International Journal of Minerals, Metallurgy and Materials Volume 24, Number 4, April 2017, Page 401 DOI: 10.1007/s12613-017-1420-7

Environmental boundary and formation mechanism of different types of H2S corrosion products on pipeline steel Lei Zhang1), Hui-xin Li1), Feng-xian Shi2), Jian-wei Yang3), Li-hua Hu4), and Min-xu Lu1) 1) Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 2) Aircraft Engine Corporation of China Shanghai Commercial Aircraft Engine Manufacturing Co., Ltd., Shanghai 201306, China 3) Shougang Research Institute of Technology, Beijing 100043, China 4) China National Offshore Oil Corporation Research Institute, Beijing 100027, China (Received: 5 September 2016; revised: 10 November 2016; accepted: 16 November 2016)

Abstract: To establish an adequate thermodynamic model for the mechanism of formation of hydrogen sulfide (H2S) corrosion products, theoretical and experimental studies were combined in this work. The corrosion products of API X60 pipeline steel formed under different H2S corrosion conditions were analyzed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. A thermodynamic model was developed to clarify the environmental boundaries for the formation and transformation of different products. Presumably, a dividing line with a negative slope existed between mackinawite and pyrrhotite. Using experimental data presented in this study combined with previously published results, we validated the model to predict the formation of mackinawite and pyrrhotite on the basis of the laws of thermodynamics. The established relationship is expected to support the investigation of the H2S corrosion mechanism in the oil and gas industry. Keywords: steel corrosion; corrosion products; thermodynamic calculations; mackinawite; pyrrhotite; prediction

1. Introduction With the increase in worldwide energy demand, the exploration of oil and gas fields tends to focus on locations with high contents of hydrogen sulfide (H2S) and carbon dioxide (CO2) [1–3]. H2S and CO2 corrosion of steels, which occurs in the process of oil and gas extraction, transportation, and refining, has attracted extensive attention in recent decades [4–8]. Pipeline corrosion at high temperatures and high H2S pressures has become increasingly problematic. [9]. According to a large number of studies [10−11], the corrosion products on steel play an important role in the corrosion process and might “control” the kinetics of steel degradation. H2S corrosion of steels, in particular, can result in the formation of a dozen ferrous sulfides with different crystal structures, generating a complex corrosion mechanism. Smith et al. [9] demonstrated that the type of H2S corrosion products formed depends on several critical factors such as Corresponding author: Lei Zhang

the H2S partial pressure, exposure time, temperature, and the environment pH value. According to literature reports, these products consist of mackinawite (FeS), cubic ferrous sulfide (FeS), troilite (FeS), pyrrhotite (Fe1−xS), and pyrite (FeS2), among other sulfides [12–16]. Mackinawite is a semi-stable form of FeS formed in environments with special conditions of low H2S activity [17−18]. The reciprocal lattice of mackinawite reflects obvious characteristics of a tetragonal crystal structure. Cubic FeS is considered to exist only as a metastable species and has a cubic stoichiometric crystal structure. It is gradually transformed into mackinawite, pyrrhotite, and other corrosion products after several days [11]. Troilite is the intermediate corrosion product of a carbon steel surface in an aqueous H2S environment at low temperatures [19]. Pyrrhotite is a non-stoichiometric form of FeS, which is formed in environments with higher H2S activity. It exhibits a preference toward growth along the c-axis of the hexagonal crystal. Pyrite is typical of a cubic

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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2017

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with a slope less than zero. However, most previous works are based on experimental results and assumptions; little theoretical modeling has been conducted to validate the conclusions and to illustrate the thermodynamic mechanism of H2S corrosion products. Corrosion products have been shown to strongly affect the H2S corrosion process; therefore, a thermodynamic model for predicting the type of corrosion products formed under specific corrosion conditions would be particularly useful. In the present study, stable conditions for mackinawite and pyrrhotite were determined and the chemical reactions related to their formation were studied. The thermodynamic parameters involved in mackinawite and pyrrhotite formation under different conditions were calculated on the basis of a thermodynamic model. By comparing the model results with the experimental autoclave results, the mechanism of H2S corrosion and the formation of corrosion products were further clarified.

