corrosion prevention strategies, evaluating the condition of the infrastructure, and making informed decisions regarding repair or replacement.
Enhanced asset management – By providing valuable data on the extent and location of active corrosion, cell-to-cell surveys contribute to improved asset management practices. This includes more accurate condition assessment, better lifecycle predictions, and the ability to implement proactive maintenance strategies, ultimately prolonging the service life of mortar-coated steel pipes.
At V&A, we are passionate about protecting infrastructure assets from the harmful effects of corrosion. Our corrosion specialists assist water and wastewater clients nationwide in evaluating potential corrosion activity on critical infrastructure assets and developing appropriate strategies for corrosion mitigation. We are available to answer your corrosion-related questions and provide project planning support. Feel free to use the link below to reach one of our corrosion engineers.