Detect, Correct, Protect and SAVE BIG
By John Kile and Kevin Niles
Hundreds of thousands of steel transmission structures support the delivery of bulk power across more than 200,000 miles of high-voltage lines in the United States. Although these structures were once thought to be permanent and free from maintenance and reliability concerns, it is now clear that steel structures do require periodic maintenance and repair. It's estimated that the effects of corrosion cost US electric utilities $5 billion to $10 billion dollars each year, according to EPRI P35.003.
Detailed inspections, accurate assessments, effective remediation and structural rehabilitation that restore original strength can add many years of additional service life to these aging assets.
Failure: How and Why
Original protection systems, including galvanizing and factory-applied coatings, help reduce or eliminate the effects of corrosion by creating a barrier between the steel and the surrounding environment. As structures age, however, the original barrier begins to deteriorate, directly exposing the steel to potentially corrosive conditions. In many cases, failed factory-applied coatings have actually become detrimental to structures because they not only allow moisture to come in direct contact with the structure, they encapsulate the moisture, holding it against the structure and accelerating the corrosion process.
|Pack rust--layer upon layer of corrosion resulting in a wedge of rust|
Once exposed, the steel is highly susceptible to deterioration because of corrosion activity. Corrosion activity typically begins at the ground line and proceeds downward along the surface of the structure below grade, causing the steel to thin and weaken. The rate at which steel corrodes below grade significantly varies based on the characteristics of the structure--age, material type, existing coatings, design and foundation type--as well as the environmental conditions including the physical and chemical properties of the surrounding soil. Additional contributing factors may include agricultural activity, industrial emissions and stray current from various sources. Transmission structures that share the right of way with pipelines, electric rail or other similar structures can be at risk for accelerated corrosion and should be given special attention.
|Severe corrosion damage on self-weathering steel lattice tower leg|
Detect--Comprehensive Inspection and Assessment
To fully understand the condition of a structure, a comprehensive inspection and assessment should be performed. The inspection process should include collection of detailed data for each structure while the assessment process should include both a thorough structural assessment and a comprehensive assessment of corrosion potential.
|Corrosion damage because of a failed coating on galvanized steel pole|
Structures should be excavated to a depth of approximately 18-24 inches. The below-grade surfaces should be cleaned to allow for accurate measurements of the steel's thickness to calculate section loss. If thinning is noted, it may be necessary to excavate further to determine the extent of the corrosion damage. Mechanical damage should also be evaluated to determine its effect on the structure. Key predictive corrosion indicators should be measured at each structure. This includes soil type, moisture level, soil resistivity, pH, redox potential and half-cell potential measurements.
In the event that excavation of the structure is not possible or feasible, there are various non-destructive evaluation (NDE) technologies available to help identify below-grade structural defects. It's important to understand, however, that NDE technologies are generally not as accurate or cost-effective as traditional evaluation methods mentioned earlier.
Ideally, the structural assessment should focus on the extent of existing corrosion and mechanical damage affecting strength and the structure's ability to support designed loads. This process should include categorizing structures based on priority to allow for the effective management of the remediation and repair process.
Assessment of Corrosion Potential
The assessment of corrosion potential should focus on the measurement of the environmental conditions used to determine the potential risk of corrosion to the individual structure. Understanding the potential risk of corrosion for each structure can help determine the most appropriate mitigation options as well as the most effective assessment cycle intervals.
|Environmental conditions such as soil resistivity should be measured as part of the inspection process to accurately assess the corrosion potential for a structure.|
Corrosion countermeasures such as specialized coatings and cathodic protection can be used to mitigate existing corrosion and extend the useful life of structures for many years.
Application of above-grade and below-grade coatings adds an improved measure of protection against environmental corrosion factors and can help alleviate potential corrosion concerns. There are a number of commercially available coatings from various manufacturers for this purpose. When choosing a coating system, a number of factors should be considered. These include the type of steel to be coated, the structure's current exposure and its potential for corrosion.
