The effect of H 2 S and NaCl concentration on hydrogen permeation in high strength low alloy carbon steel C110

Abstract

The sulphide stress cracking of high strength low alloy (HSLA) carbon steel C110 is related to the hydrogen absorption process. The objective of this research is to evaluate the effect of adsorbed H₂S and Cl⁻ on hydrogen permeation in C110, by studying the responses of hydrogen permeation current and characteristic inductance under environment with different H₂S activities and NaCl concentrations using electrochemical methods, including hydrogen permeation method and electrochemical impedance spectroscopy (EIS). The hydrogen permeation current measured in Devanathan–Stachurski cell suggests that the enhance effect of the adsorbed H₂S on the hydrogen absorption follows an adsorption isotherm behaviour as a function of H₂S activity in solution. The inductive response of EIS under different H₂S activities shows consistency with hydrogen permeation results. The EIS results also indicate that the coverage of H₂S was suppressed by Cl⁻. A good correlation about the effect of adsorbed Cl⁻ is found between inductive response and hydrogen permeation current.

Keywords

Hydrogen permeation NaCl concentration pH value electrochemistry high strength low alloy carbon steel

Introduction

HSLA carbon steels play an important role in oil and gas storage and transportation, where H₂S and water exist [1–4]. However, the utilisation of HSLA steels is restricted by corrosion and mechanical problems when exposing to wet H₂S environment [5–8]. Sulphide stress cracking (SSC) is a major problem which has been studied extensively [2,7,9,10]. SSC is a hydrogen embrittlement phenomenon of HSLA carbon steel in which crack failures or catastrophic damage may occur at the stresses well below the yield strength of material [9,11]. The absorbed hydrogen accumulates in high-stress area in HSLA steel. The crack initiation and propagation occur when accumulated hydrogen in HSLA steel achieves a critical concentration [8,12]. The interaction between dissolved H₂S and HSLA steel interface is the subject of many studies [13 15]. The consensus indicates that a layer of adsorbed H₂S or sulphides is formed depending on the environmental H₂S activity and pH value [8,16,17]. The results from literature propose that the adsorbed H₂S or sulphides layer can accelerate hydrogen adsorption-absorption process and decrease SSC resistance consequently [18], which are characterised by sub-surface hydrogen concentration C ₀ and threshold stress intensity factor K _ISSC respectively, either by suppressing the hydrogen recombination reaction at steel surface, or by catalyzing hydrogen absorption [19].

A recent study shows that, at the same H₂S fugacity, it is possible to obtain different values of K _ISSC by changing the brine ionic strength [6]. The explanation suggested for this behaviour can be described as following: the relationship between K _ISSC and C ₀ heavily depends on the H₂S coverage at metal surface. H₂S coverage can be affected by environmental NaCl concentration, H₂S activity and pH. According to the study of Cancio et al. [20,21], at constant brine ionic strength and pH, H₂S coverage on steel surface is controlled by H₂S fugacity or H₂S activity. Zhang et al. evaluate the effect of Cl⁻ accumulation on H₂S corrosion, the results show that Cl⁻ has small radius and strong penetration, can penetrate corrosion scales and be adsorbed on the metal surface [22]. The study of Case et al. also reports an adsorption interaction between H₂S and Cl⁻, in which Cl⁻ can suppress hydrogen adsorption by competing with H₂S for the adsorption sites on metal surface [6].

Although the detail of the adsorption isotherm of H₂S on HSLA carbon steel and the effect of Cl⁻ on H₂S adsorption have not been understood completely, the results reported in the literatures clearly shows that hydrogen uptake by steel, characterised by C ₀, increases with increasing environmental H₂S activity and decreasing environmental pH, which increasing SSC susceptibility subsequently [23 25].

The objective of this study is to quantitatively describe the effect of adsorbed H₂S and Cl⁻ on hydrogen adsorption-absorption process in HSLA carbon steel C110, by evaluating the response of hydrogen permeation and the formation of characteristic inductive response by electrochemical methods. This study will help to understand the SSC mechanism and contribute to the improving of SSC predictability.

