A MATCOR customer recently inquired about mixed metal oxide (MMO) anodes for low temperature seawater environments amid a concern over the formation of chlorine hydrates.
This article describes a special low temperature MMO anode formulation that addresses these concerns. MATCOR is currently working on two projects that will potentially benefit from this modified MMO anode.
What concerns are there over the use of MMO anodes in cold water environments?
As a general rule, MMO anodes perform exceptionally well in chlorinating environments such as seawater. Almost all MATCOR anodes are manufactured using a standard MMO coating formulation that is designed to operate efficiently in a wide range of electrolytes and environments.
In low temperature seawater, however, there is the possibility that high localized concentrations of chlorine can be formed in conjunction with a significantly reduced pH environment.
Chlorine hydrate is a chlorine molecule caged in 8 molecules of water. The generation of chlorine hydrate is strongly related to localized mass transport limitations which are greater at lower temperatures. There is no direct impact of chlorine hydrates to the titanium anode substrate or the anode coating. However, the presence of significant amounts of chlorine hydrate may result in inhomogeneous current distribution due to the differences in the conductivity. This could impact both the effectiveness of the current distribution being applied to the cathodically protected structure, and, over the long term, the wear rate or life of the coating.
Localized factors have a significant effect on the conditions that result in chloride hydrate formation. In addition, the properties of the MMO coating can have an impact.
MATCOR’s standard MMO formulation is suitable for low temperature chlorinated environments.
A modified formulation provides the best anode for low temperature seawater applications
By modifying the formulation, we can lower the oxygen overpotential to promote greater oxygen generation and less chlorine. This modified formulation reduces the quantity of chlorine gas while favoring the generation of oxygen.
This “cold” MMO formulation in low temperatures can be expected to reduce chlorine and boost oxygen generation by as much as 15%.
The other factor that has a large influence is current density, as it is this current density that determines the quantities of both oxygen and chlorine gas that are evolved. Reducing current density reduces the quantity of chlorine gas being generated.
In summary, MATCOR’s standard MMO coating formulation is a broad-based formulation that works in a wide range of electrolytes.
For areas of significant concern due to a combination of low temperature electrolyte and high current density, a special low temperature formulation is available. This formula reduces chlorine generation in favor of oxygen, helping to reduce any concerns over low temperature and chlorine hydrate formation.
If you have questions, or would like information on MATCOR’s special MMO anodes for low temperature applications, please contact us at the link below.
In a recent Tanks & Terminals article, Ted Huck discusses a tank cathodic protection retrofit project in the Middle East utilizing horizontal directional drilling technology.
Originally constructed in 1995, the original design of this critical service ethylene storage tank included a cathodic protection system to protect the external tank bottom in contact with the ground. Over time, the system stopped providing enough current to meet NACE criteria for the control of corrosion.
Discrete Anodes Along the Tank Perimeter Not Satisfactory
The first retrofit cathodic protection system consisted of installing discreet anodes around the perimeter of the tank. While relatively easy to install, this method of retrofit installation often struggles to drive current to the full tank bottom. The results were not satisfactory so another method was needed.
Linear Anodes Installed Using Horizontal Directional Drilling
MATCOR had proposed an alternate approach, successfully being performed in the US but not tried previously in the Middle East. It involves the installation of multiple strings of linear anodes directly below the tank using horizontal directional drilling (HDD) technology. By drilling under the tank, it is possible to install anodes spanning the entire length of the tank. This method also allows for a testing device to measure the effectiveness of the cathodic protection system.
Click below to read the full article regarding this tank retrofit cathodic protection system, installed successfully in December 2019.
But that is not all, MATCOR is very excited to roll out a new capability to our service line. We have purchased two state of the art Vacuum Excavators.
These two new vacuum excavators are dual air and water units allowing MATCOR’s crew to operate efficiently in a wide range of environments. “Big Air” compressor capability provides 300 lbs. of cutting capability, enabling us to excavate in many areas without the need for water.
The excavated material is vacuumed up in a dry form with significantly less mess and environmental impact. This allows for the expeditious backfilling of the excavation with the same material that had been removed. It also eliminates the need to haul off any spoils or haul in fresh fill materials.
Our new excavators are dual capable units that also incorporate a hydro excavation capability. High pressure water is more aggressive and can cut through virtually any soil type including more compact, rocky and dense soil that may preclude the use of air. The hydro capability is also great for cleaning the pipeline once excavated to enable physical inspections and making test station or negative connections.
MATCOR is excited to be adding vacuum excavation to our in-house construction capabilities allowing us to provide a wide range of additional services to our customers.
