Corrosion Evaluation of SAW Welded API 5L X-80 Joints in H₂S-Containing Solution

Abstract

The H₂S corrosion resistance of API 5L X80 steel and its welded joint (WJ) were evaluated using the weight loss method in a 5%wt brine solution and different corrosive environments based on a sodium thiosulphate solution (10^–3 and 10^–2 mol/l). The weight loss method, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) were applied to measure the effects of different H₂S concentrations and pH (3 and 5) on the corrosion process and formation of corrosion product films. The results showed that the obtained corrosion rate and corrosion product films for both API 5L X80 steel and its submerged arc welding (SAW) WJ depend on the pH, H₂S concentration and metal surface microstructural characteristics. The corrosion product film consists of two layers with different morphologies. The heat affected zone (HAZ) shows severe localized corrosion attack relative to the base metal (BM) and weld metal (WM), which is attributed to the microstructural characteristics of this region.

corrosion; H₂S; steel; welded joint; localized attack

1 Introduction

The corrosion evaluation of metallic materials in oil and gas refinery environments is particularly important because corrosion can cause economic and human losses. It is well known that H₂S originating from different processes in the oil and gas industry can accelerate the anodic and cathodic reactions of the corrosion process: iron dissolution and hydrogen evolution reaction, respectively^1-10. Studies focusing on improving the resistance of pipeline steels to H₂S corrosion to avoid failures during oil and gas transport have been reported^9-14. In Brazil, the manufacture and use of high-strength low-alloy steel pipelines have been implemented in recent decades; these pipelines are nationally produced using thermomechanical controlled processing (TMCP) without accelerated cooling and fabricated using the UOE process (pressing a U-shaped pipe between two semi-circular dies to form an O-shaped pipe, then welding the pipe closed and circumferentially expanding it to obtain a highly circular shape) for the conduction of liquid and gaseous hydrocarbons. These materials have been developed with specific chemical and mechanical properties, such as mechanical strength, weldability, ductility, resistance to corrosion and hydrogen embrittlement, by adding microalloying elements, such as niobium, vanadium and titanium^15-16.

H₂S is extremely soluble in water. Gaseous H₂S dissolved in an aqueous phase behaves as a weak acid and often causes pitting corrosion of the pipeline. The corrosion process is generally accompanied by the formation of several iron sulphide phases, as the weak acid formed attacks the iron and forms insoluble iron sulphide. The reactions that occur according to the mechanism proposed by Cheng et al.⁵ are

$Fe\mathrm{}\mathrm{}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{}\mathrm{}{Fe}^{+2}+2e^{-}$ (1)

$2H^{+}+\mathrm{}{2e}^{-}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{}\mathrm{}{H}^0\mathrm{}$ (2)

$Fe+\mathrm{}{H}_{2}S+\mathrm{}{H}_{2}O\mathrm{}\mathrm{}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}(FeS{H}^{-}{)}_{ads}+H_3{O}^{-}$ (3)

$\mathrm{}(FeS{H}^{-}{)}_{ads}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\to \mathrm{}\mathrm{}\mathrm{}\mathrm{}FeS{H}^{+}+\mathrm{}{e}^{-}\mathrm{}\mathrm{}$ (4)

$\left(FeSH\right)\mathrm{}\mathrm{}\mathrm{}\leftrightarrow \mathrm{}\mathrm{}\mathrm{}\mathrm{}FeS{H}^{+}+\mathrm{}{e}^{-}\mathrm{}\mathrm{}$ (5)

$FeS{H}^{+}+H_{3}0\mathrm{}\mathrm{}\mathrm{}\mathrm{}\leftrightarrow \mathrm{}\mathrm{}\mathrm{}\mathrm{}F{e}^{+2}+\mathrm{}{H}_{2}S+\mathrm{}{H}_2)$ (6)

The H₂S corrosion resistance properties of steels depend on the steel cleanliness and microstructure^11-21. The welding procedures adopted during tube production in pipeline construction can modify the microstructural and consequently the mechanical properties of the heat affected zone (HAZ). As a result, there is an increasing incidence of different types of corrosion in these regions^15,19-24.

