Influence of O₂ on the Erosion-Corrosion Performance of 3Cr Steels in CO₂ Containing Environment

Abstract

With the introduction of O₂ during oil and gas production, the erosion-corrosion rate of tubing steels increases; the objective of this report is to explore the reason for this. Erosion–corrosion experiments were performed in environments of CO₂ and CO₂–O₂, respectively. Macrographs, microstructures, and the compositions of erosion-corrosion scales were investigated using a digital camera, scanning electron microscope (SEM), Kevex-SuperDry energy spectrometer (EDS) and X-ray diffraction (XRD). The results show that the erosion-corrosion products are composed of large FeCO₃ particles and some amorphous product in the CO₂ environment, while they are made up of FeCO₃, Fe₂O₃, Fe₃O₄, and bits of amorphous product in the CO₂–O₂ environment. The interface between erosion-corrosion scales and the substrate of 3Cr steel is smooth, and Cr enrichment obviously exists in the erosion-corrosion products in the CO₂ condition. However, the erosion-corrosion scale is loose and porous with little Cr enrichment in the CO₂–O₂ environment, which makes the protectiveness of the erosion–corrosion scale weak, and pitting corrosion occurs. The addition of O₂ may destroy the protective FeCO₃ scale and Cr enrichment in the erosion-corrosion scale, which may be the main reason for the decline in the level of protectiveness of the erosion-corrosion scale, making it weak in terms of preventing the corrosive medium from diffusing to the substrate.

1. Introduction

In the later stages of oilfield development, the addition of an oil displacement agent into the oil well is a common technology used to increase oil recovery [1]. Carbon dioxide (CO₂) has been studied for many years as a widely-used oil displacement agent [2]. It can expand the volume and reduce the viscosity of crude oil [3]. Much equipment employed in the production of oil and gas is made up of steel, such as pipelines. Dry CO₂ does not corrode steel. However, there is formation water in oil and gas extraction, and this can significantly promote electrochemical corrosion in steel in humid or water-containing environments, leading to uniform and/or local corrosion [4].

CO₂ corrosion is an important degradation mechanism for tubing in the oil and gas industries. It could shorten the service life of pipelines and affect normal production [5,6,7,8,9]. In flow systems, erosion occurs between corrosive media and pipeline materials due to their relative motion. Corrosion and erosion will affect each other, resulting in faster failure of the pipeline. A certain amount of alloying elements can be doped in the steel to enhance erosion-corrosion resistance. The first choice should be elements with good corrosion resistance and low price. Steel doped with chromium (Cr) is frequently employed to significantly increase the anti-CO₂ corrosion ability of the tubing material and decrease the probability of local corrosion and/or pitting. The service time of the material can be significantly extended by increasing the concentration of Cr. When the concentration of Cr reaches 3%, the corrosion rate decreases significantly [10]. Therefore, 3Cr steel has been widely used as pipeline steel.

O₂ contamination will inevitably occur during oil and gas production by adding chemicals, shutdowns of newly-installed piping, leaky fittings, and oil recovery [11,12]. In addition, in the process of thickened oil thermal extraction, a high-temperature fluid which contains a great deal of oxygen (O₂) as well as water (H₂O), CO₂, and nitrogen (N₂) is injected into oil wells [13]. O₂ coexisting with CO₂ can make environmental corrosiveness more severe and cause an increase in the erosion-corrosion rate of tubing steels [14]. Great efforts have been made to minimize CO₂ corrosion [5,6,7,8,9], but research on the erosion-corrosion performance of tubing steels under a CO₂–O₂ condition is far from adequate, especially in terms of erosion-corrosion performance comparisons between aqueous CO₂ systems with and without O₂. In this paper, the erosion-corrosion performance of 3Cr steel in a CO₂–O₂ environment was investigated in comparison with that of a CO₂ environment. The erosion-corrosion rates were obtained by observing weight loss. Macrographs of the erosion-corrosion scale were obtained using a digital camera. The microstructures and compositions of the erosion-corrosion scale were analyzed by scanning electron microscopy (SEM), energy spectrometer (EDS), and X-ray diffraction (XRD).

2. Materials and Methods

The erosion-corrosion test was conducted using high-temperature and high-pressure equipment (HTHPE) in environments of CO₂ and CO₂–O₂, respectively. Figure 1 displays a schematic diagram of the HTHPE. It is principally composed of a gas control system, a heating system, a temperature control system, and a rotating system. The gas control system comprises an inlet pipe, an exhaust pipe, and a pressure gage. CO₂ and/or O₂ can enter the HTHPE through the air inlet pipe, and they can be discharged from the HTHPE through the exhaust pipe. The pressure of CO₂ and/or O₂ in the HTHPE is controlled by the pressure gage. The heating system can heat the HTHPE to elevate the system temperature; the temperature is controlled by a temperature control system. There is a rotating rod in the rotating system, and there are two stirring blades at the bottom of the rotating rod to stir the solution in the HTHPE. Specimens can be fixed on the rotating rod and rotate with it. The shape and size of the specimens are shown in Figure 2; they comprised a 45° stepped circular arc with an inner diameter of 96 mm and a thickness of 4 mm. The width of the outer ring was 15 mm and that of the inner ring was 12 mm.

