Pipeline Corrosion
Prof (Dr) A S Khanna
IIT Bombay(Retd.), and Chairman,
The Society for Surface Protective Coatings - India

Pipelines are important constituent of our day to day life. They control our daily life cycle to a greater extent. The very basic requirement of life, the water, reaches our home through pipelines. The essential ingredients of our life such as petrol, diesel, and many other important hydrocarbons reach us through pipelines. Natural gas, which is used in running several power plants and fertilizers plants, is transported through pipelines. Cooling water for condenser of a power plant requires a robust pipeline to transport sea water at a very high volume. Sewage disposal from residential buildings and industrial plants is also carried out effectively using a very strong network of pipelines. We cannot think of a system without a pipeline link to take care of our essential requirements at home. Industry transportation systems such as ships and commercial vehicles also use complicated pipeline network to supply fuel as well as cooling water to run the engine. All piping systems have one common problem of corrosion. The author, in this article, has strived to unfold the corrosion related problems of the pipeline systems restricting the scope to transportation of crude, gas, hydrocarbons, and water.

Abstract: Transportation of fluids by pipelines has become an essential requirement for large number of products. However water, crude, and natural gas are the most important one. Water pipelines are both underground and above ground, while pipelines for crude and gas transportation are mainly underground. One of the reasons for the same is the safety. Earth acts as the first defense of safety. Corrosion is a serious problem. Internal corrosion of pipelines depends upon the type of fluid, while external corrosion is dependent upon the climatic conditions. For underground pipelines, the internal corrosion is not a very serious problem; and is usually tackled by injection of inhibitors. However, external corrosion is a real threat which is controlled by a combination of coatings and cathodic protection.

Underground Cross Country Pipelines (UCCP): There is a huge network of underground pipelines across the globe. As a matter of fact, the length and volume of UCCP decide the economy of that country. Larger the pipeline length, more extensively it is spread in that country to reach large number of household and industrial establishments. From the very basic point of view, UCCP differs from above ground pipelines in terms of the safety aspects as major use of UCCP lines is made to transport crude and gas which are inflammable and can catch fire, if released due to some accidental leak or damage. That is why the first and prime safety factor considered for UCCP is the earth itself, which acts as the first defense to contain fire or release of the inflammable material to the environment. Usually, the pipelines are kept 6 feet below the earth level and nicely protected by gravel and mud to insulate them from outside environment. Different countries have different lengths and capacity of these pipelines spread across various land terrain varying in softness, hardness, sandy, rocky, or full of minerals or moisture. These terrains can be undersea, river beds, brackish water, etc. Thus the first step in laying underground pipelines is the selection of laying route as per the right of way permissions from several agencies in that state or country.

There are more than 6500 offshore oil and gas installations in some 53 countries around the world. More than 50 percent of these installations are in the US Gulf of Mexico [1]. The next five largest areas in descending order are: Asia, Middle East, North Sea, West Africa coast, and South America [2]. These six regions account for the majority of the existing offshore oil and gas installations.

India has more than 45000 km of UCCP, with more than 50 percent transporting crude and gas and balance the refined product. These pipelines are spread over several states and passing through various cities, villages, towns, and riverbeds. Figure 1 gives a network of various pipelines for crude and gas respectively, spread across the country[3]. This does not include a network of compressed natural gas distribution lines, already working in many big cities such as Mumbai, Delhi, Ahmadabad, Baroda, Chennai, and Bangalore.

Corrosion of UCCP: Though steel has excellent properties in terms of its strength, ease of fabrication, as well as its low cost, it has a very imperious drawback of getting corroded. Thus, steel needs corrosion protection wherever it is used. The corrosion protection of steel is a very important subject and methods like galvanization, coating by paints, use of inhibitor chemicals are the main methods to protect steels from corrosion. A pipeline made of steel is exposed to environment either above the ground or below the earth as underground pipeline, and therefore it corrodes. Many people understand corrosion of steel when exposed to open environment, as in the case of above ground pipelines; but they are unable to imagine why a pipeline corrodes when buried in earth as underground pipeline. The reasons are obvious. The earth or terrain under which a pipeline is buried, varies considerably from place to place. The soil under earth can be soft mud, sand , rocky, soil with lots minerals and moisture, brackish water, or marshy land, or a river bed. Corrosion is one of the major cause of pipeline failure. Data from an American gas pipeline in 2011 showed that about 22 percent failure of pipelines were due to corrosion alone, slightly less than the material failure.

