From JPCL February 2019
Fig. 1: Many solar projects are located in arid, desert regions where the soil may present corrosive conditions for underground steel structures. Photo: Getty Images / Eduard Lysenko.
Utility-scale solar array construction projects continue to grow in number annually. Large, open spaces with consistent UV exposure make excellent locations for solar array fields, but these sites do not always provide ideal soil conditions for the steel H-beam pile supports that are embedded into the ground. For example, many solar array projects are located in arid, desert regions or land historically used for agriculture, where the land often consists of disturbed soils, or soils that are naturally corrosive in nature.
Corrosion of these piles due to disturbed or corrosive soils is a concern for design engineers and owners. Protective measures must provide sufficient length of service to protect H-beams and ultimately prevent structural failure over the structure’s designed service life. Photovoltaic (PV) solar equipment is designed for a 25-year service life, and the supporting infrastructure should be designed to maximize construction and installation investment costs.
Today’s high-performance coatings can provide an excellent solution for protecting steel piles driven into corrosive soil conditions. This article will discuss the various corrosion mechanisms and inhibitors present in different soil conditions and the protective properties of different coatings types to meet the service life requirements for steel solar structures.
Many studies have been carried out to identify corrosive soil conditions and their effect on corrosion rates on steel embedded below-grade. These include the soil’s physical characteristics, aeration within the soil, moisture content, chemical content, pH and the electrical resistivity of the soil, which all have an effect on how corrosive a particular soil is1. These conditions influence the corrosivity of soil in a complex relationship that requires multiple test procedures and qualified corrosion engineers to fully understand individual project conditions.
The physical characteristics of soils are broken down by particle size and by soil texture. The “Unified Soil Classification System,” used by foundation engineers, classifies soil from “gravel” on one extreme to “clay” on the other end.
Moisture acts as the electrolyte of the corrosion cell, providing the conduit for electrons. Moisture content is recorded as a percentage based on the dry weight of the soil. Moisture content in soils may vary based on weather conditions and water table locations. High water tables may result in buried structures behaving as if they were immersed in a liquid environment. Water from rainfall will soak into the soil as determined by its permeability. Heavy rainfall may remove soluble salts that contribute to accelerated corrosion but in turn renders those soils acidic2. Desert areas where rainfall is irregular would have naturally high salt levels and will be more corrosive to buried metals when moistened. Dry lakes and salt sinks are usually hygroscopic in nature and will absorb any available moisture and continue to hold moisture for a prolonged period of time. Corrosion can become a problem in a short time when moisture content is 15 percent or more.
Aeration is a measure of available oxygen in the soil surrounding the metal support structure. Along with atmospheric oxygen, oxygen in soil comes from the wetting and drying process, which provides active oxygen content for accelerated corrosion effects at or near the surface. Highly aerated soils may provide high initial corrosion rates; however, poorly aerated soils will generally be found to be more corrosive. Poorly aerated soils have slower initial corrosion rates that continue at a relatively constant rate which tends to lead to failure earlier than with high aeration.
A driven pile travelling through clay and sand would encounter different aeration conditions, as the action of driving the piling into the ground will introduce oxygen into and disturb the surrounding soil above the water table. Aeration increases in disturbed soil conditions as oxygen is introduced into soil during construction or farming activity. Aeration can also increase in expansive or swelling soils, which contain a high percentage of clay particles. Clays can absorb large amounts of water and do not completely dry out after exposure to moisture, allowing for increased permeability of soils so that oxygen and moisture can penetrate deeper into the ground.
Electrical resistivity is the most common method to measure soil corrosivity, providing a quantitative value on how quickly electrons can flow through the soil to complete the corrosion process. The lower the value, the more corrosive potential a soil has as its resistance to electrical current is weak. The resistivity of soil depends on its chemical content, moisture content and temperature.
The electrical resistivity of a soil will decrease as moisture content increases until the saturation point. Beyond the point of saturation, the resistivity of the water becomes the dominant factor. Pure (deionized) water has high resistivity to electrical current flow (less corrosive). However, the resistivity of the water is influenced and reduced (more corrosive) according to the amount of salts and chemicals absorbed into the water. Higher amounts of salts and chemicals will increase the corrosivity of water.
