Corrosion behaviour of X52 pipeline steel in high H 2 S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS⁻ species.

Keywords

X52 Pipeline steel High temperature Corrosion products

Introduction

H₂S (hydrogen sulphide) corrosion of pipeline steel is always a significant problem for the oil and gas industries.1 ^– 4 Although many investigations have been carried out to understand the corrosion mechanism since the 1940s, there are still different and even contradictory understandings and theories.1 Along with deep exploitation of oil and gas, H₂S corrosion becomes much severer as the water salinity, the service temperature and the H₂S partial pressure increase.2 The content of hydrogen sulphide in some wells is more than 10%, where the H₂S partial pressure reaches about 3 MPa, and the temperature reaches 95-115°C in the Sichuan sour gas field in China.5 Although more expensive corrosion resistant alloys (CRA) have been developed to resist the H₂S corrosion under high temperature and high H₂S partial pressure conditions, carbon steel is still the most cost effective material used in H₂S-containing environments.6 However, due to the danger of H₂S toxicity, only a few research studies have been carried out under these severe conditions. Through high temperature and high pressure corrosion simulation experiments, Zhang et al.5 concluded that under 1-2 MPa H₂S, the corrosion rate increased with increasing temperature up to 60°C and then decreased gradually. Serious pitting corrosion was found at 30°C. Temperature and partial pressure directly affected the morphology and composition of corrosion products, which in turn caused the change of corrosion rate and occurrence of localised corrosion. The corrosion mechanism of carbon steel under these severe conditions is poorly understood.1 As a result of this limitation, it is difficult to carry out anticorrosion design, so it became an urgency to investigate the corrosion behaviour and mechanism of carbon steel under these severe conditions. Generally speaking, the formation of an iron sulphide film on the metal surface appears to play an important role in the corrosion behaviour of carbon steel in H₂S solutions.7 ^– 9 Various iron sulphides can form during the corrosion of carbon steel in aqueous solution at different conditions, and they show different corrosion resistance.7,10 ^– 12 Mackinawite is a major corrosion product on the surface of carbon steel and is believed to form in almost all aqueous solutions containing H₂S. Mackinawite is formed through both solid-state reaction and precipitation process and has little protective effect as a result of its porous structure and easy transformation to troilite. Cubic FeS has a cubic crystal structure and is considered as only a metastable species. Needle-like troilite appears as a more stable corrosion product on carbon steel surface through precipitation and is a more corrosion resistance product. Pyrrhotite is more stable than mackinawite and it is an iron-deficient iron sulphide with a composition ranging from Fe₇S₈ to stoichiometric troilite. Pyrite has a cubic crystal structure, and it is the most stable and corrosion resistant iron sulphide. Several reports are available regarding the sulphide film formation of carbon steel in the H₂S solution. For instance, Shoesmith et al.13 found that initially a solid-state reaction produces a layer of mackinawite, which readily cracks and spalls from the metal surface. High local release rates of soluble iron from these cracks and pits lead to the precipitation of cubic ferrous sulphide or the more stable phase troilite. Neither the mackinawite base layer nor the upper layer of precipitated material can passivate the surface, so the nucleation and growth of cubic ferrous sulphide and troilite continue steadily. In their study Kvarekval and Nyborg14 suggested that the formation of multilayered films took place due to mass diffusion and potential gradients across the films, which supported the co-existence of different iron sulphides. Iron oxides were probably formed close to the steel surface due to the depletion of sulphide ions through reactions in the outer film layers. The results mentioned above are mainly obtained at low temperatures and low H₂S partial pressure.15 Whether it is applicable at high temperature and high H₂S partial pressure conditions is still not clear. In this paper the corrosion behaviour of a commercial X52 pipeline steel was investigated in pure water solved with a high concentration of H₂S (almost 1M/L) at high temperatures. Weight loss analysis was carried out to evaluate the corrosion resistance. Morphology of corrosion products was obtained by SEM observation. X-ray diffraction was also used to identify the compounds present in the corrosion products. The cross-section of the corrosion product was analysed using SEM and EDS. The corrosion mechanism was also discussed.

Experimental

Material

The test material is a commercial X52 pipeline steel with a 108 mm outer diameter and 10 mm thickness. The chemical composition of this material is presented in Table 1. After being wet-ground with silicon carbide papers up to 2000 grit, the sample was polished and etched in a 2% Nital (nitric acid–ethanol) solution.

