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Vol.57, No. 3, June 2004, pp. 277-281
TP 1893
1. INTRODUCTION
Dissolved CO2
in water or aqueous solution causes
severe corrosion of pipeline steel and process
equipments used in the extraction, production and
transportation of oil and gas in the petroleum industry.
Many variables are associated with the CO2
corrosion
process such as pH, temperature, pressure and surface
films.1 Present study focuses on the role of CO2
in
both anodic and cathodic reaction for the pipeline
steels. Significant work has been reported in this
direction by de Waard and Milliams.1,2 However,
very few studies have been carried out on the long
term corrosion behaviour of pipeline steel in CO2
environment. Typical laboratory tests carried out
continuously for 48 hours under static condition in
CO2
containing solution at pH 4.5 and 5.8. The
formation of surface films, mainly of FeCO3, and
their influence on the corrosion rate has significant
role in the CO2
aqueous solutions.2,3 Iron carbonate
(FeCO3) formation is temperature dependent and
important in the formation of protective layers over
the metal surface.2,4
1.1 Theoretical Background of CO2Corrosion
Aqueous CO2
corrosion of carbon steel is an
electrochemical process involving the anodic and
cathodic evolution of hydrogen.4 The overall reaction
is:
Fe+CO2+H
2O = FeCO
3+ H
2(1)
The electrochemical reactions are often accompanied
by the formation of films of solid FeCO3
(and/or
Fe3O
4), which can be protective or non protective
depending on the condition under which they are
formed. One of the most important individual reactionis the anodic dissolution of iron:
Fe = Fe2+ + 2e- (2)
It is believed that the presence of CO2
increases the
rate of corrosion of mild steel in aqueous solution
by increasing the rate of the hydrogen reaction. The
presence of H2CO
3enables hydrogen evolution at a
higher rate even at pH greater than 5.5 Thus at a
given pH as the partial pressure of CO2
increases
the solubility of CO2
in the solution increases leading
CORROSION BEHAVIOUR OF PIPELINE STEEL IN
CO2 ENVIRONMENT
G.S. Das and A.S. KhannaCorrosion Science and Engineering, Indian Institute of Technology, Bombay, Mumbai-400076
E-mail : [email protected]
(Received 5 October 2003 ; in revised form 7 April 2004)
ABSTRACT
The influence of temperature (30-120oC) on the corrosion behavior of low carbon pipeline steels in the CO2
saturated solutions in the closed autoclave system has been studied. At lower temperatures, the surface films
have an open porous structure and hence the FeCO3
film formed dissolved continuously in the CO2
saturated
solution. Between 60 to 90oC, the FeCO3 film accumulated more in the outer part, which is more porous, less
dense and nonprotective in nature and hence the corrosion rates of samples increase with temperature.
Incontrast, above 90oC, a dense protective FeCO3 film is formed and the corrosion rate decreases significantly
at 120oC.
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to a higher corrosion rate than would be found in a
solution of a strong acid.6
2. EXPERIMENTAL METHODS
The materials used for the experiment were supplied
by ONGC Panvel and Juhu Helibase (Pipeline Group)
Bombay, India. The chemical composition of alloys,
as obtained by inductively coupled plasma and atomic
emission spectroscopy (ICP-AES) technique, are
shown in Table 1.
The as received materials were cut into the
rectangular specimens of dimension 15X12X3.5 mm
and 12X10X2 mm with a hole of 1.5 mm diameter
drilled near the top edge of each sample to facilitate
suspension of the sample inside of an autoclave of
the capacity of 2.2 liters. All faces of the samples
were initially coarse ground on SiC belt grinder
machine then consequently machine polished in the
successive grades of emery papers up to 600 grit.
The polished samples were washed and subsequently
cleaned in acetone. Experiments were carried out at
four temperatures (30, 60, 90 and 120oC) and at
pressures ranging from 50 to 300 PSI under static
condition in a multiphase dynamic loop machine.
Initial weight of the samples were measured and
then kept inside of the autoclave for 48 hours
continuous test. Initially the vessel was deaerated by
using a vacuum pump and purging argon continuously
for 1-2 hour for removing the oxygen impurity. Then
deaerated solution was poured into the vessel. Thetemperature was raised to the testing condition then
Fig. 1 : Corrosion rate of API grade steels at 30oC Fig. 2 : Corrosion rate of API grade steels at 60oC
Fig. 3 : Corrosion rate of API grade steels at 90oC Fig. 4 : Corrosion rate of API grade steels at 120oC
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DAS AND KHANNA : CORROSION BEHAVIOUR OF PIPELINE STEEL IN CO2
ENVIRONMENT
Table 1CHEMICAL COMPOSITION OF THE ALLOYS USED (IN WT%)
GRADE C Mn Si S P Cr Mo
API X-52 0.20 1.23 0.47 0.12 0.17 0.065 -
API X-56 0.16 1.19 0.19 0.22 0.29 0.047 -
L-80 0.22 1.38 0.22 0.21 0.28 0.013 -
API X-60 0.10 0.74 0.014 0.20 0.26 0.067 0.06
Fig. 5 : ESEM micrographs showing surface morphology of (a) API X-52, (b) API X-56 exposed at 90oC and 300 PSI and
(c) API X-52 and (d) API X-56 exposed at 120oC and 300 PSI
CO2
and argon were charged to maintain the pressure
and observed from the digital display unit (DDU).
