Pipeline Corrosion Detection and Management – An Essential Overview

Pipeline Corrosion Detection and Management – An Essential Overview

Blogs

Innovation, Integrity, and Expertise in Pipeline Sustainability

Pipeline corrosion is a and inescapable phenomenon that represents one of the most significant challenges for many industries. Manifesting in a variety of forms, it impacts longevity and integrity of liquid and gas transportation systems. Consequently, significant investment is made in the detection and identification of pipeline corrosion and its management.

In this article, we provide an overview of what is – and will continue to be – a critical field in pipeline management.

Why Is Management of Pipeline Corrosion So Important?

When poorly managed, the ramifications of pipeline corrosion can be extensive and influence safety, environmental, and economic aspects of an operation.  

Economically, pipeline corrosion leads to immense costs related to maintenance, repairs, and lost efficiency. For example, there are estimated to be around 2.15 million kilometres of oil and gas pipelines around the world, and corrosion is the most important factor in pipeline integrity. In the United States alone, the annual yearly cost of pipeline corrosion is estimated at $7 billion.

A real-life example of the potential economic impact of pipeline corrosion was demonstrated in the Alaska Pipeline system where, in 2011, severe corrosion led to a spill of approximately 200,000 gallons of oil – despite pipeline corrosion caused by faulty coating having been identified 20 years earlier.

From a safety perspective, the stakes are incredibly high. Corrosion can compromise the structural integrity of pipelines, leading to failures that might cause explosions, fires, and oil spills. These incidents not only endanger human lives, but also have devastating effects on the environment. In a study of 1,063 pipeline accidents, 21% were found to have been caused by corrosion.

Effective detection and management of pipeline corrosion is not simply a case of technical and regulatory compliance. Fundamentally, it is an ethical imperative regarding the safeguarding and wellbeing of employees, surrounding environments, and local communities. Consequently, regulatory bodies across the globe impose stringent requirements for corrosion management to mitigate these risks.

Key Methods for Detecting Pipeline Corrosion

Detecting, measuring, and monitoring pipeline corrosion involves a multifaceted approach that includes non-destructive testing, electrochemical probes and sacrificial coupons

Corrosion coupons – sacrificial metal strips exposed to pipeline conditions – are analysed to determine corrosion rate within the pipeline.

·       Non-Destructive Testing (NDT) Methods

NDT methods help to provide a more comprehensive view of pipeline corrosion. Techniques such as acoustic emission (AE), magnetic flux leakage (MFL), and liquid penetrant inspection enable early detection without damaging pipelines. Advanced NDT methods include:

  • Visual Inspection, despite being the most basic form of corrosion detection is a very valuable inspection tool. It involves regular surveys and manual inspections of accessible pipeline sections.  Of course this can only be used on the external surfaces of pipelines.

Techniques that allow both internal and external inspection:

  • Radiographic Testing (RT), is an extension of visual testing involving the use of X-rays or gamma rays to capture sub-surface images that reveal corrosive damage.
  • Ultrasonic Testing (UT), which employs high-frequency sound waves to detect imperfections or changes in material properties similar to the use of UT in medical applications.
  • Electromagnetic Testing (ET), which utilises electromagnetic induction to detect surface and sub-surface irregularities

In addition, Intelligent pigs – often referred to as ‘Smart Pigs’ – can travel internally along pipelines to detect anomalies using methods like ultrasonic testing and magnetic sensors, providing comprehensive data regarding the condition of the pipeline.  They can often provide inspection of almost 100% of both the internal and external surfaces of a pipeline.

Other forms of robots such as drones and crawlers can also be fitted with the above technologies to carry out inspections in locations that are difficult to reach.

The integration of modern technologies such as the Internet of Things (IoT) and predictive analytics represent a revolutionary step in corrosion management. Data is continuously transmitted in real time to be analysed so that potential failure points can be identified. This will allow pre-emptive repairs and significantly reduce downtime.

Corrosion coupons – sacrificial metal strips exposed to the internal pipeline conditions – are analysed to determine corrosion rate within the pipeline.

Electrochemical Probes can be inserted into a pipeline to measure the corrosivity of the fluid inside it.  They can provide a continuous output of corrosion rates that can be fed to a pipeline control room and corrosion engineers.

