Category Archives: Cathodic Protection

Vapor Corrosion Inhibitors

Are Vapor Corrosion Inhibitors Magic Dust or a Viable Corrosion Prevention Tool?

This article is intended to provide a basic primer on vapor corrosion inhibitors for use in corrosion prevention for above ground storage tanks and address where this technology stands.

There has been a significant effort within the oil and gas world to either promote or repudiate the use of Vapor Corrosion Inhibitors (VCIs) for tank bottom plate corrosion control. As a leader in the above ground storage tank corrosion control industry, MATCOR has partnered with Zerust® Oil & Gas to make VCI options available to our customers that are interested in applying this technology as part of their corrosion mitigation approach.

How Vapor Corrosion Inhibitors Work – Video Courtesy Zerust® Oil & Gas


What are vapor corrosion inhibitors and how do they prevent corrosion?

Zerust Vapor Corrosion Inhibitors can be used in conjunction with CP systems.VCIs are chemical compounds that are released into a confined space, such as the underside of a tank bottom, and diffused through the sand pad material to reach the metal surface. These compounds are adsorbed onto the metal surface forming a strong bond that promotes and maintains a passive oxide layer on the metal and blocks other contaminant molecules from reaching the surface.

Are VCIs a non-permanent solution?

Corrosion protection using VCIs requires sufficient chemical concentration to thoroughly diffuse across the entire tank bottom surface area. The VCI has a finite life, after which it ceases to remain active. When this occurs, further chemical is required to replenish the spent VCI. The frequency of VCI replacement will vary depending a range of factors:

  • The rate of leakage through the tank chime
  • The operating temperature of the tank
  • The sand properties
  • The amount of chemical initially applied
  • Other factors

As VCI technology is still in the early phase of adoption, the typical replenishment frequency remains one of the big unknowns. A conservative estimate would be a minimum of 3-5 years’ service life before replenishment although a least one source has reported upwards of 15 years of effectiveness.

How is VCI applied initially for above ground storage tanks?

There are a variety of application technologies depending on the application and whether the tank is new construction, existing tank during inspection, a tank that is in-service or a double floor tank. Other considerations include the substrate material or concrete pad. The VCI chemical can be provided in a powder or liquid form. Whatever system is utilized to deploy the VCI, consideration should be given to how it will be replenished over the life of the tank.

Can vapor corrosion inhibitors be used in lieu of cathodic protection?

Practically speaking, most tank operators are not looking to replace cathodic protection but are considering VCI as a supplement to cathodic protection or as a short-term solution for inadequate or depleted CP systems until a replacement CP system can be installed.

Can VCI be used as a complement to cathodic protection?

This is where VCI provides an exciting opportunity to supplement cathodic protection. While cathodic protection has a proven track record in corrosion prevention for tank bottoms, there are limits to the effectiveness of cathodic protection. Cathodic protection only works when the tank bottom is in intimate contact with the sand bottom. Localized corrosion can occur wherever there are air gaps under the tank bottom. These can occur due to flexing of the tank bottom, imperfections in the plate steel, lapping of the plate steel, poor compaction of the sand bottom, presence of aggregate or non-conductive materials such as asphalt or oil, and at crevices in the tank ring wall. These are all areas where cathodic protection may not be effective and the proper application of VCI would be an excellent means of providing corrosion protection in these localized areas. Cathodic protection and vapor corrosion inhibitors are symbiotic. CP current distribution has been shown to improve in the presence of VCI.

How do I monitor that the VCI is working?

When applying VCI to a tank bottom, coupons, ER probes or UT probes installed under the tank are used to measure the effectiveness of the VCI and to alert the owner when the VCI requires replenishment. One of the concerns with using ER probes to measure corrosion rates under tanks is that ER probes provide an average corrosion rate and not localized pitting rates. It is understood that pitting corrosion is the dominant factor in tank bottom corrosion related failures and pitting rates can be significantly higher than average corrosion rates. There is a distinct correlation between average corrosion rates and pitting corrosion rates and the ER probes can be used to infer changes in the pitting rates.

Where do vapor corrosion inhibitors stand with industry standards and regulations?

