Protecting Pipelines from Corrosion – One Layer at a Time

Posted on Monday, July 22, 2019

By building pipeline coating performance one layer at a time, operators can greatly increase their potential for moving pipeline commodities safely, efficiently, and cost effectively. This approach may involve up to three layers of advanced coating technologies (Figure 1). The first coating layer prevents the steel pipes from corroding. The second layer protects the first anti-corrosion layer from damage. Finally, the topcoat preserves the two lower layers against ultraviolet (UV) degradation.

Each of the above layers is typically comprised of fusion-bonded powder coating technologies. The first two layers are commonly plant-applied fusion-bonded epoxy (FBE) coatings that protect pipes against corrosion and wear related to underground soil stresses, bacteria and fungus attacks, soil acids, and alkalis. The third layer blocks UV-light waves from reaching the FBE layers below and may be a fusion-bonded polyester (which would be applied at a plant like the first two layers) or a water-based acrylic coating (which would be applied in the field). This article will peel back each coating layer to demonstrate how they support the longevity of pipeline assets.

Figure 1.
A multi-layered approach to coating pipelines for long-term performance includes a base anti-corrosion layer to prevent pipeline corrosion, an intermediate layer to protect the anti- corrosion layer from wear, and a UV-blocking layer to preserve the first two layers before burying or submerging pipes.

Advanced Coatings Address Emerging Pipeline Trends
Exterior FBE pipeline coatings can help operators both maximize asset performance – by protecting steel pipes from corrosion and damage – and minimize ownership costs – by delivering long-term performance combined with a reduced cost for cathodic protection. These characteristics are helping pipeline operators address three emerging trends related to today’s harsher drilling environments, including:

Deeper Extraction.
Deeper wells increase the heat-resistance requirements of protective coatings due to their closer proximity to geothermal hot spots. In addition, deeper wells also increase pipeline heating needs, as the high-viscosity substances extracted from deep in the earth must remain at elevated temperatures to move fluidly through pipes. Therefore, pipeline coatings need to withstand the elevated temperatures anticipated throughout the entire process stream.

Hotter Conveyance.
Hotter drilling byproducts moving through pipes also cause challenges related to varied temperature cycles, soil stresses, and water ingress. These challenges are particularly evident at offshore-to-onshore transitions where pipe movement from temperature cycling may cause rocks and dirt to scratch the exterior coating. A temperature drop may also cause water to penetrate some pipeline coatings, trapping water between the coating and steel substrate. The trapped water will boil and rapidly dissipate from the area when the pipe heats up, potentially reducing primary adhesion. To address these scenarios, a protective coating layer should offer flexibility and abrasion resistance to mitigate the effects of pipeline movement, as well as moisture-barrier properties to prevent water ingress.

A protective, moisture-resistant coating layer may reduce water from penetrating the coating and causing reduced primary adhesion.

Delayed Installations.
Lengthy pipeline installation delays can create problems for pipes coated with FBEs, as they are designed to perform underground or under insulation. When exposed to UV light, epoxies can turn chalky and experience property changes. By specifying an additional preservation coating layer – a topcoat of fusion-bonded polyester or water-based acrylic latex coatings – pipeline owners can help mitigate the costs associated with installation delays.

How to Specify Pipeline Coatings:
Based on the above trends, pipeline coating manufacturers are focused on developing formulations that deliver enhanced exterior pipe coating performance and more efficient applications. In particular, they are working to improve flexibility, durability, porosity resistance, thermal properties and cathodic disbondment resistance.

Flexibility and Durability.
The flexibility and durability of pipeline coatings must be sufficient enough to handle all the stresses a pipe may encounter, including:

• Storage, as coated pipes await installation.
• Transportation, as pipes are transported to and around job sites.
• Installation, as pipes are flexed, dragged across the ground or horizontal directionally drilled.
• Operation, as pipes encounter high and varied temperatures.

To meet the competing requirements of durability and flexibility, the FBE coating’s glass transition temperature (Tg) must exceed the temperature of the pipeline contents. If not, the coating can become too pliable, leaving it susceptible to degradation. By specifying coatings with a (Tg) that’s above the highest anticipated operational temperature of the pipe, operators can enhance outcomes.

Following a multi-layered approach to coating pipes can help operators realize better results in the field.

