In modern urban infrastructure and energy transmission networks, steel pipes buried deep underground bear the critical responsibility of transporting oil, natural gas, tap water, and thermal energy. Although steel is the material of choice for pipelines due to its high strength and good toughness, if bare steel pipes are buried directly underground, they will quickly be “consumed” by the complex soil environment.
To extend the service life of pipelines and ensure long-term transmission safety, all underground steel pipes must undergo rigorous coating treatment before installation. This is particularly true for corrosion-resistant spiral-welded steel pipes, which serve as the backbone of transmission trunk lines; the quality of their coating directly determines the safety of the entire pipeline system.
So, why must underground steel pipes be clad in this “protective coating”? And in practical engineering applications, how should we scientifically select the appropriate type based on project requirements?
I. Why Do Catastrophic Accidents Occur Without Coating?
The underground environment is far harsher than we imagine; soil itself is a vast and complex electrolyte environment. When bare steel pipes are buried directly in it, they face the following critical threats:
Electrochemical Corrosion: Soil contains water, dissolved salts, oxygen, and various microorganisms. When bare steel comes into contact with the soil, microscopic electrochemical differences on the pipe’s surface create countless tiny “localized cells.” Iron atoms continuously lose electrons and become iron ions, causing the pipe wall to gradually thin, perforate, and ultimately leak.
Rapid Damage from Urban Stray Currents: Beneath urban areas, infrastructure such as subways, light rail, and high-voltage power grids generates invisible stray currents. When these currents penetrate the ground and pass through exposed steel pipes, the steel suffers exponentially accelerated electrolytic corrosion at the points where the current leaves the pipe and enters the soil—a destructive force far exceeding that of natural soil corrosion.
Physical and Chemical Barriers: A high-quality anti-corrosion coating acts like a sturdy “insulating raincoat” for steel pipes. It not only completely blocks water and oxygen but also uses its high dielectric strength to prevent damage from stray currents.
II. The “Dynamic Duo” of Coatings and Cathodic Protection
In engineering practice, relying solely on coatings is often insufficient to ensure decades of protection, as pipelines may sustain minor scratches during transportation and backfilling. Therefore, underground pipeline networks typically employ a dual-protection approach combining “coatings and cathodic protection.”
Cathodic protection technology uses an external direct current or sacrificial anodes (such as magnesium or zinc blocks) to transform the entire steel pipe into a “cathode” in an electrochemical reaction, thereby protecting it from corrosion.
Significantly Reduced Operating Costs: Without a coating, the cathodic protection system would require astronomical amounts of current to protect the entire length of bare steel pipe, which is neither economically nor technically feasible.
Precise, Targeted Protection: The coating isolates over 99% of the steel pipe’s surface area. As a result, the cathodic protection system only needs to focus on potential minor coating defects (such as pinholes or minor construction scratches), achieving full-line corrosion protection with minimal current. This saves on long-term operational electricity and material costs.
III. How to Choose Among Mainstream Coating Options for Underground Steel Pipes?
To meet the requirements of different terrains, transported media, and budgets, the industry has developed corrosion-resistant spiral-welded steel pipes with a variety of coatings. When selecting a product, purchasers and design institutes can refer to the following comparison of mainstream coating options:
| Coating Type | Structure & Characteristics | Application Scenarios |
|---|---|---|
| 3PE Anti-corrosion Coating | Three-layer composite structure: bottom layer is fusion-bonded epoxy (FBE) with strong adhesion; middle layer is adhesive; outer layer is high-density polyethylene (HDPE) providing excellent mechanical damage resistance. | Long-distance oil and gas transmission pipelines; harsh environments with high soil stress and complex terrain conditions. |
| FBE (Fusion Bonded Epoxy) | Single or dual-layer epoxy coating with excellent adhesion, strong chemical corrosion resistance, and good resistance to cathodic disbondment; however, the outer surface is relatively brittle. | Horizontal directional drilling (HDD) sections; high-salinity and alkaline soil areas; pipelines requiring extremely high coating adhesion performance. |
| TPEP Anti-corrosion Coating | Dual protection system: outer layer uses 3PE anti-corrosion coating, inner layer uses fusion-bonded epoxy (FBE). Combines strong external impact resistance with internal flow reduction and hygienic performance. | Long-distance urban water supply and drainage systems; drinking water transmission networks. |
| Liquid Epoxy / Coal Tar Epoxy | Easy to apply and relatively low cost; however, environmental performance and mechanical damage resistance are relatively weaker. | Field joint coating repair; temporary pipelines; non-critical pipeline systems with limited budgets. |
IV. How to Ensure the Coating Lasts for Over 50 Years?
Selecting high-quality anti-corrosion spiral steel pipes does not mean the job is done. To ensure the coating lasts for 50 years or longer, the following three key points must be strictly adhered to throughout the entire construction lifecycle:
- Strictly Control “Factory Quality”: Steel pipes must undergo rigorous spark testing for leaks before leaving the factory. A high-voltage probe is used to scan the entire coating surface to ensure there are no microscopic pinholes invisible to the naked eye.
- Prioritize the “Transportation and Backfilling” Process: During pipe hoisting, lowering into trenches, and backfilling with soil and rock, avoid any violent impacts caused by human error. During backfilling, strictly control the quality of the backfill material to prevent large, sharp rocks from directly striking the pipe body and scratching the polyethylene outer coating.
- High-standard “on-site joint repair”: After on-site welding of the pipeline, the exposed areas at the welds must undergo on-site joint repair. Whether using heat-shrinkable tape or liquid epoxy, the corrosion protection grade at the repair site must match that of the pipe body. The corrosion protection lifespan of the entire pipeline often depends on this “weakest link” at the joint repair site.