Experimental procedures

Experimental design

Following the objective of this study, an experimental design was developed to evaluate the individual effects of H₂S and NaCl in solution on hydrogen permeation behaviour in HSLA carbon steel C110. The experiment sequence contemplated for the present study is summarised in Table 1. As it can be appreciated, the evaluation is centred on the separate effects of H₂S content in solution and NaCl concentration in the brine.

Table 1

The sequence of experimental conditions.

Experiment	H₂S (mol-%)	NaCl (wt-%)	pH
Effect of H₂S	0.7-35.0	1	3.5 & 5.5
Effect of NaCl	3.5	1-20	3.5

In the experiments evaluating the effect of H₂S on hydrogen permeation, NaCl concentration in brine is constant, the independent variables are the H₂S concentration in gas phase, which can be converted to H₂S activity in solution using thermodynamical speciation calculation, and pH of the solution. For the study of the Cl⁻ concentration effect, the independent variable is NaCl content in solution, whereas the environmental pH and H₂S concentration are kept constant. The dependent variables in our study are the hydrogen permeation rate and the parameters obtained from the analysis of electrochemical tests performed on the corroding surface of the OCTG grade HSLA carbon steel C110.

Materials

The steel studied is OCTG grade HSLA carbon steel C110 with a minimum yield strength of 758 MPa (110 ksi). The chemical composition is shown in Table 2. Before testing, HSLA carbon steel C110 is baked at 200 °C for 8 h [26]. as stated in ASTM B850-98 standard, to remove the trapped hydrogen in steel. The surface of specimen was polished to 800 grits to eliminate the effect of surface roughness on hydrogen permeation.

Table 2

Chemical composition of C110 (wt-%).

	C	Mn	Mo	Cr	Ni	P	S
Min.	–	–	0.250	0.400	–	–	–
Max.	0.350	1.200	1.000	1.500	0.990	0.020	0.005

Hydrogen permeation measurement

Hydrogen permeation is conducted at 1 atm and 25 °C using Devanathan–Stachurski system. As shown in Figure 1, Devanathan–Stachurski system is composed by twin cells separated by HSLA carbon steel C110 membrane with thickness of 0.02 ± 0.005 cm and exposed area of 7.06 cm². The left cell contains test solution with different pH values, H₂S and NaCl concentrations. Environmental pH is adjusted by sodium acetate and acetic acid (CH₃COONa + CH₃COOH) buffer solution. H₂S is generated in the left cell by a chemical reaction between Na₂S and HCl as shown in Equation (1). The H₂S concentration in gas phase is adjusted by controlling the addition of Na₂S·9H₂O and HCl in solution as shown in Table 3 following the calculation results of thermodynamical speciation calculations. These calculations are based on the equilibrium conditions of the decomposition reaction of the sodium sulphide in Equation (1). (1)

Figure 1

Devanathan–Stachurski system.

Table 3

Chemical composition of test solution.

Chemicals Conditions			Sodium Chloride (g L⁻¹)	Sodium Acetate (g L⁻¹)	Sodium Sulphide (g L⁻¹)	Acetic acid (mL L⁻¹)	Hydrochloride acid (mL L⁻¹)
C_NaCl (wt-%)	pH	CH2S (mol-%)	g L⁻¹	g L⁻¹	g L⁻¹	mL L⁻¹	mL L⁻¹
1	3.5	0.7	10	4	0.2	30	0.2
3.5	10	4	1	30	1
5.8	10	4	1.67	30	1.3
10	10	4	3.42	30	2.9
24.8	10	4	7.5	30	6.7
35	10	4	11	30	8.4
5.5	0.02	10	4	0.005	0.6	/
0.35	10	4	0.1	0.3	0.1
3.4	10	4	1	0.3	0.8
30	10	4	9	0.3	7
5	3.5	3.5	50	4	0.895	30	1
10	3.5	100	4	0.78	30	1
15	3.5	150	4	0.69	30	1
20	3.5	200	4	0.625	30	1

The procedure for performing the hydrogen permeation testing conforms to the ASTM G148-97 standard procedure. Before testing, the right side of the polished C110 membrane is electroplated by nickel in Watt Bath (NiCl₂·6H₂O: 45 g L⁻¹, NiSO₄·6H₂O: 240 g L⁻¹, H₃BO₃: 40 g L⁻¹) using a cathodic current of 0.0043 A cm⁻² is applied on the membrane to produce a nickel-plating layer on the steel membrane exposed to the hydrogen reduction side of the Devanathan–Stachurski cell. Previously to the start of the hydrogen permeation testing, both cells are de-aerated by N₂ for 1 h according to ASTM G148-97 (2011) standard before introducing the HCl acid and generate the H₂S in the left cell [25]. A voltage of +300 mV (vs. SCE) is applied to C110 in the right cell. And the current output is recorded as hydrogen permeation current.