crystal. It is considered the most stable form of FeS; as a corrosion product, it appears after long exposure times. Because the crystal structures and properties of the aforementioned sulfides differ from each other, they might play different roles in the H2S corrosion process [17,20–22]. In most situations, different corrosion products formed on the steel surface are defined as the rate-limiting step of the corrosion reaction. The evolution and transformation among different H2S corrosion products can also proceed under a specific combination of temperature and H2S concentration [23]. Several studies have shown that the corrosion products that change with the H2S concentration lead to an acceleration or an inhibition of iron corrosion, resulting in the formation of a FeS protective film on the electrode surface [24]. Mackinawite formation as an initial product is driven by the rapid kinetics of the corrosion reaction and the sluggish kinetics of the rest of the solid FeS species [25−26]. The layer is loose, less adherent, and rich in defects, thus contributing to good electron conduction and sometimes also to inhibition of anodic dissolution [27−28]. Phase transitions may occur at the initial corrosion stage [21,29−30]. The growth and rupture of the metastable mackinawite film can lead to its conversion into other sulfides, such as troilite and pyrrhotite, with greater stability and better protective properties [25]. The protective or inhibitive effect of H2S depends on the stability and compactness of the transformed sulfide. Several recent reports that address FeS formation have appeared in literatures. Smith and Miller [31] and Morse et al. [32] described the protectiveness and harmfulness of polymorphous FeS on the metal substrate by considering the thermodynamic stability of corrosion products. Yang [33] indicated that mackinawite was the main H2S corrosion product at lower temperatures and lower H2S partial pressures and that mackinawite gradually transformed into pyrrhotite with increasing temperature and H2S partial pressure. Ning et al. [11] constructed the Pourbaix diagrams of the H2S–H2O–Fe system at 25°C. These diagrams indicated that, under typical conditions in aqueous H2S-containing solutions (potential and pH range), mackinawite is expected to form during shorter exposures; however, pyrrhotite should be the key corrosion product formed during longer exposures. Smith and Pacheco [23] reported that the boundary between mackinawite and pyrrhotite is a straight line

2. Experimental API-X60 pipeline steel specimens were exposed to controlled autoclave tests to obtain different types of H2S corrosion products. The chemical composition of the steel is listed in Table 1. All specimens were mechanically ground with up to 800-grit silicon carbide paper, washed with distilled water, and degreased with acetone prior to immersion. An autoclave with a volume of 3 L and made from Hastelloy C276 (UNS N10276) was filled with 5wt% NaCl solution, and the autoclave was purged with high-purity N2 for at least 24 h before the test. The testing temperatures and H2S partial pressures ranged from 30°C to 150°C and from 0.15 to 3 MPa, respectively. Fig. 1 shows a schematic of the experimental setup. The surface morphology of the corrosion product scale formed on the specimen after a certain period of time in the autoclave immersion test was characterized by scanning electron microscopy (SEM, LEO-1450). Microanalysis of the corrosion product scale was carried out by energy-dispersive X-ray spectroscopy (EDS, Rigaku 12 kW), and its crystal structure was analyzed by rotating anode X-ray diffraction (XRD).

Table 1. Chemical composition of API-X60 pipeline steel

wt%

C

Si

Mn

S

P

Nb

Ti

Mo

V

Cu

Ni

Fe

0.071

0.26

1.1

0.004

0.002

0.042

0.006

0.003

0.025

0.01

0.024

Bal.

L. Zhang et al., Environmental boundary and formation mechanism of different types of H2S corrosion…

403

Fig. 1. Schematic of the high-temperature and high-pressure autoclave testing setup.