If you have questions, or for information on MATCOR’s construction capabilities, please contact us at the link below.
Looking for a better tank cathodic protection system?
Cathodic Protection of the external tank bottom for large diameter above ground storage tanks has been adopted as good engineering practice around the world.
Unfortunately, many existing grid anode systems have experienced premature failures, resulting in excessive tank bottom corrosion and costly replacement.
A recent MATCOR article published in Storage Terminals Magazine provides an overview of these grid CP systems and an alternative concentric ring linear anode system (link to the full article below). Here are just a few key points:
Grid Tank Anode Systems
Consist of field assembled MMO ribbon anodes and titanium conductor bars
Require flawless design and installation
Subject to poor welding and other concerns
Failures can be catastrophic
Concentric Ring Linear Anode System
Factory assembled—no field cutting or splicing required
Easy, fast and reliable installation
Coke backfilled sock protects the anode
Redundant—each ring segment has two feeds
Long life compared to the grid systems of the 1990s
Josh Johnston, MATCOR’s director of sales, wanted to share a recent conversion of a new customer to the growing list of satisfied Durammo Deep Anode System users. As Josh explained, “the Durammo is a salesman’s dream product. It offers our customers an innovative product that has an amazing track record. Its design makes it safer and easier to install because it comes preassembled ready to immediately lower down the hole.”
A complete description of the Durammo deep anode system is available here.
As Josh continued, “The hardest part about selling the Durammo, is that it is different than what they have used and what others are offering. There is a certain leap of faith that we ask customers to take when trying something different. I can explain to them that it is less expensive, has a longer operating life, is safer and easier to install, that several thousands of these are installed across the country and around the world, some with more than 20 years in service. It all sounds great, but it is different. That is my job, to convince people to do something different.”
One such company is Enerfin Resources Company, a midstream company operating natural gas and crude oil field services assets in Oklahoma, Texas and Louisiana. MATCOR met with them in the Fall of 2019 and explained to the Enerfin team the benefits of the Durammo deep anode system. As Josh noted, “Enerfin was willing to try this “new to them” technology, based on the value we offered.”
In March, MATCOR installed three of the Durammo systems for Enerfin. Tony Gustin, Project Development & Construction Manager noted, “the installation of these systems was very professional and the factory assembled system dropped in place as easy as advertised. We are sold on this product and look forward to using MATCOR and the Durammo system on many future projects.”
If you are ready to try something better, but different for your next deep anode system project, contact MATCOR and we would be happy to help you take the next step.
A client recently raised the concern about the cathodic protection reaction causing a drying out of the sand under a large diameter above ground storage tank. This is a very interesting question. We recently developed a stoichiometric analysis to assess the cathodic protection carbon footprint of a deep anode system by calculating the amount of carbon dioxide produced. The same methodology can be used to assess the risk of drying out of the tank bottom.
For this analysis, let’s assume a typical 150 ft diameter above ground storage tank with a bare tank bottom and a 1-foot sand bed resting atop a non-permeable liner. Based on a common design criteria of 2 mA/ft2 of bare surface area, this tank would nominally require a total of 17.7 amperes of current.
How much water does a cathodic protection system consume?
For every 2 electrons generated, one H2O molecule is required. One amp-year is equal to 3.1536 x 107 amp seconds or coulombs. One Faraday or 96.487 coulombs is equal to one mole of electrons therefore, one amp-year is equal to 326.84 moles of electrons. With the 2 to 1 ratio of electrons to H2O molecules that means that for every mole of electrons, 0.50 moles of H2O are generated. H2O has a molar mass of 18.0 g/mol so for each amp year a total mass of 2,941.6 grams of H2O is generated – that is approximately 0.78 gallons of water per amp year.
For our 17.7 ampere, 150 ft diameter tank anode system, that would mean 13.8 gallons of water is consumed as part of the cathodic protection reaction each year. Assuming that there is no new water being added into the tank foundation (a perfect chime seal and a completely non-permeable liner), then over a 30-year operating life the CP system would consume a little more than 400 gallons of water. While that might seem like a lot of water consumption, what is the percentage of drying out that is occurring with the sand over that time frame?
Will the Tank Bottom Dry Out?
Well, typical sand has a bulk density of approximately 100 lb/cubic foot and the typical moisture content for commercial sand is between 2% and 6%. For purposes of this exercise, let’s assume that the moisture content is on the low end at 2%. This means that there are approximately 2 lbs of sand per cubic foot. A 150 ft diameter tank has 17,671 cubic feet of sand bedding which equates to 35,342 lbs of water or about 4,241 gallons of water. So, if no new water is added over the thirty-year operating life, the typical CP system will consume about 10% of the sand moisture for very dry sand.