From the hydrogen permeation experiments, the sub-surface hydrogen concentration C ₀ is calculated from hydrogen permeation current density by Equations (2)–(4). (2) (3) (4) where J _ss is hydrogen permeation flux J(t) at steady state (mol s⁻¹ cm⁻²), L _C110 is thickness of sample (cm), D _eff is effective diffusivity of atomic hydrogen (cm² s⁻¹), I(t) is hydrogen permeation current detected in right cell, A is immersion area (cm²), F is Faraday constant and t _lag is the time to achieve a value of J(t)/J _ss = 0.63 (s) [27].

Electrochemical impedance spectroscopy (EIS) measurement

EIS measurement is carried out in a three-electrode electrochemical cell with a solution capacity of 250 mL at 1 atm and 25 °C. The environment in the cell is the same as the environment of left cell for hydrogen permeation. The three-electrode system is utilised in EIS measurement comprising working electrode C110 cylinder (length 4.37 ± 0.05 cm, diameter 0.64 ± 0.05 cm and immersion area 6.07 cm²), a W/WO₃ electrode system was used as reference electrode and Pt wire as counter electrode. Testing solution is de-aerated by N₂ for 1 h before introducing the HCl and generate H₂S within the electrochemical cell. The EIS measurement is carried out after letting the H₂S producing reaction reach completion, typically after 30 min. The EIS experiments are carried out using a perturbation with voltage of 10 mV (vs. OCP) and frequency of 10,000 Hz to 0.01 Hz was applied to the testing system.

Results

Effect of H₂S and pH on hydrogen permeation

The hydrogen permeation current density curves obtained from hydrogen permeation testing in 1 wt-% NaCl solution at pH 3.5 and pH 5.5 with varying H₂S concentrations are shown in Figure 2. As it can be observed from Figure 2, a decrease in H₂S concentration and increase in pH value results in a decrease in hydrogen permeation current density within the HSLA carbon steel C110. This indicates H₂S can enhance the hydrogen uptake of HSLA carbon steel C110. And this effect is more noticeable in the environment at pH 3.5 than at pH 5.5.

Figure 2

Effect of environmental H₂S concentration and pH value on hydrogen permeation current density.

The results from the EIS testing, in the form of the corresponding Nyquist diagrams are shown in Figure 3. These experiments were conducted in the same environmental conditions as considered in Figure 2 for hydrogen permeation testing. As it can be observed in Figure 3, all Nyquist diagrams show indications of contribution from a capacitive loop at high and medium frequency and an inductive loop at low frequency. The apparent diameter of capacitive loop can be verified that it increases with increasing pH and decreasing H₂S activity of environment, which suggests that the environment with higher pH value and lower H₂S activity benefits the formation of an iron sulphide protective layer which can increase the faradaic polarisation resistance [27 29]. This is consistent with the hydrogen permeation results in Figure 2, where higher pH value and lower H₂S activity result in lower hydrogen permeation current which is associated with cathodic reaction counterpart of the corrosion reaction taking place at open circuit potential.

Figure 3

Evolution of Nyquist diagram with environmental H₂S concentration and pH value.

Effect of NaCl on hydrogen permeation

The effect of NaCl concentration in solution on hydrogen adsorption-absorption is evaluated by hydrogen permeation and EIS. The hydrogen permeation current density curves at pH of 3.5, H₂S concentration of 3.5 mol-%, and different NaCl concentrations are exhibited by Figure 4. As it can be appreciated a decline of the hydrogen permeation current density is detected when environmental NaCl concentration increases. In addition, the steady-state current density i _ss in 15 wt-% NaCl is similar to the result at 20 wt-% NaCl, suggesting that the incorporation of Cl⁻ on the steel surface follows an absorption isotherm.

Figure 4

The effect of environmental NaCl concentration on hydrogen permeation current density.

