The US is constructing an increasing number of very large solar power generation farms, which brings about the question–what about corrosion? This new article explores solar farmsteel pile corrosion. Do the buried galvanized steel piles supporting solar arrays meet service life requirements?
We continue to construct an increasing number of solar power generation facilities. The United States plans even more as it continues to pursue policies encouraging renewable energy development.
The economies of scale make these facilities more competitive with other electrical generation technologies. These utility-scale solar farms have a capacity of anywhere from 1 MW to 1000+ MW. One feature of these solar farms is their physical footprint. The average solar farm requires 6-8 acres of land to support the tens of thousands of PV cells necessary to generate electricity at this scale.
For example, the Mammoth Solar Farm project in northern Indiana, once completed, will have a generation capacity of 1650 MW. And it will cover an area of 13,000 acres and use more than 2.85 million solar panels. This is the largest project in the US and should become fully operational in 2024.
Steel Piles That support Solar Arrays: What About Corrosion?
Given these facilities’ size, cost, and anticipated service life, a fair question to ask during the design phase is: what about corrosion? Specifically, the buried support structures that hold the solar arrays in place.
Typically, construction crews drive or screw galvanized steel piles into the soil to support the solar panels’ frames. Galvanized steel piles generally have a good service life in most environments for this application. However, as with all steel structures, they are subject to corrosion. Eventually, steel pile corrosion will adversely affect the support structure’s integrity.
Therefore, solar farm operators should consider the impact of corrosion on the service life of the galvanized piles during the project’s design phase.
Start with The Piling Details
The first step in the corrosion assessment process is to know your piles.
Any sizable solar farm project will require thousands of steel piles. The type of pile and the galvanizing thickness significantly impacting the project cost.
Unfortunately, structural engineers often do not consider corrosion when sizing the piles. They are keenly aware of loading concerns and select the pile to use based on a detailed soil load-bearing analysis and wind load analysis. They consider how big and how deep the piles need to be to support the predicted mechanical loads. This is often the only consideration. As a result, any conversation regarding corrosion is often relegated to – don’t worry, the piles are galvanized.
How Corrosive is the Environment?
But even galvanized piles are subject to corrosion, and in a highly corrosive environment, that service life might be much lower than expected. Therefore, additional corrosion mitigation measures, such as cathodic protection, might be warranted in some cases.
Given the massive footprint covering hundreds to thousands of acres these sites require, a thorough corrosion assessment for the entire area is warranted.
Typically, Geotech firms are contracted early in the project to perform detailed soil testing across these sites. This testing provides the requisite soil load capacity data to design the structural supports properly. While performing the site-wide testing, these firms will often perform some representative soil resistivity testing. While these sample soil resistivity tests may indicate the soil’s corrosiveness, they are often insufficient to evaluate correctly.
MATCOR Study: A Soil Testing Analysis
In one study performed by MATCOR, a comprehensive soil testing analysis found that much of a solar farm was in moderately corrosive soils. Still, a significant part of the facility was in very corrosive soils. In addition, the service life calculations varied significantly for the piles depending on their location within the solar farm. Estimated life ranged from 17 years to 30+ years.
Another factor in the service life calculations is the anticipated degradation of the piles during installations. The impact on the zinc coating can vary significantly depending on the soil characteristics.
Steel Pile Corrosion: How Long Will the Steel Piling Last?
The first phase of the service life of driven steel piles is the outer zinc layer of the galvanized steel pile. Zinc acts as both a coating and a galvanic anode. As the zinc layer is consumed, the underlying steel substrate is exposed. Therefore, the estimated service life, in simple terms, is the anticipated service life of the zinc coating plus the expected service life of the steel substrate.
We define the service life for solar structural galvanized steel piles as some allowable percentage thickness loss before compromising the pile’s integrity.