Given our assumptions and calculations, it does not appear that significant sand drying will occur due to water consumption.
Another Consideration: Electro-osmotic Drying
This analysis does not consider the effect known as electro-osmosis. Electro-osmotic drying is a process that is used in the civil engineering world to dewater sludges by creating a DC electrical flow – the flow of electrons pulls polar water molecules away from the anode. For CP applications, this is generally not considered to have a significant impact except where there are very high current densities at the anode – for example some deep anode systems operating at very high output rates in certain soil formations. For tanks, this is not considered an issue.
If you have other technical questions, or for information on MATCOR’s above ground storage tank cathodic protection solutions, please contact us at the link below.
This article explores the carbon footprint of cathodic protection deep anode systems and compares it to that of a typical passenger car.
Deep Anode Systems
Deep Anode Systems are commonly used throughout the CP industry as a cost-effective means to discharge significant amounts of current to protect pipelines over long distances or large structures in a small area. One of the common components in a deep anode system design is the vent pipe.
The deep anode system vent pipe serves two important related functions:
To prevent gas blockage that will impede the
operation of the anode system
Prevents the accumulation of chlorine
concentrations where chlorides are available
Both issues are directly related to the electro-chemical reactions that occur at the anode to coke backfill, and coke backfill to earth interfaces.
There are two basic types of anodes used in deep anode systems—conventional “massive” anodes, and dimensionally stable anodes.
The conventional “massive” anodes are those anodes that consume as part of the electro-chemical reaction and as such their mass is critical in determining the system’s performance life. The dimensionally stable anodes, typically Mixed Metal Oxide (MMO), are catalytic in nature and do not consume as part of the anodic reaction.
Cathodic Protection Reactions
The primary cathodic protection reactions all involve generating gas:
In a properly functioning deep anode system, the gases that occur from these reactions predominantly involve the coke backfill creating carbon monoxide and carbon dioxide. If chlorides are present, some percentage of chlorine gas will also be generated.
The Importance of Venting the Deep Anode System
The gases generated in the coke column typically do not rapidly diffuse into the earth around the coke column and thus will build up. These gases are not electrically conductive and once enough gas builds up around the anode, then the anode can no longer effectively discharge current—a phenomenon known as gas blockage. If Chlorides are present, the chlorine gas reacts with water to create hydrochloric and hypochlorous acids that can attack the cable insulation and cause permanent damage. This is why it is important to properly vent these gases that are a part of the electro-chemical reaction that must occur for CP to function.
What’s the Carbon Footprint?
Given that deep anode systems generate gas, an interesting, although not commonly asked question, is how much carbon dioxide a typical deep anode system generates—in other words, what is the carbon footprint of a deep anode cathodic protection system.
With a few worst-case assumptions and a little stoichiometric chemistry analysis we can answer this question. Assuming all the reactions are generating carbon dioxide and there is no oxygen generation, then for every 4 electrons generated, one CO2 molecule is generated.
One amp-year is equal to 3.1536 x 107 amp seconds or coulombs. One Faraday or 96.487 coulombs is equal to one mole of electrons, therefore, one amp-year is equal to 326.84 moles of electrons. With the 4 to 1 ratio of electrons to CO2, that means that for every mole of electrons, 0.25 moles of CO2 are generated. CO2 has a molar mass of 44.01 g/mol, so for each amp year a total mass of 3,596 grams of CO2 is generated.
For a nominal 50 amp anode system, that would mean a maximum generation of 180 kg of CO2 per year if CO2 was the only gas generated.
How much CO2 is 180 kg/year?
The EPA estimates that the typical passenger vehicle generates 4,600 kg of CO2 per year.
Therefore, your 50 amp deep anode system generates about 4%—or just 1/25th—of what a typical passenger car generates annually.
If you have other technical questions, or for information on MATCOR’s deep anode cathodic protection solutions, please contact us at the link below.
AC Corrosion Implications for New and Existing Pipelines
AC inference can result in significant and rapid corrosion and is a threat that must be considered for both new and existing pipelines. NACE provides a detailed standard practice to specifically address the threat of AC corrosion; however, it is very important for corrosion professionals to understand the guidelines and their implication for pipeline design, monitoring and risk assessment.