Pipeline Corrosion Prevention Techniques

When taking action to prevent corrosion of pipelines, utilising a combination of techniques and methods is crucial. Such methods include:

·       Cathodic Protection

An electrochemical process that reduces the oxidation within metal pipelines by making them the cathode of an electrochemical cell. The main cathodic protection techniques are to use sacrificial anodes or impressed current cathodic protection.  This technique is primarily used to protect the external surface of a pipeline – usually in combination with coatings and linings.

·       Coatings and Linings

Coatings and linings can be be made from organic, metallic and inorganic materials.  They provide   an anti-corrosive layer that acts as a barrier between the pipe material and the corrosive environment. When designing pipelines to minimise the risks of corrosion, it’s crucial to select materials that are inherently resistant to corrosion.  They can be used on both internal and external surfaces of pipelines.

·       Corrosion Inhibitors

Corrosion inhibitors are the most widely used method for protecting the internal surfaces of pipelines.  They are added to the pipeline fluids and absorb onto the metal surface to provide a barrier to corrosion.

Future Trends in Corrosion Management of Pipelines

Corrosion detection and management of pipelines has evolved rapidly in recent years, and will have  to continue to do so – to tackle new corrosion threats such as the transportation of hydrogen and carbon dioxide from carbon capture plants. From regulatory frameworks to materials science to sensing technologies, corrosion science and corrosion engineering continue to advance.

·       Advances in Material Science

The development of new alloys and composite materials is pivotal in combatting pipeline corrosion. These materials are engineered to endure harsh environments and aggressive chemicals that accelerate corrosion in conventional materials.

Additionally, the incorporation of nanotechnology into material fabrication has given birth to nano-coatings and nanocomposites that enhance durability and resilience by improving barrier properties and reducing molecular wear and tear.

A key breakthrough has been in the use of graphene, which acts as an impermeable barrier to gases and liquids, minimising oxidation, which is a key factor in corrosion.

·       Enhanced Sensing Technologies

Technological innovations in sensing technologies are revolutionising corrosion management by enabling more precise and comprehensive monitoring capabilities. Modern sensors now incorporate features like higher resolution, greater coverage areas, and advanced data analytics to detect and predict corrosion sites before they manifest into larger issues.

For example, fibre optic sensors provide real-time data on pipeline integrity by detecting changes in temperature, pressure, and structure. These sensors are immune to electromagnetic interference, making them versatile for diverse environments. Moreover, drones equipped with hyperspectral imaging sensors can perform aerial surveys to detect corrosion under insulation or in inaccessible areas.

·       Regulatory and Standards Evolution

The evolution of international standards and regulatory requirements is crucial in pushing the envelope of what is technologically feasible and economically viable in corrosion management. Updating of standards ensures that the newest technological advances and best practices are implemented to safeguard public and environmental health.

Advancing Knowledge and Expertise in Corrosion Management

As the complexities of pipeline corrosion evolve, so too must the expertise of those tasked with managing it. The Institute of Corrosion (ICorr) plays a crucial role in this dynamic landscape. By sharing knowledge, expertise, and best practices, ICorr ensures that corrosion professionals are well-prepared to tackle current and future challenges. We are also at the forefront of policy discussion, giving our members a voice in the future regulation of the industries in which they operate (as demonstrated by our sponsorship of the Reuse, Repair, Replace Conference).

Our commitment to education and continuous professional development is evident through our industry-leading training programs. These are designed and delivered to elevate the technical competence of personnel across the corrosion industry. Through initiatives such as our membership pages, social media, Corrosion Management magazine and more, we provide a collaborative environment in which all our members (and the wider corrosion community) can benefit from insight and innovation in corrosion science and corrosion engineering.

In an era where technological advancements are critical to economic viability and environmental stewardship, the role of institutions like the Institute of Corrosion is indispensable. Through its efforts, ICorr not only contributes to the global economy but also fortifies the industry’s capacity to manage corrosion effectively, safeguarding infrastructure and ecosystems alike.

To learn more about the Institute of Corrosion, our membership schemes, and our comprehensive training packages, email admin at ICorr.