There are a lot of factors that affect the pile’s corrosion rate, including:
Presence of copper grounding bonded to the pile
Degradation of the zinc coating during the driving of the pile
Differential soil resistivity strata
Differential oxygen levels/aeration
Location of the water table in the area of buried piles
One of the biggest drivers of accelerated corrosion for piles is the presence of copper grounding. When steel piles are connected to a copper grounding grid, their service life can be significantly reduced as the zinc coating layer and the underlying steel substrate act as galvanic anodes. This can be very impactful in low soil resistivity environments.
Significant Steel Pile Corrosion Can Occur in as Little as Five Years
In some extreme cases, corrosion of the galvanized piles can be structurally significant in less than five years of service. In most cases, the service life in corrosive environments can be anywhere from 10-25 years of service. A service life of more than 50 years in low-corrosivity soils might be achievable.
Solar farms are a growing part of our electrical generation portfolio and will continue to see substantial investments in the next several decades. Given the footprint and cost of these facilities, design engineers should seek corrosion assessments from qualified corrosion engineering firms such as MATCOR to confirm that the piling systems used to support the solar arrays meet the facilities’ service life requirements.
If you need assistance with galvanized steel pile corrosion protection, please contact us. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.
Exothermic welding and pin brazing are two methods to connect a cathodic protection system to the protected steel structure. These connections route back to the rectifier to complete the circuit for an impressed current cathodic protection system. Or they connect to the anode lead cable in a galvanic anode system. They are an essential part of any cathodic protection system.
Exothermic welding and pin brazing cathodic protection connections resulted from historical needs in the railroad industry. In addition, both have a long history of use in the cathodic protection industry.
MATCOR has the experience and capability to use either connection technology depending on the client’s specifications or requirements. In the absence of a customer preference, MATCOR generally defaults to pin brazing for CP applications.
Exothermic Welding for CP
The older of the two technologies is Exothermic or Thermite welding. More prevalent in United States specifications, this technology utilizes the heat generated from the reaction when you ignite a mixture of Aluminum powder and Iron Oxide III (ferric oxide Fe2O3). The resulting reaction is vigorously exothermic, generating temperatures more than 2000 C – sufficient to create molten iron.
Initially developed by German Chemist Hans Goldschmidt in 1893, exothermic welding connected steel rails on the Essen train line. In the 1930s, the technology gained widespread use for connecting bonding cables to railroad ties. This was thanks to the efforts of Charles Cadwell, a physicist for the Electric Railroad Improvement Corporation (ERICO.)
The Cadweld connection process has changed very little over time. It involves cleaning the structure surface down to the bare metal and laying the connector attached to the structure in a graphite mold. Next, you place an appropriately sized cartridge containing the aluminum powder and ferric oxide ready for igniting in the mold. Finally, using an ignitor sparks the reaction. As a result, an iron slug melts and flows over the copper conductor, welding it to the steel surface.
Pin Brazing for CP
Like exothermic welding, the railroad industry developed the second standard cable-to-structure technology—pin brazing.
In Sweden in the 1950s, high heat from thermite welding caused grain growth in the copper cable. As a result, connections to the rails were subject to fatigue failures from cyclical stresses associated with the movement of the rails as trains passed.
To solve this issue, the railroad industry developed lower-temperature joining technology using brazing. Brazing uses a range of silver-based filler metals to achieve the bond. These filler metals have a melting temperature between 620 and 970 C – well below the temperatures reached during exothermic welding.
Commonly specified in European standards, the pin brazing process has remained fundamentally the same since the 1950s, with some refinements to the equipment.
The pre-assembled welding pin and the pin brazing gun are the keys to pin brazing. The pin consists of a stud with a defined amount of flux encapsulated in the brazing metal. When you press the trigger, current flows through the pistol via the pin to the steel pipe. At the same time, an electromagnet is energized, drawing the pin holder and pin away from the steel surface, forming an electric arc. The arc heats the steel and starts to melt the tip of the pin. As a result, it causes the flux to melt and flow onto the steel. The electromagnet de-energizes when the current flow ceases, and the spring forces the molten stud onto the fluxed pipe surface. With the arcing stops, solidification is very rapid.