Table 1

Chemical composition of X52 pipeline steel

Element	C	Si	Mn	P	S	Al	Ni	Cr	Nb
wt-%	0·17	0·32	1·26	0·014	0·010	0·012	0·017	0·024	<0·01

Test procedure

The immersion test was carried out in a 5 l high temperature and high pressure autoclave CJF-5 with designed temperature 350°C and designed pressure 22 MPa. The testing solution was prepared by dissolving H₂S into deionised water (18·2 MΩ cm). The test conditions are summarised in Table 2. The pressure needed to accomplish the concentration of H₂S was calculated according to Henry's law.16 ^– 18 After deoxygenation by bubbling N₂ for 12 h, the test autoclave was heated to the test temperature, and then charged with H₂S to the target pressure.19,20

Table 2

Test temperature and H₂S concentration of experiments

Test temperature	T±3°C	25	60	90	120	140	90	90	90	90
H₂S concentration	g/l	22	22	22	22	22	4·87	9·71	19·27	27
H₂S partial pressure	MPa	0·63	1·23	1·61	2·18	2·38	0·36	0·72	1·41	1·97

All the test specimens, sized 30 mm×20 mm×3 mm, cut from the material were wet-ground with silicon carbide papers up to 1000 grit and then ultrasonically cleaned with alcohol for 5 min. After being weighed, they were immersed in the test solution. The immersion time lasted for 96 h. After immersion tests, the samples were cleaned by water and dried, and then analysed by SEM and XRD. For each test three replicated samples were ultrasonically washed in a 0·2% EDTA solution for 5-30 min in order to remove the corrosion products. The samples were weighed again after the corrosion products were removed and dried. In order to know the corrosion of the substrate during the corrosion product removal process, a non-corroded sample with the same size and surface finish as mentioned above was also ultrasonically washed in the 0·2% EDTA solution for the same time, and the weight loss of this sample was less than 0·005 g, indicating that the corrosion of the substrate is negligible. Then, general corrosion rates were calculated based upon the weight loss of the samples. The morphology of the corrosion products was observed by ESEM FEI XL30. The crystal structure of the corrosion products was analysed by XRD X'pert PRO PANalytical. The cross-section of the corrosion film was also analysed using SEM and EDX.

A layer of gold sized 5 mm×10 mm×20 nm was initially sputtered on the specimen surface using Gatan 682 model precision etch coating system to investigate the corrosion film formation process in the test carried out at 90°C under H₂S concentration 22 g/l.

Results

Microstructure

Figure 1 shows the typical microstructure of the X52 pipeline steel studied. The microstructure consisted of ferrite and pearlite with randomly distributed elongated manganese sulphides inclusions and small globular oxide inclusions. Analysis of the local chemical composition revealed that the inclusions contained varying amounts of Ca, Mn, S, and Al. The ferrite grain size was in the range of 5-30 μm with curved grain boundaries. The microstructure at low magnification showed banding with ferrite rich and pearlite rich areas elongated along the rolling direction as shown in Fig. 1a.

Figure 1

Microstructure of X52 pipeline steel consisted of banded structure of ferrite and pearlite (a banded structure, b oxide and elongated manganese sulphide inclusions).

Corrosion rate

The influence of temperature on the corrosion rate of X52 pipeline steel at constant H₂S concentration of 22 g/l is shown in Fig. 2. As shown in Fig. 2, the corrosion rate is very severe at all test temperatures.21 The corrosion rate first increases as the temperature increases, then decreases at 120°C and then slightly increases at 140°C. Fig. 3 displays the relationship of the corrosion rate and the H₂S concentration at 90°C. It shows that as the H₂S concentration increases the corrosion rate increases.

Figure 2

Corrosion rate of X52 pipeline steel as a function of temperature at a constant H₂S concentration of 22 g/l.

Figure 3

Corrosion rate of X52 pipeline steel as a function of H₂S concentration at 90°C.

Corrosion products

Figure 4a–e shows the SEM images of the iron sulphides formed on surface of the X52 pipeline steel. The corrosion scales uniformly cover the surface. At 25°C and with a H₂S concentration of 22 g/l, the corrosion products were mainly small tetragonal particles and a few big hexagonal crystals, and the integrity of corrosion scale was poor with many cracks and micro-holes. As the temperature increased the corrosion products were mainly big hexagonal crystals with few small tetragonal particles. Also the corrosion product became much denser. The SEM images of the corrosion products formed at 90°C with different H₂S concentration are shown in Fig. 5a–e. The corrosion products were mainly small tetragonal particles and big hexagonal crystals at low H₂S concentration. The corrosion products mainly changed to big hexagonal crystals as the H₂S concentration increased.

Figure 4

Scanning electron microscope images of corrosion products at different temperatures under an H₂S concentration of 22 g/l (a 25°C;b 60°C; c 90°C; d 120°C; and e 140°C).

