Alternating current (AC) is the lifeblood of modern society, powering homes and industries through a vast network of high-voltage transmission lines. Yet, this same energy poses a silent, insidious threat to the steel pipeline infrastructure often buried alongside it. AC corrosion is a complex and aggressive form of metal degradation that can compromise pipeline integrity far more rapidly than conventional corrosion mechanisms. Understanding this threat is no longer optional; it is a critical necessity for ensuring the safety and reliability of our essential energy transport systems.

Pipelines are the arteries of the global economy, safely transporting oil, gas, and water across vast distances. This critical infrastructure, along with other vital assets like offshore platforms, is predominantly constructed from steel, a material chosen for its strength and durability. However, steel's fundamental nature makes it susceptible to corrosion—an electrochemical process that seeks to return refined metal to its more stable, natural state, like iron ore. This degradation is a persistent battle, costing industries billions annually in maintenance, repairs, and lost production, and posing significant environmental and safety risks.

AC corrosion is an accelerated form of localized corrosion that occurs on buried metallic structures, like pipelines, when they are subjected to AC interference from nearby power sources. Unlike more familiar direct current (DC) corrosion, which involves a steady, one-way current flow, AC corrosion is driven by the rapid, cyclical reversal of current. For years, it was believed that the alternating nature of AC would cancel out any corrosive effect, but this assumption has been proven dangerously false. The unique corrosion mechanism at the metal surface leads to a net loss of material, often in the form of deep, localized pits that can perforate a pipeline wall with surprising speed.

The proliferation of shared utility corridors means that pipelines and high-voltage powerlines are increasingly located in close proximity, making AC interference a growing concern. The rapid, localized nature of AC-induced pitting can lead to pipeline failure with little warning, posing a greater immediate risk than slower, more uniform corrosion. Furthermore, AC interference can disrupt traditional cathodic protection systems and can hide their effectiveness. This makes standard corrosion prevention methods insufficient. For this reason, asset managers and corrosion technicians must understand this corrosion type. This knowledge helps them assess risks accurately, reduce damage, and protect public safety.



For AC corrosion to occur, an electrical circuit must be established that allows alternating current to flow from a power source, onto the pipeline structure, and then off the pipeline at specific locations back to the source. This transfer of energy doesn't require direct physical contact; it occurs through electromagnetic principles that couple the power line and the pipeline, even when they are separated by soil. Understanding these coupling mechanisms is the first step in diagnosing and mitigating the threat.

The most common source of AC interference is high-voltage overhead power lines running parallel to a pipeline. The powerful electromagnetic fields generated by the current flow in these lines induce a voltage onto the nearby steel pipeline. Other significant sources include:

During normal operation, these sources create a steady-state level of interference. However, fault conditions, such as a short circuit on a power line, can cause a massive, instantaneous surge of current onto the pipeline, posing extreme corrosion and safety risks.

There are three primary ways AC energy is transferred from a source to a pipeline's metal structure:



At its core, all corrosion is an electrochemical process involving the flow of electrons and ions. It requires an anode (where metal is lost), a cathode (where a protection reaction occurs), an electrolyte (the soil), and a metallic path. AC complicates this classic model by introducing a rapidly oscillating potential that fundamentally alters the reactions occurring at the metal surface.

In standard DC corrosion, an electrochemical cell forms due to a potential difference between two points on a metal structure. At the anode, iron atoms lose electrons and dissolve into the electrolyte (Fe → Fe²⁺ + 2e⁻). These electrons travel through the metal to the cathode, where they are consumed by a reaction, typically involving oxygen and water (O₂ + 2H₂O + 4e⁻ → 4OH⁻). This steady, one-way current flow results in gradual metal loss at the anode. The entire process of corrosion protection is designed to halt this anodic reaction.

AC introduces a dynamic where every point on the metal surface alternates between being an anode and a cathode 50 or 60 times per second. It was initially thought these rapid reversals would result in zero net corrosion. However, the electrochemical reactions are not perfectly reversible or efficient. The process of dissolving metal (anodic reaction) and the protective reaction (cathodic reaction) do not behave symmetrically in response to the changing voltage. This asymmetry at the steel-soil metal interface means that over each full AC cycle, more metal is dissolved during the anodic half-cycle than is "plated" back or protected during the cathodic half-cycle. This imbalance creates a net DC corrosion current, resulting in significant metal loss.

