effect of small amount of h2s on the corrosion behavior

8/17/2019 Effect of Small Amount of H2S on the Corrosion Behavior

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Accepted Manuscript

Title: Effect of small amount of H2S on the corrosion behaviorof carbon steel in the dynamic supercritical CO2 environments

Author: Liang Wei Xiaolu Pang Kewei Gao

PII: S0010-938X(15)30150-5

DOI: http://dx.doi.org/doi:10.1016/j.corsci.2015.11.009

Reference: CS 6547

To appear in:

Received date: 15-7-2015Revised date: 29-10-2015

Accepted date: 5-11-2015

Please cite this article as: Liang Wei, Xiaolu Pang, Kewei Gao, Effect of small amount

of H2S on the corrosion behavior of carbon steel in the dynamic supercritical CO2

environments, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.11.009

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Effect of small amount of H2S on the corrosion behavior of carbon steel in the

dynamic supercritical CO2 environments

Liang Wei, Xiaolu Pang, Kewei Gao*

Department of Materials Physics and Chemistry,University of Science and Technology Beijing,

Beijing 100083, China* Corresponding author: E-mail: [email protected]

Tel: +86 1062334909; Fax: +86 1062334909


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Highlights

(1) Effect of H2S on the corrosion of carbon steel in supercritical CO2 was studied.

(2)

H2S changed the adsorbability of H2O on the steel surface in supercritical CO2

phase.(3) H2S presented more significant effect on the corrosion of steel in the aqueous

phase.

(4) H2S did not change the dominant corrosion type in supercritical CO2 phase.

(5)

H2S changed the dominant corrosion type of carbon steel in aqueous phase.


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Abstract

The effect of small amount of H2S on the corrosion of carbon steel in both dynamic

supercritical CO2 and aqueous phase was investigated. H2S changed the adsorbability

of H2O on the steel surface, causing the adsorption of H2O on the whole steel surface,

which accelerated the general and localized corrosion of carbon steel in supercriticalCO2 phase. The small amount of H2S presented more significant effect in the aqueous

phase - it changed the microstructure and compositions of the corrosion product scale,

ultimately changed the dominated corrosion type of carbon steel from localized to

general corrosion.

Keywords: A. Carbon steel; B. SEM; B. Raman spectroscopy; B. XRD; C. Acid

corrosion

1. Introduction

Carbon capture and storage (CCS) technology is regarded as one of the most

effective methods to significantly reduce CO2 emission and mitigate globing warming.

CCS process generally consists of capturing CO2 from different emission sources (e.g.,

coal-fired power stations, oil refineries), compressing CO2 into supercritical or liquid

states, then transporting and injecting it into storage sites (such as underground

aquifers or depleted oil and gas reservoirs). The transportation and injection of CO2,

which is important in CCS application, is usually accomplished by pipelines made by

carbon steel. For abating climate change, approximately 10 Gt/year of CO2 must be

transported for sequestration underground by 2050, which requires the construction of

3000 twelve-inch or 1000 twenty-inch carbon steel pipelines [1]. With the rapid

growth for energy demand, the exploitation of high pressure gas fields with large

quantities of CO2 (from 25% to 89%) becomes increasingly valuable [30]. However,

the large quantities of CO2 must be separated from the gas, captured and transported

to storage sites, which presents similar challenges as CO2 transportation in CCS

application. Meanwhile, the captured CO2 can also be injected into oil and gas fields

for enhanced oil/gas recovery (EOR/EGR). Generally, water can present in the

processes stated above, which resulted in potential supercritical CO2 (SC CO2)

corrosion risks to the carbon steel pipelines.

In the past few years, the corrosion problems of steels under SC CO2 conditionshave been given increasing attention [2-8]. It is well known that dry SC CO2 is not

corrosive [9]. When the H2O content in SC CO2 phase was less than its solubility limit,

the corrosion rate of carbon steel was very low [8-10]. If the H2O content exceeded its

solubility limit (steels remaining exposed in SC CO2 phase) or carbon steels

completely immersed in SC CO2-saturated water (aqueous phase), the corrosion rate

significantly increased [9, 11]. Notably, the addition of small amounts of SO2 in the

SC CO2/H2O system could noticeably enhance the corrosion rate of carbon steels,

even if the water content in the SC CO2/H2O/impurity system was much lower than its

solubility limit [12]. This suggested that the presence of impurities may drastically

enhance the corrosivity of SC CO2 to steels.


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The captured CO2 from various sources (anthropogenic and natural) commonly

contains different impurities. Though the compositions of CO2 streams are rarely

available in the open literature, Table 1 lists a summary of complied data from various

researchers [13-17]. Unfortunately, the content of impurities such as SOx, NOx and O2

was unclear. While to date there are still no recognized specifications of the CO2 quality required for transportation, some tentative CO2 specifications and CO2 quality

tolerances have been suggested in the literature as listed in Table 2 [1]. Combined

with Table 1, 2 and the data summarized by Lee et al. [18], it is worth noting that the

primary impurities, except for H2O, concerned in the CO2 streams were O2, SO2, NO2,

CH4 and H2S. The effects of SO2 and O2 on the corrosion behaviors of carbon steels in

the SC CO2/H2O/impurity system have been investigated in depth by researchers [12,

19-23], nevertheless, studies on the effect of H2S on the corrosion behavior of steels

in the SC CO2/H2O/impurity system was very limited. Most studies for the effect of

H2S on the corrosion of steels in the CO2/ H2S system were at low CO2 partial

pressure conditions or in CO2 environment with high H2S content [24-33].In deep water oil and gas production, there is also small amounts of H2S existing in

the high pressure CO2 stream [34]. When the water content in the SC CO2 system was

far below its solubility limit, corrosion was very slight in SC CO2/H2O/H2S systems

(


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in Fig. 1, the phase of X65 steel mainly consisted of Ferrite (F) and carbide (Fe3C).

The test specimens were machined with dimensions of 10×10×3 mm3. Specimen

surfaces were ground with SiC papers up to 800 grit, cleaned with deionized water,

followed by alcohol, and dried with cold air. Specimens were then weighed on an

electronic balance with a precision of 0.1 mg and stored in a desiccator until use.During exposure experiment, the samples were fixed in a specimen holder made of

polytetrafluoroethene (PTFE) to prevent galvanic effects, and six samples were placed

in the autoclave for every test.

2.2 Exposure experiments

Exposure experiments were conducted in a high temperature and high pressure

autoclave with a 3 L capacity and its schematic diagram is shown in Fig. 2. According

to Hua et al. [5] and Sim et al. [8], there were two main corrosive systems under

supercritical CO2 conditions: the SC CO2-saturated water system (water-rich phase,

i.e. aqueous phase) - samples completely expose in water, and the water-saturated SCCO2 system (SC CO2-rich phase, i.e. SC CO2 phase) - samples expose to the SC CO2

phase. Therefore, the corrosive environments considered in this study included these

two systems.

The corrosion medium was 3.5 wt.% NaCl (the content of Cl- is ~ 21g/L), which

was made from deionized water and analytical reagents. During the exploitation of

high pressure CO2 fields or the injection of CO2, formation water normally presents in

the environments, which mainly consisted of corrosive ions such as Cl-. Researchers

[39-40] have confirmed that the most detrimental chloride content in the corrosion

solution for CO2 corrosion of carbon steel was around 15-25 g/L. As a result, the

chloride content (~21 g/L), was selected in this study to evaluate the corrosion

behavior of carbon steel in the worst corrosion medium.