The evolution of the EIS response with the NaCl concentration in solution at pH of 3.5 is shown in Figure 5 in the form of the Nyquist diagram, the environmental H₂S concentration of 3.5 mol-% in the gas phase is kept constant during these experiments. Similar to the results shown in Figure 3, the Nyquist diagrams also are composed of a capacitive loop and an inductive loop at different frequencies. The diameter of capacitive loop increases with increasing NaCl concentration, implying the hydrogen evolution is impeded by NaCl.

Figure 5

Evolution of Nyquist diagram with environmental NaCl concentration.

Discussion

Effect of H₂S and pH on hydrogen uptake

Hydrogen permeation

The hydrogen uptake of HSLA carbon steel C110 is characterised by sub-surface hydrogen concentration C ₀ calculated from Figure 2 by Equations (2)–(4). The effect of environmental H₂S activity and pH value is presented in Figure 6 in log-scale. As it can be observed in the results presented in Figure 6, with increasing environmental H₂S concentration the value of C ₀ increases at both pH of 3.5 and 5.5, which indicates that hydrogen absorption is enhanced by the presence of H₂S in solution. This is consistent with the results showing that at the same H₂S concentration, C ₀ at pH 3.5 is higher than at pH 5.5, due to higher H⁺ concentration available for hydrogen evolution in solution at pH 3.5. Besides, a less acidic environment benefits the formation of a compact protective iron sulphide layer [29].

Figure 6

The effect of environmental H₂S Activity and pH value on C ₀, the highlighted region is associated with the limiting C ₀ value which becomes independent of the H₂S concentration at pH 3.5.

According to the literature [14,30 32]. a layer of adsorbed H₂S or containing sulphides is formed, depending on the H₂S concentration and pH, during the hydrogen adsorption-absorption process resulting from the interaction between H₂S contained with the corrosive environment and HSLA steel interface. The relationships between the environmental factors and C ₀ can be described by the general expression defined in Equation (5) [19]: (5) where T is temperature, H₂S and H⁺ stands for hydrogen sulphide and proton concentration in environment respectively, θ_S is the adsorbed H₂S coverage on metal surface through which the catalytic effect on hydrogen adsorption-absorption occurs. Following the relationship shown in Equation (5), it is implicit that the particular relationship between the H₂S concentration in solution and the amount of absorbed H₂S on the steel surface which defines the θ_S isotherm limits the value of C ₀ that can be reached at a given pH and temperature.

The results of the C ₀ value obtained from the permeation curves shown in Figure 6 indicate the presence of a limit value of the hydrogen content with the H₂S activity in solution, at both pH values evaluated. This behaviour suggests that the active sites to receive adsorbents on metal surface are fully occupied in environment with high H₂S activity. Therefore, further increasing environmental H₂S activity cannot increase θ_S and adsorbed hydrogen. Subsequently, according to Equation (5), C ₀ becomes constant. This means the effect of H₂S on C ₀ depends on a specific adsorption isotherm of H₂S. However, the explicit form of the C ₀ dependency with the H₂S concentration in solution can only be assessed if the absorbed Hydrogen coverage can be experimentally measured under the present conditions.

Evaluation of the adsorbed hydrogen coverage

The EIS results in Figure 3 are analyzed using the equivalent circuit in Figure 7 which is used to model the hydrogen evolution and adsorption–desorption process on steel surface [29]. In the equivalent circuit: R _sol is the resistance of solution, R _ct is the charge transfer resistance associated with double-layer capacitance Q _dl, Q _film combined with R _film is utilised to describe the iron sulphide film forming on the surface, and the inductance L _ad is associated with the interaction of the absorbed reduced species which in our case we consider to be hydrogen given. According to the literature [24], the presence of this inductive term in the equivalent circuit is associated with the presence of an adsorbed species as part of electrode kinetics involved in either cathodic or anodic current process at near open circuit potential (OCP) condition. Assuming only one specie is adsorbed on the surface, the relationship between surface coverage and inductive term is detailed as Equation (6). (6) where θ_H is surface coverage of the adsorbed item, which is hydrogen in this study, β is the symmetry constant for the adsorption reaction σ = FΓ ^∞ which is the charge for θ = 1, R is the universal gas constant, T is the temperature (K), k_f and k_b are the adsorption and desorption rate constants, and L is the inductive term of the equivalent circuit.

