1

Effect of Elemental Sulfur on the Corrosion Behavior of X80 Steel under a

Supercritical CO2 Environment

Qingjun Gong1, Yong Xiang2,*, Wenguan Li3, Zhenlin Li4

1 Beijing Key Laboratory of Process Fluid Filtration and Separation, China University of

Petroleum, Beijing 102249, China, 1449611564@qq.com 2 Beijing Key Laboratory of Process Fluid Filtration and Separation, China University of

Petroleum, Beijing 102249, China, xiangy@cup.edu.cn 3 Beijing Key Laboratory of Process Fluid Filtration and Separation, China University of

Petroleum, Beijing 102249, China, 928438113@qq.com 4 Beijing Key Laboratory of Process Fluid Filtration and Separation, China University of

Petroleum, Beijing 102249, China, zhenlinli@263.net

Abstract

The carbon dioxide (CO2) captured from power plants will contain flue gas impurities, such as sulfur

oxides, nitrogenoxide, hydrogen sulfide and oxygen. During the process of transportation, these

impurities can interact with each other, resulting in the formation of elemental sulphur (S) which may

deposit on the inner wall of the pipe and induce localized corrosion. In the tests, S was thawed and

poured onto a polished specimen to stimulate the deposit S, and then the specimen was installed into an

autoclave to be exposed in a supercritical CO2 (SC-CO2) environment. The results showed that S can

lead to uniform and localized corrosion of X80 steel. Scanning electron microscopy, energy dispersive

X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy three-dimensional

profilometry were employed to analyze the morphology and chemical composition of the corrosion

product scales. The corrosion product scales were mainly composed of ferrous carbonate, gamma-ferric

oxide, ferrous sulfate and hydroxyl oxidize iron (FeOOH) (alpha-FeOOH or gamma-FeOOH) in water-

saturated SC-CO2 and SC-CO2-saturated water environment with S for one and seven days. The

introduction of S decreased the uniform corrosion rate while induced severe localized corrosion. The

mechanism of localized corrosion induced by S in the SC-CO2 environment was also discussed.

Keywords: elemental sulfur; deposit; localized corrosion; supercritical CO2

Introduction

In recent years, global climate change issues have received increasing attention from all walks

of life. Carbon capture, utilization and storage (CCUS) technology is considered to be one of

the effective measures to mitigate the global climate change. The captured carbon dioxide can

be used to enhance oil/gas recovery before permanent storage, which can help reduce the total

cost of CCUS. However, the captured CO2 will inevitably contain flue gas impurities, such as

sulfur oxides (SOX), nitrogenoxide (NOx), oxygen (O2) and hydrogen sulfide (H2S), which can

also corrode the SC-CO2 transport pipeline and the injection well [1-3].

Under deposit corrosion (UDC) may occurs when corrosion products deposit on the steel

surface. In the supercritical CO2 with flue gas impurities, the interaction of the impurities can

lead to the formation of elemental sulphur (S), which has been verified by the previous

experiments [4-6]. The S always be found at the bottom of the autoclave after test, and

sometimes it can be detected on specimen surface. The presence of S can also result in UDC.

The mechanism is: due to the influence of equipment geometry and the corrosion product

deposition, the diffusion of electrolyte from the bulk to the steel surface is restricted, causing

* Coresponding author.

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the chemical composition of the media in the blocked cavity to be greatly different from that of

the bulk. The pH value of the media in the cavity decreases greatly owing to the self-

acidification process, resulting in the occluded cell corrosion.

The formation of S deposits may be resulted from the following ways [4, 5]:

2H2S + 3O2 → 2SO2 + 2H2O (1)

2H2S + SO2 →3𝑋⁄ S𝑋 + 2H2O (2)

2H2S + O2 →2𝑋⁄ S𝑋 + 2H2O (3)

4NO2 + 2H2S → 4NO + SO2 + 2H2O + S (4)

The presence of sulfur dioxide (SO2) can lead to the formation of sulfuric acid (H2SO4), while

nitrogen dioxide (NO2) can result in the formation of nitric acid (HNO3). Both of them may

react with H2S to form S [4]:

H2SO4 + H2S → S + SO2 + 2H2O (5) 2HNO3 + 3H2S → 3S + 2NO + 4H2O (6)

The presence of S can not only result in the UDC, but also directly react with Fe or cause the

formation of acid by hydrolysis [7]:

Fe + S8 → FeS (7)

4S8 + 4H2O → 3H2S + H2SO4 (8)

It is well know that the presence of S can induce localized corrosion in sour gas systems [7, 8].

