Whether you’re in the transmission or distribution business, corrosion protection for pipelines is imperative for preventing natural gas leaks. It’s also a wise choice for prolonging the life of your system.
If your pipeline corrodes, you’ll have to grapple with safety concerns and potential lawsuits. In this article, you’ll learn what’s required to protect your natural gas pipeline against the three causes of corrosion.
Why you need multiple layers of corrosion protection
Pipe corrosion can be caused by chemical reactions, electrochemical reactions or abrasions from rock and soil. Each source of corrosion is a concern, and each requires a different method of protection.
Coatings protect against chemical corrosion, and cathodic protection defends against electrochemical corrosion. Tapes, mastics and wraps protect fittings and prevent abrasions that make your pipeline more vulnerable to the other types of corrosion.
Both coatings and cathodic protection are required by law. According to Section 192.455 of PHMSA’s Part 192, all buried or submerged pipeline installed after July 31, 1971, must be protected against external corrosion using external coatings and a cathodic protection system.
The reason for these requirements is that pipeline corrosion is a major safety hazard. If you don’t protect your pipeline, it will develop weak points that can lead to natural gas leaks.
1. Use coatings to prevent chemical corrosion
Pipeline coatings, including paints and epoxies, are the primary way to protect a pipeline from chemical corrosion. Coatings can be applied at the factory or painted on pipe in the field as a sealant.
Fusion-bonded epoxies (FBE) are one of the most commonly applied coatings for underground steel pipe. They are generally light green in color. FBE is a soft coating, so it needs to be protected with other coatings or wraps to prevent damage from backfilling or boring.
FBE is also subject to damage from ultraviolet (UV) rays, so you can’t use it for above-ground pipe without additional coatings. Special paints with UV protection are a primary choice for exposed pipelines.
Imperfections in the coating process, nicks or abrasions are known as holidays. These holidays are very important to repair and guard against because they create a concentrated path to corrosion.
An abrasion-resistant overcoat (ARO) is generally black in color and can be applied over FBE at the factory to add excellent resistance to abrasions. Additional coatings can be applied to further insulate pipe and protect it against moisture, which oxidizes the metal and creates rust.
2. Use cathodic protection to prevent electrochemical corrosion
Cathodic protection defends against electrochemical corrosion by using an electrical current to replenish lost electrons. This current can be created naturally using sacrificial anodes, or it can be aided by an impressed current from a rectifier.
It is incredibly important to have cathodic protection: without it, the rate of corrosion will be greatly accelerated if a failure, such as a holiday, occurs in the primary coating.
In cathodic protection, the pipeline acts as a cathode, or less reactive metal. More active metals, such as aluminum, zinc or magnesium, serve as anodes and are attached to the pipeline via wire.
Since the anodes are more reactive, they corrode instead of the pipeline by donating free electrons to it. This process preserves the pipeline at the expense of the anodes — hence, the name “sacrificial anodes.”
With sacrificial anodes, you don’t need any external source of electricity. Electrical current flows naturally from the anodes to the cathode through the ground. You simply need to complete the circuit using wire leads.
Impressed current cathodic protection
Alternatively, you can use impressed current cathodic protection to give the current flowing from the sacrificial anodes an extra boost. The anodes are connected to an external power source that generates a constant electrical current.
If you need your anodes to cover a wide area or last a long time, you may choose impressed current cathodic protection. An engineer can help you determine the best solution for your situation.
Either way, you need to monitor your cathodic protection by using aboveground test stations. Test stations can measure the electrical current flowing from your cathode to your anode. Once your voltage drops below an established value, it’s time to replace the anodes or the battery.
3. Add tapes, mastics and wraps to prevent abrasions
Finally, abrasion prevention is important to think about alongside corrosion prevention. If the ground around your pipeline shifts or settles, it can cause damage to the coating. This will introduce a corrosion path and ultimately lead to leaks.
While fusion-bonded epoxies coat the majority of the carbon steel, tapes and mastics are typically used at joints or fittings. Mastics are a type of sealant that you wrap around your pipe for insulation and protection against abrasions.
Abrasion-resistant wraps protect against scratches in the coating caused by ground shifting or settling. If the pipe is being installed by horizontal directional drilling (HDD), you must protect against abrasion with nonshielding wrap.
If you have rocky soil, rock shield is especially important to prevent holidays upon backfilling. Check with an engineer to find out if wraps are recommended based on your soil composition.
Get the correct corrosion protection for your pipeline
For each of these corrosion prevention methods, specific recommendations depend on your pipeline’s size, metal, soil composition and other factors.
Border States has Application Engineers and Account Managers who can help you determine what you need to meet PHMSA requirements and protect your pipeline. Get in touch with your local Border States branch to find an expert near you.