Materials and Technologies in Corrosion Mitigation

Materials and Technologies in Corrosion Mitigation

Blogs

Shaping the Future of Corrosion Management

Effective corrosion mitigation is crucial to prevent economic losses, ensure safety, and enhance the longevity of infrastructure and machinery in all walks of life, but especially in industries which are most vulnerable to corrosion, such as oil and gas, maritime, automotive, and construction. The financial impact includes repair and replacement costs, downtime, and reduced efficiency. Additionally, corrosion can lead to catastrophic failures, endangering lives, and the environment.

Successful corrosion mitigation begins with understanding the fundamentals of corrosion – how it occurs and the different forms of corrosion against which we must protect. This fundamental knowledge then allows us to decide upon which materials and corrosion mitigation techniques to use in various applications and corrosive environments.

Traditional Materials Used in Corrosion Mitigation

Traditional materials used in corrosion mitigation have proven effective in various applications. Stainless steels, renowned for their high chromium content, form a passive oxide layer that significantly protects against corrosion. Among these, grades such as 304 and 316 are particularly noteworthy, each tailored for specific environments and applications.

In addition to stainless steels, corrosion-resistant alloys like Inconel, Hastelloy, and Monel stand out for their exceptional resistance to extreme conditions. These alloys perform remarkably well at high temperatures and in aggressive chemical environments, making them indispensable in industries such as aerospace and chemical processing, and also in upstream production where robust materials are crucial for safety and efficiency.

Advanced Materials for Corrosion Resistance

High-performance polymers such as PTFE, PEEK, and PVDF provide excellent chemical resistance and are used in applications where metal corrosion is problematic. These materials are lightweight and can be easily fabricated into complex shapes.

Composite materials, combining two or more distinct materials, offer superior corrosion resistance and mechanical properties. Fiberglass-reinforced plastics (FRP) and carbon fibre composites are widely used in corrosive environments.

Coating Technologies

Corrosion mitigation using protective coatings has been a tried and tested preventative measure for decades. Advances in corrosion science have delivered continuous improvements in the types of coatings we use.

For example, epoxy coatings, create a durable, impermeable barrier on metal surfaces, effectively shielding them from moisture and chemicals. This makes them a popular choice for marine and industrial applications, with zinc-rich epoxy coatings widely used to protect steel against corrosion.

Polyurethane coatings, on the other hand, offer remarkable flexibility and abrasion resistance, making them ideal for surfaces that endure mechanical stress. Their robust chemical resistance further enhances their suitability for a wide range of industrial uses.

Meanwhile, ceramic coatings excel in providing exceptional heat and corrosion resistance. These coatings are particularly beneficial in high-temperature environments and for safeguarding components that are exposed to aggressive chemicals. Some ceramic coatings are also used to prevent erosion due to their high wear resistance.

Inhibitors in Corrosion Mitigation

Inhibitors play a vital role in corrosion mitigation by forming protective layers on metal surfaces. Organic inhibitors, such as amines and azoles, create a film that prevents corrosion. These compounds are frequently used in cooling water systems and oilfield applications due to their effectiveness in such environments.

Inorganic inhibitors, including phosphates, and molybdates, also provide a protective barrier against corrosion. These substances are particularly effective in various industrial processes, such as water treatment and metal finishing, where they help maintain the integrity and longevity of metal components.

Nanotechnology in Corrosion Mitigation

Nanocoatings, consisting of nanoscale particles, provide enhanced protection due to their high surface area and strong adhesion properties. These coatings can be engineered to offer specific corrosion resistance characteristics.

Incorporating nanoparticles into polymer matrices, nanocomposites exhibit superior mechanical and corrosion resistance properties. They are used in advanced engineering applications where traditional materials fail.

Smart Materials and Sensors

Self-healing materials can autonomously repair damage caused by corrosion. They contain microcapsules filled with healing agents that release and polymerize upon cracking, restoring the material’s integrity.

Advanced corrosion sensors embedded in structures provide real-time data on corrosion rates and environmental conditions. This information is crucial for predictive maintenance and early intervention.

Cathodic Protection

Cathodic protection is a widely used in corrosion mitigation. This method operates on the principle of redirecting corrosive reactions away from the protected metal to a more reactive metal or by applying an external current.