The NaCl solution was de-aerated by CO2 bubbling in a conical flask for 12 h prior

to the exposure experiments. Then the corrosion medium was introduced into the

autoclave at ambient temperature and pressure and sealed, and continuously bubbled

CO2 to remove the oxygen within the system. Subsequently, pure CO2 or a mixture of

CO2 and H2S with certain ratio was directly injected into the autoclave and adjusted to

the required temperature and pressure. The starting point of each test was taken from

the time once the system reached the required temperature and pressure. The

experimental conditions are listed in Table 3.The solubility of water in CO2 has been investigated by Spycher [41] and Choi [11],

which presented as a function of temperature and pressure. According to Spycher et al.

[41], the solubility limit of water in SC CO2 phase at 10 MPa and 80 °C is around

10,000 ppm. Consequently, to ensure the water-saturated SC CO2 states during the test,

100 ml of solution was introduced to the autoclave. The specimens were all exposed

to the SC CO2 phase and the free aqueous phase was at the bottom of the autoclave, as

shown in Fig. 2. However, during the aqueous phase test, specimens were all

immersed in the 1000 ml of solution.


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2.3 Weight loss tests and microstructure characterization

After the exposure experiments, the six samples were taken out and cleaned by

deionized water and alcohol. Of which, three specimens were pickled in 10%

hydrochloric acid inhibited with 10g/L hexamethylenetetramine to remove thecorrosion product scale and the general corrosion rate was calculated according to the

following equation [42]:

RG =87600 ∆m

Sρt

(1)

where RG is the general corrosion rate, mm/y; Δm is the weight loss, g; S is the

exposed surface area, cm2; ρ is the density of steel, g/cm3; t is the exposure time, h.

Localized corrosion was measured by profilometer. All the scale-removed

samples were analyzed by profile meter and the pit depth analysis was in alignment

with ASTM Standard G46-94 [43]: an average of the 10 deepest pits and themaximum pit depth were used for pit damage. The localized corrosion rate was

calculated by the following equation:

RL=0.365h

t

(2)

where RL is the localized corrosion rate, mm/y; h is the pit depth, μm; t is the

exposure time, d.

The surface and cross-section morphologies of corrosion product scale were

analyzed by Scanning Electron Microscopy (SEM). The chemical composition and

the microstructure of the corrosion product scale were characterized by Energy

Dispersive Spectrometer (EDS), X-ray Diffraction (XRD) and Raman Spectroscopy.

The surface morphology of localized corrosion was also characterized by SEM.

3.

Results and discussion

3.1 Effect of H2S on the general corrosion rate

Fig. 3 shows the general corrosion rates of X65 carbon steel exposed to both SC

CO2 and aqueous phases with 50 ppm H2S and without H2S at 10 MPa and 80 °C for

10 days. The general corrosion rates exposed to the aqueous phase both with andwithout H2S were much higher than those exposed to the SC CO2 phase, which was in

agreement with the results of Choi et al. [11].

In the SC CO2 phase, the general corrosion rate with 50 ppm H2S was slightly

higher than that without H2S, indicating the presence of H2S enhanced the general

corrosion rate of X65 steel. The effect ratio (φ) of H2S on the corrosion rate of carbon

steel in the SC CO2 environments is defined as follow:

= Rwith- Rwithout Rwith

(3)

where Rwith and Rwithout are respective the corrosion rate of carbon steel in the SC CO2


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environments with and without H2S, mm/y.

According to Eq. (3), the effect ratio of 50 ppm H2S in the SC CO2 phase was

approximate 23.5%. The investigation of Choi et al. [34] indicated that the effect ratio

of 200 ppm H2S on the corrosion rate of carbon steel in the SC CO2 phase was 98%,

much higher than that in this study, which should be ascribed to the lower H2S contentin this study. The general corrosion rate of X65 steel exposed to the aqueous phase

with 50 ppm H2S was approximate two times higher than that without H2S. The effect

ratio of H2S in the aqueous phase was 83%, much higher than that in the SC CO2

phase (23.5%), which indicated that the effect of H2S in the flowing aqueous phase

was more significant than that in the flowing SC CO2 phase.

3.2 Effect of H2S in the SC CO2 phase

The surface morphologies of corrosion scale formed on X65 steel exposed to

water-saturated SC CO2 phase with and without 50 ppm H2S at 80 °C and 10 MPa areshown in Fig. 4. In the SC CO2 phase without H2S, two different characteristics on the

X65 steel surface were observed: as shown in Fig. 4(a), some regions were covered by

dense crystalline product, which was confirmed as FeCO3 by XRD measurements in

Fig. 5; on other regions (approximate 30%-50% of the surface), as shown in Fig. 4(b),

the polishing marks were still visible, only some corrosion product scattered, the

Raman spectra of this product (Fig. 6) demonstrated that it was also FeCO3. However,

as observed in Fig. 4 (c) and (d), the whole surface of X65 steel was covered by

corrosion product scale in the SC CO2 phase with 50 ppm H2S. Further, the corrosion

product scale divided into two layers, XRD spectra in Fig. 5 demonstrated it mainly

consisted of FeCO3 and small amount of iron sulfides.

Comparing the surface morphologies of X65 steel exposed to dynamic SC CO2

with and without H2S, it is obvious that the presence of H2S significantly influenced

the corrosion behavior of X65 steel. The addition of small amount of H2S may

increase the solubility of H2O in the system, subsequently increased the H2O content

condensed on the steel surface and accelerated the corrosion of the steel; or the

presence of H2S influenced the adsorbability of H2O on the X65 steel, which

influenced the corrosion of the steel. Therefore, it is very necessary to determine the

main reason effecting the corrosion of the steel. Given the lack of the H2O solubility

data in CO2/H2S/H2O mixed system under SC CO2 conditions, the solubility of H2Oin pure CO2 phase and pure H2S phase was calculated, respectively. The mutual

solubility of CO2, H2S with H2O could be calculated according to the following

equations [44], which referred to the calculation equations of Spycher et al. [41]:

yH2O

=1- ∑ yiBi

1 A⁄ - ∑ yiBi

(4)

xi= yiBi (5)

where yH2O

is the solubility (mole fraction) of H2O in i (i is CO2 or H2S) and xi is

the solubility (mole fraction) of i in H2O, yi is the mole fraction of i in the gas phase.


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Parameters A and Bi in Equation (4) and (5) are defined as follows:

A = KH2O0

∅ H2OP totexp[(P -P 0)V H2O

RT ] (6)

Bi = ∅ i P totγi

H i

T, 1

exp [- ( P - P 0)V i

RT ] (7)

where K is the true equilibrium constant; ∅ H2O is the fugacity coefficient of H2O; P tot

is the total pressure, bar; P is the partial pressure, bar; P 0 is the reference pressure, 1

bar; V H2O and V i are, respectively, the average partial molar volume of the pure

condensed component H2O and i over the pressure val P 0 to P , cm3/mol. ∅ i is the

fugacity coefficient of i; γi is the activity coefficient in mole fraction scale; H iT, 1

is the Henry’s constant; R is the gas constant; T is the temperature, K.