However, when S presents in the SC-CO2 systems, its effect on the corrosion behavior of

pipeline steel is not clear, although the effects of various parameters, such as temperature,

impurity concentration, and exposure time etc., have been investigated [5, 9-28]. The current

work is mainly focused on the impact of S on the corrosion behavior of X80 steel, which is not

a direct impurity from the CO2 stream but from the interaction of the impurities. Sun et al. [20]

investigated the corrosion of X80 steels in water-saturated SC-CO2 system environment

containing 1000 ppmv O2 and 1000 ppmv H2S, and found ferrous sulfide (FeS) crystals.

According to them, O2 can react with H2S to generate S that precipitates on the steel surface

and then further reacts with iron to produce FeS [7]. Dugstad et al [4]. studied that there may

be S formation in the presence of O2 and H2S, which may make the corrosion process of carbon

steel more complicated.

The aim of this work is to understand the synergistic effect of multiple impurities such as O2,

CO2 and NO2 on the corrosion behavior of X80 pipeline steel in water-saturated SC-CO2 system

and SC-CO2-saturated water system. To achieve this objective, the corrosion rate was

determined by weight loss tests. The morphology and composition of corrosion scales were

characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS),

X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and three-dimensional

profilometery. Accordingly, the synergistic interaction impact factors of multiple impurities

concerning the corrosion of X80 steel were proposed.

Experimental Procedure

Materials and Preparation

The American Petroleum Institute (API) 5L X80 steel was used as the test material. The

elemental composition of X80 steel is illustrated in Table 1. The specimens for weight-loss tests

and surface analysis had a size of 10×10×6 mm. The specimens were polished by 320-, 400-,

600-, and 1000-grit silicon carbide papers subsequently, then rinsed with acetone and

dehydrated with ethanol. Finally, all specimens were dried with ultrapure nitrogen gas (N2)

(99.999% in vol.) and sealed in sealing bags. As shown in Figure 1, 0.05 g of S was placed at

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the surface of specimens in the shape of cone. After the placement of S, the specimens were

placed in the vacuum drying oven for 2 h at 155°C until the S powder melted down completely.

The specimens with deposited S were taken out and sealed in the sealing bags immediately after

the internal temperature and pressure restored to ambient temperature and pressure. The

specimens were weighed by an electronic balance with a precision of 0.1 mg.

Figure 1. Prepared specimens of X80 steel before the experiment (a. blank specimen, b.

specimen with S, c. specimen after removal of S)

Table 1. Chemical compositions of API 5L X80 steel used in the present study (wt. % Fe in

balance)

Element Mn Si C Cr P Ti Mo V Nb S

Concentration 1.72 0.23 0.042 0.01 0.009 0.013 0.019 0.005 0.047 0.001

Experimental Setup and Procedures

The experiment apparatus is the same as the previous work [29, 30]. The ultrapure N2 was used

to deoxygenate for 2 h after the specimens were mounted. According to the solubility data, 3 g

purified water was added into the autoclave in order to create a water-saturated SC-CO2

environment. The autoclave temperature was set to 40°C before the impurity gases and CO2

were injected into the autoclave, which could greatly shorten the stabilization time. The

CO2/NO2 (NO2: 5.0% in volume) and CO2/O2 (O2: 5.0% in volume) mixtures were injected into

the autoclave to increase the gauge pressure to 0.07 MPa and 0.33 MPa sequentially. Finally,

the CO2 was pumped into the autoclave through a booster pump and the final gauge pressure

reached 8.0 MPa. The impurity concentrations of NO2 and O2 were 200 ppmv and 1000 ppmv

respectively. Another group of tests which contains 1.3 L water was conducted in order to

simulate the corrosion environment in SC-CO2-saturated water. The detailed test conditions

were listed in Table 2.

