Steel pipe and pipeline facilities must be protected from the effects of external and internal corrosion. Nonferrous piping materials, such as fiberglass, PVC, and polypropylene, do not undergo the same corrosive effects and require little attention. Industry codes and standards require that pipelines, and facilities be protected from the effects of corrosion.
Internal corrosion may be caused by the presence of CO2, water, H2S, chlorides (salt water), bacteria, completion fluids, or other substances in the produced hydrocarbon. When CO2 or H2S is mixed with oxygen and/or water, acids are formed that attack and destroy the steel. When CO2 or H2S is mixed with oxygen and/or saltwater, extreme corrosion occurs. Certain types of bacteria often found in producing formations can also attack and destroy the steel. Any of the internal corrosives, separately or in combination, can cause leaks and severe blowouts.
The potential corrosives usually can be identified from a chemical analysis of the produced hydrocarbons. In instances where high concentrations of CO2, H2S, or other highly corrosive chemicals are present, additional pipe wall thickness may be added in the pipe design to allow for the potential corrosive effects. This is not normally recommended, as corrosion could be localized and the rate difficult to predict. In most cases, the removal of oxygen and water from the fluid is sufficient to combat potential corrosion. Where this is not practical, corrosion-inhibition chemicals, internal coatings, and corrosion-resistant materials are used.
Internal corrosion also can be caused by erosion or wear. Excessively high velocities in liquid and multiphase fluid systems can erode or wear the internal pipe wall as well as fittings and valves. The conditions that cause mechanical erosion can be mitigated through proper pipe sizing and design.
The corrosive effects of the hydrocarbon fluid may change over time as the chemistry of the produced fluid changes or as bacteria develop that were not present earlier. Where unknown corrosives develop after operations have commenced, chemical treatment may be the best solution.
External corrosion affects buried pipe and above-ground pipe. Buried pipe is subjected to cathodic actions and galvanic actions. Above-ground pipe is subjected to atmospheric corrosion and galvanic actions.
Cathodic actions occur when steel pipe is buried below ground. Ferric and other materials, such as soils, have small electrical potentials. In the natural process of converting metals back to their elemental or native state, electrolytic conduction takes place. Unprotected, the steel pipe becomes an anode (positively charged) and transfers material, by means of electrons, to the cathode (negatively charged) material, which is the soil or surrounding medium. The pipe metal literally flows away by means of the electric current between the anode and cathode. Water contained in the soils and other media serves as the electrolyte to help promote the electron transfer.
To counteract cathodic actions, pipe is coated with anticorrosive materials and cathodic protection systems are placed on the pipeline. The coating must provide an effective “insulation” against the environment but must be tough enough to withstand the operating temperatures, be resistant to the soil, and withstand physical handling.
There are a number of coating systems that are economical and commercially available, which include:
- Extruded systems (polyethylene or polypropylene over asphalt mastic or butyl adhesives)
- Tape coats (polyethylene, polyvinyl, or coal tar over butylmastic adhesive)
- Fusion bonded epoxy (thin film)
- Coal-tar epoxy
Fusion bonded epoxy (FBE) coatings are the most popular coating systems because they:
- Are excellent insulators
- Are hydrocarbon, acid, and alkali resistant
- Are unaffected by temperature
- Do not require a primer
- Can be applied over finished welds (field joint)
Tape-coating systems and coal-tar enamel systems are becoming less and less popular. Tape coating is difficult to apply and is especially difficult to use on large-diameter pipe. A number of tape-coated systems have experienced failures over relatively short spans of time because of improper application. Coal-tar epoxy is becoming less desirable because of some health and environmental concerns caused during application.
In addition to the anticorrosion pipe-coating systems, cathodic protection systems are added to the pipeline to protect the pipe where breaks in the coating system occur. The cathodic protection system employs either an impressed current or sacrificial anode to protect the underground pipe. The cathodic protection system reverses the electrolytic conduction process and uses an impressed electrical current or another metal object (sacrificial anode) to make the pipe a cathode. In simplified terms, the impressed current reverses the natural flow of electrons from the pipe to the surrounding medium to prevent the loss of metal ions. The sacrificial anode made of a higher potential metal, such as magnesium, is in contact with the pipe and the surrounding medium. The anode gives up its electrons (metal) in place of the steel pipe.
Sacrificial-anode systems are simpler and less expensive than impressed current systems. Onshore pipelines generally use magnesium, and offshore pipelines use zinc or aluminum anodes. Impressed current systems are much more complex and require external power sources and AC/DC power inverters or rectifiers to provide the current to the pipe.
The design of cathodic protection systems requires specialized training and can be very complicated. Detailed soil surveys must be conducted to determine the electrical potential and resistivity of the soils or surrounding medium, pipe-to-soil potentials, and a number of other criteria. System design should be done by a cathodic protection expert.
Another important facet of the anticorrosion system is prevention of galvanic corrosion. Galvanic corrosion is caused by the interface of dissimilar metals with different electrolytic potentials. The dissimilar metals will gain or lose electrons from or to each other resulting in one of the metals effectively flowing away and losing material. Steel pipe that undergoes abrupt changes in the medium will behave somewhat as dissimilar metals and cause galvanic actions. Pipe transitioning from below ground to above ground may experience galvanic-like corrosion. Mating materials such as carbon steel with stainless steel will cause the carbon steel to corrode.
Insulating flanges or joints can be used to counteract the effects of galvanic actions. Efforts should be made to avoid the interface of the dissimilar materials in the system design.
The effects of atmospheric corrosion are readily apparent. Bare steel will corrode rapidly when exposed to:
- Chemicals (pollution)
- Air (oxygen)
Piping and equipment exposed daily to the elements must be protected with anticorrosion coatings. Good paint coating systems, such as epoxies, and regular maintenance will normally provide adequate protection to the above-ground facilities.
Facilities exposed to severe service, such as offshore, may require more-extensive protection systems.