Methods of Corrosion Protection of UCCP: Corrosion protection of pipeline is classified as internal and external corrosion. As per the schematic of a pipeline as shown in Fig. 3, the internal corrosion is due to the fluid flowing in the pipeline, crude or gas or water, while external corrosion is due to the soil corrosion as described in penultimate section. Internal corrosion is usually not a serious problem unless the crude is sour ( H2S level more than 500 ppm) in nature. In case of sweet crude or gas, the internal corrosion is usually taken care by addition of inhibitors into the pipelines. Sometimes, a 50 μm epoxy coating is applied to take care of the drag problems in the pipe. External corrosion problems are considered to be more serious and require high class protective coatings , followed by additional cathodic protection.

Internal Corrosion in UCCP: Corrosion on the internal wall of a natural gas pipeline can occur when the pipe wall is exposed to water and contaminants in the gas, such as H2S, CO2, or chlorides. The and extent of corrosion damage are the function of the concentration of various corrosive constituents, such as CO2, H2S, water/moisture entrapment, pH, within the pipe as well as of the operating conditions of the pipeline [5]. For example, gas velocity and temperature in the pipeline play a significant role in determining the corrosion damage and its location. In other words, a particular gas composition may cause corrosion under some operating conditions but not others. Therefore, it would be difficult to develop a precise definition of the term :corrosive gas" that would be universally applicable under all operating conditions.

Methods of combating Internal Corrosion: Usually internal corrosion problem arises when crude or gas is sour in nature. For sour gas environments, usually a corrosion resistant coating is required. These internal coatings are usually epoxy based coatings and are applied by internal spray of epoxy coatings. In sweet gas (SO2 level lower than 500 ppm ) and crude environments, usually the corrosion resistance is taken care by inhibitor dosing in the downstream pipelines. Several inhibitors are available which are used for inhibiting corrosion. The basic purpose of these inhibitors is creating a thin film of organic material on the inner steel pipe which prevents its direct reaction with the moisture or corrosive gases. Most of the inhibitors used for oil and gas applications are usually organic in nature, for example, acetylene, alcohols, quaternary ammonium salts, aldehydes, amines, etc. [6].

External Corrosion Problems
External corrosion of pipelines is due to a continuously varying environment of different kinds of soils, sand, marshy land, brackish water, riverbeds, and sea organisms and micro-organisms - especially SRB. In addition, the corrosive environment of soils due to moisture, minerals, oxygen, chloride, etc., further enhances corrosion of the pipeline external surface. Coatings are natural choice to create a barrier for the corrosive environment in soils. Over the years, there have been different coating materials and formulations used to protect pipelines. For example, in the 1940s and 1950s, coal tar, wax, and vinyl tape were used; in the 1960s, asphalts were used; and in the 1970s to the present day, several polymer based insulating coatings are being used. Polyethylene (PE) tape and extruded PE jacket material also have been used from early 1950s to the present days. Fusionbond epoxy (FBE) was one of the most successful ones since 1970-s and is equally popular even today.

It is the most important to note that the coatings alone are not a permanent solution for the corrosion protection of pipelines as the coatings deteriorate with time while interacting with environment. Hence, to take care of this degradation in the underground pipelines, an additional safety -shield using cathodic protection is given. A total pipeline protection system, thus includes consideration of steel quality, coating application, surface condition and treatments, design of coating, and cathodic protection system.

Pipeline Coatings: Though there are many types of coatings that have been applied on buried pipelines, the three main coatings commonly used for pipelines are: coal tar, FBE coating, and three-layer PE(3LPE) coatings. It is important to discuss the salient features of these coatings, while extensive information on the same can be found elsewhere.

Coal Tar and Asphalt Coatings: Coal tar enamel (CTE) protective coating systems have been used to protect underground and subsea pipelines from corrosion for decades. Its ease of application, low cost, compatibility with cathodic protection, and proven performance in the field for over 80 years, make it a popular choice of pipeline companies worldwide. Its resistance to water absorption, hydrocarbons, soil chemicals, and bacteria is excellent. However, the most serious drawback of CTE coatings is the emission of carcinogenic vapors during its application, which not only threatens the workers carrying out the application but also pollutes the environment. Asphalt coatings is a bitumen coating, a stable polymeric, aliphatic hydrocarbon, having good resistance to water and chemicals. Asphalt enamel coating, used for external surface of underground pipelines, consists of a primer and enamel coat. This is strengthened by reinforcement and wrapper.