Table 2. Test conditions of X80 steel in SC-CO2 at 40°C and 8.0 MPa

Test

No. Moisture content

NO2

(ppmv)

O2

(ppmv)

Immersion time

(day) S content

1 Water-saturated (3.0 g) 200 1000 1 0.05 g deposit

2 Water-saturated (3.0 g) 200 1000 1 0

3 Water-saturated (3.0 g) 200 1000 7 0.05 g deposit

4 Water-saturated (3.0 g) 200 1000 7 0

5 SC-CO2-saturated water (1.3 L) 200 1000 7 6.5 g in Solution

6 SC-CO2-saturated water (1.3 L) 200 1000 7 0

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The corroded specimens were analyzed by scanning electron microscopy (SEM)/energy-

dispersive X-ray spectroscopy (EDS), X-ray Diffraction (XRD) and X-ray photoelectron

spectroscopy (XPS) before removing the corrosion products. The scanning rate of XRD was set

to 4 °/min. The uniform corrosion rate was determined through weight-loss method. The

corroded specimens was cleaned with Clark solution (50 g stannous chloride, 20 g antimony

trioxide and concentrated hydrochloric acid to make 1000 mL) to remove the corrosion products

[31].

The three-dimensional (3D) profilometry was deployed to analyze the surfaces of cleaned

specimens in order to measure the pit depths. The maximum pitting depth and pitting rate was

calculated by the top 10 pitting depths according to the American Society for Testing and

Materials (ASTM) G46-94 standard [32].

Results and Discussion

Corrosion Rates

Figure 2 shows the variation of corrosion rate at different conditions at 8 MPa and 40°C. As

shown in Figure 2, the uniform corrosion rates of S-containing and S-free specimens were

similar under the condition of water-saturated SC-CO2 and the uniform corrosion rate decreased

significantly with the extending of experimental period. Under the SC-CO2-saturated water

condition, the uniform corrosion rate of the specimens in the S-free environment was

significantly higher than that in the S-containing environment, and the pitting corrosion under

both conditions was not obvious. It can be concluded from the weight-loss data that the uniform

thickness loss in the S-free environment was nearly double that in the S-rich environment.

However, the maximum pit depth of the S-free X80 specimen in aqueous phase was about half

of that with S. In addition, the corrosion rate in aqueous phase was greater than that in the water-

saturated SC-CO2, regardless of the S content.

Figure 2. The maximum pitting rates, maximum pitting depths and uniform corrosion rates of

X80 specimen under different conditions. (1, 2) with and without S (water-saturated SC-CO2

for one day); (3, 4) with and without S (water-saturated SC-CO2 for seven days); (5, 6) with

and without S (SC-CO2-saturated water for seven days)

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Morphological Studies of Corroded Specimens

Figure 3 shows the macroscopic morphologies of the X80 specimens exposed to water-

saturated SC-CO2 for seven days. As shown in Figure 3a, b and c, the surface color of the

specimen changed to brown compared to the un-corroded specimen. In order to clearly identify

the S-covered and non-S-covered areas, a circle was drawn along the outline of the S deposit.

Figure 3d and 3e were the cross-sectional macro-graphs of the corroded specimens. As shown

in Figure 3d, the S is well preserved in the corrosion process.

This experiment was illustrated with figure 3 as an example: "under the S deposit" is the part

inside the circle drawn in figure 3c, "outside the S deposit" is the part outside the circle drawn

in figure 3c, and "without S" is the case shown in figure 3a, where is no any S in the

experimental environment, and "with S" is the case in water-saturated SC-CO2 environment for

one day and seven days as shown in figure 3b. "With S" means that S powder is dissolved in

aqueous solution in the SC-CO2-saturated water environment for seven days.

Figure 4 shows the SEM images of the X80 specimens under different experimental conditions.

