Pipeline owners spend more than one-third of their budgets on steel pipe sections, valves, and other components. Protecting that investment means safeguarding its most vulnerable points — the thousands of welds that hold pipeline sections together.
Corrosion leading to steel loss is one of the most common pipeline failure modes, and it most often occurs at or near a weld joint. Proper surface preparation and the application of protective coatings are essential for pipeline risk management, and there is more to the process than meets the eye.
The Pipeline Service Environment Stresses Welds
Mainline sections of pipe, usually 40 or 60 ft, are all factory-coated with fusion bonded epoxy (FBE) before shipment to the job site. But the 6 in. or so at the ends of each section are left uncoated to allow field welders to connect one to the next.
The bare welded steel needs protection against corrosion to keep the pipeline safe from failure. Pipeline girth weld coating comes to the rescue.
There are many coating options on the market that contractors choose based on budget, application, performance properties, anticipated pipeline life, pipe size, operating temperatures, soil conditions, and operational flow pressures.
All these factors are important when you consider how abusive a pipeline’s service environment is:
High moisture in wet soils leads to aggressive corrosion.
Burial in rocky locations enhances the risk of physical damage because pipelines move during operation.
Variable soil chemistries along the length of a pipeline influence the nature and aggressiveness of corrosion.
The size, weight, and operating parameters (like temperature or pressure) of a pipeline apply stress to welds, enhancing corrosion risk.
In these environments, even the most perfect weld is a weak link. It needs an equally robust protective coating.
Matching Protection Method to Pipeline Type
Not every pipeline is protected the same way. Different technologies have emerged to suit owners’ differing operational and cost objectives.
Gathering lines are laid from individual wellheads to a collection point prior to processing. These lines are usually from 2 to 6 in. O.D. and are not expected to last as long as a mainline midstream pipeline.
A tape system wraps around the bare steel weld and features a mastic backing for adhesion to the steel. Tapes are ideal in small outside diameter gathering line service because these lines operate at lower pressures, encounter less soil stress, feature fewer regulatory obligations, and are designed for shorter life expectancies. They are inexpensive to buy and easy to apply without specialized labor.
Midstream lines transport processed hydrocarbons to market. These pipelines range from 16 to 48 in. O.D. and are usually built for a 30–40-year life expectancy. Often crossing state lines, these assets are subject to much closer regulatory scrutiny. Owing to their scope and cost to build, midstream owners usually choose a much more cohesive protective coating system, such as liquid-applied epoxy. Liquid epoxy is more durable and longer lasting than tape-type products, but the trade-off for higher performance is the need for better preparation and skilled labor.
Welds on midstream pipelines must be abrasive blasted to industry standards for two reasons. First, the surface needs a suitable anchor profile so the coating has something to cling to. Second, blasting removes rust, mill scale, or other debris that would interfere with coating adhesion.
The temperature of the weld steel in relation to ambient temperature also influences a coating’s cure, so propane torches or heated rings are commonly used to raise the surface temperature and promote faster, more uniform curing.
There are many ways to apply liquid epoxy coatings, depending on the pipeline size. Smaller pipelines are typically coated by hand with two coating technicians brushing or troweling the liquid epoxy onto the pipe. Larger pipelines are usually coated with an automated ring system. These systems feature spray nozzles mounted on rotating rings that dispense liquid epoxy onto the pipe quickly and with great precision.
Transmission lines operate cross-county, are large in outside diameter, and operate at elevated pressure to achieve high delivery volume. These pipelines usually send high volumes of hydrocarbons for large-scale processing into another fuel type, deliver fuels or feedstocks to export terminals, or provide finished fuels to power whole communities or the A1 centers that consume energy at a staggering rate.
Aside from the longer mileage and increased number of weld joints (tens of thousands for a 500-mile pipeline), there is no difference in how weld joints on transmission and midstream lines are protected. Liquid-applied epoxies are preferred because they perform better and last longer. It is more than worth the price an owner pays for the skilled labor needed to install them.
Craftsmanship Extends to Coatings
Girth welding of pipeline sections is fundamental to the performance of these assets and fundamental to the commerce and energy security of any location where a pipeline operates. It’s hard to imagine a more dangerous scenario in which to cut corners.
But good pipeline craftsmanship extends beyond welding. As demonstrated above, a pipeline’s service environment contains numerous stressors that leave welds vulnerable if they aren’t adequately protected.
The coating that protects a weld is as important as the weld itself. Specifying and applying the right one requires just as much skill and attention.
This article is based on a presentation from the AWS Pipeline, Facilities & Tanks Conference held Nov. 5 and 6, 2025, in Fort Worth, Tex. Reprinted with permission: The AWS Welding Journal.