According to API 651, there are several situations where CP is not recommended for specific tank foundation designs. In some of these designs, PHMSA recognizes that CP is not feasible.  In these cases, VCI can be a viable option. API 2610, the Tanks and Terminals standard outlines the use of VCI for tank bottoms in section 12.5. API 651, the CP standard, is being updated currently and VCI is being included as an option in this document. The State of Florida has identified that VCI can be used in tandem with CP or a standalone solution, for more than 6 years. NACE is currently working on publishing a standard “NACE TG543”, which is a comprehensive document on the application of VCI under tank floors. PHMSA is currently reviewing Special Permit requests for the use of VCI without a functioning CP system. If a non-regulated tank’s CP system is not meeting criteria, or has depleted, but the tank is still a few years from its next inspection, VCI can be applied to protect the floor until CP system repairs can be economically accomplished.

What independent published studies exist supporting VCI?

A 2018 study published by PRCI provides the strongest validation of the effectiveness of VCI and concluded that:

  • VCIs were found to be effective in mitigating pitting of steel exposed to corrosive sand but was not as effective as CP for reducing pitting corrosion. The study confirmed the importance of using the manufacturer’s recommended concentrations, as low levels of VCI was found to be ineffective.
  • ER Probes can be used to monitor the efficacy of VCIs
  • VCIs are compatible with impressed current cathodic protection; however, VCIs change the native potential of the steel and this must be considered when selecting CP criteria in accordance with NACE SP0193

Access the full study: PR-015-153602-R01 Vapor Corrosion Inhibitors Effectiveness for Tank Bottom Plate Corrosion Control

In conclusion, the application of VCI is a viable tool in our corrosion tool box that should be considered in conjunction with cathodic protection for critical service applications and as a standalone solution in some applications.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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Have Extra 2019 Budget Monies?

MATCOR can help.

This is the time of year when thoughts turn to Thanksgiving and Christmas vacations, using up all your remaining vacation and wondering what to do with any leftover 2019 cathodic protection budget monies.  More than likely, it is too late to schedule and complete new projects.  MATCOR along with most of our competitors have full construction schedules and adding additional commitments is quite difficult.

So what to do with that leftover budget monies that may reset in 2020?  Many of our clients look to stock up on materials for their anticipated 2020 projects.  It is not too late get MATCOR’s Mitigator® Engineered AC Mitigation System, SPL™ Impressed Current Linear Anodes, Durammo® Deep Anode System, MMP™ Prepackaged MMO Canister Anodes and other ancillary materials into this year’s budget.

Click HERE to get in touch with your MATCOR account manager for more information, to ask a question or get a quote. Or, complete our contact form at the link below and we will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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AST Cathodic Protection System Tank Isolation Considerations

cathodic protection tank isolation considerationsEvan Savant, EnLink Midstream reached out to the MATCOR Technical Team asking about AST cathodic protection system tank isolation:

“Can you advise on the importance of isolation for a new AST connected to a Pipeline, and can you advise on the need to isolate the tank cathodic protection from the tank grounding?

MATCOR’s Director of Engineering, Kevin Groll PE, NACE CP4 responded:

I am unaware of any papers or technical documents on the subject, but I will summarize as follows:

  1. Why can a lack of isolation hurt your cathodic protection?
    When trying to protect any type of structure from corrosion, cathodic current loss to nearby structures is always a concern. Losses can occur when the structure in question is directly bonded to other structures which may “steal” current. Offending metal structures that are close to the cathodic protection anode and structures with better resistance to earth (e.g., bare copper grounding, bare driven piles, etc.) will more likely take a significant amount of current.
  2. How do you obtain isolation without losing overvoltage protection?
    To prevent current loss, your target structure must be electrically isolated from the offending structures.  However, once you isolate a structure, you will lose grounding (if it was purposefully grounded) and you will lose protection against overvoltage events, AC faults, and lightning strikes.  Therefore, to obtain DC isolation but maintain AC continuity and overvoltage continuity, we use solid state decouplers (SSDs) and polarization cell replacements (PCRs). The primary difference between these devices is how much surge current they will carry.
  3. Tank cathodic protection design considerations.
    When we design an under-tank CP system with concentric rings, we assume that we will not have isolation from grounding and facility piping, and we also assume that most of the current will get to the tank bottom because of the proximity of the anodes.  This is not always the case, as we saw in a recent project, but for the most part concentric ring systems can be powered high enough to overcome the lack of isolation.

Horizontal directional drilling installed linear systems show approximately 1.5 to 2 times as much current is required as a concentric ring system due to current losses.  Again, we usually factor in enough current capacity to overcome these losses.