Figure 5

Scanning electron microscope images of corrosion products with different H₂S concentrations at 90°C (a H₂S 4·87g/l, b H₂S 9·71g/l, c H₂S 19·29g/l, d H₂S 22g/l, and e H₂S 27g/l).

The cross-section images of the corrosion film were also obtained by SEM. All the corrosion scales consisted of two layers: an inner fine-grained layer and an outer columnar-grained layer. The typical cross-section images of the test sample tested at temperature 90°C under the H₂S concentration of 22 g/l is shown in Fig. 6. As seen in Fig. 6a, the white line that is a gold marker and represents the original sample surface is located at the interface between the inner and outer layers of the iron sulphide corrosion products.

Figure 6

The cross-section images of the corrosion film tested at 90°C under the H₂S concentration of 22 g/l: a gold sputtering sample b without gold sputtering sample.

The phase composition of the corrosion scales at different temperature and H₂S concentration was also analysed by XRD, as shown in Figs 7 and 8. As seen in Figs 7 and 8, there are mainly four types of corrosion products: mackinawite (FeS_1−x), troilite (FeS), cubic ferrous sulphide (FeS) and pyrrhotite (Fe_{1 −x}S). Under the H₂S concentration of 22 g/l, the corrosion products were constructed mainly by mackinawite and a few troilite at 25°C. As the temperature increased the corrosion products were constructed mainly by more thermodynamically stable iron sulphide troilite and pyrrhotite. Also, with the increase of H₂S concentration at 90°C more thermodynamically stable iron sulphide troilite and pyrrhotite became the major products. Mackinawite was formed at all test conditions. Cubic ferrous sulphide as a metastable state iron sulphide only appears in certain tests at 90°C.

Figure 7

X-ray diffraction patterns for the corrosion products formed at different temperatures under the same H₂S concentrations 22 g/l.

Figure 8

X-ray diffraction patterns for the corrosion products formed at different H₂S partial pressures at 90°C.

Discussion

Effect of temperature on the corrosion behaviour of X52 pipeline steels at a constant H₂S concentration of 22 g/l

It is commonly acknowledged that the specific corrosion process determines the corrosion rate of a material under a certain environment. In the H₂S corrosion of X52, the pipeline steel's temperature can affect the corrosion process from two opposite aspects: on the one hand it accelerates the electrochemical reaction of the iron with H₂S and the diffusion process of the reaction species through the corrosion film to reach the reaction sites, these effects increase the corrosion rate and has already been discussed elsewhere by other researchers.22 ^– 24 On the other hand it affects the formation of different types of iron sulphides which show an important effect on the corrosion rate,25 ^– 27 because the corrosion products is able to slow down the corrosion process, which will be discussed in detail in the following part.

During the H₂S corrosion of carbon steel in an anaerobic environment, different types of iron sulphides can form depending on the physical and chemical factors of the solution such as temperature, pH, H₂S concentration, inorganic ions, organic compounds and so on. The corrosion film can slow down the corrosion process by presenting a diffusion barrier for the species involved in the corrosion process which prevents the underlying steel from further dissolution. As more iron sulphides form, the corrosion products grow in density as well as in thickness. The corrosion rate will decrease if the corrosion products show perfect protective effect. The common iron sulphides encountered in H₂S corrosion and their structure and chemical composition are listed in Table 3. The solubilities of different iron sulphides at 0·1 MPa and 1·8 MPa H₂S pressures between 25°C and 125°C have been investigated by Tewarl et al.28 Their results show that the solubilities are in the ratio ≈6000:80:10:1 for mackinawite, troilite, hexagonal pyrrhotite and pyrite respectively. As temperature increases, the solubilities for mackinawite decreases about 80 times but for troilite and hexagonal pyrrhotite it only decreased by a factor of two. This means troilite and pyrrhotite are much more thermodynamic stable than mackinawite. Cubic FeS is a thermodynamically metastable phase. When cubic FeS forms, it will normally transform into either mackinawite at lower temperature or pyrrhotite at higher temperature over a period of a few days.13,29 The corrosion resistance of iron sulphides follows a sequence of mackinawite<troilite<pyrrhotite<pyrite.13 ^– 15

Table 3

Chemical composition and structure of common iron sulfides10

Species	Stoichiomentry	Structure
Mackinawite	Fe_1+xS	Tetragonal
Cubic ferrous sulphide	FeS	Cubic
Troilite	FeS	Hexagonal
Pyrrhotite	Fe_1-xS	Hexagonal or monoclinic
Marcasite	FeS₂	Orthorhombic
Pyrite	FeS₂	Cubic

