Resistance to corrosion is a key advantage of fiber-reinforced polymer (FRP) materials. 

It makes FRP an ideal choice for tanks, pipes and other equipment in industries like pulp and paper and chemical processing where corrosion is an everyday challenge.

However, despite significant advances in FRP materials science, corrosion is still the leading cause of FRP equipment breakdown as exposure to harsh substances, chemicals and gases, as well as high temperatures and pressures, take a toll over time.

In the 1960s, as FRP composites became commonplace in chemical processing and other industries, the corrosion barrier emerged as a key design feature to improve reliability of FRP equipment.

Monitoring the corrosion barrier condition is therefore critical to ensuring FRP Fitness For Service.

However, another challenge remains: the current lack of standards and codes for the inspection of in-service FRP equipment. 

Standards for in-service inspection are on the horizon, and the publication of WRC Bulletin 601 provides a technically valid, quantitative and repeatable process for evaluating in-service fiber-reinforced polymer (FRP) assets, including the corrosion barrier.

In this article we cover frequently asked questions about FRP corrosion barrier assessment, including scientific information about failure modes, and inspection methods used to determine Fitness for Service.

What is a corrosion barrier?

A FRP corrosion barrier is made from multiple layers of resin-saturated glass fibers. It is usually integrated into the process side (or inner surface) of an FRP vessel or pipe that will be exposed to corrosive or caustic chemicals. 

The purpose of the corrosion barrier is to prevent leaks and protect the structural FRP from damage by the operating environment. Although it is bonded to the structural FRP, the corrosion barrier is not normally considered to contribute any structural strength to the FRP.

Types of FRP corrosion barriers

A corrosion-resistant barrier is normally composed one of two ways: using layers of reinforced thermosetting polymer or using a thermoplastic sheet. 

A thermosetting polymer is a polymer that is applied in liquid form with curing agents added that react with the polymer to form bonds between the polymer chains, known as cross-linking. Examples of thermosetting polymers include epoxy, vinyl ester, and polyester resins. 

A thermoplastic sheet is a polymer that can be molded into different shapes by some combination of heat and stress. Examples of thermoplastic materials include: polypropylene; polyvinyl chloride; polyethylene; polyvinylidene fluoride, and many others. 

The image below shows a typical cross-section of FRP, with a corrosion barrier on the mold side or process side. The innermost surface might be made of 90% resin and 10% reinforcement (usually glass), but the resin/glass ratio can be adjusted to suit different service requirements. 

In most process industries, the corrosion barrier consists of three to six layers of resin-saturated glass, with the polymer resin making up 70% to 85% of the total volume. Most specifications and codes call for a minimum thickness of 2.5 mm;  however, thicker corrosion barriers may be required in some types of service, while other FRP equipment might have no corrosion barrier.  

Corrosion barrier construction specifications, standards and codes

Corrosion barriers have been included in most specifications, standards and codes related to construction of chemically-resistant FRP since the late 1960s. 

For FRP designers and engineers, choosing the right polymer is a critical concern to ensure the equipment is leakproof and corrosion-resistant.

ASTM Standard Practice C581 provides guidance for choosing thermosetting polymers. This allows engineers or resin manufacturers  to test different combinations of polymer and reinforcement in a controlled service environment for up to 12 months. Specimens are evaluated every three months for:

• Changes in hardness of the resin surface
• Changes in weight and thickness
• Changes in flexural modulus 

The practice also allows for additional observations to be made about changes to the polymer’s appearance, but it doesn’t provide any objective criteria. 

Thermoplastics can also be evaluated in this way by exposing cutouts or specimens to service conditions in a controlled environment and taking similar measurements.

Corrosion barrier inspection and repair

Regardless of the original corrosion barrier design, FRP corrosion barriers are not immune to damage. 

Although the quantitative results from ASTM C581 testing are used to form recommendations for resins to use in FRP construction, they are NOT used to provide specifications for in-service FRP inspection and repair.

It’s also important to note that results from corrosion testing of thermosetting and thermoplastic polymers are not widely published or available for reference. Data from these tests are not used to provide guidance for assessment of corrosion barriers that are in service.

Monitoring corrosion barrier condition

As design and construction standards for corrosion barriers evolved, asset owners and reliability engineers began to focus on monitoring corrosion barrier condition as a key factor in maintaining FRP reliability. 

It’s important to remember that the underlying damage mechanisms affecting FRP composites, including the corrosion barrier, are invisible to the naked eye and are completely different from the mechanisms that damage metal alloys. 

These invisible damage mechanisms (weakened reinforcement / weakened polymer / loss of polymer volume) lead to failure modes or overt damage that CAN be observed visually.