Porosity Resistance.
FBE coatings with higher porosity resistance provide better corrosion protection by minimizing the penetration of moisture to the steel pipe substrate. Such coatings have minimal voids, or pores, in their surface, which is driven by the coating technology itself, as well as an optimized coating process. Using the appropriate application temperature and coating technology is critical to ensuring a spray-applied powder coating will melt and flow into the nooks and crannies of a heated pipe’s surface profile and cure to form a durable exterior coating without pores.

If the application process is too hot for the specific chemistry, pores are likely to occur. Such voids indicate areas where the coating film is noncontinuous, which shortens the pathway for water to reach the steel pipe substrate. If the application process is too cool, the sprayed FBE powder may not melt sufficiently, which reduces its adhesion. By fine-tuning application temperatures based on the thickness of the pipe, its diameter, how fast it moves through the line, and other factors, applicators can reduce the porosity of applied FBE coatings. To help, applicators should consider FBE coatings that have an intrinsically broad application window, as this flexibility can help to reduce costs and electrolytes. Subsequently, this adversely affects the while ensuring better coating applications.

Thermal Properties.
To accommodate the rigors of deeper well drilling, pipeline coatings must have sufficient thermal properties. While much of today’s market faces maximum pipeline temperatures of 120 degrees Celsius, growing segments of the market are reaching temperatures of 150 degrees Celsius and even 180 degrees Celsius. Coatings manufacturers are responding with technologies that are pushing the boundaries of science to potentially handle pipeline temperatures exceeding 200 degrees Celsius.

Cathodic Disbondment Resistance.
FBE coatings work synergistically with cathodic protection (CP) systems to provide a two-pronged approach to corrosion prevention. First, the FBE coatings protect the metal pipe substrate from corrosion. In addition, they allow electrons from a backup CP achieve excellent porosity resistance during application. system to reach that metal substrate. The resulting electron flow allows sacrificial anodes connected to the CP system to corrode preferentially to the pipe. If the FBE coating shielded that electron flow instead of allowing it to reach the pipe, the backup CP system would be almost useless, creating an environment ripe for corrosion and potential pipeline failure.Cathodic disbondment typically occurs when a break forms in an FBE coating, allowing for a localized increase in alkalinity due to the CP current’s interaction with water hydrogen-bonding based coating adhesion. Cathodic disbondment can be mitigated by selecting FBEs with strong adhesion potential and ensuring proper pipe surface preparation and heating during applications.

How to Protect Pipelines
Following a multi-layered approach to coating pipes can help operators realize better results in the field. That means focusing on preventing pipeline corrosion, protecting anti-corrosion coatings from wear and preserving FBE coatings before pipes are buried in the ground.

An anti-corrosion FBE coating is the base layer. It prevents corrosive elements from reaching the steel pipe substrate. It can effectively resist chemicals, saltwater, solvents, corrosive gases, and more when applicators As the layer in contact with the steel pipe substrate, anti-corrosion FBEs should deliver excellent adhesion to steel, as well as excellent resistance to cathodic disbondment. In addition, they should feature outstanding resistance to cracking, cold flow, and softening over a wide temperature range.

The next layer is a protective, abrasion-resistant FBE previously impossible to extract. Formulators are currently coating designed to resist the scratches and scrapes a pipe may encounter from transportation to installation. It protects the first layer from being damaged, so the bare steel substrate does not become exposed to corrosive elements. It is helpful for this protective layer to also provide a moisture barrier that prevents water from penetrating through to the anti-corrosion coating.

The preservation layer is optional. However, this final coating layer is often a beneficial insurance policy against project delays because FBEs don’t age well under UV light. If a pipeline installation project would become severely delayed, there is potential for deleterious degradation of the FBE-coated pipe staged onsite waiting to be installed. Applying an outer protective coating layer of a fusion-bonded polyester or water-based acrylic coating will prevent UV light from reaching the FBE layers, prolonging their lives to enable later pipe installations.

Multiple Coating Layers Deliver Comprehensive Performance
As underground and underwater drilling environments become hotter and harsher, coatings manufacturers are answering new challenges with new formulations that allow pipeline operators to tap energy resources that were previously impossible to extract. Formulators are currently meeting operating temperature ranges topping 150 degrees Celsius, and coatings rated for 180 degrees Celsius and 200 degrees Celsius will conceivably enter the market soon. To realize long-term performance and reduced ownership costs, pipeline operators are encouraged to build their coating performance one layer at a time, focusing on preventing corrosion, protecting against wear, and preserving the integrity of their assets.

Dr. Jeffrey David Rogozinski, Ph.D. is global product director – 
fusion-bonded epoxy/pipe for Sherwin-Williams Protective & 
Marine Coatings.