Soluble salts, namely chloride ions are generally detrimental because they decrease soil resistivity. The presence of salts in soil primarily occurs from agricultural activities, or is naturally occurring based on the region where the project is located. Different salts have different conductive properties and corrosivity will increase or decrease based the amount and type of salts present in the soil. A soil containing sodium chloride would have lower resistivity (more corrosive) than a soil containing an equal amount of sodium sulfate.
The measure of acidic or alkaline conditions, or pH, also has varying effects on corrosion rates of buried metals. Ferrous metal corrodes quickly in acidic environments, and slowly or not at all as alkalinity increases. Soils that are highly acidic (pH < 4.5) are considered highly corrosive for ferrous metal.
Many of these conditions can interact to provide compounding degrees of corrosivity, such as high-moisture-content soils having higher acidity or dryer soils with high chloride content, both leading to higher corrosion rates. From the studies conducted, there is a consensus that the following soil conditions are considered aggressive or highly corrosive, and would require steel piles to be given additional protection3. The corrosion rates or loss of base metal is severe in these conditions.
Mildly corrosive soil conditions, shown below, would also require additional corrosion protection, but the assumed corrosion rates of carbon and galvanized steel are low — 12µm/year (0.308 oz/ft2) for carbon steel and 15µm/year (0.308 oz/ft2) for zinc4.
Project-specific corrosion rates can be determined by a qualified corrosion engineer assessing each of the mentioned conditions. One corrosive soils study demonstrated that corrosion of carbon steel in the aforementioned conditions will yield section loss of 2-to-3 ounces per square foot per year5. Other studies have shown that zinc in similar conditions will yield section loss of 1-to-2 ounces per square foot per year. In terms of mil thickness, the high range for steel metal loss is 6-to-7.87 mils per square foot per year and galvanized metal loss is 0.60-to-0.98 mils or more in corrosive soil conditions6. Section loss of 25 percent would be considered serious for a pile — a beam that lost 25 percent of its load bearing capacity would be considered structurally unsound7.
Corrosion can be broken into two types: localized pitting or uniform corrosion over the entire surface. Although pitting corrosion is a primary concern for the pipeline industry, driven piles are intended to carry axial loads, and thus localized pitting may not significantly reduce the load-carrying capacity of the pile. However, uniform corrosion over a large surface, or the clustering of many small pits, may lead to reduced capacity of the pile8.
Several proven methods are currently available to mitigate the risk in corrosive soils. These include concrete encasement, “upsizing” beam size, covering the beams with galvanizing, using cathodic protection and installing protective coatings. In determining suitable protective methods, an owner or design engineer must consider installation costs, additional time requirements for installation, protective characteristics of the method used (both in-place and during installation), and estimated design life once the protective method is put in place.
One of the most common methods to protect steel piles is to apply hot-dipped galvanization to the steel beam. Hot-dipped galvanizing exhibits low corrosion rates in atmospheric service and in mildly corrosive soil conditions, and is applied at a thickness range of 3-to-5 mils or by weight of 1.7-to-3 ounces per square foot. Galvanized structures typically exhibit a low corrosion rate because a continuously passive film, known as a zinc patina, forms on the pure zinc top layer of the galvanized surface when it is exposed to the atmosphere. As the patina starts to develop, a layer of zinc oxide quickly forms as the zinc reacts with oxygen in the air. The zinc oxide layer, when exposed to moisture, converts into a thin layer of zinc hydroxide, which reacts with atmospheric carbon dioxide over time and becomes a dense, insoluble layer of zinc carbonate that slows corrosion of the underlying zinc. Full formation of the protective film generally occurs in 6-to-12 months and is dependent upon storage methods and location after initial application.
In the case of corrosive soil conditions, the expected service life of zinc is greatly reduced. Dissolved chloride content in water is highly corrosive to zinc. Zinc is considered to be an amphoteric material, one that reacts in both highly acidic and highly alkaline environments; pH conditions below 4.5 are highly corrosive to zinc and show the effect of pH on corrosion of zinc or loss rate of 0.8-to-1.2 mils per year8. Acid-reducing soils result from the loss of chlorides from constant rain, from natural soil content, or from bacterial growth. When moisture in the soil is above 15-percent, salts will become solubilized in the moisture and further increase the corrosion rate of zinc.