As calculated by Eqs. (4) – (7), the solubility of H2S in the aqueous phase in this

study is 2.1924 mol/kg, which is much higher than that of CO2 in the aqueous phase(~ 0.7778 mol/kg). However, the solubility of H2O in H2S phase is 10,200 ppm (mole)

at the experimental condition in this study, similar to the solubility of H2O in CO2

phase (10,500 ppm). That is the addition of H2S in the SC CO2/H2S mixed system

barely effected the solubility of H2O. Therefore, the presence of small amount of H2S

in the system changed the adsorbability of H2O on the X65 steel surface, which led to

H2O adsorbing on the whole steels surface. This was one reason that the general

corrosion rate of X65 steel exposed to the SC CO2/H2S mixed system was higher than

that exposed to the pure SC CO2 system.

The cross-section morphologies of corrosion product scale formed on X65 steel

exposed to SC CO2 phase with and without 50 ppm H2S at 80 °C and 10 MPa are

shown in Fig. 7 (a) and (b). There were two layers of corrosion product scale under

both conditions of H2S content. In the SC CO2 phase without H2S, the EDS analysis

of the inner layer formed on X65 steel (Fig. 7 (c)) confirmed that this layer possessed

similar compositions with that of the outer layer. Raman spectra of the inner layer

indicated that it also consisted of FeCO3, as shown in Fig. 8. In the SC CO2 phase

with 50 ppm H2S, EDS result indicated the dense outer layer and relatively loose

inner layer all contained Fe, C and O, while S element existed in the inner layer only,

as shown in Fig. 7 (e). Combined with the XRD spectra (Fig. 5), it can be confirmed

that the outer layer consisted of FeCO3, while the inner layer consisted of FeCO3 andiron sulfides. Notably, the inner layer of corrosion product scale formed in SC CO2

phase without H2S was dense, while the inner layer formed in SC CO2 phase with H2S

was loose, which was in alignment with that observed by Banas et al. [45].

Once the water condenses on the steel surface and forms a thin water film (Eq.

(8)), CO2 can immediately dissolve in this film and produce H2CO3, then H2CO3

dissociates in two steps to produce H+, HCO3- and CO3

2- [46]:

H2Odi ↔ H2Oaq (8)

CO2g + H2Ol ↔ H2CO3(aq) (9)


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H2CO3 ↔ H+ + HCO3- (10)

HCO3- ↔ H++CO32- (11)

During the initial corrosion period of steel in the SC CO2-saturated thin water film,the main cathodic reactions are the direct reduction of H+, the reduction of H2CO3 and

HCO3- [47]:

2H+ + 2e- ↔ H2 (12)

2H2CO3 + 2e- → H2 + 2HCO3- (13)

2HCO3- + 2e- → H2 + 2CO32- (14)

The anodic reaction is the oxidation of Fe:

Fe → Fe2+ + 2e- (15)When the concentrations of Fe2+ and CO3

2- exceed the solubility limit K sp of

FeCO3, FeCO3 will initiate to precipitation:

Fe2+ + CO32- ↔ FeCO3 (16)In the SC CO2 phase, the water film condensed on the steel is normally very thin,

reaction ions such as H+ can easily reach the steel surface, which leads to the fast

dissolution of Fe (Eq. (15)), and a mass of Fe2+ forms quickly and accumulates on the

steel surface. In addition, the effect of dynamic SC CO2 flow on the diffusion of Fe2+

in the thin water film is very limited. As a consequence, a high concentration of Fe2+

is obtained near the steel surface and leads to the nucleation of FeCO3 crystals

dominating, a dense FeCO3 layer thus forms on the steel surface. This scale impedes

the reactants reaching the steel surface and decreases the corrosion rate effectively.

Correspondingly, the concentration of Fe2+ decreases, which results in a low relative

supersaturation of FeCO3 above the previous formed FeCO3 layer, so an outer FeCO3

layer with larger grain size forms, as shown in Fig. 4 (a).

However, if H2S is contained in the SC CO2 phase, it dissolves in the water film

more easily than CO2, and produces H+, HS- and S2- according to the dissociation

reactions of H2S [48]:

H2S ↔ H+ + HS- (17)

HS- ↔ H+ + S2- (18)The dissolution of H2S further increased the concentration of H+ according to Eq.

(17) and (18), promoting the cathodic reaction process (Eq. (12)) and subsequently

increased the corrosion rate of X65 steel. HS - has greater chemical adsorption ability

than HCO3- and OH- on the steel surface [50], it accelerates the reactions of corrosion

and inhibits other corrosive species (such as H2CO3, HCO3-) from reacting with Fe.

Iron sulfides such as mackinawite (FeS1-x) firstly forms on the steel surface according


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to the solid state reactions from (19) to (21) [31]. Compared to the steel matrix, the

iron sulfides had a more positive potential and possessed certain electrical

conductivity, as reported by researcher [49]. The potential difference between the

inner layer and the steel promoted the corrosion of steel. However, as the researcher

confirmed [24], the initial formed iron sulfide layer was loose and had some defects,which resulted in the poor protective ability of the iron sulfide layer. Corrosive ions

could reach the steel surface due to the defects in the iron sulfide layer, thereby more

Fe2+ formed. With corrosion proceeding, once the concentrations of Fe2+ and CO32- in

the iron sulfide layer exceeded the K sp of FeCO3, FeCO3 precipitated in the layer and

a mixed layer (i.e. the inner corrosion product scale presented in Fig. 7 (b)) contained

FeCO3 and iron sulfides formed. Although this inner layer was relatively porous, it

could still retard the movement of corrosive ions and Fe2+ through the film [25],

decreased the dissolution of Fe and hence the concentration of Fe2+ in the solution. On

the other hand, as listed in Table 4, the solubility limits of FeCO3 and FeS under the

conditions of this study could be calculated, which were also listed in Table 4; theconcentrations of CO3

2- and S2- in the solution in this study could be calculated

according to the corresponding literatures [47, 50], and were respective 5.8×10-5

mol/L and 2.71×10-32 mol/L. As calculated above, FeCO3 was relatively easy to

precipitate according to Eq. (16), while FeS barely precipitated according to

precipitation reaction (22). Therefore, an outer layer which only contained FeCO3

formed on the inner layer.

Fe+ HS- → FeSHads

- (19)

FeSHads- → FeSHads+ + 2e- (20)

FeSHads+ → FeS1-x+xHS

-+ (1-x)H+ (21)

Fe2+ + S2- → FeS (22)It is worth noting that, as show in Fig. 7 (a) and (b), localized corrosion was

observed on the steel exposed to SC CO2 phase both with and without H2S, the effect

of H2S on the localized corrosion of steel would be depth discussed in the later

section.

3.3 Effect of H2S in the aqueous phase

The surface morphologies of corrosion product scales formed on X65 steel

immersed in SC CO2-saturated NaCl solution at 80 °C and 10 MPa with and without

H2S are shown in Fig. 9. In the aqueous phase without H2S (Fig. 9(a)), a relatively

dense FeCO3 scale with some pores was observed. However, when 50 ppm H2S

presented, a loose corrosion product scale, covered by some granular corrosion

products, was observed on the X65 steel surface, as shown in Fig. 9(b). Similar scale

structure was reported by Valdes et al. [53]. XRD results in Fig. 10 indicated that this

corrosion product scale formed in the aqueous phase with H2S also consisted of


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FeCO3 and iron sulfides, the same as the microstructure of scale formed in the SC

CO2 phase with H2S.