One approach is to use sacrificial anodes. In this technique, a more reactive metal, such as zinc or magnesium, is attached to the structure needing protection. This reactive metal, known as the sacrificial anode, corrodes in place of the primary metal, thus preventing the primary metal from degrading. This method is particularly beneficial in environments where metal structures are exposed to harsh conditions, such as in marine applications or underground pipelines.

Another approach is to use the impressed current method. Here, an external power source provides a continuous current to the protected structure, counteracting the electrochemical reactions responsible for corrosion. This method is exceptionally effective for large and complex structures like pipelines and storage tanks, where uniform protection is essential. By maintaining a constant current, the impressed current method ensures comprehensive protection, extending the life of critical infrastructure and reducing maintenance costs.

What is the Future for Corrosion Mitigation?

The future of corrosion mitigation is set to be transformed by emerging materials and innovative technologies, driven by ongoing research and development efforts. The Institute of Corrosion, a leading authority in the field, plays a leading role in corrosion management and policy influence.

Emerging materials, such as high-entropy alloys and graphene-based coatings, are at the forefront of corrosion resistance. High-entropy alloys, with their unique composition and structure, offer unprecedented durability and resistance to a wide range of corrosive environments. Graphene-based coatings, known for their exceptional strength and impermeability, provide a revolutionary solution for protecting surfaces against corrosion.

Innovative technologies are also making significant strides. Additive manufacturing, commonly known as 3D printing, is enabling the production of complex components with tailored corrosion-resistant properties. This technology allows for precise control over material composition and structure, resulting in parts that are not only stronger but also more resistant to corrosive elements.

Furthermore, artificial intelligence (AI) and machine learning are being harnessed to predict corrosion behaviour and optimize mitigation strategies. By analysing vast amounts of data, AI systems can identify patterns and predict areas at risk of corrosion, allowing for proactive maintenance and more efficient resource allocation.

The Institute of Corrosion continues to lead the charge in these advances, promoting research, disseminating knowledge, and fostering collaboration among industry professionals. As these trends continue to evolve, the future of corrosion mitigation looks promising, with new materials and technologies poised to deliver more efficient and sustainable solutions for industries worldwide.

Stay ahead of the curve in your career in corrosion – join the Institute of Corrosion and enjoy all the benefits of membership.

PFP – An Introduction to Intumescent Coatings

PFP – An Introduction to Intumescent Coatings

Blogs

The Role of Intumescent Coatings in Passive Fire Protection

Intumescent coatings have been a key strategy in the protection of buildings and other structures against fire for several decades. They form part of a passive fire protection strategy that might also include other materials such as concrete, mineral fibre boards, vermiculite, and cements.

In this article, we explore the effectiveness of intumescent coatings in providing passive fire protection for diverse types of structure, including offshore platforms and commercial buildings.

What Is Passive Fire Protection?

Fire protection in buildings and structures is crucial for safeguarding lives by delaying structural collapse which allows enough time for safe evacuation. It does this by either preventing fire or delaying the escalation of fire.

Active fire protection provides an immediate response to either suppress or extinguish a fire. Key components include fire alarms systems; sprinklers; suppression systems (such as foam and dry chemical systems); smoke control systems; and emergency lighting.

Passive fire protection (PFP) helps to prevent, withstand, and contain fire within a structure. These measures are integrated into the structure, and designed to protect the structural integrity of the building and provide occupants with sufficient time to evacuate safely. Key components of PFP include fire resistant materials; compartmentation (division of a building to contain the spread of fire and smoke using tactics like fire walls, fire doors, and fire-rated partitions); firestopping sealants; fire resistant insulation and intumescent coatings.

A Brief History of the Use of Intumescent Coatings

Intumescent coatings have been integral in protecting steel structures from fire for about 40 years. When exposed to fire, these coatings swell to create a protective barrier that can last up to four hours, significantly delaying the steel from reaching its critical failure temperature of approximately 400°C.

In the 1980s, the use of intumescent coatings saw a significant rise in Europe. Major oil companies recognised their ability to protect structural steel from the intense heat of hydrocarbon fires. This period also saw an increase in the use of exposed steel in commercial and high-rise buildings, which further boosted the demand for aesthetically pleasing, but simultaneously safe, coatings.