Deep anode systems and semi-deep anode systems suffer the worst losses. These systems will sometimes require isolation of the tanks to prevent critical current loss.  If a system is already in place, testing can be performed to determine how much loss there is to existing structures by measuring the current returned on ground rods and pipes. This is accomplished by using clamp-on current meters around wires/rods and Swain meters around pipes.

It is important to note that tank terminal isolation and grounding are factors in these complex tank terminal applications that must be considered in the proper design of Cathodic Protection.  MATCOR’s experienced team of engineers can evaluate your specific application and make the appropriate recommendations.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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Oil Cooled Cathodic Protection Transformer Rectifiers: Why, when and where…

Oil Cooled Cathodic Protection Transformer RectifiersThis article describes the components of a cathodic protection rectifier, and when to use oil cooled cathodic protection transformer rectifiers vs. air cooled rectifiers.

When it comes to cathodic protection power supplies, conventional transformer rectifier circuits have long been employed by the cathodic protection industry for impressed current CP systems.  These power supplies (commonly referred to as rectifiers in the CP world) consist of three main components; the transformer, the rectification stack, and a cabinet to house these components.  The transformer takes the input AC voltage on the primary side and controls the output AC voltage on the secondary side.  The rectification stack, typically silicon diode stacks which have largely replaced older less efficient selenium stacks, convert the AC input wave form into a DC wave form by cycling the AC flows in one direction and blocking in the other.  Additional components typically include circuit breakers, fuses, voltage and current output meters, lightning arrestors, surge suppressors, transformer tap bars, and monitoring systems.

Air-cooled Rectifier
Typical Air-cooled Rectifier

The majority of these Rectifiers are housed in air-cooled NEMA 3R enclosures – these enclosures are typically constructed of hot dipped galvanized steel, aluminum, stainless steel or painted steel.  NEMA 3R enclosures are intended for outdoor use.  They provide a degree of protection against falling rain and ice formation but are not completely water tight or weather proof and could be subjected to beating rain or streams of water, under certain conditions, entering the enclosure.  This is the most common type of rectifier enclosure in the industry.

When and Where to Use Oil Cooled Cathodic Protection Transformer Rectifiers


Oil Cooled Rectifier
Oil Cooled Rectifier

For some applications; however, the use of air cooled NEMA 3R enclosures is not recommended or not suitable.  The three most common reasons not to use air-cooled NEMA 3R enclosures are:

  1. Rectifier transformer size is too large to support an air cooled enclosure. For a small percentage of impressed current CP systems where the power requirements (measured in DC Watts) are sufficiently high that the cooling capacity of the enclosure is insufficient for the heat generated by the transformer (typically anything more than 12kW for single phase and 18kW for three phase.)
  2. Severe environment locations where high humidity, dust or other situations could shorten the life of a standard air cooled rectifier. Marine and near shore applications often fall into this category.
  3. The enclosure must be in a hazardous classified location requiring Class 1 Div. 2, Group D compliant enclosure – commonly referred to as Explosion Proof.
Oil Cooled Rectifier for Hazardous Locations
Oil Cooled Rectifier for Hazardous Locations

For these applications, oil cooled cathodic protection transformer rectifiers are typically specified.  As implied in the name, the oil cooled rectifier utilizes an enclosure that has a sealed reservoir which houses the transformer and transformer tap bars and is filled with a special transformer oil.  The transformer oil provides better heat transfer and dissipation and the larger case facilitates improved heat removal.

It is very important to note that standard oil cooled rectifiers are NOT explosion proof.  For an oil cooled rectifier to be considered Explosion Proof, the components that are not immersed in the transformer oil reservoir must be housed in special Explosion Proof fixtures.  Simply specifying oil cooled when ordering a rectifier does not satisfy the requirements for locating the rectifier in a hazardous Class 1 Div.2 location without also including the additional provisions required for the explosion proof fittings.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote for cathodic protection materials or related construction services, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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Helium Leak Testing for Cathodic Protection Anodes

MATCOR is pleased to announce that we are now capable of performing Helium leak testing on our full range of linear anode products as an optional testing service. This is a common practice among companies and product developers that provide products that could potentially leak gas or that require water tightness. Products commonly leak tested include refrigeration lines, vehicle brake lines, and devices that contain potentially harmful or deadly substances. Helium is the second smallest element (Hydrogen is the smallest), which means that it is valuable for leak testing. Smaller molecules naturally can find smaller gaps or defects from which to leak. Unlike hydrogen, however, helium is a noble gas and is therefore unreactive due to its complete valence electron shell. As a result, helium is the most viable gas for use in leak testing.

