Page 192 - CW E-Magazine (12-12-2023)

Special Report

oil pipeline indicating the halo effect stream. Oxygen is more soluble in oil Erosion corrosion
(12, 13). than in water. Consequently, a slight Erosion corrosion increases the
ingress of oxygen will not necessarily corrosion reaction rate by continuously
There are a few key ways to miti- result in a high corrosion rate because removing the passive layer of corro-
gate MIC. These include: the oxygen in the water will be low. sion products from the pipe wall. The
1. If a biofi lm is formed inside the If the water that entered the pipeline passive layer is a thin fi lm of corrosion
pipeline, pigging can scrape it out. were saturated with oxygen at 8-ppm products that stabilizes the corrosion
However, it cannot remove the (the saturation concentration of oxygen reaction and slows it down. As a result
entire bacterial colonisation, and the in water at ambient temperature), the of the turbulence and high shear stress
bacteria may grow and develop the concentration in the 0.5% water phase in the line, this passive layer can be
biofi lm again. in a crude oil pipeline would be below removed, causing the corrosion rate to
2. Biocides are most effective to ter- 40-ppb. The appearance of the oxygen increase. The mechanism of erosion cor-
minate MIC. They are chemical corrosion damage is pitting or general rosion is depicted in Figure-10. Erosion
compounds that kill living micro- corrosion as given in Figure-8 showing corrosion is always experienced with
organisms, and are of two types – the appearance of oxygen pitting found a high turbulence fl ow regime with a
oxidizing biocides and non-oxidiz- in internal crude oil pipelines. Figure-9 signifi cantly higher corrosion rate and
ing biocides. shows magnified appearance of depends on fl uid fl ow rate and the density
3. It is also essential to have a con- oxygen pitting found in internal crude and morphology of solids present in the
tinuous protective coating that can oil pipeline in longitudinal and axial fl uid. High velocities and the presence
tolerate aggressive chemicals such cross-sections(9, 10, and 11). of abrasive suspended material and the
as sulphuric acid and acetic acid corrodents in drilling and produced
produced by some bacteria as part The use of gas blanketing, vacuum fl uids transported further through
of the corrosion attack. deaeration, and oxygen scavengers can pipelines contribute to this destructive
4. The most recent scheme for miti- help mitigate oxygen corrosion in pipe- process especially in the bends of pipe-
gating MIC uses different types of lines. lines as shown in Figure-11. The effect
bacteria against the primary bacteria
responsible for MIC in a specifi c
industrial environment.

Oxygen corrosion
Oxygen can enter a pipeline through
seals on the suction side of pumps and
storage tanks if the oil is exposed to the
air (for example, fl oating roof tanks).
Generally, the concentration of oxygen
in the crude oil will be low because
there is suffi cient dissolved gas in the Fig. 8: Internal corrosion of a crude oil pipeline Fig. 9: Appearance of oxygen pitting found in
due to of presence of Oxygen.
Internal crude oil pipeline
oil at equilibrium to prevent a high
concentration of oxygen dissolving in
the crude oil. This is because CO and
2
H S are more soluble in crude oil and
2
water than oxygen. If there is any trace
of H S present, this will react with the
2
oxygen.
Corrosion by oxygen will typi-
cally occur at the inlet of the pipeline
and reduce further downstream as the
oxygen concentration is depleted. The
ingress of oxygen will be low, and the Fig. 10: Erosion Corrosion Mechanism in the internal surface of crude oil pipeline
oxygen will be dispersed into the oil Corrosion resistance depends on the development of a surface fi lm ( passivity).

192 Chemical Weekly December 12, 2023

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