Metallic Corrosion: Methods for Assessing Structure Condition
Metallic surfaces and structures commonly found in the water and wastewater industry include pipelines, gates, treatment plant basins, reinforcing steel embedded in concrete structures, access lids, and steel tanks. Metallic assets are critical components of water and wastewater systems; however, these assets are susceptible to corrosion which can be detrimental to system operations.
The mechanism of corrosion is an electrochemical reaction between the metallic surface and its environment. Electric current is discharged from the metallic surface at an anodic location through the water or soil environment and back to the metallic surface at a cathodic location. This flow of electric current is referred to as a corrosion cell, with corrosion occurring at the anodic location. Any metallic surface can have an infinite amount of corrosion cells.
Depiction of Corrosion Cell
Engineers employ various methods to prevent electrochemical reactions from causing corrosion damage to metallic surfaces. Coatings and linings such as concrete, epoxy, polyurethane, PVC, and polyethylene encasement often provide the first line of defense. Cathodic protection, either using galvanic anodes or impressed current systems, can prevent corrosion damage to metallic structures. Regular intervals of condition assessment are critical to evaluating metallic structure assets to identify and counteract corrosion before it becomes a costly and devastating problem.
Methods of Condition Assessment for Metallic Surfaces
Visual Assessment
Qualitative visual evaluations are performed from both inside and outside of structures, focusing on the condition of the metal surfaces. It should be noted that much of a visual assessment is subjective and based on the evaluator’s experience working with metallic structures in the water and wastewater industry. V&A created standardized ratings to characterize conditions and provide consistent reporting of corrosion damage based on objective criteria. You can read more about our VANDA® Concrete and Metal Condition Indices here . While visual evaluation is the first step to identifying defects, assessment testing provides quantifiable data to determine the level of deterioration and remaining useful life.
Dry Film Thickness (DFT)
Dry film thickness (DFT) is the thickness of a coating after it has cured. A DFT gauge uses electromagnetic induction or eddy current technology to measure the thickness of a wide variety of coatings on metal surfaces. Point measurements taken identifies potential areas of inadequate coating protection and/or coating damage.
Ultrasonic Testing Process
Ultrasonic Thickness Testing (UT)
Ultrasonic testing (UT) is a non-destructive evaluation technique to determine metal wall thickness. High-frequency sound waves transmit through one side of a metal wall from a transducer. When the sound waves reach the other side of the metal wall, a fraction of the waves echo back to the transducer. The metal thickness is determined by recording the time it takes for the sound waves to travel through the metal and return.
UT requires direct contact with the metallic surface being measured; therefore, cementitious coatings or linings must be removed. Epoxy and polyurethane coatings or linings less than 1/8-inch thick can remain intact as they do not interfere with measurements. UT is a tool for taking point measurements, typically performed around the circumference of a metallic pipeline and referenced by clock positions with 12:00 located at the pipe crown. If pitting or extremely localized corrosion is found, a pit depth gauge is necessary to quantify the deterioration since UT will not provide accurate readings due to the rough surface.
Measuring pit depth of severely corroded metal surface
Pit Depth Measurements
Measuring pit depth is complementary to UT measurements as it is performed on metallic surfaces where excessive pitting has occurred. Inconsistent metal surfaces due to severe corrosion prevent accuracy in UT data measurements; therefore, corrosion severity is determined with a pit depth gauge.
Broadband Electromagnetic (BEM) Testing
Broadband electromagnetic (BEM) testing is a non-destructive frequency-independent application of electromagnetic or eddy current systems that produce a thickness profile of a pipe. The BEM scan is unaffected by background electromagnetic interference, and test frequencies can be adjusted to the specific pipe material and site conditions. Metallurgic changes in pipe composition, as formed by corrosion processes such as graphitization, can be identified with BEM testing. This assessment method can be done externally or internally to the pipe and does not require intimate contact with the metal surface. Removal of coatings, cement mortar, or insulation from pipe surfaces up to two inches is not required.
BEM testing of metallic pipeline in excavation pit
Data collected from BEM testing can be used to determine apparent wall thickness as well as provide an indication of internal or external corrosion. With BEM information correlated to the nominal pipe wall thickness, a rate of corrosion can be established and using a mechanistic model, an estimated remaining life of the pipe can be predicted.
Half-cell potential testing of reinforcing steel in concrete wall
Half-Cell Potential Testing
Half-cell potential testing is performed on uncoated reinforcing steel embedded in concrete to determine corrosion activity. Testing should be in accordance with ASTM C876, “Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete.” An electrical potential difference, or voltage, exists between the anodic (corroding area) and cathodic (non-corroding area). Each of these areas, anode and cathode, represents a half-cell of the corrosion circuit. The electrical potential of the reinforcing steel can be non-destructively compared to the electrical potential of a standard reference half-cell. Testing involves creating a connection from a voltmeter to the reinforcements (exposed bar) and from the half-cell to the voltmeter.
In summary, visual observations help identify potential issues, but visual data alone may not always provide a complete picture of a metallic structure’s condition. By performing field-based condition assessments to obtain quantitative data, engineers can better predict overall structure conditions, estimate remaining useful life, and ultimately make better-informed decisions for rehabilitation, repair, or replacement of critical water and wastewater infrastructure.
Stay connected for follow-up articles that further discuss the benefits of condition assessment and factors that impact the cost of field-based condition assessment. And as always, if you have a question and want to talk to one of V&A’s technical experts, please contact us using the link below.
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