Figure 7

Equivalent circuit utilised to analyze EIS results.

The fitting results of inductive response L _ad as changing environmental H₂S activity and pH are reported in Figure 8 in log-scale. As it can be observed from the results shown in Figure 8, the increase in the H₂S activity in solution reduces the value of L _ad which is consistent with the expectation that the hydrogen adsorption-absorption is limited by the amount of adsorbed H₂S on the metal surface. Since as shown in Equation (6), there is a reciprocal relationship between L _ad and surface coverage of hydrogen. Therefore, increasing H₂S activity can benefit the hydrogen adsorption on metal surface, consequently, increases hydrogen uptake which is characterised by C ₀.

Figure 8

The effect of environmental H₂S Activity and pH value on L _ad.

The correlation between C ₀ and L _ad, from the results shown in Figures 6 and 8 respectively, is presented in Figure 9. As it can be appreciated in Figure 9, the relationship between C ₀ and L _ad is consistent with Equation (6) and indicates that at the higher H₂S concentrations where the C ₀ values reach their maximum coincide with asymptotical values of the L _ad, this also is consistent with the proposed relationship indicated in Equation (5) on the environmental H₂S activity, i.e. hydrogen uptake can be accelerated by catalytic effect of adsorbed H₂S.

Figure 9

Correlation between C ₀ and L _ad depending on adsorbed H₂S.

The resistance of iron sulphide layer R _film calculated from Figure 3 is shown in Figure 10. The results suggest that with decreasing environmental H₂S concentration and increasing pH, R _film increases, which implies that compact iron sulphide layer is more likely to form under environment with higher pH value and lower H₂S concentration. This is consistent with the hydrogen permeation results shown in Figure 6.

Figure 10

Effect of environmental H₂S activity and pH on resistance of FeS film.

Effect of NaCl on hydrogen uptake

Analysis of the hydrogen permeation results

The impeditive effect of environmental NaCl concentration on hydrogen uptake in environment with 1 mol-% H₂S and pH of 3.5 derived from Figure 4 is shown in Figure 11. An increase in environmental NaCl concentration leads to an exponential decrease in C ₀. This means there should be a critical concentration above which further increasing NaCl concentration would have little impact on hydrogen absorption. However, many studies report that Cl⁻ can accelerate anodic dissolution because that Cl⁻ has a small radius and can penetrate the corrosion scale and change the interface environment [6,33,34]. The possible explanation for this opposite effect of Cl⁻ on anodic dissolution and hydrogen absorption is that Cl⁻ can be adsorbed on the surface, which suppresses the catalytic effect of adsorbed H₂S, hence, impedes the hydrogen adsorption-absorption process. Therefore, the hydrogen absorption decreases at high NaCl concentration though the electrochemical reaction is accelerated. However, to justify this explanation, the relationship between NaCl concentration in solution and the adsorbed hydrogen coverage needs to be measured experimentally.

Figure 11

The effect of NaCl concentration in C ₀.

Effect of Cl⁻ on H₂S and hydrogen adsorption

As previously stated, the correlation between NaCl concentration and adsorbed hydrogen coverage is evaluated from EIS results in Figure 5 to explain the role of Cl⁻ in hydrogen adsorption-absorption process. The fitting results of L _ad point out a negative relation between NaCl and L _ad as reported in Figure 12. This implies that NaCl can impede hydrogen adsorption on metal surface according to the reciprocal relation between L _ad and θ_H shown in Equation (6). These results of EIS show consistency with hydrogen permeation results related to the effect of NaCl concentration.

Figure 12

The effect of NaCl concentration in L _ad.

In addition, a decent in R _ct is distinguished as increasing NaCl concentration as shown in Figure 13, which implies that NaCl can accelerate anodic dissolution. This is consistent with the literature study mentioned before about the catalytic role of Cl⁻ in electrochemical reaction of steel.

Figure 13

Effect of environmental NaCl concentration on R _ct.