Corrosion barrier failure modes

With respect to the corrosion barrier, there are several failure modes that can lead to a gradual loss of FRP reliability and which can be detected by visual inspection, including:

Oxidation: Resins used in FRP are all organic polymers and therefore they can be oxidized by agents such as sodium hypochlorite, chlorine and chlorine dioxide. Oxidation reduces the thickness of the corrosion barrier. 

Oxidation is quite easy to detect visually. In the image below, the resin has been yellowed by exposure to chlorine dioxide. The oxidized resin on the surface is softened and can be abraded off with little effort.

Microcracking: Microcracking is difficult to detect until the cracks are fairly large – more than 40 microns wide – or they have been stained by the process fluid. 

The most common cause of this failure appears to be mechanical in origin, related to the stresses created from different rates of thermal expansion of resin and glass: resin experiences more rapid changes than the structural glass. 

These cracks originate in the exposed resin-rich surface and progress into the laminate until they are stopped by the reinforcement or imperfect bonding at exotherm interfaces. These cracks may also be due to chemical causes such as incomplete curing of the resin.

Chemical Diffusion: This occurs when the corrosion barrier is attacked by the substances contained in the vessel or pipe. It can be differentiated from oxidation in that the resin will appear to be intact, although there may be discoloration or changes in glossiness of the surface. These changes occur as a result of chemical reactions taking place within some areas of the resin as a result of aggressive chemical reactions. Chemicals that are known to cause these changes include sodium hydroxide (caustic), sodium hypochlorite, solvents and some acids. The image above shows different surface textures and colours inside a tank — a sign of chemical attack.

Process Temperatures: thermal expansion of resins and glass differ markedly. For this reason, process temperatures can cause significant changes in expansion. In the corrosion barrier, with a higher resin fraction than the structural layers, the difference can create cracks. These cracks allow process fluid to gain access deeper into the laminate and can form into inter-layer cracks that may result in separation. The photo above shows cracks on the interior surface of a scrubber at a pulp and paper mill.

Blisters and Diffusion: blisters are formed in some corrosion barriers as a result of two main mechanisms – heating and diffusion. Blisters are caused by heating when local areas separate from underlying areas because of thermal expansion. This can result in permanent deformation. In many cases, process fluids will diffuse through the blister skin and collect in the cavity underneath. Blisters caused by diffusion result when process fluids diffuse into the corrosion barrier and collect in a local area where resin is weakly bonded to underlying areas. Diffusion is common and it should be expected to occur in all corrosion barriers.

It’s not as bad as it looks. Or is it?

Recognizing the importance of the corrosion barrier in maintaining the Fitness For Service of FRP equipment, many asset owners and reliability engineers incorrectly assume that visible damage (failure modes) that extends to the boundary of the corrosion barrier signals the end of service life for the FRP. 

In our experience, this incorrect assumption often leads to premature asset replacement or unnecessary corrosion barrier relining at significant expense.

How can asset owners know if visible damage actually affects fitness for service, or if there are unseen damage mechanisms at work that spell trouble ahead?

Below we discuss three approaches to inspecting FRP corrosion barriers: visual inspection, destructive testing, and UltraAnalytix non-destructive testing to understand its condition and take appropriate steps to repair or replace it.

Corrosion barrier assessment methods

Conventional approaches for corrosion barrier inspection usually require access to the interior surface of an asset so it can be assessed based on visual appearance, as well as surface hardness and the depth of damage using methods such as divot tests described in TAPPI standard practice 04021.

In general, these methods allow reasonable evaluation of oxidation, some micro-cracking, process temperature damage and surface attack failure modes explained above.

When surface damage is detected, it has probably also occurred to the surfaces that are not accessible, such as small nozzles. In such cases, alternative methods can be used. For small diameter pipe, some success has been achieved using digital x-ray to determine the thickness of the corrosion barrier. 

However, while there are some resources available to help inspectors identify damage, specifications for acceptable levels of different damage types are subjective, leading to poor consensus among inspectors and engineers, and confusion for asset owners. In fact, available guidelines often refer inspectors to standards for new FRP, which do not apply for use with in-service equipment as noted above.

Visual inspection of the corrosion barrier: is it necessary?

Visual inspection of the corrosion barrier surface is often used along with other simple non-destructive tests like a Barcol hardness impressor or acetone rub. These methods typically require a shutdown, clean out and confined-space entry. 

Visual inspection of the corrosion barrier is inherently subjective and leans heavily on the judgment and ability of individual FRP inspectors to actually “see” the damage.

The underlying assumption is that as long as the corrosion barrier appears to be intact, no damage is occurring to the structural layers. 

However, just because the corrosion barrier looks okay, doesn’t mean that it is.