Comparing Exothermic Welding and Pin Brazing for Cathodic Protection Connections
Both methods are safe procedures when trained personnel follow the correct procedures. Neither method poses any environmental threat, although users should be sure to properly store and handle the thermite powder charges. For thermite welding, the process can be sensitive to moisture which could vaporize on contact with the molten iron slug. As a result, the potentially dangerous hot metal can be spat out of the mold. For this reason, you should conduct the pin-brazing process in damp environments and offshore applications.
Cathodic Protection Connection Reliability
Both connections have been used extensively and are widely accepted in cathodic protection. Unfortunately, no published data detailing the reliability of either connection technology exists, and reports of Cathodic Protection connection failures are infrequent and anecdotal. Lab testing on tensile load indicates that pin brazing is a slightly stronger bond; however, the loads at failure far exceeded any load possible in regular service. Nevertheless, both techniques will provide reliable, low-resistance connections when properly performed.
Both processes are thermal and will affect the metallurgical condition of the pipe. Many piping codes typically advise that the design consider the impact of any changes in the parent metal due to localized heating during the attachment process. Microhardness testing has shown that both connections are safe for the normal range of carbon steel pipe; however, some consideration must be given to thin-walled structures. Pin brazing results in lower temperatures and greater process control and should be considered for all thin-walled steel and alloyed piping.
Effects of Cathodic Protection Connections on Internal Coating and Fluids
Using thermal bonding to the exterior pipeline wall of a pipeline filled with highly flammable hydrocarbons requires some consideration. In addition, where internal coatings exist, it is reasonable to question whether or not thermite welding or pin brazing might damage the interior coating. Based on testing, the inner wall temperature rises more with thermite welding than with pin brazing; however, neither method’s results were sufficient to give any reason for concern.
If you need assistance with a cathodic protection assessment, please contact us. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.
MATCOR is excited to announce the acquisition of a new drill rig to our existing fleet of HDD and vertical drill rigs.
Our newest rig is designed to be a cost-effective option for drilling shallow holes. The rig features a much smaller footprint than the conventional deep anode drill rigs used for installing Durammo® and other deep anode systems.
Drill Rig Features
The smaller and more agile auger rig allows MATCOR to be able to maneuver the rig in tighter areas than the full-scale vertical rig would allow. Additionally, the unit is available with a hollow stem drill pipe allowing us to lower anodes in place in environments where an open hole may not be feasible. The rig is capable of drilling holes down to 100 feet deep, but for hollow stem purposes, we are limited to a depth of only 50 feet.
What This Means for Our Future
MATCOR is excited to add this new rig to our industry-leading inventory of cathodic protection installation enabling us to better compete for:
Shallow conventional anode beds
Distributive anode beds around tanks and congested facilities
Mobility is increased since it is loaded on to a semi-trailer
For more information, please contact us at the link below, or reach out to your local MATCOR account manager.
This article explores the answer to a question posed by a student about the length of pipeline protected by a cathodic protection system.
We recently received a question from our website from someone who self-identified as a Student. We love when people ask technical questions and are pleased that students visit the MATCOR website–we have always strived to have a content-rich website to help share CP knowledge. The question is as follows:
“For installed impressed current CP systems with 15 anodes, what would be the approximate radius/length of a 200-mile petroleum metal pipe that would be protected?”
So before diving into the answer, let’s frame this question with an assumption, identify some unknowns and provide a definition.
The 15 anodes are part of a single anode bed. The anodes are electrically remote from the pipeline and connect to an appropriately-sized DC power supply (transformer/rectifier, solar power/battery unit, thermoelectric generator, etc.)
Unknown #1: Pipeline Details
Before doing any detailed engineering, there are a few details that must be specified:
Pipeline diameter and material of construction
Coating type and condition
The layout of the pipeline (location of pumping stations, valve stations, and metering stations)
a lessening in amount, force, magnitude, or value according to Merriam-Webster
When discussing at what distance cathodic protection continues to be effective along a pipeline, you must consider the attenuation of the CP current. At some point, the current diminishes along the length of the pipeline, becomes insufficient, and can no longer protect the pipeline.
