Page 186 - CW E-Magazine (1-4-2025)

Special Report

When the CO /H S ratio is bet- assessment can be performed for Once the key corrosion issues are
2
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ween 20 to 500, the corrosion tends a new pipeline, when updating an identified and quantified, mitigation
to be uniform. The uniform corro- existing pipeline, or when process procedures can be designed. Regular
sion rate can be lower than for sweet fluid and/or contaminants have been monitoring and documentation for
hydrocarbons because of the co- changed. Next, the hazards are eval- the identified risks need to be incor-
precipitation of iron carbonate and uated for their likelihood and seve- porated into the mitigation plan.
non-continuous iron sulphide scales. rity of causing a leak or rupture. Fac-
tors to consider include corrosion, Mitigation and prevention strategies
cracking caused by H S external damage, operational issues,
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HIC in carbon steel pipelines is and environmental conditions. Cathodic protection (CP)
caused by the dissociation of H S CP is an electrochemical process
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to produce atomic hydrogen that The key aspects of therisk assess- that slows or stops corrosion cur-
collects at defects in the steel. The ment include: rents by applying a direct current to
uptake of hydrogen and formation Assessment of relative amounts of the metal. CP systems control corro-

of molecular hydrogen can then contaminants such as sulphides, sion through either sacrificial anodes
potentially cause blister formation or chlorides, CO , and water. or impressed current(7, 8):
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crack initiation. Crack propagation However, modeling the corrosion
is not extremely rapid but can propa- rate may not be reliable. With galvanic protection, apro-
gate in due course. Fig. 10 shows Determination of potential varia- tective anode(such as a zinc or mag-

hydrogen induced cracking initiated tion in these contaminants with nesium rod/coating) sacrificially
and propagated in a steel pipeline. varying service conditions. corrodes to protect the steel until
Assessing and measuring tem- it can no longer provide protection
perature and pressure of fluids at (Fig. 11). An impressed current pro-
various pipeline locations. Pre- tection system (ICPS) consists of
sence of condensate and water anodes that are connected to a power
at these locations, and the extent source (DC rectifier) that provides
of liquid water should be deter- much longer protection (Fig. 12).
mined.
Assess flow conditions between Corrosion inhibitors
fluids resulting from variations in Corrosion inhibitor scan be used
Fig. 10: Hydrogen induced cracking initiated wall shear stress. either continuously or in batches.
and propagated in a steel pipeline. Determine metallurgy of pipe- However, an accurate chemical dos-
line. ing and an understanding of the requi-
SSC involves wet H S, with the
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hydrogen atoms diffusing into the
lattice of the steel. This reduces
the ability of the steel to withstand
deformation from either residual or
applied stress. SSC can initiate and
propagate to failure much faster than
HIC. Additionally, it often initiates
at weld heat-affected zones.

Integrity Management Analysis
and Strategy(4,5 and 6)
To minimize the potential of pipe-
line failures, one can use a quantita-
tive risk assessment methodology.

First, a detailed assessment of
potential hazards is identified. Such an Fig. 11: Cathodic protection of buried pipeline with magnesium (sacrificial) anode.

186 Chemical Weekly April 1, 2025

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