Fig. 11 presents the cross-section morphologies of corrosion product scales on X65

steel. In the aqueous phase without H2S, localized corrosion pits were clearly

observed, shown in Fig. 11(a). The corrosion product scale was comprised of twolayers (Fig. 11(b)): the outer layer consisted of FeCO3 crystal grains; the inner layer

consisted of dark grey corrosion products and granular substances with a light contrast,

which was similar to the steel matrix. Localized Raman spectroscopy analysis at the

dark grey corrosion product locations (Fig. 12) confirmed that it was FeCO3. EDS

result of the granular substance showed that it only contained Fe and C, and the

corresponding atomic ratio was near 3:1, which indicated that these granular

substances were Fe3C. Therefore, the inner layer scale consisted of FeCO3 and Fe3C,

this was in agreement with the results of Gao [42].

FeCO3 and Fe3C are the typical compositions of corrosion product scale of carbon

steel during CO2 corrosion process [31, 42], while Fe3C is not the corrosion product but the remainder of carbon steel after the selective dissolution of ferrite. As shown in

Fig. 1, the microstructure of X65 steel consists of ferrite and Fe3C. Fe3C has a positive

potential comparing with ferrite, which leads to the formation of a galvanic cell

between ferrite and Fe3C in the steel. Therefore, ferrite, as an anodic phase, dissolves

preferentially; Fe3C, as a cathodic phase, accumulates on the surface during initial

corrosion process [54]. This remaining Fe3C is a metallic conductor with low

hydrogen overvoltage and accelerates the corrosion of carbon steel. The initial formed

porous Fe3C layer influences the concentration gradient of the solution in it, leading

the local increase of pH and the concentration gradient of Fe2+, allowing the formation

of FeCO3 in the Fe3C layer. Meanwhile, Fe3C can act as the nucleation points of

FeCO3 crystal precipitation [55]. FeCO3 precipitated around Fe3C and first formed the

inner layer scale, which protected the steel and decreased the corrosion rate. This

protective inner layer scale hindered the diffusion of Fe2+ from the scale/matrix

interface to the scale/solution interface effectively and decreased the concentration of

Fe2+ above the inner layer scale. The flowing liquid promoted the diffusion of Fe2+

and further decreased the concentration of Fe2+ above the inner layer scale. Therefore,

the relative supersaturation of FeCO3 in the solution near the inner layer was low, the

growth of FeCO3 crystals was thus dominating and a relatively porous outer FeCO3

layer formed above the inner layer, as shown in Fig. 11 (a) and (b).As observed in Fig. 11 (c) and (d), the corrosion product scale formed in the

aqueous phase with H2S also consisted of two layers: the porous outer layer consisted

of FeCO3 crystals; the loose inner layer mainly contained FeCO3 and iron sulfide

(mackinawite), which was confirmed by the EDS and XRD results. Meanwhile, the

gaps between the inner layer scale and the steel surface were obviously visible,

indicating the adhesion stress of the inner layer to the steel matrix was poor, so the

inner scale easily spalled by the wall shear stress in the dynamic aqueous phase. The

distance between the original steel surface and the scale/matrix interface was much

larger than the thickness of the corrosion product scale, which demonstrated that the

scale spalled seriously in the dynamic aqueous phase dissolved H2S.


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Similar to the effect of H2S on the corrosion of carbon steel exposed to SC CO2

phase, according to Eq. (17), H2S dissolved in the solution and formed HS-, which

could adsorb on the steel surface and temporarily block the action of CO2, therefore,

FeS preferentially formed on the steel surface [49]. Reactive ions such as H+, HCO3-

and CO32-

could penetrate through the porous iron sulfide layer and continuouslycorroded the steel, meanwhile iron sulfide could also promote the dissolution of steel

due to its potential being more positive than the steel matrix. As a consequence, large

amounts of Fe2+ was produced and diffused through the layer to the bulk solution.

Fe2+ combined with CO32- in the iron sulfide layer and formed FeCO3, leading to

tension in the layer and resulted in the formation of cracks. On the other hand, in the

initial CO2 corrosion period, the corrosion rate was much higher than the formation

rate of the layer, which resulted in the poor adhesion stress of scale to steel matrix

[56]. Therefore, the defective FeCO3/FeS mixed layer easily spalled in the dynamic

aqueous phase.

It is worth noting that Fe3C could remain on the steel surface due to the selectivedissolution of ferrite. When no H2S presented in the solution, FeCO3 could nucleate

and grow quickly around Fe3C and formed a FeCO3 scale, which presented good

adhesion stress to the steel and anchored Fe3C on the steel surface [42]. However,

when H2S dissolved in the solution, the preferentially adsorbed HS- resulted in the

formation of porous and loose iron sulfide layer, and impeded the formation of FeCO3.

As a result, the remained Fe3C was easily scoured off in the dynamic aqueous phase.

Moreover, the porous and cracked iron sulfide layer, which may contain some Fe3C,

also easily spalled off on account of the poor adhesion stress of the scale to steel

matrix. Therefore, almost no Fe3C existed in the corrosion product scale on X65 steel

immersed in the dynamic aqueous phase dissolved H2S.

As mentioned before, when small amount of H2S presented in the system, the iron

sulfide formed mainly via the solid state reactions (19) to (21) rather than the

precipitation reaction (22). So an outer layer only containing FeCO3 formed above the

inner FeCO3 and iron sulfides mixed layer in SC CO2 phase with H2S. However, in

the aqueous phase dissolved H2S, the effect of flow velocity on the diffusion of Fe2+

was much more significant than that in the SC CO2 phase, and a much lower

concentration of Fe2+ was expected to obtain near the inner iron sulfide layer. As a

result, the growth of FeCO3 was dominated and some FeCO3 crystals with large grain

size precipitated above the inner layer. However, the diffusion of Fe2+ in the thinwater film in the dynamic SC CO2 phase was restricted due to the small volume of

water film, the higher Fe2+ concentration was expected to obtain, which accelerated

the formation of a relative compact FeCO3 layer with small grain size. The grain size

of FeCO3 crystals formed in the aqueous phase with and without H2S was

approximately 50 μm, one order of magnitude larger than that formed in the SC CO2

phase (approximate 5 μm), as shown in Fig. 4 and 9.

3.4 Effect of H2S on localized corrosion

Fig. 13 and 14 show the surface morphologies of X65 steel exposed to different


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systems after scale removal and the corresponding surface topographies measured by

profile meter, respectively. It can be observed that the surface morphologies after

scale removal seemed to correspond with the surface characteristics of corrosion

product scales.

In the SC CO2 phase without H2S, as shown in Fig. 13 (a), some localized corrosion pits were observed on the steel surface, and on other regions the polishing marks were

still visible. The surface topography of X65 steel in Fig. 14 (a) also shows that partial

surface area was smooth and not corroded. This implied that the corrosion of X65

steel exposed to SC CO2 phase without H2S was very localized. However, large

amounts of localized corrosion pits appeared on the whole steel surface in the SC CO 2

phase containing H2S, as shown in Fig. 13 (b). A much rougher steel surface was

observed on X65 steel (Fig. 14 (b)), which indicated that the whole surface of X65

steel suffered different degrees of corrosion. The maximum depth of localized

corrosion pits on X65 steel exposed to SC CO2 phase with H2S (20.4 µm) was

approximate two times larger than that without H2S (10.6 µm), suggesting thatlocalized corrosion was more serious when H2S existed in the SC CO2 system. The

calculated localized corrosion rates are presented in Fig. 15, the localized corrosion

rate of X65 steel exposed to SC CO2 phase with H2S was also approximate two times

higher than that without H2S. In pure SC CO2 phase, H2O only condensed on the local

surface of the steel [5, 10], this was the reason resulting in localized corrosion. In SC

CO2 phase, flow velocity barely effects the formation of corrosion product scale, so

the difference of localized corrosion in SC CO2 phase between with and without H2S

obviously ascribed to the addition of H2S. As confirmed above, when H2S existed in

the SC CO2 phase, H2O could condense on the whole steel surface and formed a water

film. HS- preferentially adsorbed on the steel surface and led to the formation of

porous iron sulfide layer. The corrosive ions such as HCO3- and HS- could penetrate

this layer through the defects in the layer and resulted in localized corrosion. On the

other hand, HS- enriched at the steel surface above which the corrosion scale was

defective, and resulted in the accumulation of the iron sulfides at these areas. The

accumulated iron sulfides on these areas could couple with the steel and promote

further dissolution of steel, ultimately aggravating localized corrosion.

