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Fe - CO2 Corrosion
Rahmad Budi Arman
Muhammad Pribadi
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Internal Corrosion
The internal corrosion of vessels, equipment and piping in
the offshore / onshore facilities depends on internal
process fluids and operating conditions. The process fluid
may vary from corrosive liquid or gaseous hydrocarbon,
corrosive chemicals, produced water and brine. Based on
this, the corrosion occurring under these conditions can be
broadly classified as:
Sweet corrosion ( CO2 present )
 Sour corrosion (H2S or combination of H2S and CO2
present);
Amine corrosion and cracking.
Chloride corrosion.
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CO2 Corrosion
Wet CO2 corrosion is generally referred to in
the oil and gas industry as ‘sweet corrosion’
when H2S is absent. When CO2 is present
in the gas phase, any water in contact with
this gas will dissolve CO2 to a concentration
proportional to the partial pressure PCO2, of
the CO2 in the gas.
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CO2 Corrosion
In the well fluid, CO2 will be in equilibrium
between the three phases, water, oil and
gas. The quantities of CO2 present in each
of these phases are therefore interrelated,
with differences in concentration and activity
coefficient. In multiphase effluents, which
are generally highly turbulent, even if the
CO2 is rarely in perfect equilibrium between
the three phases, the deviations can never
be very large.
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CO2 Corrosion
Hence,
 The CO2 is at its solubility equilibrium in
the water and hydrocarbon phases.
 The CO2 content of the water is
determined by its fugacity in the gas
phase in contact with the water, or by
default, the last gas phase.
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CO2 Corrosion
Based on CO2 concentration proportion,
partial pressure and temperature, the CO2
corrosion rate will vary. The type of
corrosion that takes place in the topside
facilities due to CO2 corrosion is usually in
the form of localized attack (Mesa) pitting
and preferential weld corrosion.
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CO2 Corrosion
The overall reaction by which CO2 corrodes
steel is as follows:
CO2
+ H2O → H2CO3 (Carbonic acid)
Fe + H2CO3 → Fe CO3 + H2
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CO2 Corrosion
Based on this a monogram, which provides
a relationship between corrosion rate of
steel, temperature and CO2 partial pressure
was developed by deWaard and Milliams of
Shell International. This was later modified
to take care of various parameters, such as
high system pressure, velocity, presence of
crude, pH, glycol in the system and
corrosion protective film formation at high
temperatures.
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CO2 Corrosion
The empirical relationship developed
between PCO2 and the corrosion rate is:
Log υ = 5.8 – 1710 + 0.67 log (ƒ CO2)
T
Υ = predicted corrosion rate for CS (mm/yr)
T = Temperature (°K)
ƒ CO2
=
a x PCO2
a
=
Fugacity Coefficient
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CO2 Corrosion
The
use of fugacity, rather than partial
pressure, allows for the non-ideality of the
gas with increasing pressure and
temperature.
Starting with the worst-case corrosion rate
prediction by the deWaard-Milliams
equation, correction factors can be applied
to quantify the influence of environmental
parameters and of corrosion product scales
formed under various conditions.
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CO2 Corrosion
From field studies, it has been found that
carbonic acid can form protective scale above
60°C.
The correction factor Fscale for the basic
corrosion rate equation can be calculated:
Log Fscale = 2400 – 0.6 Log (ƒ CO2) – 6.7
T
With a maximum value of Fscale of 1.
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CO2 Corrosion
From field studies, it has been found that
carbonic acid can form protective scale above
60°C.
The correction factor Fscale for the basic
corrosion rate equation can be calculated:
Log Fscale = 2400 – 0.6 Log (ƒ CO2) – 6.7
T
With a maximum value of Fscale of 1.
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CO2 Corrosion
Similarly, pH level has an influence on the
corrosion rate.
With increase in pH the
corrosion rate decreases and this is particularly
significant in cases where bicarbonate is
present in produced water.
Wet CO2 corrosion may be mitigated by the
use of inhibitors. The effect of inhibitors varies
depending on the type and operating
conditions. Usually inhibitors can provide about
90- 95% efficiency in reducing the corrosion
rate.
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CO2 Corrosion
Taking into account all the above factors the
Corrected Corrosion Rate (CCR) is as follows:
CCR=
UCR x Fc x Ft x Fi x FPH
mm/yr
UCR=
Ft =
Fc =
Fi =
FPH =
Uninhibited corrosion rate
temperature factor (scaling)
Water Condensation factor
Inhibitor factor
(0.05 for 95% inhibitor efficiency)
pH factor
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CO2 Corrosion
This approach to carbon steel corrosion due
to wet CO2 has been incorporated into
software known as ‘The Electronic Corrosion
Engineer’ and this program has been used
for corrosion rate calculations
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