Page 193 - CW E-Magazine (12-12-2023)

Special Report

or crevices in the metal and the fl uid 2. Use sealants to avoid moisture
getting stagnant in the gap. This is penetrating the crevice;
caused by concentration differences of 3. Design to remove stagnant areas
corrodents over a metal surface. Electro- where moisture and deposits can
chemical potential differences result in collect;
selective crevice or pitting corrosion 4. Increase maintenance to include
attacks. Oxygen dissolved in drilling removal of deposits by high-pres-
Fig. 11: Impingement (Erosion Corrosion) of fluid promotes crevice and pitting sure washing(9, 10, and 11).
Elbow in the bend of a Crude Oil Pipeline attacks of metal in the shielded areas of
of fl uid fl ow is an important factor for the drill string and is the common cause Stress corrosion cracking (SCC)
corrosion and thus high corrosion rates of washouts and destruction under SCC is a form of localised corro-
are often observed in transfer lines. rubber pipe protectors. sion, which produces cracks in metals
Some studies have correlated a linear by simultaneous action of a corrodent
relationship with fl ow velocity or wall CC is a form of localized corrosion and tensile stress. It propagates over
shear stress, while others have sug- that occurs at, or immediately adjacent a range of velocities from 10- to 10
3
gested that there is a threshold value for to, discrete sites where free access to mm/h, depending upon the combina-
onset of fl ow induced corrosion. the bulk environment is restricted. This tion of alloy and environment involved.
form of corrosion, normally, can be
High velocity fl ow can also remove identifi ed visually and is recognized by The impact of SCC on a material
the iron sulphide (FeS) layer and the pitting or etching near or adjacent seems to fall between dry cracking and
accelerate the corrosion of steel. API to locations of restricted fl ow. Com- the fatigue threshold of that material.
RP 939-C recommends a velocity of mon sites for CC are under loose-fi tting SCC in an oil and gas pipeline is a type
less than 60m/s to maintain any FeS washers, fl anges, or gaskets. However, of environmentally-associated crack-
scale. It can be noted that wall shear this form of corrosion is not limited ing. This is because the crack is caused
stress – the mechanical force produced to crevices formed by mated surfaces by various factors combined with the
on the metal surface by the fl owing of metal assemblies. For example, environment surrounding the pipe. The
media – is a more correlative factor for Figure-12 shows an oil & gas pipeline most obvious identifying characteristic
other types of corrosion phenomena. of SCC is high pH of the surrounding
environment. Appearance of patches
This form of corrosion is often over- or colonies of parallel cracks on the
looked or recognized as being caused surface of the pipe indicates SCC of
by wear(9, 10, and 11). the pipeline(9, 10, and 11). Figure-13
shows an oil and gas pipeline after
There are a few key methods for being attacked by SCC.
mitigating erosion corrosion. They in-
clude:
1. Increasing the pipe diameter to
reduce velocity, streamlining bends
to reduce impingement, and using Fig. 12: Oil and Gas pipeline under crevice
replaceable impingement baffl es; corrosion – crevices formed due to improper
and fi tting of fl anged joints
2. Using more corrosion-resistant under CC, with crevices formed due to
alloys or altering the process improper fi tting of fl anged joints. CC
environment to reduce corrosivity, can also occur under scale and surface
such as by deaeration, condensate deposits (termed ‘under-deposit’ corro- Fig. 13: Oil & Gas pipeline after being
attacked by stress corrosion cracking
injection, or adding inhibitors, as sion).
applicable. There are a few key ways to miti-
There are a few key ways to miti- gate SCC. They include:
Crevice corrosion (CC) gate CC. They include: 1. Lower the stress below threshold
CC is usually localised corrosion 1. Use alternative joining techniques value by annealing or thickening
taking place in the narrow clearances such as welding or brazing; the pipeline section under stress, or

Chemical Weekly December 12, 2023 193

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