In the aqueous phase, the localized corrosion of X65 steel without H2S was clearly

more serious than that with H2S, as observed in Fig. 13 (c) and (d). The surface

topographies shown in Fig. 14 (c) and (d) also demonstrated that X65 steel sufferedmore serious localized corrosion when no H2S existed. The maximum depth of

localized corrosion pits without H2S (316 µm) was nearly 4 times larger than that with

H2S (81 µm). Fig. 15 shows that the localized corrosion rate in the aqueous phase

without H2S was much higher. This was contrary to the phenomenon observed in the

SC CO2 phase. The wall shear stress was weak in the SC CO2 phase and had

essentially no effect on the formation of corrosion product scale, though it was strong

in the aqueous phase where it had a significant effect on the formation of corrosion

product scale. In a dynamic aqueous phase, without H2S, the wall shear stress could

cause cracking or spalling of a relatively compact scale due to defects inside the scale

or at the scale/matrix interface, as described in Fig. 16 (a) and (b). The lower pH and


15/43

local flow intensity resulted in the localized dissolution and further destruction of the

scale, as a result, aggressive ions could reach the fresh steel surface and localized

corrosion occurred. The aggressive solution was also capable of reaching the steel

surface through cracks and induced localized corrosion, as presented in Fig. 16 (c). In

addition, the acceleration of the localized anodic reaction rate produced more Fe

2+

, promoting the precipitation of FeCO3 in the destructive scale area, as shown in Fig. 16

(d). When trace H2S dissolved in the aqueous phase, a defective iron sulfide scale

preferentially formed on the steel surface due to the preferential adsorption of HS- on

the steel surface (Fig. 17 (a)). This scale had poor protectiveness and corrosive ions

still easily reached the steel surface through this scale and accelerated the dissolution

of Fe. The dissolved Fe2+ combined with CO32- in the iron sulfide layer and resulted in

the formation of a cracked FeCO3/FeS mixed layer, as shown in Fig. 17 (b). The

mixed layer presented poor adhesion stress to the steel surface and easily spalled

under the action of the wall shear stress, resulted in serious general corrosion, as

presented in Fig. 17 (c). The corrosive ions could penetrate through this cracked andspalled scale and accelerated the general corrosion of the whole steel surface, thereby

localized corrosion was slight on X65 steel.

4. Conclusions

1 In the SC CO2 phase, the presence of small amount of H2S slightly enhanced the

general and localized corrosion rates of X65 steel. In the aqueous phase, the small

amount of H2S greatly increased the general corrosion rate of X65 steel, while

significantly decreased the localized corrosion rate.

2 The effect of H2S on the corrosion behavior of X65 steel in the dynamic aqueous

phase was much larger than that in the flowing SC CO2 phase. H2S did not change the

dominated corrosion type in the SC CO2 phase, localized corrosion was still the main

corrosion type. However, in the aqueous phase, H2S changed the dominated corrosion

type of X65 steel from localized corrosion to general corrosion.

3 In the SC CO2 phase without H2S, the local condensation of H2O on the steel

surface resulted in localized corrosion of the steel. However, the addition of small

amount of H2S in the SC CO2 phase changed the adsorbability of H2O on the steel

surface, resulting in the corrosion of the whole steel surface; the defects of the

siderite/iron sulfide mixed scale and the local enrichment of iron sulfides acceleratedin the localized corrosion of the steel.

4 In the dynamic aqueous phase without H2S, a dense FeCO3 scale with some defects

formed on the steel, which cracked and spalled easily from the location of defects and

led to severe localized corrosion. However, when small amount of H2S dissolved in

the dynamic aqueous phase, a porous and cracked siderite/iron sulfide mixed scale

formed on the steel surface. This layer had poor adhesion stress to steel matrix and

easily spalled completely from the steel surface, led to severe general corrosion.


16/43

Acknowledgement

This research was supported by the Natural Science Foundation of Beijing (major

project) (No. 2131004) and the National Natural Science Foundation of China (No.

51271024).

References

[1] A. Dugstad, M. Halseid, Internal corrosion in dense phase CO2 transport pipelines

– state of the art and the need for further R&D, CORROSION/2012, NACE

International, Houston/TX, 2012, paper no. 0001452.

[2] Z. D. Cui, S. L. Wu, S. L. Zhu, X. J. Yang, Study on corrosion properties of

pipelines in simulated produced water saturated with supercritical CO2, Appl. Surf.

Sci. 252 (2006) 2368-2374.

[3] S. L. Wu, Z. D. Cui, G. X. Zhao, M. L. Yan, S. L. Zhu, X. J. Yang, EIS study of

the surface film on the surface of carbon steel from supercritical carbon dioxidecorrosion, Appl. Surf. Sci. 228 (2004) 17-25.

[4] Y. C. Zhang, X. L. Pang, S. P. Qu, X. Li, K. W. Gao, Discussion of the CO2

corrosion mechanism between low partial pressure and supercritical condition, Corros.

Sci. 59 (2012) 186-197.

[5] Y. Hua, R. Barker, A. Neville, Comparison of corrosion behavior for X65 carbon

steel in supercritical CO2-saturated water and water-saturated/unsaturated supercritical

CO2, J. Supercrit. Fluids 97 (2015) 224-237.

[6] Y. C. Zhang, X. L. Pang, S. P. Qu, X. Li, K. W. Gao, The relationship between

fracture toughness of CO2 corrosion scale and corrosion rate of X65 pipeline steel

under supercritical CO2 condition, Int. J. Greenh. Gas Control 5 (2011) 1643-1650.

[7] S. Hassani, T. N. Vu, N. R. Rosli, S. N. Esmaeely, Y. S. Choi, D. Young, S. Nesic,

Wellbore integrinanoty and corrosion of low alloy and stainless steels in high pressure

CO2 geologic storage environments: An experimental study, Int. J. Greenh. Gas

Control 23 (2014) 30-43.

[8] S. Sim, F. Bocher, I. S. Cole, X. B. Chen, N. Birbilis, Investigating the effect of

water content in supercritical CO2 as relevant to the corrosion of carbon capture and

storage pipelines, Corrosion 7 (2014) 185-195.

[9] Y. C. Zhang, K. W. Gao, G. Schmitt, Effect of water on steel corrosion under

supercritical CO2 conditions, Mater. Performance 50 (2011) 62-68.[10] Y. Hua, R. Barker, A. Neville, Effect of temperature on the critical water content

for general and localized corrosion of X65 carbon steel in the transport of

supercritical CO2, Int. J. Greenh. Gas Control 31 (2014) 48-60.