How Do Intumescent Coatings Work?

The chemistry of intumescent coatings involves several components:

  • An organic binder resin (epoxy for hydrocarbon fires or acrylic for cellulosic fires)
  • An acid catalyst (such as ammonium polyphosphate)
  • A carbonific source (like pentaerythritol), and a spumific agent (e.g., melamine)

Upon exposure to fire, these ingredients react to form a low thermal conductivity carbon char that expands to provide a protective barrier. This char layer insulates the steel, reducing heat transfer and extending the time before the steel reaches its critical failure temperature.

Types of Intumescent Coatings

Intumescent coatings are categorised as thin-film or thick-film:

  • Thin-film intumescents, often single-component solvent or water-based products which have dry film thicknesses (DFTs) of less than 5 millimetres.
  • Thick-film coatings are typically solvent-free and epoxy-based which have DFTs up to 25 millimetres.

Recent advances have introduced multi-component methacrylate or ‘hybrid’ products, providing specific advantages over traditional formulations.

Which Type of Intumescent Coating Should Be Used?

Thin-film intumescents are typically used for aesthetic applications in commercial buildings, providing a paint-like finish. In contrast, thick-film coatings are used in more industrial settings, offering robust protection.

To specify the correct intumescent coating, it is essential to identify the item to be protected, such as structural steel or fire-resistant bulkheads.

The coating thickness depends on the steel’s weight and type, with lighter sections requiring thicker coatings for adequate protection. Determining the appropriate coating thickness involves calculating the steel’s shape and considering any irregularities. Manufacturers provide guidance for this and third-party certification ensures compliance with standards, ensuring optimal protection.

Common Applications for Intumescent Coatings

Intumescent coatings can be applied using various methods, including spraying. New hybrid products have expanded application possibilities, offering enhanced performance and flexibility. Typical situations in which intumescent coatings are used include:

·       Commercial and Residential Buildings

Intumescent coatings are widely used in commercial and high-rise buildings to protect exposed steel, blending safety with architectural aesthetics.

·       Industrial Facilities

In industrial settings, especially offshore platforms and floating facilities, intumescent coatings protect structural steel from the extreme heat of hydrocarbon fires, ensuring the integrity of critical infrastructure.

·       Transportation and Marine Industries

These coatings are also used in transportation and marine industries, protecting against cellulosic and hydrocarbon fires, including jet fires resulting from high-pressure fuel releases.

Testing Intumescent Coatings

Given the variability in fire conditions, standardised tests offer a reproducible method to evaluate coating performance, ensuring reliability under different scenarios.

Intumescent coatings are subjected to standardised fire tests to ensure their effectiveness. Standards like BS 476 and EN 13381 for cellulosic fires, and UL 1709 and ISO 22899-1 for hydrocarbon fires, provide benchmarks for performance.

Further, to provide effective fire protection, intumescent coatings must be durable and intact when exposed to fire. Therefore, formulating these coatings requires careful consideration of moisture sensitivity and environmental resistance to prevent corrosion and ensure long-term protection.

Different resins are used based on the application environment. For example, water-based acrylics are suitable for dry, internal locations, while solvent-based epoxies are used for more demanding conditions. Standard tests like NORSOK M 501 and UL 2431 ensure these coatings meet durability requirements.

ICorr’s Passive Fire Protection Courses

To enhance understanding and application of passive fire protection, the Institute of Corrosion (ICorr) offers specialised Passive Fire Protection (PFP) courses. These courses provide in-depth knowledge on the principles of PFP, including the selection and application of intumescent coatings. Participants learn about the latest standards, testing methods, and best practices in ensuring the durability and effectiveness of PFP systems.

For example, the PFP Coating Inspector (Epoxy) Level 2 trains and examines Inspectors of Epoxy Intumescent Passive Fire Protection on the inspection of common types of epoxy coatings used to protect against hydrocarbon fires on installations for both on and offshore facilities.

ICorr’s courses are designed for professionals involved in fire safety and providepractical insights and certification to ensure the highest standards of fire protection in various industries.