The Answer: Impressed Current CP Systems are Complicated
We can effectively use attenuation calculations for signals generated on a uniform conductor and transmitted through a uniform environment.
In this case, the pipeline is not a uniform conductor; unless it is bare, it is anything but uniform. The coating has less than perfect effectiveness and an unknown number of defects distributed in an unknown manner. The environment is equally non-uniform; soil resistivities change based on location and weather changes. The more non-uniformity, the more inaccurate the results will be for any attenuation calculations.
It is virtually impossible to model mathematically for older pipelines with insufficient coatings. The only effective strategy is to collect data by installing a temporary current source to measure the effective current throw in each direction in multiple locations along the pipeline.
For new pipelines with very good coatings, it is possible to perform some attenuation calculations and empirically determine a reasonable separation distance between anode stations.
The math starts with determining something called the propagation or attenuation constant. To calculate this, take the square root of the resistance per unit length of the structure divided by the leakage conductance per unit length.
In Simple Words…
How hard is it for the current to travel along the pipeline versus how easy it is for the current to jump onto the pipeline?
The smaller this number, the further current will spread. Key factors affecting the attenuation constant include earth resistivity (higher resistivity soils mean further current spread) and coating quality (better coating means further current spread). Armed with this, there are six simultaneous equations that we can use, and that include hyperbolic sine and cosine functions.
Larger, new construction pipeline projects require you to consult with a professional engineer. A brief newsletter article will not adequately cover the mathematical gymnastics involved. We did say that the math is complex.
Well-coated, newer pipelines in moderate to high-resistivity soils can typically be protected for 20+ miles in each direction from an anode bed. Poorly-coated or bare pipelines in low-resistivity soils may require anodes every quarter mile or less.
Need more information? Please contact us at the link below.
Ted Huck, Director of Manufacturing and QA/QC at MATCOR, recently published an article in the summer edition of Tanks and Terminals Magazine titled “Understanding Cathodic Protection Systems.” He explains how to assess the performance of cathodic protection systems for above-ground storage tank bottoms (Tank CP Systems).
When asked to summarize these performance assessments, Mr. Huck commented, “Tanks are pretty easy to test, except for those rare occasions when they are not. At that point seek professional help.”
MATCOR shipped four of the heaviest customer sled anodes we have ever fabricated this month – over three tons each. Headed to Beaumont, TX, the anodes are part of a Gulf Coast refinery expansion project. The anodes will protect a variety of marine piling structures as part of a light crude processing expansion at the refinery.
Each anode is rated for 75 amps and weighs approximately 6300 pounds. Each anode assembly consists of a pair of two-inch diameter Mixed Metal Oxide coated titanium tubular anodes, five-foot-long. The anodes utilize MATCOR’s proprietary tubular anode connection technology for larger diameter anode tubes. Each sled anode has two concrete bases resting on a common wood base fabricated with stout 6” x 6” pressure-treated wooden beams. The concrete ends include lifting lugs to support installation by crane either from land or on a work boat. The function of the treated wood beams is to sink into the sea floor.
The anode lead cables are dual insulated HMWPE/ Kynar 1/0 cables housed in a proprietary HDPE jacket and held in place along the sea floor using concrete weights.
Sled Anodes are an exceptionally cost-effective means of protecting large near-shore structures such as dolphins, jetties, pilings, and sheet pile walls. They are easy to install and even easy to remove in advance of dredging operations.
More Information About MATCOR’s Sled Anode Products
This article explores a deep anode system gone wrong and guidelines for properly sizing the system coke column.
Earlier this year, we received a call from a pipeline customer with whom we have a solid relationship.
Historically, they used graphite anodes for their deep anode installations. But over the past few years, they began trying the MATCOR Durammo® Deep anode System with success.
When one of their new Durammo anode installations started having strange operating data, it was time to get MATCOR on the phone pronto and figure things out.
MATCOR reviewed the RMU operating data on the wayward installation and found that the data was indeed strange.