Each experiment was conducted using the same
procedures for a total period of 48 hours continuouslywith four samples and corrosion rates were measured
in mils per year (mpy). In order to analyze corrosion
products X-ray diffraction (XRD) and environmental
scanning electron microscope (ESEM) were used in
this study.
3. RESULTS AND DISCUSSION
Corrosion rates of samples as a function of pressure
at different temperatures are shown in Figs. 1 to 4.
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At low temperature, corrosion rate of samples slowly
increases due to continuous dissolution of Fe2+ion
in the solution as a result of formation of porous
FeCO3, which is not protective in nature, however
as the temperature increases from 30 to 60oC, the
FeCO3
film becomes less porous, more adherent to
the metal surface and protective in nature and hence
the corrosion rate decreases. Beyond 60oC, the
corrosion rate increases and it is higher at 90 0C due
to accumulation of more porous inner FeCO3
film
on the metal surface which initiates formation of
cracks and finally spallation of FeCO3
film. The
corrosion rates of all the samples are higher at 90oC
as shown in Fig.3. In all the cases, corrosion rate
of the pipeline steel increases as the partial pressureof CO
2increases due to local depletion of HCO
3-,
which is favoring the cathodic reaction. Crolet and
co-workers6 have reported that FeCO3
can precipitate
on the steel surface with higher rate of dissolution
of Fe2+ ion and the additional HCO3- anions produced
by the cathodic reduction of CO2
It has been also
reported that FeCO3
precipitation is temperature
dependent and for its precipitation super saturation
with the Fe2+ ion is required which is 5-10 times
higher than the thermodynamically calculated values
of solubility.7-9 The surface morphology of API X-
52 and API X-56 as shown in Fig. 5 indicates
cracking and spallation of FeCO3
filmat 90oC and
300 PSI. However, at 120oC and 300 PSI, the FeCO3
film is showing protective nature and good adherence
on the metal surface as shown in Fig. 6. Similarly
API X-60 and L-80 grade steels at 90oC and 300
PSI indicate crack formation and less adherence of
the protective film with the base metal and thus
corrosion rates are higher, but at higher temperaturethe oxide layer is more protective in nature and
adheres on the metal surface with exception of L-80
grade steel. The phases formed on the metal surface
were obtained by XRD analysis as shown in Fig.7
indicates the formation of FeCO3,
Fe3O
4, and Fe
2O
3
Fig. 6 : ESEM micrographs showing surface morphology of (a) API X-60 (b) L-80 exposed at 90oC and 300 PSI and (c)
API X-60 and (d) L-80 exposed at 120oC and 300 PSI
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DAS AND KHANNA : CORROSION BEHAVIOUR OF PIPELINE STEEL IN CO2
ENVIRONMENT
4. CONCLUSIONS
1. At lower temperature the FeCO3 film getsdissolved continuously and hence the corrosion
rate somewhat increases but at higher
temperature the dense protective layer of FeCO3
film formed on the metal surface which is
adherent and more protective in nature.
2. The Corrosion behaviour of line pipe steel is
related to the formation of FeCO3, which is a
corrosion product in CO2
environment.
3. At high temperature a solid protective film of
FeCO3 formed on the metal surface, which actsas a corrosion barrier against corrosion.
REFERENCES
1. C. de Waard, and Milliams D E, Prediction of carbonic
acid in natural gas pipelines, First International
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2. C. de Waard, Lotz U, and Milliams D E, Predictive
model for CO2 corrosion engineering in wet natural gas
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3. C. de Waard and Lotz U, Prediction of CO2
corrosion
of carbon steelin the Oil and Gas Industry, Institute of
Materials Publisher, UK (1994) pp. 3049.
4. Palacios C A, and Shadley J R, Characteristics of
corrosion scales on steel in a CO2-saturated NaCl brine.
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5. C. de Waard and Milliams D E, Carbonic acid corrosion
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Electrochemical properties of iron dissolution in the
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containing carbon dioxide. Corrosion 42 (1986), pp.
7178.
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formation. Mats. Perf. 28 (1989), pp. 4650.
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Corrosion91, NACE, USA, paper 268 1991.
Fig.7 : XRD Patterns of all the four samples exposed at 120oC and 300 PSI