There is no published science to support assumptions about the relationship between the appearance of FRP and its reliability.

Destructive testing

Traditionalists often also call for destructive testing to assess the FRP’s mechanical properties, which involves taking cutouts or coupons and testing tensile, shear and compression strength in the lab. 

The total cost to the operation can be significant, including downtime, engineering, and repairs even before any evaluation of the pipe has started. Any conclusions about the asset condition hinge on the specific samples removed.

Destructive testing may also include examining the material microscopically for signs of chemical diffusion into the corrosion barrier.

Objective, data-driven corrosion barrier inspection

Inconsistent and subjective assessment of the corrosion barrier condition has left many asset owners and reliability engineers looking for a better way.

UTComp’s UltraAnalytix® NDT method is an evidence-based alternative to visual inspections and destructive testing, based on data from thousands of FRP inspections in many types of service over the past 15 years. 

This method (also known as attenuation-based ultrasonic testing or UAX), assesses FRP reliability and Fitness For Service by quantifying polymer damage through the full thickness of the material — including the corrosion barrier — without the need for shutdown or confined space entry.

Corrosion barrier assessment: the UAX method

Many industries rely on ultrasound technology as their go-to non-destructive methodology for detecting corrosion, loss of thickness, cracking and other defects in steel infrastructure. 

However, while FRP has a long record of success safely transporting and storing corrosive acids, caustic chemicals, petroleum products and other materials, ultrasonic testing of FRP composite equipment — including the corrosion barrier —  is relatively new.

While conventional ultrasonic testing can be used to measure thickness and find defects in materials, UltraAnalytix uses ultrasound differently.  Like conventional ultrasonic testing, UltraAnalytix can identify thickness changes, cracks, delaminations and other defects. 

UltraAnalytix data can also reveal the effects of oxidation, oxidation depth,  diffusion depth, process temperature damage and surface damage. It can also discern microcracking on the surface when the corrosion barrier is submerged.

Corrosion barrier assessment: PDS

Unlike destructive testing, UltraAnalytix results parallel the ASTM C581 standard for measuring — in an unstressed state — the chemical resistance of thermosetting resins used in FRP laminates.

UltraAnalytix uses ultrasonic readings taken from the exterior surface of FRP equipment to assess the condition of the corrosion barrier based on two key parameters:

1. Damage Depth — the depth that damage penetrates the corrosion barrier resin
2. Percentage of Design Stiffness (PDS) — the stiffness (strength) of the resin within the damaged depth

The image above shows scans from a typical corrosion barrier analysis. 

UltraAnalytix uses patented algorithms that analyze ultrasonic data to calculate changes in stiffness, a value expressed as Percentage of Design Stiffness (PDS).

Generally,

• PDS values above 50% indicate the FRP is in good overall condition and fit for service until the next scheduled inspection (recommended at 3-year intervals)

• PDS values between 40% and 50% indicate potential problems that may require an engineering review

• PDS values below 40% indicate end of service life or a high risk of failure requiring immediate action

Corrosion barrier assessment: damage depth

By measuring changes in corrosion barrier stiffness (PDS), thickness and the depth of damage, UltraAnalytix quantifies the condition of the FRP to provide reliable Fitness For Service assessments that allow asset owners to make data-based decisions about their FRP assets.  

The UltraAnalytix method also includes a systematic visual inspection when conditions allow. However, the reality is that visual inspections are not always practical or safe and, as noted above, damage to the FRP resin is often invisible.

Learn more about FRP testing methods.

Publications

The UltraAnalytix system is powered by AI, drawing on millions of data points from FRP assets in a wide range of industries all over the world, to provide reliable and repeatable results.

To  learn more about the science behind this approach, see the following following publications:

1. ASTM, “ASTM C 581, Standard Practice for Determining Chemical Resistance of Thermosetting Resins Used in Glass-Fiber-Reinforced Structures Intended for Liquid Service,” ASTM, Conshohoken, 2001.

2. G. Clarkson, “FRP Corrosion Barrier Inspection: Non-destructive and Non-intrusive Technique,” Inspectioneering Journal, Houston, 2020.

3. G. Clarkson, “Toward Objective Evaluation of FRP Corrosion Barrier Condition,” in AMPP Corrosion 2022, San Antonio, TX, 2022. 

4. W. N. Findley, J. S. Lai and K. Onaran, Creep and Relaxation of Nonlinear Viscoelastic Materials, New York: Dover Publications, 1976. 

5. G. Clarkson, Assessment of Existing Fiber Reinforced Polymer Equipment for Structural Damage, WRC Bulletin 601, 2nd Edition, New York, NY: Welding Research Council, Inc., 2023.