Intumescent Coatings: The Bottom Line

Intumescent coatings provide critical passive fire protection, offering up to four hours of defence against fire. Applications range from commercial buildings to industrial facilities, ensuring safety and structural integrity across various environments. With advances in formulation and application techniques, intumescent coatings continue to evolve, enhancing fire protection capabilities in diverse settings.

If you’re looking to start or advance your career in PFP inspection, reach out to the admin team at ICorr or email IMechE Argyll Ruane to discover which of our specialised PFP courses is best for you.

Designing, Monitoring, and Integrating Cathodic Protection with Coatings

Designing, Monitoring, and Integrating Cathodic Protection with Coatings

Blogs

Key Knowledge to Ensure Longevity of CP Systems

In the first two articles in this mini-series, we explored the basics of corrosive reaction and using cathodic protection to counteract the corrosive reaction. In this third and final instalment we cover the design, monitoring, and maintenance of CP systems and their interaction with protective coatings. Understanding these aspects is essential to ensure the long-term effectiveness and reliability of CP in preventing corrosion.

Design of Cathodic Protection Systems

Designing an effective CP system requires a detailed assessment by a cathodic protection engineer.

The engineer must determine the need for corrosion protection, decide on the appropriate CP system, and calculate the current requirement and optimal locations for anodes. The goal is to ensure that the CP current is uniformly distributed to protect all intended surfaces and to achieve the most economical and reliable solution.

In complex structures, mathematical modelling may be employed to optimise anode locations, spacings, and sizes.

Monitoring and Maintenance of CP Systems

Regular monitoring, and maintenance are crucial to the performance of CP systems. All anodes, whether galvanic or impressed current, degrade over time and need to be replaced periodically. CP systems can be designed to last up to 50 years, but this requires routine inspections and potential adjustments to ensure continued effectiveness.

Monitoring involves measuring the steel/electrolyte potential using fixed monitoring locations or portable survey equipment. This data helps the CP engineer determine if the current delivery and distribution are adequate, optimal, or need adjustment. Ensuring proper CP performance involves maintaining the balance between current density and distribution to prevent corrosion effectively.

Interaction of CP with Protective Coatings

CP is often used in conjunction with protective coatings to provide enhanced corrosion protection:

  • The coatings act as a barrier, reducing the overall current demand of the CP system and improving current distribution by isolating the metal from the corrosive environment.
  • CP then protects the steel at damaged areas of the coating where the underlying metal is exposed.

Not all coating systems are suitable for use with CP:

  • Coatings must be compatible with CP operation and withstand cathodic reactions, which can generate alkaline conditions.
  • Coatings used with CP should undergo ‘cathodic disbondment’ testing to ensure they are not damaged by these reactions.
  • Additionally, excellent adhesion to the metal surface is crucial to prevent disbonded coatings from shielding the steel from CP current, which could allow corrosion to occur under the coating.

CP and Concrete Structures

CP is widely applied to protect steel reinforcement inside concrete, particularly when normal passive conditions for steel in alkaline concrete fail or are at risk of failing due to chloride contamination from seawater or de-icing salts.

In North America, Europe, and other regions, extensive impressed current CP systems are applied to reinforced concrete structures, either from new construction or as part of repairs.

Impressed current systems for concrete use mixed metal oxide/titanium anodes in mesh, tube, or ribbon form, either cast into new concrete or fixed onto or into existing structures. For buried and immersed reinforced concrete, anodes may be positioned somewhat remote from the structures. Conductive carbon-filled organic coatings and metal sprays of zinc and aluminium alloys have also been used as impressed current anodes.

Galvanic anodes have been used in reinforced concrete, though their performance in dry environments is debated. These can include small cast anodes in concrete repairs or drilled holes, and zinc or aluminium alloy thermal sprayed coatings. In tidal applications, immersed galvanic anodes are used on piles and columns.

In Summary

Cathodic protection is a  a powerful tool in reducing corrosion damage – especially when combined with suitable protective coatings. The key points to remember are:

  • Properly designed and maintained CP can prevent corrosion damage to uncoated structures and on coated structures where the coating becomes damaged
  • CP does not prevent corrosion on internal surfaces when applied to external surfaces, and is ineffective above mid-tide level in marine environments
  • Combining CP with compatible coatings provides economical and long-term protection for buried or immersed structures.