Fast-track construction schedules should also be a concern for design engineers and owners when using galvanized metals. The proper formation of the calcium carbonate protective layer with galvanized metals would require longer lead times and sufficient landscape to store materials prior to installation. The resulting performance of zinc is greatly diminished without the formation of the protective film.
Fig. 2: Individual H-beams are laid down in the shop and individually blast-cleaned and spray coated before curing and installation. Photos courtesy of the authors unless otherwise noted.
When normal galvanizing will not provide adequate protection in corrosive soils, design-build firms have proposed upsizing the steel support beam and targeting the dry thickness of galvanizing to a maximum of 5 mils. However, this approach may be deemed cost-prohibitive, as upsizing beams not only adds to material cost but will also add to transportation costs because of the additional load requirements. Upsizing beams, weights and thickness of the zinc essentially provide additional base metal to allow for the higher corrosion rates by offering more sacrificial material, but does not increase or improve the barrier protection and corrosion-resistant characteristics of the process. Rising costs of base metal, zinc and the additional transportation costs continue to make this option less viable and a more expensive solution.
Coatings have a long history of use in protecting steel piles. Coatings are designed to provide barrier protection of the coated surface and have dielectric properties that reduce the corrosion current. The American Water Works Association (AWWA) and American Petroleum Institute (API) rely on coatings to protect buried steel piping and supports and have developed numerous standards defining acceptable performance characteristics for coatings in buried conditions.
Today’s current coating solutions for buried steel piles exceed the performance capabilities of previously used coating materials and meet the design criteria needed for industrial PV solar farms when placed in corrosive soil conditions. Current coating technology also removes scheduling concerns, as many coatings can be buried in as little as 24 hours after application.
The two generic coating types best suited for buried applications are high-build versions of epoxy and aromatic polyurethane. These products are formulated as ultra-high-solids coatings (up to 100-percent-solids formulations) that provide a thick, dense, low-permeability dried film. These coatings are resistant to abrasion and impact, have high dielectric strength and can protect edges in order to maximize the corrosion-resistant capabilities of the coating. Coatings that have permeability values less than 0.18 U.S. perms provide excellent resistance to moisture migration through the film. Dielectric strength is measured in volts per mil. Coatings with higher dielectric strength will have better resistance to electrical current arising from corrosive conditions. Ultra-high-solids epoxy formulations will have a dielectric strength up to 770 volts per mil and polyurethane will have a strength of 430 volts per mil. Appropriate epoxy coatings that have successfully passed pile driving and extraction tests have impact resistance greater than 20 inch/pounds and abrasion resistance of less than 25-mg loss. Polyurethane coatings exhibit impact resistance greater than 60-inch/pounds and abrasion resistance of less than 110-mg loss. Epoxy coatings can be formulated to have higher edge-retention results of 70 percent to maximize protection along edges in accordance with military performance specification MIL 23236.
It is generally accepted that either epoxy or aromatic polyurethane coatings with the above physical characteristics and formulation for steel will provide a 1-mil-per-year thickness loss (when applied above 20-mils dry-film thickness) in corrosive soil conditions. The minimum desired film thickness of 20 mils should be applied in a single spray coat application. The maximum dry film thickness should be closely related to the surface profile achieved during abrasive blasting; however, up to 50-to-80 mils of coating can be successfully applied to properly prepared steel surfaces in a single coat to increase the total protection capability. State and national design parameters confirm the consensus approval of the 1-mil-per-year corrosion rate utilizing coating systems with lesser performance values than those cited in this article. Historical studies have shown that these coatings are highly resistant to moisture-laden soil with chlorides, not only for piling applications but for 20-year exposures to wet dry cyclic conditions in marine vessels9. The performance capabilities of these coatings match the long-term design service life requirements for buried piles in corrosive soils.
Industrial-scale solar PV projects often take years of planning, permitting and design, but the actual construction activity is performed with extreme efficiency in mind. Hundreds of structural piling units are installed per day. Fabricators are often given very short delivery deadlines and finished fabricated items do not remain in lay-down yards for extended periods of time.
Because H-piles are mechanically driven into the ground when installed, any corrosion protection method must be able to withstand soil shear, impact and the stresses associated with pile driving. Piles and their accompanying corrosion protection must also provide resistance to pull-out and axial loads to ensure stability in changing environmental conditions.