In the present work, weight loss tests, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD) were applied to measure the effects of different H₂S concentrations and pH (3 and 5) on the corrosion process and corrosion product films that were formed on API 5L X80 steel (base metal (BM) and its welded joint (WJ)). The results show that the obtained corrosion rates and characteristics of the corrosion product films for all regions of the WJ (BM, WM and HAZ) and the BM alone depend on the solution pH, H₂S concentration and steel microstructure; for example, a severe attack in the HAZ was evidenced, which indicates the effect of the microstructure on the corrosion attack mechanism.

2 Experimental Procedures

2.1 Materials

Samples were obtained from two different regions: the longitudinal joint, which was obtained using submerged arc welding (SAW) and therefore contained all three regions (BM, WM and HAZ), as shown in Figure 1, and the fully BM. The average values of the chemical composition sampled from two different points (Figure 2) of the BM and WJ were obtained using the optical emission spectroscopy technique according to ASTM E 415-2008²⁵, as shown in Table 1.

Figure 1
Specimen dimensions in mm.

Figure 2
Sampling regions for chemical composition using optical emission spectroscopy.

Table 1
Chemical composition of the base metal and weld metal in wt%.

Element	Base Metal (wt %)	Weld Metal (wt %)	API 5L Standard Maximum (wt %)
(C)	0.058	0.056	0.22
(Si)	0.203	0.273	-
(Mn)	1.71	1.59	1.85
(P)	0.023	0.027	0.025
(S)	0.0031	0.0062	0.015
(Cr)	0.131	0.100	-
(Ni)	0.020	0.023	-
(Mo)	0.192	0.178	-
(Al)	0.044	0.021	-
(Cu)	0.011	0.050	-
(Co)	<0.0015	<0.0015	-
(Ti)	0.015	0.014	c, d
(Nb)	0.064	0.037	c, d
(V)	0.023	0.017	c, d
(B)	0.0007	0.0027	-
(Ca)	0.0032	0.0016	-
(Fe)	Balance	Balance

c: Niobium, vanadium, titanium, or combinations thereof may be used at the discretion of the manufacturer.
d: Sum of the niobium, vanadium, and titanium contents do not exceed 0,15% (API 5L, 2007).

The microstructural characterization of the WJ is presented in Figure 3. The BM consists of a ferritic matrix with martensite and austenite microconstituent (MA); the WM microstructure is mainly acicular ferrite and grain boundary ferrite. The HAZ usually exhibits a ferrite bainitic microstructure with decomposed MA microconstituent regardless of region; in some regions of the fine-grain HAZ, polygonal and almost polygonal ferrites are observed.

Figure 3
Microstructural characterization of the welded joint. a) Base metal (BM). b) Weld metal (WM). c) Coarse grain heat affected zone (CGHAZ). d) Fusion line (FL). e) Fine grain heat affected zone (FGHAZ).

2.2 Weight loss tests

The effects of pH, H₂S concentration and microstructural characteristics on the corrosion rate and corrosion film morphology were studied for the BM and a longitudinal SAW WJ of API 5L X80 steel. Weight loss tests were conducted in a 5%wt NaCl brine and different corrosive environments based on a sodium thiosulphate solution, as listed in Table 2. H₂S was indirectly generated from the reaction of the solution with the metal surface at atmospheric pressure and room temperature, without removing oxygen from the system. The specimens with dimensions of 40×15×5 mm were produced in triplicate to determine the reproducibility of the test. The samples were washed, polished with sandpaper (220-600 mesh) and weighed before the test began. At least three samples from each region (BM and WJ) were submitted to identical conditions for 30 days. After exposure to the testing environment, the samples were removed from the solution, and the corrosion products were removed from the specimen surfaces by scrubbing with a non-metallic bristle brush. After the removal, the specimens were cleaned, dried and re-weighed according to the ASTM G1-03(2011) standard²⁶. The corrosion rate was then calculated as the average of the results obtained from three different samples using the following equation:

$Corrosion\mathrm{}Rate\mathrm{}\left(mpy\right)=\mathrm{}\left(\frac{K\mathrm{*}W}{A\mathrm{*}T\mathrm{*}D}\right)$ (7)

where K = a constant = 3.45 × 10⁶, W = mass loss in g, A = area in cm², T= time in hours and D = density in g/cm³ (7.85).

Table 2
Chemical composition of the solutions used in the weight loss tests.

Solution	Sodium Thiosulphate (Na2S2O3)	Sodium Chloride (NaCl)	pH
Brine	0	5%wt	5.0
Solution 1	10–3 mol/l
Solution 2	10–2 mol/l
Brine	0		3.0
Solution 1	10–3 mol/l
Solution 2	10–2 mol/l

Visual, optical microscopy (OM) and SEM analyses of the samples surfaces were performed to determine the type and morphology of the corrosion film.

3 Results and Discussion

The average corrosion rates obtained in the loss weight tests of both materials (BM and WJ) in different H₂S-containing solutions are recorded in Table 3 and Figure 4. The corrosion rates increase with decreasing pH of the solution, and this effect is more significant for solution 2. In aqueous environments, hydrogen sulphide dissociates and produces different chemical species with different concentrations depending on the pH, temperature and H₂S concentration. For each pH value, the predominant chemical species determines the oxidation and reduction reactions and characterizes the corrosive process^1-3. According to Kane²⁷, for most steels in H₂S-containing environments, the corrosion rates tend to increase with decreasing pH of the aqueous solution. The stable form of H₂S depends on the solution acidity (pH). H₂S is only stable as a dissolved species in aqueous solutions at low pH levels (pH ≤6); at higher pH levels (7-11), the predominant form is HS^-, and for alkaline pH values, the stable form is S^2-.

Table 3
Average corrosion rate obtained from the weight loss tests in different environments

Solution	Material	Rcorr (mm/y)	Rcorr (mpy)
Brine pH=5.0	BM	0.12	4.94
	WJ	0.146	5.76
Brine pH=3.0	BM	0.23	8.99
	WJ	0.25	9.73
Sol 1. 10–3 mol/L Na2S2O3, pH=5.0	BM	0.086	3.39
	WJ	0.093	3.69
Sol 1. 10–3 mol/L Na2S2O3, pH=3.0	BM	0.112	4.41
	WJ	0.109	4.33
Sol 2. 10–2 mol/L Na2S2O3, pH=5.0	BM	0.143	5.63
	WJ	0.148	5.82
Sol 2. 10–2 mol/L Na2S2O3, pH=3.0	BM	0.394	15.04

Figure 4
Average corrosion rates obtained by weight loss tests in different solutions.

According to Shoesmith³ and Ma et al.⁵, in alkaline solutions, the corrosion rate decreased because of the formation of FeS, which is favoured by the reduced solubility of the solid sulphur and the less prevalent hydrogen reduction reaction. For pH higher than 7, a sulphide layer capable of promoting surface passivation is formed. For pH between 4 and 7, the sulphide layer breaks down and allows the iron dissolution to proceed, which increases the formation of precipitates from the same sulphide on the initial layer. Below pH 4, the solubility of the sulphide is increased by reducing the amount of precipitates on the surface^3,5. At lower pH values (less than 3), the amount of precipitate on the steel surface is small, and some solid product of the reaction may appear.