This opposite role of NaCl in hydrogen coverage and anodic dissolution can be explained by the competition between NaCl and H₂S. As shown by N. Zhang et al., Cl⁻ also can be adsorbed on the surface, competes with HS⁻, S²⁻ and H₂S for the active sites on metal surface. Assuming both H₂S and Cl⁻ follow a Langmuir adsorption isotherm, an expression is proposed by Case et al. to model the interactive adsorption of H₂S and Cl⁻ as following [6]: (7) where θ_s represent the coverage of H₂S, and are the adsorption equilibrium constant for H₂S and Cl⁻ respectively, and are the activity of H₂S and Cl⁻ in solution, which are related to the concentration of H₂S and Cl⁻, α characterises the ratio of active adsorption sites occupied by H₂S to those of Cl⁻. Equation (7) suggests that as increasing environmental NaCl concentration increases, more active sites are occupied by Cl⁻, which decreases H₂S adsorption. As mentioned, adsorbed H₂S accelerate hydrogen uptake by increasing adsorbed hydrogen coverage on steel surface as shown in the results presented in Figures 6 and 9. Therefore, though the anodic dissolution is accelerated, increasing NaCl concentration still leads to a decline in hydrogen adsorption-absorption.

These results indicate that the major contributor of the hydrogen content at the metal solution interface, C ₀, is the dependency of the hydrogen permeation on H₂S absorption isotherm, this is shown by both the behaviour of the C ₀ as function of the H₂S concentration in solution and its relation to the actual hydrogen absorption coverage inferred from the inductive component in the EIS spectra, also by how the competition with chloride absorption into the steel surface affects both the C ₀ and the H₂S absorption coverage.

The analysis of the experimental data can successfully be performed considering a simple Langmuir type isotherm as proposed in Equation (7), however this not necessarily supported by the experimental data so far. The present study however is limited to only indicating the H₂S absorption effect, although the shape of the respective isotherm is the focus of further studies.

Conclusions

The effect of environmental H₂S activity, pH and NaCl concentration is evaluated for HSLA carbon steel C110 through electrochemical methods including hydrogen permeation and electrochemical impedance spectroscopy. The major conclusions for this study are described below.

H₂S increases hydrogen uptake by increasing hydrogen adsorption on C110 in 1 wt-% NaCl solution.

The catalytic effect of H₂S on hydrogen uptake is more remarkable in solution with pH 3.5 than pH 5.5.

Environmental NaCl concentration decreases hydrogen uptake exponentially by decreasing hydrogen adsorption on HSLA carbon steel C110 in environment with 1 mol-% H₂S at pH 3.5.

A quantitative description of accelerative effect of adsorbed H₂S on hydrogen absorption is developed. It helps to improve the predictability of fracture related to hydrogen embrittlement of C110 in wet H₂S environment. However, more conditions with varying H₂S activity and NaCl concentration need to be measured to develop an isotherm of adsorbed H₂S and NaCl. Besides, additional tests, such as SEM and XRD should be conducted to evaluate the effect of corrosion production on the surface. The authors are currently working on the isotherm of adsorbed H₂S. The results will be reported in future publications.

Footnotes

Acknowledgements

The authors would like to kindly thank ConocoPhillips, ExxonMobil, Shell, Tenaris for their financial support. Michael Elverud and Ubaldo Lopez are also warmly acknowledged for sample making. The help of Lin Chen, Alan Martinez and Yuan Ding in the analysis of impedance data and process of measurement are greatly recognised.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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The effect of H 2 S and NaCl concentration on hydrogen permeation in high strength low alloy carbon steel C110

Abstract

Keywords

Introduction

Experimental procedures

Experimental design

Materials

Hydrogen permeation measurement

Electrochemical impedance spectroscopy (EIS) measurement

Results

Effect of H2S and pH on hydrogen permeation

Effect of NaCl on hydrogen permeation

Discussion

Effect of H2S and pH on hydrogen uptake

Hydrogen permeation

Evaluation of the adsorbed hydrogen coverage

Effect of NaCl on hydrogen uptake

Analysis of the hydrogen permeation results

Effect of Cl− on H2S and hydrogen adsorption

Conclusions

Footnotes

Acknowledgements

Disclosure statement

References

Effect of H₂S and pH on hydrogen permeation

Effect of H₂S and pH on hydrogen uptake

Effect of Cl⁻ on H₂S and hydrogen adsorption