Understanding ARO Coating Systems: Protecting Pipelines Where Damage Risk Is Highest
By James (Jim) Kunkle, PCS
When pipeline owners discuss corrosion protection, the conversation often begins with fusion bonded epoxy (FBE), polyethylene systems, or other primary external coatings. While these coatings provide exceptional corrosion protection, some pipeline installations expose the pipeline coating system to mechanical stresses that exceed the capabilities of traditional coatings alone.
This is where Abrasion Resistant Overlay (ARO) coating systems become critical.
For pipelines buried and installed through challenging environments, particularly horizontal directional drilling (HDD) crossings, rocky terrain, river crossings, and areas containing cobble or gravel formations, ARO systems provide an additional layer of protection designed specifically to withstand physical abuse during construction while preserving the integrity of the underlying corrosion-control coating.
What Is an Abrasion Resistant Overlay?
An Abrasion Resistant Overlay is a supplemental coating layer applied over a primary corrosion protection coating, most commonly fusion bonded epoxy. The purpose of the ARO is not to replace the primary coating's corrosion protection function, but rather to protect it from mechanical damage during handling, transportation, installation, and service.
Think of the ARO as the armor surrounding and protecting the pipeline coating beneath it.
During HDD installations, pipelines may be back pulled thousands of feet through an underground borehole while being subjected to friction, drag forces, impact loads, shear, gouge, and contact with rock formations. Without adequate mechanical protection, pipeline coating damage can occur during pullback operations, creating defects that may increase future cathodic protection demand and long-term corrosion risk, or pipe structural integrity.
Why HDD Installations Demand Additional Protection
Horizontal directional drilling has become one of the most widely used installation methods for environmentally sensitive crossings and congested infrastructure corridors. While HDD minimizes surface disturbance, it creates a unique set of challenges for coating systems.
As the pipeline is pulled through the underground borehole, the coating may encounter:
● Continuous sliding abrasion
● Contact with rock and gravel formations
● High compressive forces
● Increased pullback tensions
● Surface gouging and scratching
● Repeated impact loading
Traditional corrosion coatings are primarily designed to prevent corrosion, not necessarily to resist severe underground mechanical abuse. ARO systems fill this performance gap.
Common Types of ARO Systems
Several ARO technologies are available today, each designed to address specific installation risks and project requirements.
Epoxy-Based ARO Systems
Epoxy AROs are among the most common solutions used for HDD applications. These systems typically consist of a reinforced epoxy layer applied over the primary FBE coating.
Advantages include:
● Excellent abrasion resistance
● Good impact resistance
● Strong adhesion to FBE
● Compatibility with existing coating systems
● Proven performance in HDD projects
These coatings are frequently specified for long crossings where pullback forces and abrasion risks are significant.
Dual-Layer FBE Systems
Dual-layer FBE systems utilize a primary corrosion-resistant FBE layer followed by a second, more durable outer layer designed to enhance impact and abrasion resistance.
Benefits include:
● Simplified coating architecture
● Excellent coating adhesion
● Improved handling durability
● Reduced repair requirements
These systems are often selected when moderate abrasion protection is required without adding substantial coating thickness.
Polyurethane-Based ARO Systems
Polyurethane ARO coatings offer exceptional flexibility and toughness while maintaining strong resistance to gouging and abrasion.
These systems are often considered for:
● Severe HDD crossings
● Rocky terrain installations
● Areas susceptible to coating impact damage
● High-stress construction environments
Their elasticity can help absorb mechanical loads that might otherwise damage more rigid coating systems.
Multi-Layer Composite Systems
Some specialized pipeline projects require advanced multi-layer systems combining several protective technologies.
These may incorporate:
● FBE corrosion protection
● Adhesive tie layers
● Reinforced intermediate coatings
● Abrasion-resistant outer layers
Such systems are often used on large-diameter transmission pipelines where coating integrity is considered mission-critical.
Selecting the Right ARO System
The best ARO system is not always the thickest or most expensive.
Selection should be based on several factors:
● Borehole underground geology
● HDD crossing length
● Pipe diameter
● Anticipated pullback forces
● Environmental conditions
● Construction methods
● Long-term asset integrity requirements
A successful specification should balance mechanical protection, corrosion resistance, constructability, inspectability, and lifecycle economics.
Inspection and Quality Assurance
Even the best coating system can fail if quality control is overlooked.
ARO inspection programs typically include:
● Surface preparation verification
● Coating thickness measurements
● Holiday detection testing
● Adhesion testing
● Cure verification
● Visual inspection
● Repair validation
Quality assurance during application often determines whether a coating system performs as designed during a demanding HDD installation.
Looking Beyond Construction
While ARO coatings are often justified based on construction risk reduction, their value extends far beyond the installation phase.
Every coating defect avoided during construction represents:
● Reduced exposure of steel substrate
● Lower cathodic protection demand
● Fewer repair activities
● Improved long-term coating performance
● Lower lifecycle maintenance costs
● Increased confidence in pipeline integrity
In many respects, abrasion-resistant overlays represent an investment in the future reliability of the entire corrosion protection system.
Takeaways
As pipeline projects continue to expand into more challenging environments, coating specifications must evolve to address both corrosion protection and mechanical durability requirements. Abrasion Resistant Overlay systems have become an essential component of modern pipeline protection strategies, particularly for HDD installations where traditional coatings may be exposed to extreme construction stresses.
Understanding the strengths, limitations, and appropriate application of various ARO technologies allows owners, engineers, inspectors, and contractors to make informed decisions that improve installation success and protect critical infrastructure for decades to come.
Because when it comes to pipeline integrity, the goal is not simply applying a coating. The goal is ensuring that coating survives the journey from the coating plant to the end of the asset's service life.
About the Author
James (Jim) Kunkle, PCS, is a Protective Coatings Specialist, corrosion-industry educator, and technical communicator with decades of experience supporting asset integrity, corrosion prevention, and protective coating technologies. He serves as a professional representative for Henkel's protective coatings portfolio, is a Founding Member and Vice-Chair of the Coatings Committee for the Corrosion Technician Association (CTA), and is the Technical Principal of ProCoatTec, LLC. Through industry publications, educational programs, and The Digital Revolution with Jim Kunkle podcast, he helps bridge the gap between traditional corrosion-control practices and emerging digital technologies that are reshaping the future of infrastructure protection.