Helium leak testing is now available for all MATCOR linear anode products, however our Kynex connection technology has zero reported failures since it was introduced a decade ago.
Helium leak testing is now available for all MATCOR linear anodes, however our patented Kynex technology has zero reported failures since it was introduced a decade ago.

MATCOR has enjoyed an outstanding record as the world’s leading supplier of MMO anodes/Titanium linear anodes with over 25 years of linear anode experience supplying our industry leading SPL™ family of linear anode products for pipelines, tanks and other applications around the world. Our patented automated injection molded Kynex® connection technology has an outstanding track record with no known connection failures since this technology was introduced in 2009.

We do, however, see some client specifications calling for 100% connection testing and helium leak testing is the most effective means to test an entire anode assembly.


For more information, please feel free to contact your local MATCOR representative or contact us at the link below.

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Manufacturing Quality: What Does It Mean To Be Exceptional?

Manufacturing Quality for Anode SystemsAt MATCOR, we pride ourselves on being a world class manufacturer of unique cathodic protection systems and AC mitigation systems. Our anode systems offer you longer life, lower total installed cost, and are safer and easier to install than many conventional anode solutions. We have earned a reputation for exceptional manufacturing quality—but all companies say their products are world class and have exceptional quality, right? What makes MATCOR different? What does it mean to be exceptional?

Manufacturing Quality: ISO Certified

At our state of the art Chalfont, Pennsylvania manufacturing facility we have developed a culture of quality. That is not to imply that we are perfect or that we don’t occasionally make a mistake; we are not perfect. However, we HAVE embraced, through our ISO Certified Quality Management System, a systematic approach towards excellence. So, while everyone aspires to do a quality job, our manufacturing team’s quality culture is based on perspiration—we work relentlessly to do a quality job for YOU by embracing the key tenets of quality.

Through our Manufacturing Quality Management System, we:

  • Document procedures for what we do
  • Train our team on the proper processes
  • Hold ourselves and our suppliers to high quality standards
  • Self-audit to ensure we are doing what we say we will do
  • Measure our performance daily through KPIs (key performance indicators)
  • Strive to continuously improve
  • Collect and act on YOUR feedback, comments and complaints

We’d love to hear from you about our manufacturing quality, please comment or contact us at the link below.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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Cathodic Protection Remote Monitoring

This article provides a brief overview of the important role of cathodic protection remote monitoring systems in today’s pipeline operations. We will cover the CP equipment and features that can be monitored and how data is transmitted.

cathodic protection remote monitoring
Advanced cathodic protection remote monitoring systems are critical for today’s pipeline operator.

Modern pipeline operations face increasing pressures to incorporate advanced technologies to:

  • Drive down operating costs
  • Improve system reliability
  • Comply with regulatory requirements
  • Monitor the health of their pipeline networks
  • Monitor the critical systems that are integral to pipeline integrity

The use of advanced cathodic protection remote monitoring systems has become a critical component in the pipeline operator’s toolbox to meet these challenges.

CP remote monitoring (and control) has proven to be a reliable and cost-effective means to oversee the proper functioning of cathodic protection systems and AC Mitigation systems that are critical to assuring pipeline integrity and the proper protection against pipeline corrosion. Where operators in the past would have to send technicians out to remote pipeline locations to collect snapshot data on a frequent basis, the smart deployment of cathodic protection remote monitoring systems can provide continuous real time data that can be accessed from any cloud connected handheld or desktop device. Additionally, a remote monitoring unit for cathodic protection is well-insulated; this construction affords them excellent protection against lightning strikes. The financial, environmental and safety impact of eliminating hundreds of thousands of windshield hours is staggering across the vast pipeline industry.

Cathodic Protection Remote Monitoring – What can you monitor?

  • Cathodic Protection Rectifiers – the installation of RMUs with built in interruption capabilities should be standard on all new pipeline installations and retrofitting older units can provide significant cost savings and improve CP system reliability.
  • DC Cathodic Protection Test Stations – with today’s continuing advances in remote monitoring technology and costs, it is quickly becoming very cost effective to install remote monitoring units on all test stations. When combined with the ability to easily interrupt all of the influencing current sources on a pipeline, regularly scheduled testing of the CP system can be performed quickly and at virtually no cost.
  • AC and DC Coupon Test Stations – the latest NACE guidelines for AC Mitigation (SP21424-2018*) emphasize that the localized DC current density has a significant impact on AC corrosion and gathering data on both AC and DC current densities at areas of interest/risk is critical to a successful AC Mitigation strategy. Effectively doing so requires the ability to monitor these values over time as AC loads vary during the day and seasonally.
  • Critical Bonds – monitoring the effectiveness of critical bonds is necessary (and in many cases required by local regulatory bodies) to assure pipeline integrity.