Galvanic Cathodic Protection
Cathodic protection is typically used as a secondary mitigation method to target specific structures that require additional protection. Cathodic protection involves attaching sacrificial anodes to the steel structure and placing them in the same soil profile. Cathodic protection does not eliminate corrosion, but instead transfers the corrosion activity from the steel structure to the anodes, which sacrifice themselves to protect the steel. It is important to note that this type of cathodic protection system is often referred to as passive because it relies on the surrounding site conditions to provide protection.
Whether you apply coatings, install cathodic protection or both, it's imperative to implement a regular monitoring and maintenance program to ensure the continued performance of the coatings and/or anodes.
Comprehensive corrosion assessment programs are designed to mitigate corrosion on in-service structures and identify structures in need of repair so costly replacements can be avoided.
|Repairs can be implemented to avoid the high cost of replacing the entire structure.|
Structures that are significantly weakened by section loss of their supporting structural members or foundations can often be repaired at a fraction of the cost of replacement. One utility's average cost to repair a tower, for example, is less than 10 percent of the cost to replace it. In many cases, these repairs can be engineered to restore original strength and increase capacity when greater strength is required for line uprates or upgrades.
Investing in a comprehensive program to proactively address corrosion and corrosion-related issues can have long-term financial benefits. This investment can also significantly limit safety and reliability risks and reduce emergency and unscheduled maintenance costs system wide. In addition, because these coating and repair programs significantly extend the useful life of the system, many utilities choose to capitalize their programs. We encourage utilities that do not have the internal resources to develop and support a program to seek a turnkey solution provider with industry expertise in implementing and managing steel infrastructure programs.
About the authors: John Kile is a senior product manager and Kevin Niles is a corrosion engineer at Osmose Utilities Services Inc. John and Kevin have more than 50 years of combined experience with bulk transmission system management and corrosion assessment and remediation. For more information on managing aging steel infrastructure, please contact John at firstname.lastname@example.org or Kevin at email@example.com.
Chemical and Physical Soil Properties that Influence Corrosion
Soil type (particle size)
Particle size and composition help dictate the degree of aeration, moisture content and subsequent time of wetness in a given soil environment. As a general rule, the larger particle size of sandy soils tends to promote aeration. Soils of smaller particle size, such as clay and silt, tend to restrict aeration and retain higher moisture content. Soils that are well aerated typically have a shorter time of wetness and, therefore, a lower rate of corrosion. Soils that are poorly aerated have a much longer time of wetness and are more likely to promote corrosion.
Soil resistivity is a key indicator in determining how corrosive a soil environment is. Soils with low resistivity allow for the easy flow of electric current, creating a greater potential for corrosion activity. Conversely, soils with high resistivity tend to restrict the flow of current, limiting corrosion activity. Soil type and moisture content play a significant role in soil resistivity. Coarse dry soil, for example, tends to have high resistivity while fine wet soil tends to have low resistivity.
The pH of soil can vary from 2.6 to 10.2. Most soil, however, has a pH of 5 to 8, which does not represent a serious risk to corrosion. Steel performs at its best in soils that are neutral (6.5 - 7.5) or slightly basic/alkaline (>7.5). Extreme corrosion and deep pitting are associated with soils that have a very low pH (highly acidic, <4.0). Such conditions can be caused by industrial waste, microbiological activity, or the decomposition of plant or animal waste.
Redox measures the content of dissolved oxygen in the soil. Oxygen content can indicate the potential for microbial corrosion activity, otherwise known as microbial induced corrosion (MIC). Although bacteria do not attack metals directly, they can influence the corrosion process by creating corrosive byproducts. Redox measurements collected in close proximity to the structure can help determine the level of risk for microbial induced corrosion.
Structure-to-soil (half cell) potential
The structure-to-soil potential measurement or "half cell" is the measurement of a structure's electro-chemical potential within its environment. Negative measurements (< -0.850 mV) typically indicate a reduced potential for corrosion, while larger measurements (> -0.400 mV) typically indicate a higher potential for corrosion.
While each of these properties can individually influence the corrosivity of the surrounding soil, it's the evaluation of these collective properties that provides the most accurate assessment of the structure's corrosion potential.