Criteria for Control of AC Corrosion
Approved in December of 2017, NACE SP21424-2018-SG “Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation, and Monitoring” provides supplemental guidance for the control of corrosion for cathodically protected pipelines that are subject to influence from close proximity high voltage AC transmission systems. This standard practice expands significantly on the earlier standard SP0177 “Mitigation of Alternating Current and Lightning Effect on Metallic Structure and Corrosion Control Systems” and introduces new criteria for addressing AC Interference for cathodically protected pipelines.
The criteria detailed in Section 6 of SP21424 allow for two means of assuring that effective AC corrosion control has been achieved:
Document that the corrosion rate is less than the common benchmark for effective corrosion control of 0.025mm/y (1 mil per year). This can be achieved using weight loss coupons, corrosion rate probes or through in-line metal loss inspection tools—provided the inspection tool resolution is sufficient to detect small-diameter attacks such as AC corrosion. This approach is great for areas where AC corrosion risk is considered minimal. Essentially this says we don’t expect AC corrosion and we will demonstrate that AC corrosion is not occurring with a modest testing program. In those areas where AC corrosion can be reasonably anticipated; however, a second criteria is provided.
For areas where AC corrosion mitigation can be anticipated, the criteria for effective control is based on reducing the time weighted average AC current density below a specific threshold that varies depending on the DC cathodic protection current density as follows:
Where the DC current density is controlled to less than 1A/m2, the AC current density should be controlled to less than 100 A/m2
Where the DC current density is not controlled to less than 1A/m2, the AC current density should be controlled to less than 30 A/m2
This first criteria, much like the first criteria for
cathodic protection in SP0169-2013, allows for a prove-it type criteria based
on documenting that corrosion is not occurring.
The second criteria, unlike the criteria for cathodic protection, is not based on a measured potential, but is instead based on measuring current density on a time weighted basis. Not just one type of current density must be considered, but instead the criteria requires evaluation of the time weighted average of both AC and DC current densities.
Current Density vs. Polarization
While conventional criteria associated with control of corrosion through the application of cathodic protection is based on shifting potentials on the pipeline, the control of AC induced corrosion is based on limiting current density criteria on a time weighted basis. These requirements are quite different—and when AC corrosion control is a concern this will require a change in how pipelines are monitored, a shift in CP design philosophy in those areas where AC corrosion is a concern and some understanding of the impact of AC mitigation.
Pipelines are typically designed to monitor polarization levels with the installation of test stations at frequent intervals to support measuring polarization levels at the test station and to facilitate continuous close interval polarization surveys. When AC corrosion is a threat, the monitoring provisions need to shift from providing connections to the pipeline for polarization measurements to the installation of coupon test stations to facilitate current density measurements.
CP System Design Philosophy
The primary concern with cathodic protection design is typically making sure that more than enough current is available to ensure minimum polarization levels (either 100mV shift or -850mV off potential) are met along the length of the pipeline. This often means the CP system is over-designed and overdriven—there is little cost associated with over-polarizing some segments of the pipeline to ensure that the entire pipeline meets the minimum requirements. If the pipeline does not meet criteria in some locations, the first step was to push more current over the entire system until those low potential sections also met the polarization criteria. Little consideration is given to concerns with areas receiving too much current.
However, when we overlay the concerns with AC induced corrosion and the desire to control the DC current density below 1A/m2 or face the requirement to mitigate to a much lower threshold for AC current density, it becomes a more challenging CP system design. Now the CP system designer must:
Understand the interaction between cathodic protection system design and its impact on AC mitigation requirements
Provide provisions to monitor (on a time-weighted basis) both AC and DC current densities
Give consideration to being able to intentionally control DC current densities in those AC corrosion risk corridors—this might require additional CP stations to reduce over-polarization, the strategic use of isolation devices to create DC current density control zones, and the use of auto-controlled rectifiers to vary current output to control DC current densities. Improving the control of DC current density can significantly reduce the amount of AC mitigation that might be required.
For existing pipelines, the AC mitigation requirements should be based on some actual data on the CP current density in specific areas of concern. Current densities are typically highest closer to a CP station and in areas of low soil resistance. Another factor that can impact current density is the quality of the coating. Poorly coated pipelines have more uniform and lower CP current densities while well coated pipelines may have higher localized current densities because of the small size and infrequent nature of the coating defects. For new pipelines, the AC mitigation designer should be careful to presume that the higher AC mitigation threshold based on controlling DC current density can be applied without consultation with the CP system designer to assure that the design provides for sufficient control of CP current density.