With respect to installation of coated piles, both epoxy and aromatic polyurethane coatings have successfully passed pile-drive tests in rocky soils. Both coating types demonstrate little or no material loss at the driven head or along the edges of the beam. In all cases, no base metal was exposed, confirming that the coatings remained intact during the pile-driving process and resulting in the long-term protection needed for piles located in corrosive conditions.
Fig. 3: Finished, coated H-beams ready for installation in the field.
Modern sophisticated formulations in conjunction with plural-component spray equipment provide increased film thickness application in a single coat with rapid-curing cycles. The installation process includes surface preparation, application of coating, curing of the applied coating and handling of the equipment and coating material. With surface preparation, common automated mechanical abrasive equipment can blast-clean three steel piles to an SSPC-SP 10/NACE No. 2, “Near-White Blast Cleaning,” finish in a matter of 30 seconds — an extremely efficient process of preparing the surface. The surface profile of blast-cleaned steel should have a large angular profile of 4-to-6 mils to adjust for higher film-build thickness of the coating. For coating application and curing, H-beams are typically lined up on supports and a single coat is spray-applied to one side of each beam. After the coating has cured, the beams are flipped and the opposite sides are coated. The coated piles are then removed from the spray area and allowed to fully cure. Epoxy coatings are typically a bit slower than polyurethane to reach dry-to-handle (flip) time. An epoxy coating’s dry-to-handle minimum time limit is typically two hours, compared to modern aromatic polyurethanes at 20 minutes. Both products are cured and ready for service in 24 hours.
Ultimately, not all soils are extensively corrosive. However, when soil conditions do present corrosion challenges, today’s advanced coatings provide an improved solution when compared to the alternative protective measures of galvanizing, upsizing steel beams and concrete encasement. Protective coatings provide the barrier protection needed for aggressive and mildly aggressive soil conditions. A qualified corrosion engineer can determine the corrosivity of the soil, predict corrosion rates of base metals, and confirm the suitability of corrosion protection methods for an individual project that will meet the designed service-life requirement.
The advanced surface processes and application methods associated with today’s high-build epoxy or polyurethane coatings offer incredible efficiencies compared to previous generations of coating product solutions. These coatings cure quickly and are ready for service in 24 hours, which is significant when compared to the amount of time required for zinc coatings to properly form a carbonate-based protective layer. With these efficient processes and products in place, protective coatings installation can dramatically reduce the material impact on construction schedules.
The advances in coating technology and installation processes increase the viability of solutions for solar projects. When working with qualified corrosion engineers, the data from an individual project’s soil condition assessment will provide key insights into the need for dielectric coating protection. In any case, coatings should be one of the primary considerations when designing corrosion protection systems for solar piling projects.
Paul Trautmann is a project development manager for the protective coatings division of The Sherwin-Williams Company. He has been with the company for 18 years after originally working as a paint and coatings contractor. Trautmann has a B.S. degree in business administration from CSU Bakersfield and is a NACE-certified Coating Inspector.
Matthew Markowski has been employed by The Sherwin-Williams Company since 2001, most recently as a senior protective coatings sales representative in the central California region. He is a NACE-certified Coating Inspector who holds a B.S.B.A. degree from Bowling Green State University.
Brien Clark is a corrosion technical lead for HDR, with 19 years of corrosion control and condition assessment experience. He has a B.S. degree in chemical engineering from California State Polytechnic University, Pomona and is a NACE Cathodic Protection Specialist.
James Keegan is the corrosion and lab services section manager for HDR, overseeing the corrosion and condition assessment group and soil and water laboratory. Keegan has been in the corrosion and condition assessment industry since 1993 and has performed failure investigations in the wastewater, drinking water and general plumbing and construction industries.
Mersedeh Akhoondan is a corrosion engineer for HDR with over 12 years of experience in condition assessment and rehabilitation. She holds an M.S. degree in civil engineering and a Ph.D. in corrosion engineering from the University of South Florida and is the chair of the NACE San Diego Section.
Tagged categories: Coating performance; Coating selection; Corrosion inhibitors; Corrosion protection; Features; Photovoltaic coatings; Quality Control; Sherwin-Williams; Solar