[11] Y. S. Choi, S. Nesic, Determining the corrosive potential of CO2 transport

pipeline in high pCO2-water environments, Int. J. Greenh. Gas Control 5 (2011)

788-797.

[12] Y. S. Choi, S. Nesic, Effect of water content on the corrosion behavior of carbon

steel in supercritical CO2 phase with impurities, CORROSION/2011, NACE

International, Houston/TX, 2011, paper no. 11377.

[13] F. W. Schremp, R. G. Roberson, Effect of supercritical carbon dioxide on


17/43

construction materials, SPE paper (1975) 4667.

[14] A. Oosterkamp, J. Ramsen, State-of-the-art overview of CO2 pipeline transport

with relevance to offshore pipelines. Polytech Report No: POL-O-2007-138-A.

[15] J. Gale, J. Davison, Transmission of CO2-safety and economic considerations.

Energy 29 (2004) 1319-1328.[16] E. D. Visser, C. Hendriks, M. Barrio, M. J. MØlnvik, G. D. Koeijer, S. Liljemark,

Y. L. Gallo, Dynamis CO2 quality recommendations. Int. J. Greenh. Gas Control 2

(2008) 478-484.

[17] D. E. McCollough, R. L. Stiles, Operation of the central basin CO2 pipeline

system. Society of Petroleum Engineers, 1987.

[18] J. Y. Lee, T. C. Keener, Y. J. Yang, Potential flue gas impurities in carbon dioxide

streams separated from coal-fired power plants, Journal of the Air and Waste

Management Association 59 (2009) 725-732.

[19] Y. S. Choi, S. Nesic, D. Young, Effect of impurities on the corrosion behavior of

CO2 transmission pipeline steel in supercritical CO2-water environments, Environ. Sci.Technol. 44 (2010) 9233-9238.

[20] Y. Xiang, Z. Wang, C. Xu, C. C. Zhou, Z. Li, W. D. Ni, Impact of SO2

concentration on the corrosion rate of X70 steel and iron in water-saturated

supercritical CO2 mixed with SO2, J. Supercrit. Fluids 58 (2011) 286-294.

[21] Y. Xiang, Z. Wang, Z. Li, W. D. Ni, Effect of exposure time on the corrosion

rates of X70 steel in supercritical CO2/SO2/O2/H2O environments, Corrosion 69 (2012)

251-258.

[22] Y. Xiang, Z. Wang, X. X. Yang, Z. Li, W. D. Ni, The upper limit of moisture

content for supercritical CO2 pipeline transport, J. Supercrit. Fluids 67 (2012) 14-21.

[23] Y. Hua, R. Barker, A. Neville, The influence of SO2 on the tolerable water

content to avoid pipeline corrosion during the transportation of supercritical CO2, Int.

J. Greenh. Gas Control 37 (2015) 412-423.

[24] J. W. Tang, Y. W. Shao, J. B. Guo, T. Zhang, G. Z. Meng, F. H. Wang, The effect

of H2S concentration on the corrosion behavior of carbon steel at 90 °C, Corros. Sci.

52 (2010) 2050-2058.

[25] Y. S. Choi, S. Nesic, S. Ling, Effect of H2S on the CO2 corrosion of carbon steel

in acidic solutions, Electrochim. Acta 56 (2011) 1752-1760.

[26] Z. F. Yin, W. Z. Zhao, Z. Q. Bai, Y. R. Feng, W. J. Zhou, Corrosion behavior of

SM 80SS tube steel in stimulant solution containing H2S and CO2, Electrochim. Acta53 (2008) 3690-3700.

[27] B. F. M. Pots, R. C. John, I. J. Rippon, M. J. J. S. Thomas, S. D. Kapusta, M. M.

Girgis, T. Whitham, Improvements on de waard-milliams corrosion prediction and

application to corrosion management, CORROSION/2002, NACE International,

Houston/TX, 2002, paper no. 02235.

[28] M. Singer, B. Brown, A. Camacho, S. Nesic, Combined effect of CO2, H2S and

acetic acid on bottom of the line corrosion, CORROSION/2007, NACE International,


[29] S. N. Smith, J. L. Pacheco, Prediction of corrosion in slightly sour environments,

CORROSION/2002, NACE International, Houston/TX, 2002, paper no. 02241.


18/43

[30] W. He, O. Ø. Knudsen, S. Diplas, Corrosion of stainless steel 316L in simulated

formation water environment with CO2-H2S-Cl-, Corros. Sci. 51 (2009) 2811-2819.

[31] G. A. Zhang, Y. Zeng, X. P. Guo, F. Jiang, D. Y. Shi, Z. Y. Chen, Electrochemical

corrosion behavior of carbon steel under dynamic high pressure H2S/CO2

environment, Corros. Sci. 65 (2012) 37-47.[32] H. Ma, X. Cheng, G. Li, S. Chen, Z. Quan, S. Zhao, L. Niu, The influence of

hydrogen sulfide on corrosion of iron under different conditions, Corros. Sci. 42

(2000) 1669-1683.

[33] P. Bai, S. Zheng, H. Zhao, Y. Ding, J. Wu, C. Chen, Investigations of the diverse

corrosion products on steel in a hydrogen sulfide environment, Corros. Sci. 87 (2014)

397-406.

[34] Y. S. Choi, S. Hassani, T. N. Vu, S. Nesic, Effect of H2S on the corrosion

behavior of pipeline steels in supercritical and liquid CO2 environments,

CORROSION/2015, NACE International, Houston/TX, 2015, paper no. 5927.

[35] H. E. Alami, C. Augustin, B. Orlans, J. J. Servier, Carbon capture and storage projects: material integrity for CO2 injection and storage. EuroCorr 2011. Stockholm,

paper no. 4741.

[36] W. F. Li, Y. J. Zhou, Y. Xue, Corrosion behavior of 110S tube steel in

environments of high H2S and CO2 content, Journal of Iron and Steel Research,

International, 19 (2012) 59-65.

[37] F. Farelas, Y. S. Choi, S. Nesic, Corrosion behavior of API 5L X65 carbon steel

under supercritical and liquid carbon dioxide phases in the presence of water and

sulfur dioxide, Corrosion 69 (2012) 243-250.

[38] K. W. Gao, F. Yu, X. L. Pang, G. A. Zhang, L. J. Qiao, W. Y. Chu, M. X. Lu,

Mechanical properties of CO2 corrosion product scales and their relationship to

corrosion rate, Corros. Sci. 50 (2008) 2796-2803.

[39] Q. Y. Liu, L. J. Mao, S. W. Zhou, Effects of chloride content on CO2 corrosion of

carbon steel in simulated oil and gas well environments, Corros. Sci. 84 (2014)

165-171.

[40] F. F. Eliyan, F. Mohammadi, A. Alfantazi, An electrochemical investigation on

the effect of the chloride content on CO2 corrosion of API-X100 steel, Corros. Sci. 64

(2012) 37-43.

[41] N. Spycher, K. Pruess, J. Ennis-King, CO2-H2O mixtures in the geological

sequestration of CO2. I. Assessment and calculation of mutual solubilities from 12 to100 °C and up to 600 bar, Geochim. Cosmochim. Acta 67 (2003) 3015-3031.

[42] M. Gao, X. Pang, K. Gao, The growth mechanism of CO2 corrosion product

films, Corros. Sci. 53 (2011) 557-568.

[43] ASTM Standard G1-03, Standard Practice for preparing cleaning and evaluating

corrosion test specimens. ASTM International, West Conshohocken, PA, 2003.