The primary corrosion mechanism under AC influence is driven by this "electrochemical rectification" effect. This is especially dangerous at small defects, or "holidays," in the pipeline's protective coating. These defects act like lenses, concentrating the AC current flow from a large surface area onto a tiny point of bare steel. This creates extremely high local AC current densities, dramatically accelerating pitting corrosion. The pits are often deep and crater-like. Furthermore, the high current densities can disrupt or damage the naturally forming passive film — a thin, protective oxide layer that normally slows down corrosion on metals like stainless steel and can offer some protection to carbon steel. By damaging this film, AC exposes the reactive metal beneath, allowing the aggressive pitting process to proceed unchecked.



The risk of AC corrosion is not uniform; it is a function of a complex interplay between the environment, the pipeline's characteristics, and the nature of the AC interference itself. A thorough risk assessment must consider all these contributing factors to determine the likelihood and potential severity of damage to the infrastructure.

The soil acts as the electrolyte in the corrosion circuit. Its properties are paramount. Low-resistivity soils (e.g., wet clays) provide an easy path for current flow, increasing the severity of AC corrosion. Soil chemistry, including pH level and the presence of aggressive ions like chlorides and sulfates, also significantly influences the rate of the electrochemical reactions. Furthermore, environmental factors like moisture content and temperature can fluctuate, changing the soil's properties and the corresponding corrosion risk.

The physical properties of the pipeline structure itself play a crucial role. The quality of the external coating is perhaps the most critical factor. A high-quality, holiday-free coating provides excellent protection. However, even small coating defects, scratches, or areas of disbondment become focal points for intense corrosion. The size and geometry of these defects matter; smaller, deeper defects lead to higher current densities and more aggressive pitting. The type of steel and its surface condition can also affect its susceptibility to the AC corrosion mechanism.

The characteristics of the induced AC are direct drivers of corrosion. The most important parameter is the AC current density at the coating holiday — the amount of current flowing per unit area of exposed metal surface. Industry standards, such as those from NACE International (AMPP), suggest that densities above 20-30 A/m² pose a significant corrosion risk. The magnitude of the induced AC voltage on the pipeline is also a key indicator, with levels above 15V AC considered a safety hazard for personnel in contact with the structure. The frequency of the AC (typically 50 or 60 Hz) and the duration of the interference also contribute to the overall metal loss.



Identifying and quantifying the risk of AC corrosion in the field is more complex than for standard DC corrosion. The dynamic nature of AC interference, which can vary with powerline load, requires specialized equipment and survey methodologies to capture an accurate picture of the threat.

AC and DC signals often coexist on a pipeline, making it difficult to isolate and measure the AC component accurately. Standard measurement tools may be susceptible to AC interference, leading to erroneous readings for cathodic protection surveys. Furthermore, power consumption on adjacent lines fluctuates daily and seasonally, meaning that the induced AC voltage and current flow are not constant. A single measurement may not represent the worst-case scenario. Also, we cannot directly measure the AC current density at tiny coating defects buried underground. Instead, we must calculate and model it indirectly.

Corrosion technicians employ several techniques to assess AC interference. Direct measurement of the AC voltage between the pipeline and the soil (pipe-to-soil potential) using a reference Electrode and a high-impedance multimeter is a primary step. Coupons — small, buried pieces of steel electrically connected to the pipeline — are used to measure AC current density by simulating a coating holiday. Techniques like Close Interval Potential Surveys (CIPS) are adapted to measure both AC and DC potentials along the entire length of the pipeline, helping to identify areas of high interference.

For more detailed analysis, the field of Corrosion Science offers advanced techniques. One of the most powerful is Electrochemical Impedance Spectroscopy (EIS), a non-destructive method that applies a small AC signal to the pipeline and measures the impedance response. The results can be used to characterize the corrosion processes at the metal-soil interface and even estimate corrosion rates. Researchers use lab tests that mimic field conditions. These tests help them understand reactions better and confirm ways to reduce corrosion. These advanced methods provide deeper insights than routine field measurements, helping to validate mitigation strategies and understand complex corrosion scenarios.



Effective corrosion prevention in an AC environment requires a multi-faceted approach. It begins with proactive design choices and extends to the installation of dedicated mitigation systems and the careful management of existing corrosion protection systems.