NACE SP21424-2018 “Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation, and Monitoring”

How does a CP remote monitoring system transmit data?

remote monitoring unit cathodic protection
Mobiltex Cathodic Protection Remote Monitoring Unit (CP RMU)

Today’s operators have a range of options to assure that remote monitoring systems can regularly communicate data to their host data collection systems. The availability of conventional cellular networks combined with various commercial satellite systems assures pipeline operators of the ability to communicate with devices in even the remotest of locations. Your monitoring system provider can work with you to select the appropriate communications technology for each cathodic protection remote monitoring unit (CP RMU) location.

In addition to choosing how the communication is to occur, another key factor to consider is whether the communications are to be one way (monitoring only) or two-way (monitoring and control). For test station applications where data collection is the goal, one way transmission of the monitoring unit’s data is all that is required. For rectifier units, the ability to control the system output and/or the ability to initiate an interruption cycle for close interval surveys or test station polling purposes necessitates the ability of the remote monitoring unit to receive and act on communications as well as to transmit data.

Software Interfaces – Installing the appropriate CP RMU hardware is just one step in implementing a successful remote monitoring (and control) program. The data must be collected, stored, and accessible for the operator. Sophisticated cloud-based interfaces have been developed that incorporate critical features including firewall-friendly, password protected internet browser access. These systems allow for multiple client user accounts with configurable permission levels and automated alarm and status information including email and text alerts for designated alarm conditions.

In summary, the use of remote monitoring technology is a key component to the successful operation of any modern pipeline integrity management program. While MATCOR has extensive experience with all of the major RMU manufacturers, we have recently teamed up with Mobiltex, a leader in the field of remote monitoring, to bring state of the art technology to the pipeline and cathodic protection industry. Mobiltex’s CorTalk® line of CP RMU units combined with their CorView interface offers all the features necessary to implement a comprehensive, cost-effective, and highly robust cathodic protection remote monitoring program.


Please contact us at the link below if you have questions about cathodic protection remote monitoring, or if you need a quote for services or materials.

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New Pipeline Cathodic Protection Design | 12 Things to Consider

Pipeline cathodic protection design for new pipelines may appear to be a rather easy task for anyone with a basic understanding of cathodic protection. However, as with all design efforts there are a wide number of factors that need to be considered for a sound design that meets generally accepted industry practices.

Pipeline cathodic protection design considerations.

This article highlights 12 things that the pipeline cathodic protection system designer needs to consider when developing a CP system design. This is not intended to be a comprehensive list as every project has its own unique challenges, but these 12 items would all typically have to be addressed during the design phase. It is assumed that the basic pipeline information is already available to the CP designer including pipeline length, pipeline routing and pipeline characteristics (material, wall thickness, coating type, operating temperature, etc.). Armed with this basic information the CP designer should also consider the following in their design efforts.