Figure 5 shows the surface appearance of some of the specimens that were tested for weight loss before the removal of corrosion products. The growth of a thicker, more discontinuous layer was observed for solutions with pH = 5.0 relative to the layers formed for solutions with pH = 3.0 for all studied conditions. After the corrosion products were removed from the metal surface, preferential attack was observed in the form of pitting. In solutions with pH = 5.0, in most tests, the BM and WM show uniform corrosion, and localized attack was observed for the HAZ, as shown in Figure 6a. For solutions with pH=3.0, localized attack was observed for all regions of the WJ, and it was more severe for the HAZ in solution 2 than in solution 1, as shown in Figures 6b and 6c.

Figure 5
a) Welded joint in brine, pH=5.0; b) base metal in brine, pH = 3.0; c) welded joint in solution 1, pH= 5.0; d) welded joint in solution 1, pH=3.0; e) welded joint in solution 2, pH=5.0; f) welded joint in solution 2, pH=3.0.

Figure 6
OM images of the surface appearance of the specimens after the removal of corrosion products in different environments. (a) WJ in brine, pH=5.0. b) WJ in solution 1, pH=3.0. c) WJ in solution 2, pH=3.0.

In general, iron corrosion in H₂S-containing solutions leads to the formation of a FexSy film, where the type of sulphide depends on the H₂S concentration in the solution.

Considering the H₂S concentration of the solutions in this work, the corrosion rate was lower for the brine with solution 1 (10^–3 mol/L Na₂S₂O₃) and higher for solution 2 (10^–2 mol/L Na₂S₂O₃). This effect is thought to be caused by the formation of a partially protective film on the samples in solutions with low H₂S concentration.

In general, higher H₂S concentrations have been found to correspond to higher steel corrosion rates^{1-9,15,24,28-33} because both anodic dissolution and hydrogen evolution processes are accelerated. However, under certain special operating conditions with low H₂S concentrations (<0.04 mmol/l, <15 ppm or <0.05 psi), pH values of 3-5 and immersion times exceeding two hours, the corrosion process can be inhibited, which significantly decreases the corrosion rate^1-9, as shown in the results obtained for solution 1 in this study.

The H₂S corrosion process tends to form iron sulphide (Fe_xS_y) films. Although this film can facilitate hydrogen permeation, it can also act as a physical barrier between the metal and the medium, reducing the corrosion rate²³. Because these films can vary widely in composition and stoichiometry, the protective effect depends on the conditions of the environment in which they are formed (pH, sulphide concentration, presence of chlorides, formation time, liquid or gas phase and microstructure of the substrate)^{2,6, 8,9,13,19,21,23}. In the present case, an increase in the corrosion rate was observed for solution 2, which indicates that the formed film was not protective under these conditions. For solution 1, a decrease in the corrosion rate was observed for the two studied pH values, which indicates the formation of a film that inhibits the corrosion process of the metal substrate under these conditions. These results suggest that the H₂S concentration has a stronger effect than pH on the protective level of the film that forms in different test environments.

It has been found that for unsaturated H₂S solutions, a type of sulphur called mackinawite (Fe₁ + _XS) is initially precipitated and later becomes troilite (hexagonal FeS). Next, a cubic ferrous sulphide is formed. These phases continually precipitate until reaching a pH value of 4. Below this pH, the solubility of these sulphides is sufficiently high for the corrosion rates to also be high. Above pH = 7, a passivating film of mackinawite is developed^1-3. At pH values of 3-5, an inhibitive effect of H₂S is observed, as a protective ferrous sulphide film (FeS) forms on the electrode surface^3,9. Because the corrosion rate is significantly affected by the corrosion product film, the corrosion products were observed visually (Figure 5) and using SEM (Figures 7-8). The structure and composition of the films were determined using EDS and XRD (Figures 7-10).

Figure 7
SEM and EDS characterization of the corrosion product film on the surface of the welded joint tested in solution 2, pH=3.0.

Figure 8
SEM and EDS characterization of the corrosion product film of the welded joint tested in solution 1, pH=3.0.

Figure 10
XRD spectrum of the corrosion product film of the welded joint tested in solution 1, pH=3.0.