As shown in Fig. 7 and Fig. 4a, at 25°C the major corrosion product is mackinawite, and mackinawite shows little protection because it forms a loose and porous corrosion film. As a result the diffusion of electrochemical reaction species such as Fe²⁺ and H₂S is easier through this kind of corrosion film. Some investigations30 indicate that mackinawite can even accelerate the corrosion rate. As temperature increases the major corrosion products change to more thermodynamically stable products such as troilite and pyrrhotite. On the one hand with the increase of temperature, the rate of electrochemical reaction increases, this in turn increases the corrosion rate. On the other hand, the formation of more protective iron sulphide such as cubic ferrous sulphide, troilite and pyrrhotite is accelerated, which decreases the corrosion rate. Therefore, as shown in Fig. 2, as temperature increased, the corrosion rate first increased and then decreased. This is in agreement with the experiment results of L. Zhang et al.5 The pH of the solution and supersaturation of mackinawite were calculated using the method suggested by Sun,27 and the results are shown in Table 4. As shown in Table 4, at 120°C the pH reached a minimum, without other ions except H⁺, HS⁻, minus Fe²⁺and S²⁻ in the solution, it indicates that the conductivity of the solution and the concentration of HS⁻ reached a maximum. The precipitation reaction between aqueous Fe²⁺ and dissolved H₂S can be interpreted in terms of two competing reactions.31 The first was donated as a H₂S pathway. The second was named as bisulphide pathway. The formation of Fe(HS)₂ is beneficial to the formation of more thermodynamically stable iron sulphides, such as pyrite. Although the dominant reaction is the H₂S pathway at low pH in the test conditions, the reaction rate of the bisulphide pathway is much faster than that of the H₂S pathway. So the higher concentration of HS⁻ means more precipitation of the protective iron sulphides. This could cause the reduction of the corrosion rate at 120°C.

Table 4

pH and supersaturation of mackinawite at different temperatures and H₂S concentrations (assuming that the bulk solution concentration of Fe²⁺ is 50 ppm)

H₂S concentration (g/l)	22	22	22	22	22	4·87	9. 71	19·29	27
Temperature T±3°C	25	60	90	120	140	90	90	90	90
pH	3·58	3·41	3·34	3·32	3·33	3·66	3·51	3·36	3·29
Supersaturation	0·29	1·32	4·90	17·94	42·32	4·90	4·90	4·90	4·90

Effect of the H₂S concentration on the corrosion behaviour of X52 pipeline steels at 90°C

As shown in Fig. 2, the corrosion rate at 90°C was greater than that at the other four test temperatures, which means the protective effects of the corrosion products at 90°C are still not strong enough to obviously slow down the corrosion rate. As shown in Table 3 the pH decreased as the H₂S concentration increased. Lower pH usually means more severe corrosion.13,26 As shown in Fig. 3, with the increase in H₂S concentration the corrosion rate increased and in the mean time, the corrosion products became more thermodynamically stable and corrosion resistant as shown in Fig. 8. The increasing rate of corrosion decreased with the increase of H₂S concentration before the H₂S concentration reached 27 g/l.

Mechanism of the corrosion process

Although the mechanism of iron sulphide formation in the H₂S environment is still in controversy, in anaerobic water solutions at low pH it is assumed that the iron sulphide forms by reaction processes as follows:3,13 HS⁻ ions chemisorbed on iron to form an approximate monolayer through reaction (1); the anodic discharge occurred through reactions (2)and (3); The species FeSH_ads ⁺ may directly form mackinawite through reaction (4) or may be hydrolysed to yield Fe²⁺ ions through reaction (5); the cathodic reaction include the H₂S dissociation reaction (6) and hydrogen ion reduction reaction(7).

The anodic reactions are as follows (1) (2) (3) (4) (5) The cathodic reactions are as follows (6) (7) At low temperatures a layer of mackinawite initially formed on the steel surface by a solid-state reaction. This process occurred fast and immediately when the sample was immersed into the solution containing H₂S. As the corrosion film grew thicker it cracked and peeled off due to the stress as a result of the volume effect. Mackinawite is a thermodynamically semi-stable form of FeS that forms on the steel surface under conditions where iron sulphide is still soluble in the bulk solution; i.e. the concentrations of Fe²⁺ and H₂S are still below the saturation point of the bulk solution.27 As can be seen from Table 4, mackinawite is still under saturation at 25°C at the constant H₂S concentration of 22 g/l. When more ferrous ions were released from the steel surface especially in the area where the corrosion film peeled off, cubic ferrous sulphide and troilite precipitated on the steel surface because of the high local supersaturation of iron sulphide.13,25,26 So at 25°C the major corrosion product was mackinawite with minor troilite as shown in Figs 7 and 4a.