3. Corrosion product data from autoclave tests Depending on the combination of the selected experimental parameters (30–150°C, 0.15–3.00 MPa, 72–216 h) used for a given autoclave test, two types of H2S corrosion products, specifically, mackinawite and pyrrhotite, were obtained. Fig. 2(a) presents the XRD pattern of the corrosion products formed at 60°C and a H2S partial pressure of 0.3 MPa. The pattern indicates that mackinawite is clearly the dominant product under this test condition. Its crystalline morphology is shown in Fig. 3(a). Among the fine grains, a profusion of flake-like mackinawite with regular crystalline features is formed and grows on the top surface. Fig. 2(b) shows the XRD spectrograms of corrosion products formed at 90°C and a H2S partial pressure of 1.2 MPa. Under these conditions, mackinawite and pyrrhotite were found to be the main products; the micro-morphology of the corroded sample is shown in Fig. 3(b). Integrated hexagonal pyrrhotite crystals with discoid structure covered a mixture of disordered and closely packed FeS grains. Fig. 4 shows the EDS analysis results for samples corroded under different conditions. Fig. 4(a) demonstrates that the flake-like mackinawite contains 49.58at% sulfur atoms, whereas the pyrrhotite contains 55.81at%. Therefore, the concentration of S2− increases with the increase in H2S partial pressure and temperature, and then leads to sulfur-richer pyrrhotite forms. H2S corrosion products formed at various temperatures and H2S partial pressures are plotted in Fig. 5; the corresponding data are listed in Table 2 (entries 1–14). Fig. 5 demonstrates that the data points for mackinawite are located in the area of lower temperatures and lower H2S partial

pressures. By contrast, the data corresponding to pyrrhotite exhibit the opposite trend, thus 1.2 MPa and 60°C were selected as the dividing lines; however, a large area in the figure contains no information. Additional data from literature

Fig. 2. XRD patterns of corrosion products formed under different conditions: (a) 60°C and a H2S partial pressure of 0.3 MPa; (b) 90°C and a H2S partial pressure of 1.2 MPa.

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Fig. 5. H2S corrosion products formed according to the selected experimental parameters.

Fig. 3. Crystal morphologies of flake-like mackinawite and hexagonal pyrrhotite under different conditions: (a) 60°C and a H2S partial pressure of 0.3 MPa; (b) 90°C and a H2S partial pressure of 1.2 MPa.

reports [34–38] are also included in Table 2 (entries 15–21) and are integrated in Fig. 6 with the experimental data from Fig. 5. Fig. 6 shows two boundaries for the corrosion product formation: Line 1 for mackinawite and Line 2 for pyrrhotite. A gap of lacking data exists between Line 1 and Line 2. Notably, “Y” in Table 2 represents “Yes” and indicates the corrosion product that exists under the specified test conditions.

Fig. 6. H2S corrosion products formed according to experimental (■ and ▲) and published (□ and △) data (I: mackinawite; II: mackinawite and pyrrhotite coexist; III: the gap between Line 1 and Line 2).

4. Thermodynamic model 4.1. Model description

Fig. 4. EDS analysis results for mackinawite and pyrrhotite formed under different conditions: (a) 60°C and a H2S partial pressure of 0.3 MPa; (b) 90°C and a H2S partial pressure of 1.2 MPa.

Mackinawite, a type of thermodynamically semi-stable product, is formed when the Fe2+ ion concentration is below the FeS saturation limit in the bulk fluid. With progressive formation and dissolution of mackinawite, the Fe2+ ion concentration in the solution reaches or exceeds the FeS saturation limit. Pyrrhotite deposits on the steel surface when the ther-

L. Zhang et al., Environmental boundary and formation mechanism of different types of H2S corrosion…

modynamic conditions for its formation are reached; meanwhile, mackinawite is slowly transformed into pyrrhotite. Mackinawite is formed and deposited before pyrrhotite because the concentration of Fe2+ ions in the solution has not reached the saturation limit. The reaction to form mackinawite (M) in solution can be summarized as follows. H 2S(g) → H 2S(aq)

(1)

Fe 2 + + H 2S(aq) → FeS(M) + 2H +

(2)

405

For reactions (1) and (2), their reaction equilibrium constants, K H 2Saq /H 2Sg and K FeS/Fe2+ , can be respectively defined as K H 2Saq /H 2Sg = K FeS/Fe2+ =

aH2Saq

(3)

r × PH 2Sg

aH2 + × aFeS

(4)

aFe2+ × aH 2Saq

Table 2. Summary of H2S corrosion products formed under different experimental conditions No.