12 Things to Consider for New Pipeline Cathodic Protection Design

  1. Soil Resistivity is a factor in many of the design calculations and assumptions (e.g. current requirement, anode resistance, attenuation, AC interference, etc…) Actual soil resistivity data should be collected along the proposed route. Learn about soil resistivity testing.
  2. Attenuation calculations should be performed in accordance with accepted industry standard equations and practice, such as NACE International CP-4 Cathodic Protection Specialist Course Manual 2000 Figure 2.18.
  3. Design current requirement is selected based on the soil type(s) using some accepted industry guidelines taking into consideration the coating manufacturer’s recommended coating efficiency or other industry accepted guidelines. Additional current requirements for mitigating interference currents should be considered based on the designer’s experience.
  4. Distribution of CP System Stations should take into consideration the total current required, the pipeline attenuation characteristics, the availability of power for impressed current cathodic protection systems, varying soil regimes, isolation valves and other factors to determine how many, what size and where each CP System will be located.
  5. Foreign pipelines and other DC interference sources should be evaluated as part of the CP system design efforts and generally warrant immediate mitigation measures or testing and monitoring provisions for observation and assessment.
  6. AC Interference assessment should be performed to determine if there are one or more high risk categories for AC Interference. Should the initial assessment confirm that there is potential for AC Interference an experienced AC Interference and Mitigation specialist would typically use sophisticated AC modeling to assess the risk and propose appropriate mitigation. From a CP perspective, there is a relationship between DC current density and AC induced corrosion risks where too much cathodic protection accelerates the AC induced corrosion rate so care must be exercised by the CP designer to avoid high DC current densities in AC risk areas.
  7. CP Station design includes the type of anode configuration, anode selection, installation methodology, etc… The CP designer will typically provide detailed Bill of Materials as well as CP System issued for construction drawings and construction details showing the location of equipment and providing installation instructions.
  8. Isolation of MLVs and Stations is a key design criterion that impacts the pipeline cathodic protection system design. Some owners are strongly in favor of isolation of MLVs and Stations from their main pipeline while other owners prefer not to isolate and have to maintain isolation and instead require the that CP system be sized to account for losses to current drains.
  9. Power supply type, sizing and selection is another of the decisions that is determined by the CP designer with consideration given to the pipeline owners specifications and preferences. For most pipeline applications, impressed current systems are typical and these require a DC power source. Electrical AC to DC power supplies (“rectifiers”) are the most common power supply but for remote areas with limited AC power availability, alternate power supplies such as solar, wind, fuel cells, thermo-electric generators or other sources may be required.
  10. Terminal piping is often associated with a new pipeline construction project and the pipeline CP system designer must often provide a supplemental design specifically for the terminal or station piping, or account for these in the primary pipeline CP system design
  11. Use of temporary CP systems is often recommended when permanent power may not be available for some time. These typically involve the installation of galvanic anodes strategically along the pipeline.
  12. Provisions for testing and monitoring are critical components to any successful pipeline CP system design. This often includes the use of remote monitoring systems for all of the system power supplies, specialized test coupons for AC and DC Interference, and numerous cathodic protection test stations placed at the appropriate strategic locations to be able to properly test and monitor the CP system performance.

As noted earlier, this is far from a comprehensive list of all of the factors for a specific pipeline CP System design. Every project may have its own unique challenges; however, the 12 items listed above represent a great starting point for any new pipeline cathodic protection system design challenge.


Please contact us at the link below if you have questions about pipeline cathodic protection design, or if you need a quote for services or materials.

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Soil Resistivity Testing

This article discusses the most common soil resistivity testing method and provides some guidelines for properly collecting sufficient data for the cathodic protection system designer.

Soil Resistivity TestingOne of the most important design parameters when considering the application of cathodic protection for buried structures is the resistivity of the soil. Soil resistivity testing is an important consideration for assessing the corrosivity of the environment to buried structures. It also has a tremendous impact on the selection of anode type, quantity, and configuration. Thus, it is critical that the CP designer have accurate data on the soil conditions at both the structure and at any proposed anode system locations. The lack of sufficient soil resistivity data can render a cathodic protection system (CP system) design ineffective and can result in costly remediation efforts during commissioning.

Soil Corrosivity

Soil resistivity is the principal diagnostic factor used to evaluate soil corrosivity. When performing soil resistivity testing, there are numerous factors that can be assessed, including soil composition, moisture content, pH, chloride and sulfate ion concentrations, and redox potential.  These are all common components of a lab or in-situ soil testing program and all have an impact on soil resistivity. While a comprehensive soil testing program may be warranted, especially when performing failure analysis, for most environments the soil resistivity testing data provides an outstanding basis for assessing soil corrosivity. Below is a typical chart correlating soil resistivity with soil corrosivity.

Soil Resistivity (ohm-cm)Corrosivity Rating
>20,000Essentially non-corrosive
10,000 to 20,000Mildly corrosive
5,000 to 10,000Moderately corrosive
3,000 to 5,000Corrosive
1,000 to 3,000Highly corrosive
<1,000Extremely corrosive

SOURCE: Corrosion Basics: An Introduction, NACE Press Book, 2nd edition by Pierre Roberge

Soil Resistivity Testing

Soil Resistivity Testing
Wenner four-pin soil resistivity testing method

While there are several methods for measuring soil resistivity, the most common field testing method is the Wenner four-pin method (ASTM G57). This test uses four metal probes, driven into the ground and spaced equidistant from each other. The outer pins are connected to a current source (I) and the inner pins are connected to a volt meter (V) as shown in Figure 1.