It could be seen from Figure 4a and b that under the condition of water-saturated SC-CO2 for

one day, when S presents, there was some bubble-like product on the corroded specimen surface.

The surface of specimen was covered by ruptured bubble-like and strip-shaped corrosion

products in S-free water-saturated SC-CO2 environment. For the cases of seven day exposure

in water-saturated SC-CO2 phase, the corrosion products with S were also in the form of large

ruptured bubble-like products, but in the S-free condition the fragmental corrosion products

were formed. Under the condition of SC-CO2-saturated water for seven days, the corrosion

products of the specimen containing S were mostly in the form of stacked sheets, while for the

case without S the corrosion products seems to be flat on the surface.

As shown in Figure 5, the pitting depth of X80 in water-saturated SC-CO2 and SC-CO2-

saturated water was measured by 3D profilometry. Figure 5a and b illustrated the 3D-

morpoholgies of specimens with S in water-saturated SC-CO2 for one day. Figure 5a shows the

3D profilometry under the S deposit, which was relatively flat. Figure 5b shows the appearance

of the outside of the S deposit, which has a rough surface and a number of small pits. In the S-

free environment for one day (Figure 5c), the surface of the corroded specimen was rougher

compared to the surface covered by S deposit. Under the condition of water-saturated SC-CO2

for seven days, the morphology of S covered area was similar to the same area for one day.

There were fewer shallow pits in the S-free for one day (Figure 5c), compared with the result

of seven day that more severe pitting corrosion was observed on the specimens with S deposit.

Under the condition of SC-CO2-saturated water for seven days, more severe pitting corrosion

was observed on the specimens for the case with S powder. When the S was absent in the water

phase, the surface of the specimen was relatively flat, indicating that there was no obvious

pitting corrosion.

This experiment was illustrated with figure 3 as an example: "under the S deposit" is the part

inside the circle drawn in figure 3C, "outside the S deposit" is the part outside the circle drawn

in figure 3c, and "without S" is the case shown in figure 3a, where is no any S in the

experimental environment, and "with S" is the case in water-saturated SC-CO2 environment for

one day and seven days as shown in figure 3b. "With S" means that S powder is dissolved in

aqueous solution in the SC-CO2-saturated water environment for seven days.

Figure 6 and 7 show the cross-sectional SEM morphologies of the corroded specimens and the

corresponding EDS line scanning for different test conditions. It could be seen that under the

condition of S in water-saturated SC-CO2 for one day, a compact single-layer of corrosion

product was observed, while under the condition of S-free, the texture of corrosion product

scale became relatively loose, and the thickness of the corrosion scale was thicker than that with

elemental S.

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As shown in Figure 6c and 7c, a double-layered corrosion scale could be observed and the inner

layer had a thinner thickness compared to the outer layer. The corrosion scale seemed to be

compact and was thicker than the scale formed under the condition with S for one day. Under

the same conditions, the corrosion product scale had a more irregular texture, and its thickness

was not only thicker than the case with S for seven days but the cases with and without S for

one day.

The XRD of corrosion products under different experimental conditions were observed that the

peak of iron matrix could be detected in all conditions, while the peak of iron carbide (Fe3C)

was also detected under the condition of SC-CO2-saturated water with S.

Figure 3. Macroscopic morphologies of X80 steel after seven days' exposure in water-

saturated SC-CO2. (a) blank specimen, (b) specimen deposited with S, (c) corroded X80

specimen after removal of deposition S, (d, e) cross-section of corroded specimens deposited

with and without S.)

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Figure 4. SEM morphologies of corroded X80 steels: (a, b) with and without S (water-

saturated SC-CO2 for one day); (c, d) with and without S (water-saturated SC-CO2 for seven

days); (e, f) with and without S (SC-CO2-saturated water for seven days)

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Figure 5. 3D profilometry images of corroded specimens after removal of corrosion

products:(a-c) in water-saturated SC-CO2 for one day: (a) under the S deposit, (b) outside the

S deposit and (c) without S; (d-f) water-saturated SC-CO2 for seven days: (d) under the S

deposit, (e) outside the S deposit and (f) without S; (g, h) in SC-CO2-saturated water for seven

days: (g) with S and (h) without S powder.