Experimental parameters t/h T / °C PH2S / MPa

Mackinawite

Products Pyrrhotite Troilite Pyrite Cubic FeS

Remark

1

120

60

0.15

Y

⎯

⎯

⎯

⎯

PH2S / PCO2 = 1.7

2

120

60

0.3

Y

⎯

⎯

⎯

⎯

PH2S / PCO2 = 1.7

3

120

60

1.0

Y

Y

⎯

⎯

⎯

PH2S / PCO2 = 1.7

4

120

60

1.5

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 1.7

5

120

60

2.0

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 1.7

6

120

60

2.5

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 1.7

7

120

130

3

Y

Y

Y

⎯

⎯

8

96

30

1.2

Y

⎯

⎯

⎯

⎯

Pure H2S PH2S / PCO2 = 2

9

96

60

1.2

Y

⎯

⎯

⎯

⎯

PH2S / PCO2 = 2

10

96

90

1.2

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 2

11

96

120

1.2

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 2

12

96

150

1.2

Y

Y

⎯

⎯

⎯

PH2S / PCO2 = 2

13

72

90

1.5

Y

Y

Y

⎯

⎯

PH2S / PCO2 = 7.5

14

216

90

1.5

Y

Y

Y

Y

⎯

PH2S / PCO2 = 7.5

15 16 17 18 19 20 21

72 72 ⎯ 72 ⎯ ⎯ 168

35 100 80 100 110 110 120

1.5 1.5 0.1 0.01 0.56 0.86 0.35

Y Y Y Y Y Y Y

Y Y ⎯ ⎯ Y Y Y

Y ⎯ ⎯ ⎯ ⎯ ⎯ ⎯

⎯ ⎯ ⎯ ⎯ ⎯ ⎯ ⎯

⎯ ⎯ ⎯ ⎯ ⎯ ⎯ ⎯

Wikjord et al. [34] Wikjord et al. [34] Thomason [35] Ren et al. [36] Choi et al. [37] Choi et al. [37] Kvarekval et al. [38]

where aH2Saq , aFeS , aFe2+ , and aH + are the activity of H2S, FeS, Fe2+, and H+ in the solution, respectively, PH2Sg is the partial pressure of H2S, and r is the H2S fugacity coefficient in the gas phase. Eq. (3) multiplied by Eq. (4) results in Eq. (5). However, the activity of a solid, such as FeS, is assumed to be 1, as well as the activity of Fe2+ near or at the metal surface. Thus, Eq. (5) can be simplified into Eq. (6), which can be rearranged as Eq. (7). Because aH+ = 10− pH , Eq. (7) can be transformed into Eq. (8). K H 2Saq /H 2Sg × K FeS/Fe2+ =

aH2Saq r × PH 2S g

×

aH2 + × aFeS

aFe2+ × aH2Saq

(5)

K H 2Saq /H 2Sg × K FeS/Fe2+ =

PH 2Sg = PH 2Sg =

aH2 +

r × PH 2Sg

aH2 +

r × K H 2Saq /H 2Sg × K FeS/Fe2+ 10−2pH r × K H 2Saq /H 2Sg × K FeS/Fe2+

(6)

(7) (8)

By using Eq. (8), we can obtain the critical partial pressure of H2S to form mackinawite in a particular system. The values of r and pH are properties of the production system and can vary. By contrast, equilibrium constants K H 2Saq /H 2Sg and K FeS/Fe2+ , which are dependent upon the thermodynamic

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properties of the reactants and the temperature, can be calculated using the Gibbs free energy of the corresponding reaction: ln K R =

ΔGRT − RT

(9)