By working together, coatings technologists and CP specialists can deliver optimal, long-term, and affordable corrosion protection solutions for their clients.

In conjunction with industry experts, we have developed a comprehensive set of training and certification programs for all in the corrosion industry. These include our leading Cathodic Protection, Training, Assessment and Certification Scheme and Coating and Inspection Training. For details of these courses, please contact ICorr today.

Using Cathodic Protection to Counteract the Corrosive Reaction

Using Cathodic Protection to Counteract the Corrosive Reaction

Blogs

Using Cathodic Protection to Counteract the Corrosive Reaction

How We Prevent Corrosion Using Cathodic Protection

In our previous article, we discussed the basics of cathodic protection (CP) and the fundamentals of the corrosive reaction. In this second installment, we will delve deeper into the types and mechanisms of CP, specifically focusing on the principles of CP, and the application of galvanic (sacrificial) anodes, and impressed current systems.

(Note: We’re focusing on steel and iron, though the principles are the same whatever the metal.)

The Principles of Cathodic Protection

Cathodic protection works by counteracting or reversing the corrosion current that flows off the corroding anode sites into the electrolyte. By providing an excess of electrons, CP forces the corrosion reaction to be suppressed. This protective current flows from a CP anode, which is buried or immersed in the same electrolyte as the steel or iron, onto the metal surface being protected.

The CP current can be supplied in two ways:

1.    Galvanic (Sacrificial) Anodes

An external anode made from a more active metal (one that corrodes more readily than iron or steel) is connected to the metal surface to be protected. The anode’s own corrosion generates the CP current, making the protected metal the cathode.

2.    Impressed Current Systems

An external, direct current (DC) power source is connected to the steel, along with an inert anode in the electrolyte. The current flows from the anode,  onto the steel, turning it into a cathode.

Galvanic (Sacrificial) Anodes

Galvanic anodes are metal castings physically connected to the metal to be protected,. The anode material is chosen based on its more negative electrical potential compared to steel, as defined by their positions in the galvanic series.

The difference in potential between the sacrificial anode and the steel causes electrons to flow from the anode to the steel, making the steel surface more negatively charged and, thus, the cathode of the corrosion cell. The anode material corrodes or sacrifices itself to protect the steel.

Common materials used for galvanic anodes include alloys of aluminum, zinc, and magnesium. These anodes are widely used in the protection of ballast and cargo/ballast tanks of ships, external hulls of smaller vessels, offshore oil and gas facilities, internal and external flood defense gates, and smaller buried tanks and pipelines.

In paint technology, zinc-rich primers act similarly to galvanic anodes. The zinc particles in the primer corrode preferentially to the steel substrate, providing protection. Galvanised steel also employs this principle, where the zinc coating corrodes to protect exposed steel areas if the coating is damaged.

Impressed Current Systems

For larger structures, the current demand might be too high for galvanic anodes alone, making them uneconomical. In such cases, impressed current systems are used, where an external DC power source is used to force electrons onto the steel surface.

Impressed current anodes are typically made from materials such as mixed metal oxide-coated titanium (MMO/Ti), platinum-coated titanium (Pt/Ti), niobium (Pt/Nb), or cast iron/silicon/chrome alloys (SiFe anodes).

In Summary

Both galvanic and impressed current CP systems operate on the principle of providing an excess of electrons to the metal surface, making it the cathode, and preventing corrosion. Key points to remember are:

  • CP can minimise corrosion damage to uncoated structures and to coated structures if the coating becomes damaged.
  • CP applied to external surfaces will not reduce corrosion on internal surfaces.
  • CP is ineffective above mid-tide level and does not affect atmospherically exposed steel.
  • The choice between galvanic and impressed current CP systems depends on the specific requirements and scale of the structure being protected.

In conjunction with industry experts, we have developed a comprehensive set of training and certification programs for all in the corrosion industry. These include our leading Cathodic Protection, Training, Assessment and Certification Scheme and Coating and Inspection Training. For details of these courses, please contact ICorr today.

In the last of this mini-series of articles covering cathodic protection and the corrosive reaction, we look at designing, monitoring, and integrating cathodic protection with coatings.