In the case of S-containing SC-CO2-saturated water for seven days (Figure 6e and Figure 7e),

it was observed that the corrosion product scale was loose and discontinuous that the corrosive

media could easily reached the substrate and caused severe pitting corrosion. It could be

inferred from Figure 6e that new small pits occurred at the bottom of large pit indicating

possibility of the further growth of pits in pit. In Figure 7f, the corrosion product scale in SC-

CO2-saturated water without S for seven days was very thick and dense.

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Figure 6. SEM cross-sectional morphologies of X80 carbon steel after corrosion: (a-d) in

water-saturated SC-CO2, (a, b) with and without S for one day, (c, d) with and without S for

seven days; (e, f) in SC-CO2-saturated water with and without S for seven days.

Figure 7. EDS line scanning of X80 carbon steel after corrosion: (a-d) in water-saturated SC-

CO2, (a, b) with and without S for one day, (c, d) with and without S for seven days; (e, f) in

SC-CO2-saturated water with and without S for seven days.

Chemical Composition Studies of Corrosion Products

For the XPS analysis, the elements of interests were carbon (C), oxygen (O), sulphur (S) and

iron (Fe). The binding energy of the C 1s peak after calibration with residual carbon was 284.6

eV. The peak of nitrogen (N) was not detected, indicating that there might be no N compounds

in the corrosion products. Figure 8 shows the high resolution XPS spectra and decomposition

of peaks for different elements of corrosion products, respectively.

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Several corrosion products were found on specimens in water-saturated SC-CO2 environment

for one day (Figure 8a1, 8b1, 8c1, 8d1). In the case with S, peaks for O 1s at 529.8 eV, Fe 2p3/2

at 710.9 eV were detected, which corresponds to the presence of gamma-ferric oxide(γ-Fe2O3)

[33]; Peaks for O 1s at 530.3 eV and Fe 2p1/2 at 725.3 eV were detected, indicating that the

corrosion products contained gamma-FeOOH (γ-FeOOH) [27]; The peak for S 2p3/2 at 164.8

eV corresponds to the presence of elemental S8 [34]; Peaks for C 1s at 288 eV, O 1s at 531.4

eV and Fe 2p3/2 at 710.7 eV were detected, indicating that the corrosion product contained

ferrous carbonate (FeCO3) [35]. The peak for Fe 2p3/2 at 714.4 eV corresponds to the presence

of Shake-up peak [36]. In addition, peaks for O 1s at 532.4 eV, S 2p3/2 at 168.6 eV and Fe 2p3/2

at 712.2 eV were also detected, implying the presence of ferrous sulfate (FeSO4) [27]. In the

case without S (Figure 8e1, 8f1, 8g1), peaks for C 1s at 288 eV, O 1s at 531.4 eV and Fe 2p3/2 at

710.7 eV were also detected, indicating that the corrosion product contained FeCO3[35]; The

peak for Fe 2p3/2 at 714.4 eV corresponds to the presence of Shake-up peak; Peaks for O 1s at

529.8 eV, Fe 2p3/2 at 710.9 eV were detected, which corresponds to the presence of γ-Fe2O3.28

Peaks for O 1s at 529.8 eV and Fe 2p1/2 at 724.5 eV were detected, indicating that the corrosion

product contained alpha-FeOOH (α-FeOOH) [27]. Several corrosion products which found on

specimens in water-saturated SC-CO2 environment with S for seven days (Figure 8a2, 8b2, 8c2,

8d2) were the same as the one day (γ-Fe2O3, FeCO3, Shake-up peak, S8 and FeSO4) except for

the γ-FeOOH. Peaks for O 1s at 530.3 eV and Fe 2p1/2 at 724.5 eV were detected, indicating

that the corrosion products contained α-FeOOH; In the case without S (Figure 8e2, 8f2, 8g2),

peaks for C 1s at 288 eV, O 1s at 531.4 eV and Fe 2p3/2 at 710.7 eV were also detected,

indicating that the corrosion product contained FeCO3; The peak for Fe 2p3/2 at 714.4 eV

corresponds to the presence of Shake-up peak; Peaks for O 1s at 529.8 eV, Fe 2p1/2 at 724.5 eV

were detected, which corresponds to the presence of γ-Fe2O3.28 Peaks for O 1s at 529.8 eV and

Fe 2p3/2 at 711.8 eV were detected, indicating that the corrosion product contained α-FeOOH

[27].