According to Eq. (9), if the Gibbs free energy of the reaction at the corresponding temperature is available, then the equilibrium constant for the reaction can be obtained. In general, the standard-state thermodynamic data (25°C, 0.1 MPa) are available in the chemistry reference literature; however, the thermodynamic data at temperatures other than 25°C need to be calculated. For a chemical reaction, the Gibbs free energy can be expressed as ΔGRT = ∑ products GT○− − ∑ reactants GT○− + RT ln

p ∏ aproducts r ∏ areactants

(10)

where GT○− is the standard Gibbs free energy of each substance at temperature T, aproducts and areactants are the ac-

tivity of products and reactants, respectively, and p and r represent the algebraic sums of the products and reactants involved in the equilibrium, respectively. A difficulty encountered in solving Eq. (10) is the calculation of the Gibbs free energy for each substance on the non-standard state because data related to entropy, enthalpy, isopiestic specific heat of reaction, and several coefficients are required. Some standard thermodynamic data involved in the formation process of mackinawite (M) and pyrrhotite (P) are listed in Table 3. The Gibbs free energy of reaction (2) can be obtained using Eq. (11), which, in turn, is equal to Eqs. (12) and (13) according to Eq. (10). This approach is a classic calculation method for non-standard thermodynamic data. When the data presented in Table 3 are substituted into Eq. (13), a temperature-dependent function in which the temperature is expressed in K is obtained as Eq. (14):

Table 3. Standard state thermodynamic data [23] Species

− ΔH ○ / (kJ⋅mol−1)

ΔS ○− / (J⋅mol−1)

− ΔG ○ / (kJ⋅mol−1)

− ΔCP○ / (J⋅mol−1⋅K−1)

a

b

c

FeS (M)

−91.91

60.67

−93.30

50.52

5.59

22.96 × 10−3

0.11 × 105

FeS (P)

−105.44

60.79

−101.59

49.88

8.25

13.57 × 10−3

0.34 × 105

H2S (aq)

−39.75

121.34

−27.87

131.80

0

106.00 × 10−3

0

−3

H2S (g)

−20.60

205.69

−33.44

34.25

6.20

5.25 × 10

0.39 × 105

Fe2+

−92.26

−105.86

−91.50

−33.05

0

−26.50 × 10−3

0

0

0

0

0

H+ Note:

0 − ΔCP○

0

0

indicates the standard molar heat capacity under constant pressure.

ΔGRT = ∑ productsGT○− − ∑ reactans GT○− + RT ln

aH2 + aH 2Saq

ΔGRT = ∑ products GT○− −

∑ reactants GT○−

+ RT ln10(2lg aH + − lg aH 2Saq )

ΔGRT = ∑ products GT○− −

∑ reactants GT○−

+ 2.303RT ( −2pH − lg aH 2Saq )

ΔGRT = 40.98 − 5.59 × 10−3T ln T + 28.3 × 10−6 T 2 − 5.5T −1 − 0.0247T + 2.303RT (−2pH − lg aH 2Saq )

(11) (12)

(13)

(14)

Thus, the Gibbs free energy can be obtained by measuring the solution pH and calculating the H2S activity of the corrosive water. The Gibbs free energy determines whether reaction (2) can occur and form mackinawite under the test conditions. At the same time, by using the equilibrium constants of reactions (1) and (2) at system temperature, the H2S partial pressure required for the formation of mackinawite in

a particular system can be known. As previously mentioned, mackinawite forms and dissolves continuously in solution, reaches the saturation limit gradually, and then pyrrhotite begins to precipitate. The reactions that allow the formation of pyrrhotite in solution can be summarized as reactions (1), (2), and following (15). FeS(M) → FeS(P)

(15)

Chemical reactions (2) and (15) can be combined and simplified into Eq. (16). The equilibrium constants of reactions (1), (2), and (15) are defined as K H2Saq /H2Sg , K FeS/Fe2+ , and K FeSP /FeSM , respectively. The activity of a solid substance is usually 1; therefore, K FeSP /FeSM = 1 . Multiplying Eqs. (3), (4), and (17) together results in Eq. (5). Notably, in the case of Eq. (5), the Fe2+ activity near or at the metal surface is assumed to be 1, resulting in a series of inferences. FeS reaches saturation in the formation process of pyrrhotite; thus, the Fe2+ activity in