[44] M. Zirrahi, R. Azin, H. Hassanzadeh, M. Moshfeghian, Mutual solubility of CH4,

CO2, H2S, and their mixtures in brine under subsurface disposal conditions, Fluid

Phase Equilibiria, 324 (2012) 80-93.

[45] J. Banas, U. Lelek-Borkowska, B. Mazurkiewicz, W. Solarski, Effect of CO2 and

H2S on the composition and stability of paasive film on iron alloys in geothermal


19/43

water, Electrochim. Acta 52 (2007) 5704-5714.

[46] I. S. Cole, P. Corrigan, S. Sim, N. Birbilis, Corrosion of pipelines used for CO2

transport in CCS: Is it a real problem? Int. J. Greenh. Gas Control 5 (2011) 749-756.

[47] L. Wei, X. Pang, C. Liu, K. Gao, Formation mechanism and protective property

of corrosion product scale on X70 steel under supercritical CO2 environment, Corros.Sci. 100 (2015) 404-420.

[48] J. Kittel, F. Ropital, F. Grosjean, E. M. M. Sutter, B. Thribollet, Corrosion

mechanisms in aqueous solutions containing dissolved H2S. Part 1: Characterisation

of H2S reduction on a 316L rotating disc electrode, Corros. Sci. 66 (2013) 324-329.

[49] C. Ren, D. Liu, Z. Bai, T. Li, Corrosion behavior of oil tube steel in simulant

solution with hydrogen sulfide and carbon dioxide, Mater. Chem. Phys. 93 (2005)

305-309.

[50] J. Ding, L. Zhang, M. Lu, J. Wang, Z. Wen, W. Hao, The electrochemical

behavior of 316L austenitic stainless steel in Cl- containing environment under

different H2S partial pressures, Appl. Surf. Sci. 289 (2014) 33-41.[51] W. Sun, S. Nesic, R. C. Woollam, The effect of temperature and ionic strength on

iron carbonate (FeCO3) solubility limit, Corros. Sci. 51 (2009) 1273-1276.

[52] W. Sun, S. Nesic, D. Young, R. C. Woollam, Equilibrium expressions related to

the solubility of the sour corrosion product mackinawite, Ind. Eng. Chem. Res. 47

(2008) 1738.

[53] A. Valdes, R. Case, M. Ramirez, A. Ruiz, The effect of small amounts of H2S on

CO2 corrosion of a carbon steel, CORROSION/98, NACE International, Houston/TX,

1998, paper no. 22.

[54] F. Farelas, M. Galicia, B. Brown, S. Nesic, H. Castaneda, Evolution of

dissolution processes at the interface of carbon steel corroding in a CO2 environment

studied by EIS, Corros. Sci. 52 (2010) 509-517.

[55] J. L. Crolet, N. Thevenot, S. Nesic, Role of conductive corrosion products on the

protectiveness of corrosion layers, CORROSION/96, NACE International,


[56] E. W. J. van Hunnik, B. F. M. Pots, E. L. J. A. Hendriksen, The formation of

protective FeCO3 corrosion product layers in CO2 corrosion, CORROSION/96,

NACE International, Houston/TX, 1996, paper no. 6.


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Figure captions

Fig. 1. Optical micrograph of X65 steel.

Fig. 2. Schematic diagram of autoclave and the location of sample in the autoclave.

Fig. 3. General corrosion rates of X65 steel exposed to both SC CO2 and aqueous

phases with 50 ppm and without H2S at 10 MPa and 80 °C for 10d.

Fig. 4. Surface morphologies of corrosion product scale on X65 steel exposed to SC

CO2 phase at 10 MPa and 80 °C without H2S (a, b) and with 50 ppm H2S (c, d).

Fig. 5. XRD spectra of corrosion product scale formed on X65 steel exposed to SC

CO2 phase with 50 ppm and without H2S.

Fig. 6. Raman spectra of the corrosion products on X65 steel in Fig. 3 (b) in SC CO2

phase without H2S at 10 MPa and 80 °C.

Fig. 7. Cross-section morphologies (a, b) and the corresponding EDS spectra (c, d and

e) of corrosion product scale on X65 steel exposed to SC CO2 phase without (a) and

with 50 ppm H2S (b): (c) is the EDS spectra of outer layer scale in (a); (d) is the EDS

spectra of outer layer scale in (b); (e) is the XRD spectra of inner layer scale in (b).

Fig. 8. Raman spectra of the inner layer scale on X65 steel formed in SC CO2 phase

without H2S at 10 MPa and 80 °C.


CO2-saturated NaCl solution at 10 MPa and 80 °C without (a) and with 50 ppm H2S

(b).

Fig. 10. XRD spectra of corrosion product scale formed on X65 steel immersed in SC

CO2-saturated NaCl solution with 50 ppm H2S at 10 MPa and 80 °C.

Fig. 11. Cross-section morphologies of corrosion product scale on X65 steel

immersed in SC CO2-saturated NaCl solution without (a, b) and with 50 ppm H2S (c,

d) at 10 MPa and 80 °C.

Fig. 12. Raman spectra of the inner layer scale on X65 steel formed in SC

CO2-saturated NaCl solution without H2S at 10 MPa and 80 °C.

Fig. 13. Surface morphologies of the corroded X65 steel exposed to different systems

at 10 MPa and 80 °C after the removal of corrosion products: (a) in SC CO2 phase

without H2S; (b) in SC CO2 phase with 50ppm H2S; (c) in aqueous phase without H2S;


21/43

(d) in aqueous phase with 50 ppm H2S.

Fig. 14. The depth of localized corrosion pits on X65 steel measured by profile meter

exposed to different systems at 10 MPa and 80 °C after the removal of corrosion

products.

Fig. 15. Localized corrosion rates of X65 steel exposed to both SC CO2 and aqueous

phases with 50 ppm and without H2S at 10 MPa and 80 °C for 10d.

Fig. 16. Schematic of the crack and spallation of corrosion product scale in the

dynamic aqueous phase without H2S at 10 MPa and 80 °C.


dynamic aqueous phase containing H2S at 10 MPa and 80 °C.


22/43

Fig. 1. Optical micrograph of X65 steel.

Ferrite

Cementite


23/43

Fig. 2. Schematic diagram of autoclave and the location of sample in the autoclave.


24/43


25/43


CO2 phase at 10 MPa and 80 °C without H2S (a, b) and with 50 ppm H2S (c, d).

(a)

(c)

(b)

(d)

Polishing marks


26/43

Fig. 5. XRD spectra of corrosion product scale formed on X65 steel exposed to SC

CO2 phase with 50 ppm and without H2S.

0

4000

8000

12000

20 40 60 80 100

0

2000

4000

6000

Intensity,CPS

50 ppm H2SFeCO

3

FeFe

1+xS

Fe1-x

S

2, deg

no H2S


27/43

Fig. 6. Raman spectra of the corrosion products on X65 steel in Fig. 3 (b) in SC CO2

phase without H2S at 10 MPa and 80 °C.

200 400 600 800 1000 1200 1400

50

100

150

200

Intensity,a.u.

Wavenumbers shift, cm-1

282

1084


28/43

Fig. 7. Cross-section morphologies (a, b) and the corresponding EDS spectra (c, d and

e) of corrosion product scale on X65 steel exposed to SC CO2 phase without (a) and

with 50 ppm H2S (b): (c) EDS spectra of inner layer scale in (a); (d) EDS spectra of

outer layer scale in (b); (e) EDS spectra of inner layer scale in (b).