The layer exhibits a discontinuous and porous morphology and was mainly formed from oxides, sulphides and chlorides, as shown in the EDS spectra recorded for different areas of the layers (Figures 7-8).

For the specimen tested in solution 2 with pH = 3.0, visual inspection (Figure 5f) suggests the presence of two layers, as shown in Figure 7: an outer layer with crystals formed on top of a porous and irregular black layer. Two different morphologies can be observed. The first black layer grows adjacent to the metal surface, may be mackinawite, and shows some degree of porosity. The second layer consists of crystal clusters with a columnar geometry of possibly a more stable iron sulphide, as suggested by the EDS spectrum.

Figure 7b shows the EDS spectra from different areas of the layer formed on the MB-HAZ interface. The first layer shows the presence of Fe, Cl, Ni, Na, Mn and O, whereas the second layer (columnar crystal) is mainly composed of S, Fe, and Na. The spectra obtained from the layers that were apparently formed first indicate the presence of oxygen in this layer; thus, it can be said that an oxide layer was initially formed and became a type of meta-stable sulphide (mackinawite). Above this layer formed crystals with columnar geometry, which appear to be a type of sulphide enriched in sulphur. This assessment is supported by the XRD spectrum shown in Figure 9.

Figure 9
XRD spectrum of the corrosion product film of the welded joint tested in solution 2, pH=3.0.

The corrosion film that formed for the WJ in solution 1 with pH=3.0 (Figure 8) has different morphologies from that formed in solution 2 (Figure 7). The solution 1 film did not feature columnar crystal clusters. This film exhibits two different morphologies: an inner layer with a continuous and compact morphology adjacent to the metallic substrate surface and an outer layer with a discontinuous, porous and granular morphology above the inner layer. The EDS spectra (Figure 8) and XRD spectrum (Figure 10) show that the inner layer may be an iron oxide (magnetite or maghemite), and the outer layer may be an iron sulphide (mackinawite). The corrosion films that formed for solution 1 exhibit a more regular and compact morphology, which contributes to the formation of protective layers and thus decreases the corrosion rate.

Regarding the effect of the microstructure, the corrosion rates are lower for the fully BM samples, which have a ferritic microstructure and martensite-austenite microconstituent (MA). The metallographic analysis shows that at the HAZ (coarse-grain HAZ (CGHAZ) and fine-grain HAZ (FGHAZ)), the microstructure consists primarily of a bainitic ferritic matrix with decomposed MA, which suffered from severe localized attack (Figure 11). This result is consistent with the fact that the highest calculated corrosion rates were obtained for the WJ. Similar results were obtained by Forero et al.¹⁵ and Garcia et al.²⁸, who studied the H₂S corrosion resistance of microalloyed steel with different microstructures. They observed lower corrosion rates for ferrite+MA microconstituents and an acicular ferritic microstructure than for ferritic-bainitic microconstituents and martensitic microstructures.

Figure 11
Optical microscopy characterization of localized attacks suffered by different microstructures on the welded joint surfaces after the electrochemical tests.

4 Conclusions

The corrosion rate increases with decreasing pH and increasing H₂S concentration generated by the thiosulphate. Solution 2, which has the highest thiosulphate concentration (10^–2 mol/l), shows the highest corrosion rate, but this rate is lower for the low-concentration solution (10^–3 mol/L Na₂S₂O₃). This effect is explained by the formation of a partially protective film on the samples in the solutions with low H₂S concentration.

The corrosion product films that were formed on the two surfaces (BM and WJ) under all test conditions consist of two layers of different morphologies, which depend on the H₂S concentration.

The heat-affected zone (HAZ) showed severe localized corrosion attack, which was attributed to the microstructural characteristics of this region.

Acknowledgements

The authors wish to acknowledge the financial support of CAPES, CNPq, and Faperj.

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Publication Dates

• Publication in this collection
  Mar-Apr 2015

History

• Received
  15 Dec 2014
• Reviewed
  01 Apr 2015