L. Zhang et al., Environmental boundary and formation mechanism of different types of H2S corrosion…

solution can no longer be assumed to be 1. Therefore, formula (8) must be amended by considering the effect of the Fe2+ activity; the result is Eq. (18): Fe2+ + H 2S(aq) → FeS(P) + 2H +

(16)

aFeSP

(17)

K FeSP /FeSM = aFe2+ × PH 2Sg

aFeSM 10−2pH = r × K H2Saq /H 2Sg × K FeS/Fe2+

(18)

Where aFe2+ = α × CFe2+ , α is the activity coefficient of Fe2+ ions in solution at the system temperature and pressure, and CFe2+ is the molar concentration of Fe2+ ions in the bulk solution. Because the value of α is usually 0.1, the activity of Fe2+ ions ( aFe2+ ) in solution required to form pyrrhotite is 10 times smaller than the molar concentration of Fe2+ ions in solution, and an error of one order of magnitude is avoided with the inclusion of α in Eq. (18). When the Fe2+ activity is included, the Gibbs free energy of various reactions involved in pyrrhotite formation can be obtained according to the calculation method proposed in the previous discussion. Specifically, it is possible to know whether pyrrhotite can be formed or not by calculating the activity of H2S, Fe2+, and the solution pH value; moreover, it can be deduced that the Fe2+ ion concentration must be reduced to enable the formation of pyrrhotite. By applying Eq. (10) for reaction (16), we can show that ΔGRT = ∑ products GT○− −

∑ reactants GT○− + RT ln

aH2 +

(19)

aH 2Saq × aFe2+

407

ΔGRT = ∑ products GT○− −

∑ reactants GT○−

− 2.303RT [2pH + lg (aH 2Saq × aFe2+ )]

(20) ΔGRT

−3

−6 2

= 27.15 − 8.25 ×10 T ln T + 32.97 ×10 T − 17T 0.0093T − 2.303 × [ RT + lg(aH2Saq × aFe2+ )]

−1

−

(21)

The Gibbs free energy of reaction (16) under different conditions can be calculated according to Eq. (19) or (20), and the temperature-dependent equation for pyrrhotite formation is obtained (Eq. (21)). 4.2. Model validation Two types of data points, i.e., solely mackinawite and the co-occurrence of mackinawite–pyrrhotite, were selected from Fig. 6 for the calculation of relevant thermodynamic parameters. According to the thermodynamic model proposed in the previous section, the activity of various ions in solution and the Gibbs free energy of all reactants are involved in the formation of mackinawite and pyrrhotite. The results are shown in Table 4. Parameters ΔGR○− (3) and ΔGR○− (15) refer to the standard Gibbs free energy of reactions (3) and (15), respectively, whereas ΔGRT (3) and ΔGRT (15) refer to the Gibbs free energies at temperature T. According to the thermodynamic equilibrium criterion that stems from the general laws of thermodynamics, when ΔGR(3) < 0 and ΔGR(15) > 0, reaction (3) can occur but reaction (15) cannot; i.e., only mackinawite can be formed. Similarly, both reactions (3) and (15) can proceed when ΔGR (3) < 0 and ΔGR (15) < 0; i.e., both mackinawite and pyrrhotite can be formed.

Table 4. H2S corrosion relevant thermodynamic calculation results under different conditions Experimental parameters

T / °C

PH2S / MPa

60 60 60 30 80 100 60 60 60 90 130 100 110 110 120

0.15 0.30 1.20 1.20 0.10 0.01 1.50 2.00 2.50 1.50 3 1.50 0.56 0.86 0.35

Thermodynamic parameters

aH2S /

aFe2+ /

−1

−1

(mol⋅L ) 0.0770 0.1581 0.0773 1.0454 0.0375 0.0031 0.7056 0.9120 1.0920 0.5100 0.7632 0.4493 0.0976 0.1483 0.0959