When a known current is injected in the soil through the outer probes, the inner probes can be used to measure voltage drop due to resistance of the soil path as current passes between the outer probes. That resistance value R can then be converted into a soil resistivity value with the formula: ρ=2×π×a×R where “ρ” is measured in ohm-cm and “a” is the spacing of the pins in cm. This value represents the average soil resistivity at the depth equivalent to the spacing of the probes so if the probes are spaced 5 foot apart, the value derived would be equivalent to the average soil resistivity at 5 foot depth.

For cathodic protection system design, it is common to take multiple soil resistivity measurements using this methodology with various probe spacings. For shallow anode placement, it is usually sufficient to take reading readings at 2.5 ft, 5 ft, 10 ft, 20 ft, 25 ft. For deep anode applications, soil resistivity measurements may be recommended at much deeper depths corresponding with the anticipated depth of the deep anode system.

Layer Effects

It is important to note that the soil resistivity values generated from the four pin testing represent the average soil resistivity from the earth surface down to the depth, and each subsequent probe spacing includes all of the shallow resistance readings above it. For cathodic protection design purposes, it is often necessary to determine the resistance of the soil at the anode depth by “subtracting” the top layers from the deep readings. This process of “subtracting” the top layers requires some form of computational adjustment. One popular approach is called the Barnes method which assumes soil layers of uniform thickness with boundaries parallel to the surface of the earth. If the measured data indicates decreasing resistance with increasing electrode spacing, this method can be used to estimate the layer resistivities.

The resistance data (R) values should be laid out in a tabular format and then converted to conductance which is simply the reciprocal of the resistance value. The change in conductance is then calculated for each subsequent spacing. That value is then converted back to a layer resistance value by taking the reciprocal of the change in conductance. Finally, the layer resistivity is calculated using ρ=2×π×a×R.

For the Barnes analysis below, the data shows that a low resistance zone exists between 60m depth and 100m depth.

TEST DATABARNES ANALYSIS

Spacing a
(m)

Resistance
(ohms)
Conductance 1/R
(Siemens)
Change in Conductance
(Siemens)
Layer Resistance
(ohms)

Layer Resistivity
(Ohm-m)

201.210.831.21152
400.901.110.283.57449
600.631.590.482.08261
800.119.097.50.1317
1000.06515.386.290.1620
1100.05817.241.860.5468

Soil Resistivity Testing Equipment Considerations

Electrically speaking, the earth can be a rather noisy environment with overhead power lines, electric substations, railroad tracks, and many other sources that contribute to signal noise. This can distort readings, potentially resulting in significant errors. For this reason, specialized soil meter equipment that includes sophisticated electronic packages capable of filtering out the noise is critical when taking soil resistivity data.

There are two basic types of soil resistivity meters: high-frequency and low-frequency meters.

High-frequency Soil Resistivity Meters

High-frequency meters operate at frequencies well above 60 hz and should be limited to data collection of about 100 feet in depth. This is because they lack sufficient voltage to handle long traverses and they induce noise voltage in the potential leads which cannot be filtered out as the soil resistivity decreases and the probe spacing increases. These are less expensive than their Low-Frequency counter parts and are by far the most common meter used for soil resistivity testing. For CP design purposes, these are frequently used to assess soil corrosivity and for designing shallow anode applications.

Low-frequency Soil Resistivity Meters

Low-frequency meters generate pulses in the 0.5 to 2.0 hz range and are the preferred equipment for deeper soil resistivity readings as they can take readings with extremely large probe spacings. Some models can operate with spacings many thousands of feet in distance. These models typically include more sophisticated electronics filtering packages that are superior to those found in high-frequency models. For CP designs involving deep anode installations, a low-frequency meter is the preferred equipment to provide accurate data at depths below 100 ft.

