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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 : gsdas@met.iitb.ac.in

(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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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

Conference on the Internal and External Protection of

Pipes paper F-1, University of Durham, September 1975.

2. C. de Waard, Lotz U, and Milliams D E, Predictive

model for CO2 corrosion engineering in wet natural gas

pipelines. Corrosion 47 (1991) pp. 976985.

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.

Corrosion 47 (1991) pp. 122127.

5. C. de Waard and Milliams D E, Carbonic acid corrosion

of steel. Corrosion 31 (1975) pp. 177181.

6. Nesic S, Thevenot N, Crolet J L, and Drazic D M,

Electrochemical properties of iron dissolution in the

presence of CO2 Corrosion96 NACE, USA, paper 3,

1996.

7. Ogundele G I, and White W E, Some observations oncorrosion of carbon steel in aqueous environments

containing carbon dioxide. Corrosion 42 (1986), pp.

7178.

8. Videm K, and Dugstad A, Corrosion of carbon steel in

an aqueous carbon dioxide environment. Part 2. Film

formation. Mats. Perf. 28 (1989), pp. 4650.

9. Johnson M L, and Tomson M B, Ferrous carbonate

precipitation kinetics and its impact CO2 corrosion,

Corrosion91, NACE, USA, paper 268 1991.

Fig.7 : XRD Patterns of all the four samples exposed at 120oC and 300 PSI