FBE coatings: Out of various organic coatings, epoxies have the strongest resistance to oxygen, moisture, and chlorides, which are important constituents of soil. Further, they are highly insulating with very low conductivity and high dielectric resistance. That is why epoxies are the preferred choice where strong corrosion resistance is the main requirement. There are many ways by which the epoxy coatings can be applied: brush, spray, using liquid epoxies, or electrostatically spraying the fine epoxy powder on a heated pipe, which immediately melts it and fuses instantly. The coating has good cathodic disbondment resistance, hot water resistance, and good flexibility (50 of pipe diameter) at -50oC. FBE coatings are thermosetting compounds, which, once set, cannot be re -melted. The most important requirements of the coating are surface cleanliness, proper heating, and sufficient cure. The first step is the blast cleaning of the pipe to Sa21/2, followed by heating the pipe uniformly using an induction furnace [7]. This is followed by electrostatic spraying of FBE powder, which immediately melts and fuses. The hot coated pipe is quenched immediately. The temperature at the pipe surface usually ranges from 180oC to 210oC. The coating thickness depends upon the pressure of the FBE powder, electrostatic voltage, and conveyor belt speed. A 350-500 μm coating is required from a pipe diameter of 8 to 36 inch.

Three Layer PE Coatings: One of the drawbacks of the thin FBE coated pipe is its damage during transportation and handling. In the mid -1980s and in the beginning of the 1990s, two additional coating systems were discovered. The first was the 3LPE system and the other was the dual -layer FBE system. The purpose of both these systems was to enhance the damage resistance of the single-layer FBE coating, described above. In 3LPE, it was achieved by the application of an extruded PE coating of 1500 to 3000 μm over the FBE primer layer of 100-150 μm. Since it is not possible to directly coat a PE layer over an already-coated FBE layer, an intermediate adhesive coat of polyolefin is made, which adheres primer to the FBE layer through its polar functional groups and to PE by its hydrocarbon groups [8]. The temperatures required to coat an adhesive layer and PE layers are, respectively, 220oC and 238oC, and the two coatings must be applied within a small time interval of 13-25 s (depending upon the pipe surface temperature ). A schematic of 3LPE system is shown in Fig. 4.

Field Joint Coatings: The field joint coatings are the coatings that are used to join individual pipes, coated in the field, just before laying them in the ditch. Such coatings are also called girth weld coatings as these are applied on the weld joint between the two coatings [9]. Since welds are vulnerable areas, prone to be attacked by corrosive species, they need to be coated with special care to enhance their resistance to corrosion while at the same time maintaining their compatibility with the main pipeline coating. Though, there are many coatings that can be applied at field joints, such as liquid epoxy, elastomeric polyurethane, coldapplied tapes, and viscoelastic coatings[10], the most common and acceptable field joint coating is the heat shrink sleeve coating. These are basically the rubberized coatings that, on application and after heating shrink, hold the pipe. The first step is full cleaning of the girth weld area, followed by application of a primer epoxy coat. This is followed by the application of heat sleeve which requires uniform heating to make an excellent uniform coating on the welded area [9].

Above Ground Pipelines: The above ground pipelines are protected by many conventional paint systems. Since above ground pipelines are subjected to pilferage, are affected by changing weather conditions, it is necessary to design a coating system that has long durability, good mechanical properties such as high bond strength, abrasion and scratch resistance and also good impact resistance. Next most important property is weathering resistance. Thus the most common coating system that can work and give a life of at least 8-10 years can be a blast clean surface with an organic zinc rich primer or inorganic zinc primer, followed by an epoxy coat of atleast 150 μm, followed by an aliphatic polyurethane coating of 50 μm. If the pipeline is onshore, the epoxy coating thickness can be increased to 250 μm or MIO-epoxy coat can be used. In offshore environment, the epoxy system can be replaced by a single coat of 500 μm of polyester glassflake which can have long durability, abrasion, and corrosion resistance and also UV weathering resistance.

Conclusions: Pipeline corrosion is of great concern. Corrosion of pipeline takes place whether it is underground pipeline or above ground pipelines. Corrosion process depends upon the type of fluid transported by pipelines. Underground pipelines carry inflammable fluid crude and gas, and hence the pipelines are protected with all care where safety is one of the biggest concern. Internal corrosion of underground pipelines is not a serious problem for sweet crude or gas; and is usually tackled by injection of corrosion inhibitors. External corrosion of underground pipeline is a serious threat and therefore the pipelines are protected with utmost care using coatings and cathodic protection. Above ground pipelines are protected by conventional paint systems.

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