2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

cps/e

C

O

Fe

Fe

MnMnNa

2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

cps/e

C

O

Fe

Fe

MnMn S

2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

cps/e

C

O

Fe

Fe

(a) (b)

(c) (e)(d)

Epoxy Epoxy

Matrix Matrix

Outer layer

Inner layer

Outer layer

Inner layer

Scale Scale


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200 400 600 800 1000 1200 14000

200

400

600

800

1000

731

1024

282Intensity,a.u.


182

Fig. 8. Raman spectra of the inner layer scale on X65 steel formed in SC CO2 phasewithout H2S at 10 MPa and 80 °C.


30/43

Fig. 9. Surface morphologies of corrosion product scale on X65 steel immersed in SC

CO2-saturated NaCl solution at 10 MPa and 80 °C without (a) and with 50 ppm H2S

(b).

(a) (b)

Pores


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Fig. 10. XRD spectra of corrosion product scale formed on X65 steel immersed in SC

CO2-saturated NaCl solution with 50 ppm H2S at 10 MPa and 80 °C.

20 40 60 80 1000

4000

8000

12000

Intensity,CPS

2,deg

FeCO3

FeS

Fe1-x

S

Fe


32/43

Fig. 11. Cross-section morphologies of corrosion product scale on X65 steel

immersed in SC CO2-saturated NaCl solution without (a, b) and with 50 ppm H2S (c,

d) at 10 MPa and 80 °C.

(a)

(d)(c)

(b)

Epoxy

Matrix

Scale

Epoxy

Matrix

Scale

Epoxy

Matrix

Scale

Epoxy

Matrix

Scale

Outer layer

Inner layer

Original steel surface


33/43

Fig. 12. Raman spectra of the inner layer scale on X65 steel formed in SCCO2-saturated NaCl solution without H2S at 10 MPa and 80 °C.

200 400 600 800 1000 1200 14000

200

400

600

800

1000

1200

1400

1600

Intensity,a.u.


182

282

731

1084


34/43

Fig. 13. Surface morphologies of the corroded X65 steel exposed to different systems

at 10 MPa and 80 °C after the removal of corrosion products: (a) in SC CO2 phase

without H2S; (b) in SC CO2 phase with 50ppm H2S; (c) in aqueous phase without H2S;

(d) in aqueous phase with 50 ppm H2S.

(a)

(d)(c)

(b)


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Fig. 14. The depth of localized corrosion pits on X65 steel measured by profile meter

exposed to different systems at 10 MPa and 80 °C after the removal of corrosion

products.

0 1 2 3 4 5-30

-20

-10

0

10

20

Dept

h,

m

Distance, mm

SC CO2 phase - no H

2S

10.6 m

0 1 2 3 4 5

-300

-200

-100

0

100

Depth,

m

Distance, mm

Aqueous phase - 50 ppm H2S

81.4 m

0 1 2 3 4 5

-300

-200

-100

0

100

D

epth,

m

Distance, mm

Aqueous phase - no H2S

316 m

0 1 2 3 4 5-30

-20

-10

0

10

20

Depth

, m

Distance, mm

SC CO2 phase - 50 ppm H

2S

20.4 m

(a)

(d)(c)

(b)


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0

2

4

6

8

10

12

Localizedcorrosionrate,mm/y

no H2S

50 ppm H2S

SC CO2 phase Aqueous phase

0.29 0.48

9.19

2.45

Fig. 15. Localized corrosion rates of X65 steel exposed to both SC CO2 and aqueous phases with 50 ppm and without H2S at 10 MPa and 80 °C for 10d.


37/43


dynamic aqueous phase without H2S at 10 MPa and 80 °C.


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dynamic aqueous phase containing H2S at 10 MPa and 80 °C.


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Table 1 Compositions of CO2 streams transported in the existing pipelines (vol%)

Pipelines A [13-15] B

[16]

C

[14-1

5]

D

[16]

E

[15]

F

[14,

17]

G

[14]

H

[14-1

5]

CO2 85-95 93-96 95 96 96.8-97.4

98.5 98.7-99.4

99.7

CH4 2-15

(C6H14)

0.5-2 1-5 0.7 1.7 0.2 0.3 -

N2 ＜0.5 3-5 4 ＜300 0.6-0.9 1.3 0.3 0.3

H2S (ppmv) ＜260 150 20 9000 - ＜26 Trace -

CO - - - 0.1 - - - -

O2 (ppmv) - - - ＜70 - ＜14 - -

SOx - - - - - - - -

NOx - - - - - - - -

H2 - 3-5 - Trace - - - -Ar - 3-5 - - - - - -

H2O (ppmv) 122 Saturat

ed

630 20 315 630 418 -

Source Anthropog

enic

Natural Natur

al

Anthropoge

nic

Natural Natur

al

Natural Natur

al


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Table 2 CO2 quality specifications

DYNAMIS

CO2 quality

recommendati

on

Alstom CO2

quality tolerances

IPCC CO2 quality specification Kinder

Morgan

specificati

on

Coal fired plants Gas fired plants

Concentration Low HighPost-combust

ion capture

Pre-combus

tion captureOxy-fuel

Post-combu

stion capture

Pre-combus

tion capture

Oxy-fu

elCO2

(vol.%)＞95.5 ＞90 ＞95 ≥95

H2O

(ppmv)500 ＜10 ＜600 ≤630

H2S

(ppmv)200 ＜100

＜

150000 100-6000 0 0 ＜100 0

≤20

ppmw

SOx

(ppmv)100 ＜100 ＜1500 ＜100 0 5000 ＜100 0 ＜100

≤35

ppmw

NOx

(ppmv)100 ＜100 ＜1500 ＜100 0 100 ＜100 0 ＜100

CO

(ppmv)2000 ＜100

＜

400000 300-4000 0 0 400 0

O2 (ppmv)

EOR:100-1000

; Aquifer: ＜4

v%

＜10 ＜1000 ＜10 ppmw

CH4

(vol.%)

EOR ＜2;

Aquifer ＜4＜4 ＜5

N2, Ar, H2 ＜4 (all ＜4 ＜4 N2: ≤4


41/43

(vol.%) non-condensab

le gases)

Others

(ppmv)

Particulates

(mg/N m3):

Low ＜0.1; High

＜10

(EOR/Storage)

N2/Ar/O2:

100

N2/Ar/O2:

300-6000

N2/Ar/O

2: 37000

N2/Ar/O2:

100

N2/Ar/O2:

13000

N2/Ar/

O2:

41000

Total

sulfur: ≤

35 ppmw;

Hydrocarb

on: ≤ 5

vol.%


42/43

Table 3 Experimental conditions

Temperature

(°C)

Pressure

(MPa)

Flow

rate

(m/s)

H2S

(ppm)

Immersion

time (d)

Water-saturatedCO2 (SC CO2

phase80 10 1

0

10

50

CO2-saturated

water (Aqueous

phase)

0

50


43/43

Table 4 Experimental formulas for calculation of the equilibrium constants

Formula Solubility

limit

Ref.

K sp, FeCO3= 10-59.3498-0.041377T K -

2.1963T K

+24.5724logT K +2.518 I 0.5-0.657 I

1.58×10-10 [51]

K sp,FeS= 102848.770

T K -6.347 + log K a,1

1.58×10-5 [52]

effect of small amount of h2s on the corrosion behavior

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