Similarly, several corrosion products which found on specimens in SC-CO2-saturated water

environment with S for seven days were the same as the one day with S (Figure 8a1, 8b1, 8c1,

8d1). In the case without S (Figure 8e3, 8f3, 8g3) were the same as the water-saturated SC-CO2

environment without S for seven days (Figure 8e2, 8f2, 8g2).

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Figure 8. XPS of X80 steel after corrosion in water-saturated SC-CO2 for one day (a1-g1):

(a1-d1) with and (e1-g1) without S; XPS of X80 steel after corrosion in water-saturated SC-

CO2 for seven days (a2-g2): (a2-d2) with and (e2-g2) without S; XPS of X80 steel after

corrosion in SC-CO2-saturated water for seven days (a3-g3), (a3-d3) with and (e3-g3) without

S.

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Discussion of Corrosion and Inhibition Mechanisms

Experiments shown that S participates in the reaction under water-saturated SC-CO2 and SC-

CO2-saturated water (200 ppmv NO2, 1,000 ppmv O2, 40°C and 8.0 MPa). The corrosion of

carbon steel in water-saturated SC-CO2 phase was an electrochemical process that could be

summarized as follows:

Initially, regardless of any conditions CO2 dissolves into water:

CO2(g) + H2O → H2CO3 (9)

The cathodic reactions may include the reduction of reducing species (hydrogen ion (H+),

carbonic acid (H2CO3), bicarbonate ion (HCO3−)) [22]:

2H+ + 2e− → H2 (10) 2H2CO3 + 2e

− → 2HCO3− + H2 (11)

2HCO3− + 2e− → 2CO3

2− + H2 (12)

Anodic reaction is the dissolution of steel:

Fe → Fe2+ + 2e− (13)

The results of SEM, line scanning, XRD and XPS showed that the corrosion products mainly

included FeCO3, FeSO4, γ-FeOOH, γ-Fe2O3 (in water-saturated SC-CO2 environment with S

for one day and in SC-CO2-saturated water environment with S for seven days). There were

FeCO3, γ-Fe2O3, α-FeOOH in water-saturated SC-CO2 environment without S for one day and

seven days, and in SC-CO2-saturated water environment without S for seven days. There were

FeCO3, FeSO4, γ-Fe2O3, α-FeOOH in water-saturated SC-CO2 environment with S for seven

days.

In the experiment of one and seven days' exposure under the condition of water-saturated SC-

CO2 containing S, the corrosion rate of the S deposit coverage position was lower than that of

the S-free area. It can be seen that the deposition of S has a certain covering effect on the surface

of the X80 specimen. The thin and dense corrosion products has a certain protective effect on

the substrate, so that there was no further pitting corrosion occurred, and the main reaction

equations were as follows [8, 20, 29, 35]:

Fe2+ + CO32− → FeCO3 (14)

Fe2+ + 2HCO3− → Fe(HCO3)2 (15)

Fe(HCO3)2𝑑𝑒𝑐𝑜𝑚𝑝𝑜𝑠𝑒→ FeCO3 + CO2 + H2O (16)

S8(s) + 8H2O(l) → 6H2S(aq) + 2H2SO4(aq) (17) Fe(s) + 2H2SO4(aq) → FeSO4(aq) + H2 (18)

FeCO3 → FeO + CO2 (19) 4FeO + O2 → 2α − Fe2O3 (20)

Some FeSO4 was further reacted in aqueous solution to form FeOOH [37]:

4FeSO4 + 6H2O + O2 → 4FeOOH + 4H2SO4 (21)

However, in water-saturated SC-CO2 environment for one day, the corrosion condition of the

area where the S deposit was not covered was similar to the one-day exposure in water-saturated

S-free SC-CO2 environment.