(mol⋅L ) 9.40 × 10−6 1.32 × 10−5 2.27 × 10−5 1.88 × 10−5 7.83 × 10−6 2.65 × 10−5 2.50 × 10−5 2.85 × 10−5 3.15 × 10−5 2.58 × 10−5 3.45 × 10−5 1.95 × 10−5 1.16 × 10−5 1.35 × 10−5 2.54 × 10−6

aH+ / −1

(mol⋅L ) 1.43 × 10−4 2.02 × 10−4 4.00 × 10−4 3.74 × 10−4 1.16 × 10−4 4.08 × 10−4 4.46 × 10−4 5.17 × 10−4 5.80 × 10−4 1.03 × 10−3 5.39 × 10−4 3.83 × 10−4 2.29 × 10−4 2.65 × 10−4 3.73 × 10−5

pH 3.85 3.71 3.40 3.42 3.94 3.57 3.35 3.29 3.24 3.37 3.27 3.41 3.64 3.57 4.43

− ΔGR○ (3) /

− ΔGR○ (15) /

ΔGRT (3) /

(kJ⋅mol−1) 23.980 23.980 23.980 25.113 23.093 22.224 23.980 23.980 23.980 22.656 20.953 22.224 21.796 21.796 21.372

(kJ⋅mol−1) 11.684 11.684 11.684 12.781 10.825 9.982 11.684 11.684 11.684 10.401 8.750 9.982 9.566 9.566 9.156

(kJ⋅mol ) −18.034 −18.241 −12.304 −15.113 −21.135 −10.894 −17.920 −17.993 −17.599 −22.176 −28.624 −24.047 −24.149 −24.539 −37.595

−1

ΔGRT (15) / (kJ⋅mol−1) 1.736 0.588 5.023 0.238 2.196 9.561 −0.860 −1.296 −1.179 −2.527 −6.384 −2.636 −0.183 −1.045 −7.687

408

We concluded that the calculated results listed in Table 4 are in agreement with the autoclave test results. Therefore, whether pyrrhotite could form in a specific environment was correctly predicted by thermodynamic calculation. Indeed, we deduced that only mackinawite could be formed in the region with low temperatures and low H2S partial pressures; however, pyrrhotite could dominate at higher temperatures and higher H2S partial pressures. However, Fig. 6 shows a gap between Line 1 and Line 2. Verification of the validity of the thermodynamic model in this gap was necessary. The calculated results of the selected data points in this region indicate that mackinawite and pyrrhotite coexist, as shown in Fig. 7; presumably, a dividing line with a negative slope exists between mackinawite and pyrrhotite, in agreement with previously published results [23].

Int. J. Miner. Metall. Mater., Vol. 24, No. 4, Apr. 2017

following conclusions: (1) The formation of H2S corrosion products obeys the laws of thermodynamics. Mackinawite is mainly formed at low temperatures and low H2S partial pressures, whereas pyrrhotite coexists at higher temperatures and higher H2S partial pressures. (2) A boundary exists between the mackinawite formation region and the pyrrhotite formation region, and this boundary was verified by the thermodynamic model. (3) On the basis of the thermodynamic model, the H2S corrosion products formed under different conditions were correctly predicted.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Nos. 51271025 and 51171022).

References [1]

[2]

[3] Fig. 7. H2S corrosion products at various temperatures and H2S partial pressures (■ and ▲: experiment results verified by the model; □ and △: results calculated by the model only). [4]

5. Conclusions [5]

To establish an adequate thermodynamic model for the mechanism of formation of H2S corrosion products, the relationship between corrosion products, temperature, and H2S concentration was investigated in this study. The corrosion products of API X60 pipeline steel formed under different H2S corrosion conditions were analyzed, and a thermodynamic model was developed to clarify the environmental boundaries for the different products formation and their transformation. On the basis of the extensive experimental data obtained from autoclave tests and from previously published results, the thermodynamic model was validated for the prediction of the formation of mackinawite and pyrrhotite corrosion products. The investigation led to the

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