Field Data Considerations

When collecting accurate soil resistivity data for cathodic protection system design, it is important that the following best practices are taken into consideration to avoid erroneous readings:

  1. Suitability of the testing location. The use of the Wenner four pin testing method requires sufficient open area to properly space the pins to collect data to the depths necessary. For deep anode cathodic protection systems this would require a minimum of three times the anticipated anode system depth.
  2. Avoidance of buried piping and other metallic objects. The presence of any buried metallic structures (piping, conduit, reinforced concrete structures, grounding systems, etc…) provides low current paths that could cause a short-cutting effect that would distort the resistance readings and yield an erroneous soil resistivity reading.
  3. Depth of the probes. It is important that the probes are properly inserted into the earth. For shallow resistivity readings, probes that are driven too deep can impact the shallow readings. Ideally, the pins should be no deeper that 1/20th of the spacing between the pins and no more than 10 cm (4 inches) deep.
  4. Avoid areas of high electrical noise. Soil testing should not be performed directly under high voltage transmission systems or near other outside sources of current in the soil such as DC light rail systems.
  5. Accurately record the test location and conditions. It is important that the location of the testing is accurately recorded along with the soil conditions and temperature at the time of testing. Testing should not be performed in frozen soil, or during periods of extreme drought or abnormally wet conditions.

Summary

Soil resistivity testing with accurate collection of data is the best indicator of the corrosivity of the soil for buried metallic structures and has a significant impact on the design of cathodic protection systems. The most common test methodology for field collection of soil data is the Wenner four pin method. When properly collected, and using appropriate analytical techniques, the soil resistance field data can provide an accurate assessment of soil resistivity values for use in designing an appropriate cathodic protection system.

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Anode Current Ratings and Soil Resistivity

We appreciate the question: “How does soil resistivity impact current rating.”  The short answer is that resistance has nothing to do with anode rating. Here is a more detailed response:

  1. Anode current rating – all anodes have a current rating based on how long they can be expected to operate at a given current rating.  All anodes have some defined expected life based on current output and time – so many Amp-Hours of service life.  For example a magnesium anode may have an expected consumption rate of 17 lb/Amp-year (7.8 kg/amp) so if a 17 lb anode is operated at 0.1 amps it would have a life of 10 years.  For MMO anodes, they too have an expected life.  For our linear anode rated at 51 mA/m it is important to know that that rating is actually 51 mA/m for 25 years.  So a 100m anode segment with this rating would have an expected life of 127.5 Amp-years.  If this anode were operated at 5.1 amps (full rated capacity) it would be expected to operate for 25 years.  IF it were operated at 2.55 amps (50% of rated capacity) it should last 50 years.  The anode life is generally linear.  Please note that resistance has nothing to do with the anode current rating – the anode current rating merely calculates the life of the anode as a function of how many amps for how long of time.
  2. Actual current output – just because you install an anode rated for 5.1 amps for 25 years (our 100m segment of 51 mA/m SPL-FBR) does not mean that the anode will output this amount of current.  It just means that at that current rating you can expect 25 years of life.  The anode is merely one component of the overall cathodic protection circuit.  The actual output of the anode is function of Ohms Law ( Voltage = Current * Resistance).  It would make sense to note that if the system Voltage were zero (the rectifier were turned off or disconnected) then the anode would not have any current output.  Likewise if the 100m anode segment were installed in a very low resistance environment and driven by a powerful rectifier, the current could be much higher than 5.1 amps which would result in a much shorter life.
  3. Why anode rating is important to the CP designer – the CP designer is tasked with protecting a specific structure for a given period of time (protect this pipeline for 30 years.)  The CP designer then calculates, based on actual testing or established guidelines, the amount of current that should be sufficient to achieve appropriate CP levels to protect the structure.  This results in an answer of some number X of amps required.  If the requirements are to protect the structure for Y number of years, then the anode life required is X * Y (# of amps times # of years).  This defines the minimum amount of anode life that is needed.
  4. The next question the CP designer must address, once it is determined how much current is needed, is how to design a system that will generate that amount of current.  Since Ohms Law dictates that Voltage = Current * Resistance (V=IR) then if we know that the Current = Voltage/Resistance (I=V/R.)  Thus the CP designer must understand how to calculate system resistance (R) and must provide sufficient driving force (V)  Several factors affect system resistance (R) including anode geometry – the longer an anode, the lower its resistance – which in many applications is a big benefit to the linear anode.  One of the great benefits of the linear anode is that because of its length, in most applications the soil resistivity plays a lesser role since the anode resistance to earth is generally low for a wide range of soil resistivities due to its length.  For extremely high resistance environments, linear anodes may be the best option since short anodes will not have a low enough resistance.
  5. There are other factors that go into CP design including current distribution and making sure sufficient current is being applied across the entire structure.

CP Design can be very complicated.  I hope that the above explanation is helpful, but if there is a specific application to evaluate, please contact us with the details.  We are also available, for a reasonable engineering fee, to develop and/or review CP system designs.

Ted Huck

VP, Technical Sales