The use of AC test stations with specific AC and DC current density coupons is necessary to ensure that localized conditions do not exist where AC corrosion risk is not properly being controlled. These coupon test stations should be equipped with remote monitoring to allow for data polling at regular intervals to allow for time-weighted averaging of the current density data. Mobiltex recently introduced a new series of Test Station RMUs specifically designed to be installed in a conventional cathodic protection test station. These remote monitoring units can record and transmit AC and DC current density information from AC coupon test stations.
The frequency and location of these coupon test stations is a design issue. It is critical to note that within areas subject to AC corrosion risk, coupon test stations should be installed at all significant “inflection” points where predictive modeling and/or AC mitigation design experience would dictate elevated risk including:
Entrance/exit points for HVAC / pipeline collocations
Low soil resistivity areas or areas with notable differential soil resistivity changes within the collocation
HVAC phase transpositions
The criteria for AC corrosion control are different than those typically associated with conventional cathodic protection to control corrosion. The requirements for monitoring both AC and DC current densities are interrelated and can have a significant impact on the AC mitigation requirements and on the cathodic protection system design and operation. Understanding this relationship between AC and DC current density and properly controlling each is critical to properly controlling AC corrosion risk.
For information on MATCOR’s AC mitigation solutions or for assistance setting up testing to prevent AC corrosion, please contact us at the link below.
MATCOR is a leading manufacturer of impressed current sled anode systems and as such we tend to get asked a lot of questions about sled anodes. Here are some frequently asked questions:
Does it matter whether sled anodes are to be installed in seawater, brackish water or freshwater? What if the water salinity varies with the season or with tidal action?
These are two related questions, and both have to do with the conductivity (or resistivity which is merely the inverse of conductivity) of the water where the anodes will be located. The conductivity of the water plays a critical role in determining the overall system resistance and current output of the system. For freshwater locations, the relatively low water conductivity requires a significant quantity of anodes to keep the overall system resistance down. In those instances, a sled anode may not be the best design option as sled anodes are most cost effective in brackish or saltwater environments. For environments where the conductivity can vary seasonally or with the tides, such as estuaries or tidal river boundaries, special consideration may be required such as constant current or auto-potential controlled power supplies.
Why would we use impressed current sled anodes as opposed to galvanic anodes?
Depending on the application, there are compelling reasons for the use of each type of system. Galvanic anodes do not require an external power supply, are less subject to interference issues, and can be closely coupled directly to the structure. The impressed current sled anodes can greatly simplify installation, reduce overall costs, typically have a longer life, and can produce a lot more current from a lot fewer anodes. The choice of anode type is very much a site-specific consideration requiring a proper engineering evaluation during the design phase.
Are there any specific concerns with marine wildlife when evaluating cathodic protection systems?
Marine wildlife is generally unaffected by the presence of a cathodic protection system. Cathodic protection systems have been used in commercial aquariums and fish hatcheries without any impact on the marine life. At the structure, cathodic protection can result in a localized environment that reduces or inhibits the growth of barnacles while changes in the pH at the structure’s surface encourage the growth of calcareous deposits which reduce the current requirements and provide a form of protective coating for the steel structure.
The MATCOR sled anodes utilize a wooden base – are there any concerns with the deterioration of the wooden base releasing in chunks of wood that could damage intake structures?
We have not experienced any such problems – the wooden base is designed to sink into the mud along the sea floor and provide an anchor. Wood holds up very well in this environment; however, over time the wood will slowly become food for cellulose processing bacteria and eventually will slowly be degraded. This process is a natural process and occurs over a long period of time. There is no expectation that the wood base would break into pieces that could damage an intake structure. MATCOR can provide an inert non-metallic plastic base that would be like wood but not subject to natural biodegradation.
How do you protect the cabling from the Sled Anode back to the system rectifier?
MATCOR utilizes an HMWPE cable that has a very robust exterior jacket that is suitable for direct burial in soil or water environments. The cable is housed inside a 1” diameter flexible drilled PE pipe that provides mechanical protection for the cabling. We recommend the use of concrete weights to secure the cable along the seafloor. The drilled PE pipe holes facilitate the cabling sinking into the seafloor mud providing additional protection for the cabling.
What about dredging operations?
For locations that are subject to occasional periodic dredging operations every few years or so, MATCOR can provide a locator float and lifting lugs to allow for the anodes to be removed prior to dredging operations. If the frequency of the dredging operations is such that this would be a regular occurrence (multiple times per year), then consideration should be given to alternate designs that would not require anode removal on regular basis.
For information on MATCOR’s Sea-Bottom Marine Anode Sleds or for assistance with marine near shore cathodic protection system design, please contact us at the link below.