A key cause of pipe failure in steel pipelines is corrosion. Without proper corrosion protection, every steel pipe will eventually deteriorate and fail, resulting in leaks.
Corrosion is the natural process in which materials made from metal deteriorate through an electrochemical reaction known as oxidation (more commonly called rusting). Corrosion results in metal loss in the pipe and, if left unmitigated, can result in reduction of the wall thickness of the pipe and weak spots known as pitting. The loss of material from corrosion can eventually result in leakage or a crack, split, or rupture of the pipeline unless the corrosion is repaired, the affected pipe section is replaced, or the operating pressure of the pipeline is reduced. Corrosion may occur inside the pipe (internal corrosion) or outside the pipe (external corrosion). According to the U.S. Pipeline & Hazardous Materials Safety Administration, approximately 12% of reported incidents on gas transmission and gas gathering pipelines were caused by internal corrosion while 8% were caused by external corrosion.
Internal corrosion occurs due to environmental conditions on the inside of the pipeline. In most cases, the corrosive materials are contaminants naturally contained within the transported commodity such as hydrogen sulfide, carbon dioxide, other chemicals, or water. To prevent internal corrosion, pipelines require that gas transported meets specific gas quality specifications. In some cases internal pipeline coatings are applied or corrosion inhibitors are injected in the gas stream.
External corrosion also occurs due to environmental conditions and is the natural result of interaction between the exterior surface of the pipe and the soil, air, or water surrounding it. Many highly localized factors can contribute to the aggressiveness and persistence of external corrosion including the presence of stray electrical currents.
Three methods are commonly used to reduce pipe corrosion:
- Corrosion inhibitors are substances that can be added to the gas running through a pipeline to decrease internal corrosion.
- Pipeline coatings and linings protect against corrosion by preventing the bare steel from coming in direct contact with corrosive environmental conditions. These are used to reduce both internal and external corrosion.
- Cathodic protection (CP) is a system that uses direct electrical current to counteract the normal external corrosion that occurs on a metal pipeline due to soil and moisture conditions. This is used to reduce external corrosion.
Corrosion is an ongoing threat to pipe integrity that must be carefully managed. Operators are required by federal and state codes to employ measures to prevent or minimize loss of pipe wall due to corrosion. Electrical test stations, called ETS, are located along pipelines at roughly one-mile intervals. An ETS is a wire accessible above ground that is bonded to the steel pipe and allows the measurement of the voltage differential between the pipe and the soil. This differential is typically measured annually. Measurements outside of tolerances indicate the need to make changes to the cathodic protection system. Where possible, pipelines are examined periodically using smart pigs that can identify the presence of internal or external corrosion. If corrosion is beyond specific tolerances, the pipe must be dug up and repaired or replaced.