Some corrosion products appear to be the same substance, but they have different orbital peaks

(e.g.γ-Fe2O3 has two orbital electron peaks of Fe which 2p3/2 at 710.9 eV and 2p1/2 at 724.5 eV;

α-FeOOH has also two orbital electron peaks of Fe which 2p1/2 at 724.5 eV and 2p3/2 at 711.8

eV et al.). In addition, α-FeOOH is more stable than γ-FeOOH [38]. According to different

crystal types, FeOOH can be divided mainly into three types: beta-FeOOH (β-FeOOH), α-

FeOOH and γ-FeOOH which can easily lead to mutual transformation due to changes of the

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reaction environment. γ-FeOOH was converted to α-FeOOH in the coexistence of ferrous ion

(Fe2+) and sulfate ion (SO42-), which is the same as in SC-CO2-saturated water environment

with S for seven days:

γ − FeOOH → α − FeOOH (22)

The formation of γ-Fe2O3 in both water-saturated SC-CO2 and SC-CO2-saturated water

environment was due to the dehydration reaction of FeOOH [39]:

2α − FeOOH𝑑𝑒ℎ𝑦𝑑𝑟𝑎𝑡𝑖𝑜𝑛→ α − Fe2O3 +H2O (23)

α − Fe2O3 + Fe3O4𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑→ γ − Fe2O3 (24)

In the experiment of seven days' exposure under SC-CO2-saturated water with S powder, since

the S was dispersed in the solution, the coverage effect of S on the substrate was limited. As

shown in Figure 6e, the corrosion product scale was thin and fragile, causing the corrosive

media to further corrode the matrix severely.

The pitting corrosion in S-containing water phase (Figure 6e) was more serious than that in S-

free water phase (Figure 6f), but the former one had a lower uniform corrosion rate. The pitting

corrosion in S-containing water phase was possibly attributed to the non-uniform deposit of S

powder on the specimen surface, which might induce the initiation and propagation of pitting

corrosion.

Conclusions

In this paper, the effect of S element on the corrosion of X80 pipeline steel was studied. The

experiment was conducted at 40°C, 8 MPa with 200 ppmv NO2 and 1000 ppmv O2 in water-

saturated SC-CO2 and SC-CO2-saturated water. The following conclusions can be drawn:

1. The type of FeOOH in corrosion products under the water-saturated SC-CO2 environment or

the SC-CO2-saturated water environment without S within seven days was α-FeOOH. It is

proved that in S-free environment is easier to generate the more stable α-FeOOH.

Comparatively speaking, there was more γ-FeOOH in water-saturated SC-CO2 environment

with S for one day and in SC-CO2-saturated water environment with S for seven days;

2. In water-saturated SC-CO2 with S, the S deposit plays a role in covering the specimens,

which hindered the diffusion of corrosive media to the substrate, and the FeCO3, γ-Fe2O3,

FeOOH (α-FeOOH and γ-FeOOH) and FeSO4 formed under the S deposit. Comparatively

dense S deposit has a protective effect on the substrate, thus the corrosion rate decreased;

3.In the case of the same experimental time of seven days, the corrosion rate of specimen in

water-saturated SC-CO2 is lower than that in SC-CO2-saturated water, and the corrosion

products are mostly amorphous structures;

4. Under the condition of SC-CO2-saturated water, the pitting corrosion of S-containing is more

serious than that without S, however the uniform corrosion rate is lower than the latter.

Acknowledgements

This work was financially supported by Beijing Natural Science Foundation (Grant No.

2172048), National Natural Science Foundation of China (Grant No. 51604289), and National

Key R&D Program of China (Grant No. 2017YFC0805800). The author also would like to

thank Chen Li, Zhengwei Long, Chicheng Song, Kai Yan, and Weiman Xie for language editing.

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