The DC output oscillated from periods of robust DC current output to periods of no discernible DC output. We also looked at the deep anode system design and noted the rather short coke column height. The height was only 100 ft of active anode in an 8-inch column. We sent a technician to the site to investigate.
First, we checked the installation and operation of the remote monitoring unit (RMU). Was a poor RMU connection causing intermittent good/bad data? This was not the case.
Next, we checked the continuity of the two anode lead cables. The Durammo® system has a top lead cable and a bottom lead cable. These two cables should be electrically continuous. In this installation, they checked out properly.
Finally, we checked the vent pipe for obvious issues.
Having confirmed that the spurious data was not the result of a poor RMU connection and that the anode system cabling appeared to be intact, we began to suspect that the problem was in the coke column and its immediate environment.
The Culprit? A Small Coke Column.
When we investigated further, we determined that the on-again, off-again readings could be the result of excessive gas generation into a rather small coke column. Both phenomena are heavily impacted by the anode system’s coke column to earth current density.
When the anode system generates more gas than can be exhausted through the vent pipe and diffused through the surrounding earth, gas molecules begin to accumulate between the column particles and at the anode to coke and coke column to earth interfaces. Gas is not electrically conductive, and with enough trapped gas in the column, the system resistance can quickly rise to a point that the anode system cannot overcome this resistance, and the current output drops quickly.
A short-term solution is to turn off the anode system for a period, allowing the gas to disperse inside the coke column and the system should return to normal operation. At least until the gas molecules build up again to block the anode system.
Coke Column to Earth Interface Current Density: The Magic Number
The magic number often cited for anode coke column to earth interface current density is 150mA/ft2. Anything above this number might cause problems. Below this number, history shows that the impact of gas blockage and drying out are generally minimal.
In our example, a 100 ft coke column with an 8-inch diameter hole means that any current output above 31 amps would be pushing that 150mA/f2 threshold.
The 150mA/ft2 current density assumes a high-quality, properly installed coke. This forms a well-compacted column that promotes electronic conduction and limits electrolytic conduction. A well-formed coke column is critical for anode systems using mixed metal oxide anodes, since MMO anodes have an inherently smaller surface area available to be in contact with the column.
It is unclear why the cp system designer recommended a short anode active length for this anode system – other than perhaps the cost saving of using less coke backfill.
While a shorter column does have a positive cost impact, the performance can become an issue, as was the case with this installation. Ultimately, this customer is planning a new Durammo® anode installation for this location with a significantly longer active area.
Need information or a quote for MATCOR deep anode systems? Please contact us at the link below.
MATCOR’s Ted Huck, cathodic protection and AC mitigation expert, is pleased to be a keynote speaker at this year’s CORCON.
CORCON is Asia’s largest corrosion conference, hosted annually by the NACE International Gateway India Section (NIGIS).
The only larger corrosion conference is the NACE CORROSION Conference held annually in the Spring by NACE – we hope to see all of you in San Antonio Texas in 2022 where next year’s CORROSION Conference is scheduled.
This year, the CORCON conference is virtual; however, the need for an opportunity to share information and experiences still exists. Even in the midst of a pandemic, show organizers are optimistic that they can resume a live conference in 2022.
MATCOR has over a decade-long history of involvement in the cathodic protection market in India. The company has participated in numerous CORCON conferences as speakers, session chairs, and as an exhibitor.
Corrosion Professionals in India
MATCOR has a small office and staff in Ahmedabad, India and we believe in the great work performed by the many corrosion professionals in India.
This year, we were pleasantly surprised when conference organizers reached out to Ted Huck, a frequent visitor and speaker at the NIGIS CORCON conference, to ask that he be a keynote speaker.
The conference is scheduled for November 18-20, 2021. For more information visit www.corcon.org.
In addition to speaking at the CORCON conference, you can also find MATCOR at the National Institute for Storage Tank Management conference in the Woodlands December 1st and 2nd. Mr. Huck will be speaking in person at that conference on Tank Bottom Cathodic Protection Systems – Replacement Options.