The most effective mitigation strategy is avoidance. During the design phase of new pipelines, routing them away from high-voltage AC corridors is the best form of protection. Increasing the separation distance between the pipeline and the power line dramatically reduces the level of inductive coupling. If proximity is unavoidable, selecting a route with fewer parallel sections and more perpendicular crossings can minimize interference.

Where significant AC interference is unavoidable, engineers install dedicated systems to safely drain the induced AC from the pipeline to the ground. This is typically achieved using a grounding conductor connected to the pipeline via a decoupling device. The conductor is buried in a low-resistance backfill, creating an effective path for AC to leave the structure. The decoupler (such as a polarization cell or solid-state device) is crucial because it allows AC to pass freely while blocking the DC current from the cathodic protection system, preventing it from being drained away.

AC interference harms Cathodic Protection (CP) systems. It causes quick potential changes that make measurements hard. It also lowers the DC current's protective power. Therefore, cathodic protection systems in high-AC areas may need to be adjusted. This can involve increasing the DC current output from an impressed current system to overcome the AC effects or placing anodes closer to the pipeline. Careful monitoring is essential to ensure the CP system remains effective in providing corrosion protection despite the presence of AC.

Corrosion inhibitors are chemical substances typically used to control internal pipeline corrosion by forming a protective film on the metal surface. For external AC corrosion, their application is generally not practical or effective. The primary defense against external corrosion is a high-performance coating combined with cathodic protection. Proper surface preparation and coating application are the most critical treatments for preventing the initiation of AC corrosion at the source.



Accurate diagnosis of a corrosion failure is essential for implementing the correct preventative measures. AC corrosion has distinct characteristics that differentiate it from other forms of electrochemical degradation, such as DC stray current or general atmospheric corrosion.

DC stray current corrosion occurs when a pipeline picks up direct current from an external source, such as a DC transit system or another pipeline's CP system. This current flows along the pipeline and discharges at another location, causing rapid, localized metal loss at the point of discharge. While both are caused by interference, AC corrosion is driven by an oscillating field and results in damage at numerous locations along the pipeline, while DC stray current corrosion causes severe damage only where the current leaves the structure.

General corrosion involves a relatively uniform loss of metal across a large surface area, often seen in atmospheric exposure. Localized corrosion, such as pitting corrosion, is confined to small, specific areas. AC corrosion is a highly aggressive form of localized corrosion. The pits are often described as clean, round, or crater-like, appearing as if the metal was "scooped out," which contrasts with the more irregular shapes often found in other types of soil corrosion.

Misidentifying the corrosion mechanism can lead to ineffective remediation. For example, simply increasing the output of a CP system might not solve a severe AC corrosion problem and could even exacerbate other issues like hydrogen embrittlement. A good failure analysis is very important. It combines field data, operational history, and metal examination of the failed part. This helps find the root cause. This ensures that the chosen mitigation strategy directly addresses the specific threat. In forensic investigations, experts examine failed pipeline parts under a microscope. They look for crater-like pits that show AC corrosion caused the failure.



AC corrosion represents a significant and escalating challenge to the integrity of our vital pipeline infrastructure, from oil and gas networks to water distribution systems. As our reliance on both electrical power and transported energy grows, the colocation of these systems will become even more common, amplifying the risk. The belief that alternating current is benign has been thoroughly debunked, replaced by the knowledge that it can drive rapid, localized metal loss capable of causing catastrophic failures.

We need to change from fixing problems after they happen to preventing them first. This includes assessing risks and using prevention methods at every pipeline stage. This starts from design and routing and continues through operation. Effective management of this threat hinges on a comprehensive understanding of its core principles — from the coupling mechanisms that induce current onto the structure to the unique electrochemical reactions that cause metal loss at coating defects.

We can protect these important assets by using special survey methods. We also install proper mitigation systems. We adapt traditional corrosion protection methods wisely. Ultimately, protecting our pipeline infrastructure from AC corrosion is an engineering challenge. It is also a key responsibility to ensure public safety, protect the environment, and secure energy. By acknowledging the threat and applying sound engineering principles, we can ensure the continued safety and reliability of this critical infrastructure for decades to come.



Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

Our biggest strength is in our flexibility for our clients. Solutions and Results.

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