At high temperatures or high H₂S concentration, more thermodynamic stable iron sulphide such as pyrrhotite, marcasite and pyrite may form on the steel surface.32 As shown in Fig. 7 with the increase of test temperature there was a progression of major product variation from iron rich to sulphur rich products according to the sequence: mackinawite→troilite→pyrrhotite. This is in agreement with the Pourbaix diagrams11,32 ^– 34 developed for the prediction of iron sulphide formation under different conditions. These diagrams indicate that the stable region for sulphur rich sulphides extend towards low pH and low potentials with the increase in temperature and H₂S concentration. So troilite and pyrrhotite are able to form through direct reaction other than through precipitation on the sample surface at high temperatures as shown in Fig. 5a. The mackinawite formation kinetics is much faster than those of other iron sulphides,10 as a result even at high temperature mackinawite is able to form as shown in Fig. 7.

The diffusion of reaction species such as Fe²⁺ and HS⁻ ions through corrosion product film is an important factor which affects the corrosion mechanism. It is assumed that Fe²⁺ ions diffuse through the corrosion film from the steel/corrosion film interface to the corrosion film/solution interface to react with HS⁻ and H₂S15. But as shown in Fig. 6a the white line which represents the original sample surface is in the middle of the corrosion film. A thin layer of mackinawite is assumed to form near the steel/corrosion film interface and big hexagonal crystals of troilite and pyrrhotite are formed mainly by precipitation.13,27 The inner fine-grained layer was constructed mainly by mackinawite and the outer columnar-grained layer mainly by troilite and pyrrhotite at 90°C as shown in Fig. 4c. Mackinawite is iron rich and pyrrhotite is sulphur rich. Cation vacancy diffusion with Fe ²⁺ ions moving from the metal through the corrosion film to the corrosion film/solution interface is the usually accepted mechanism. But this probably does not happen in the iron rich inner layer, since there are few cation vacancies in iron rich phases. Thus the diffusion of sulphide ions such as H₂S and HS⁻ dominates in the inner layer. In the contrast the Fe²⁺ diffusion dominates in the sulphur rich outer layer. It is known that Fe²⁺ ions diffuse more rapidly along the c-crystallographic direction in the pyrrhotite crystal.35 so the grains start growing along the preferred c-crystallographic direction as seen in Fig. 5a. On the basis of this consideration, it is concluded that the inward migration of sulphide ions leads to iron rich mackinawite formation near the steel/corrosion film interface eventually established in the fine-grained inner layer; the outward migration of Fe²⁺ ions leads to troilite or sulphur rich pyrrhotite formation near the corrosion film/solution interface. This is in agreement with the experiment results obtained in H₂S and H₂ mixture.36

Conclusions

The structure and thermodynamic stability of the corrosion products and their corrosion resistance had an obvious effect on the corrosion rate of X52 pipeline steel. Temperature and H₂S concentration directly affected the morphology and composition of corrosion products, which in turn caused the change in corrosion rate. The corrosion rate of X52 pipeline steel first increased with temperature up to 90°C and then decreased at 120°C and then increased at 140°C again under a constant H₂S concentration of 22 g/l. The corrosion rate of X52 pipeline steel increased with the H₂S concentration at 90°C.

Mackinawite was able to form at all test conditions due to its fast formation kinetics. As temperature increased from 25°C to 140°C under constant H₂S concentration of 22 g/l, there was a major product variation from iron rich to sulphur rich products according to the sequence: mackinawite→troilite→pyrrhotite. Troilite and pyrrhotite were able to form through direct reaction other than through precipitation on the sample surface above 90°C. The corrosion film is formed through the combination of outward diffusion of Fe²⁺ and inward diffusion of H₂S and HS⁻.

Footnotes

Acknowledgements

The authors acknowledge the financial support for this study from Nature Science Foundation No. 51025104.

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Corrosion behaviour of X52 pipeline steel in high H 2 S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

Keywords

Introduction

Experimental

Material

Test procedure

Results

Microstructure

Corrosion rate

Corrosion products

Discussion

Effect of temperature on the corrosion behaviour of X52 pipeline steels at a constant H2S concentration of 22 g/l

Effect of the H2S concentration on the corrosion behaviour of X52 pipeline steels at 90°C

Mechanism of the corrosion process

Conclusions

Footnotes

Acknowledgements

References

Effect of temperature on the corrosion behaviour of X52 pipeline steels at a constant H₂S concentration of 22 g/l

Effect of the H₂S concentration on the corrosion behaviour of X52 pipeline steels at 90°C