Page 191 - CW E-Magazine (12-12-2023)

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140°C under dry conditions, while a Usually, crude oil is separated into lized general thinning and pitting cor- of process parameters such as pH, tem- line carrying crude oil depicting forma-
normal FBE coating is used from -30°C three categories based on the API gra- rosion are possible as well. Figure-2 perature and the pressure of the pipeline. tion of heavy deposit.
to 80°C. vity scale. They are light crude, medium shows the appearance of a typical
crude and heavy crude. Specifi c gravity ‘Mesa-corrosion’ at 6 o’clock position in H S or sour corrosion If unfamiliar with MIC, corrosion
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In the case of a dual-layer polyvinyl and API gravity evolve in opposite a crude oil pipeline. Another example H S in crude oil comes from the problems may be misdiagnosed as con-
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chloride (PVC)-PE system, the PVC directions. Therefore, the smaller the of Mesa attack in internal surface of a reservoir or wells. The deterioration ventional chloride-induced corrosion
layer can fail due to its brittle nature API number, the heavier the fl uid. crude oil pipeline is shown in Figure-3. of metal due to contact with H S and or CO corrosion. One prominent
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as it loses its plasticizer due to aging. moisture is called sour corrosion, which Fig. 5: Cross section of a carbon steel pipeline indicator of MIC is a higher rate of
Disbondment from the steel surface can There are several causes of corro- is the most damaging to drill pipes. from petroleum process stream showing a attack than one would generally expect.
start early in such cases due to the loss sion in crude oil pipelines(2, 3). Although H S is not corrosive by itself, it large hydrogen blister MIC is affected presence or activity (or
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of adhesive strength. becomes a severely corrosive agent in ing a large hydrogen blister due to pres- both) of microorganisms in biofi lms on
CO or sweet corrosion the presence of water, leading to pipe- ence of H S in the process stream. In the corroding material’s surface. Many
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Even the PE with butyl rubber dual- This is a type of corrosion that line embrittlement (loss of toughness). fact, the hydrogen blister took place on materials, including most metals and
layer coating can fail due to the loss of typically occurs in pipelines when the surface where H S penetrated. some non-metals, can be degraded in
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adhesive in the carrier fi lm, and dis- dissolved CO gases in the crude oil When dissolved in water, H S this manner. MIC can result from the
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bondment can begin gradually. react with water to form carbonic acid. Fig. 2: The appearance of a typical ‘Mesa- tends to form a weak acid. As such, it There are some key ways to mini- activities of the microorganisms,
This eventually corrodes the internal corrosion’ at 6 o’clock position in a crude oil is a source of hydrogen ions and is cor- mize H S corrosion damage in pipe- including bacteria, Archaea, and fungi
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Field coating vs. factory-produced surface of the pipe. CO corrosion pipeline. rosive. The corrosion products are iron lines. They include: in biofi lms, or the local environment
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coating increases in the presence of both oxygen sulphides and hydrogen. Iron sulphides 1. Use normalized electric resistance directly connected with the corrod-
Factory coatings are produced and organic acids, which can dissolve form a scale that, at low temperature, welding (ERW) line pipe that ing material. Depending on the envi-
under standard conditions and automatic iron carbonate scale and prevent further can act as a barrier to slow corrosion. meets the industrial steel selection ronment, these microbes may include
controls to ensure a high degree of scaling. When CO is present, there are The forms of sour corrosion are uni- code requirements. metal-oxidizing bacteria, sulphate-
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reliability. However, if a coating has various types of corrosion mechanisms form, pitting and stepwise cracking(8, 9). 2. Conduct periodic pigging of pipe- reducing bacteria (SRB), prokaryotes
to be applied on site, the surface pre- that could take place. The main ones are line segments to remove liquids, (SRP), acid-producing bacteria (APB)
paration, coating, curing and other steps pitting corrosion, uniform corrosion, The appearance of corrosion dam- solids and debris. and metal-reducing bacteria (MRB).
involved need skilled manpower, ins- wormhole attack, ringworm corrosion, age from sour corrosion is usually 3. Periodically apply a batch cor-
pection, equipment and site super- erosion corrosion and mesa attack(6). general thinning. However, localized rosion inhibitor to provide a pro- Damage is often characterized by
vision. Some amount of coating for joints A typical sweet corrosion/CO pitting Fig. 3: Mesa attack in internal surface of a corrosion or localized under-deposit tective barrier on the inside surface cup-shaped pits within pits (halo effect)
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Crude Oil pipeline
and repair work may have to be done at attack in a crude oil pipeline is depicted attack can occur when air or oxygen is of the pipe(10, 11). in carbon steel or sub-surface cavi-
the worksite. The reliability of the sys- in Figure-1. CO corrosion damage related to present. ties in stainless steel. However, these
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tem in such cases would depend on the fl uid fl ow is primarily associated with Microbiologically induced corrosion pits are often indistinguishable from
quality of work done at the site, which the separation and accumulation of a Depending on the type of corrosion (MIC) under-deposit corrosion in carbon steel
acts as the weakest link in the system. water phase. Therefore, longitudinal mechanism, it can be found in various MIC in itself is not a unique corro- and chloride pitting in stainless steel.
The storage of coating material on site, corrosion grooving (6 o’clock corro- locations in the pipelines. A typical fail- sion mechanism; instead, it produces Therefore, pit morphology alone may
under adverse conditions, is also very sion) is the primary concern in stratifi ed ure (sour corrosion) of a pipeline carry- conditions that increase the susceptibi- not be a reliable indicator of the cause
challenging. Taking care of environ- gas/liquid fl ow wet gas pipelines(6, 7). ing crude oil containing H S is shown lity of materials to corrosion process- of MIC corrosion.
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mental health & safety (EH&S) con- in Figure-4. Similarly, Figure-5 shows es such as pitting, embrittlement and
cerns on site needs adequate planning. CO corrosion predominantly occurs cross section of a carbon steel pipeline under deposit corrosion (UDC). Figure-6 Figure-7 shows Pitting corro-
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at surfaces freely exposed to the from petroleum process stream show- shows MIC induced corrosion in pipe- sion found in an internal sour crude
Oil pipelines corrosive environment, for example
shielded areas such as narrow crevice
Fig. 1: Pitting due to sweet corrosion/CO
Crude oils are a highly complex attack in a crude oil pipeline 2 arcs are not attacked even though they
combination of hydrocarbons, hetero- may be wetted by the fl uid. Corrosion
cyclic compounds of nitrogen, oxygen, CO corrosion of steels usually may occur along the bottom surface of
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sulphur, organo-metallic compounds, occurs as shallow pits and grooves, which a pipe if there is a separate water phase
inorganic sediment, and water. More sometimes overlap. While some sur- or along the top surface of a pipe if con-
than 600 different hydrocarbons have faces may remain unaffected, the cor- densation in wet gas systems occurs.
been positively identifi ed in crude oil, rosion is often remarkably uniform in
and possibly thousands of compounds the attacked areas, leading to so-called CO corrosion damage is mitigated Fig. 7: Pitting corrosion found in an internal
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occur, many of which will never be “mesa-corrosion,” a characteristic of by use of effective pigging and chemi- Fig. 4: Oil and Gas pipeline under sour Fig. 6: A pipeline used in an oil & gas industry sour crude oil pipeline indicating the halo
identifi ed. CO corrosion. In carbon steels, loca- cal inhibition programmes and control corrosion attacked by MIC corrosion effect.
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190 Chemical Weekly December 12, 2023 Chemical Weekly December 12, 2023 191

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