The specimens were made of 3Cr steel. The chemical compositions of the tested material are as follows: C 0.37 at.%, Si 0.48 at.%, Mn 0.51 at.%, S < 0.009 at.%, P < 0.02 at.%, Cr 3.37 at.%, Ni 0.03 at.%, Ti 0.01 at.%; the rest is Fe. Figure 3 shows the microstructure of the specimen, which reveals acicular ferrite. Before the experiments, the working surface of the specimens was ground with silicon carbide paper to 800 grit. The specimens were cleaned with an ultrasonic cleaning device (GA008, Shenzhen Guanboshi industrial technology co. LTD, Shenzhen, China) for 20 min in acetone to remove grease, and then cleaned with alcohol and dried. They were weighed using an electronic analytical balance with an accuracy of 0.1 mg. The surfaces of the specimens, except for the working surface, were coated with 704 silicone rubber (Guangdong Hengda new material technology co. LTD, Huizhou, China), so that they would be isolated from the corrosive environment. The specimens were then fixed onto a rotating rod. Three of them are employed to study the average erosion-corrosion rate. The others were used for analyses of the structure and composition of the erosion-corrosion products. A certain amount of simulated solution was added to the HTHPE to simulate the water conditions of an oil and gas field. The component concentration of the test solution is given in

Table 1. Chemical composition of formation water drawn from the oil field.

Composition	Content (mg/L)
MgCl2.6H2O	73.8
CaCl2	49.9
Na2CO3	383.6
NaHCO3	1022.7
Na2SO4	7.1
NaCl	543.2
KCl	1.8

, which ignores oily compounds in water extracted from the oil and gas field. The specimens were submerged in the solution. The experiment conditions are given in

Table 2. Experimental parameters.

Test No.	Pressure of CO2 (MPa)	Pressure of O2 (MPa)	Temperature (°C)	Flow Velocity (m/s)	Time (Hour)
A	2.5	0	120	1	120
B	2.5	0.2	120	1	120
C	2.5	0.4	120	1	120
D	2.5	0.6	120	1	120

. The flow velocity given in Table 2 means that the linear velocity of the sample rotation was used to approximate the relative velocity between the samples and the liquid.

After the erosion-corrosion experiment, the specimens were removed from the HTHPE. The specimens were washed with deionized water and analytically pure alcohol, and then dried. Macrographs of the erosion-corrosion scales and the metal substrate after removal of the erosion-corrosion scales were obtained using a digital camera. The erosion-corrosion products of three specimens were removed by chemical cleaning. The cleaning solution was made up of 500 mL deionized water, 500 mL HCl, and 3.5 g urotropine (C₆H₁₂N₄). The specimens were weighed after wiping; the average erosion-corrosion rate was calculated using Equation (1):

$${\mathrm{C}}_{\mathrm{R}}=\frac{8.76\times {10}^4\times \left({\mathrm{m}}_0-{\mathrm{m}}_1\right)}{\mathrm{St}\mathsf{\rho }}$$

(1)

where C_R is the average erosion-corrosion rate, mm/a; m₀ is the weight of original specimen, g; m₁ is the weight of the specimen after removing the erosion-corrosion product film, g; t is the erosion-corrosion time, h; ρ is the density of the specimen, g/cm³; and S is the erosion-corrosion surface area of specimens, cm².

The microstructure and elemental composition of the erosion-corrosion scales on the specimen surface under different conditions were analyzed using a SEM (ULTRA 55 SEM, Carl Zeiss AG, Jena, Germany) and EDS (INCA X-MAX 50 EDS, Oxford Instruments, Oxford, UK). The erosion-corrosion product powder was prepared by scraping the erosion-corrosion product off the surface of the specimens with a blade, and a semiquantitative analysis of the erosion-corrosion product film was carried out by XRD (D8 Advance XRD, Bruker Corporation, Karlsruhe, Germany) with the internal standard of ZnO [15]. The mass ratio of ZnO and erosion-corrosion products was 3:7.

3. Results and Discussion

The average erosion-corrosion rate of the 3Cr steel was calculated after the erosion-corrosion experiments; the results are shown in Figure 4. It turns out from the data in Figure 4 that the erosion-corrosion rate increased as the O₂ partial pressure went up. There was a significant difference in the erosion-corrosion rate of 3Cr steel comparing the CO₂ and the CO₂–O₂ conditions. It was only about 0.5 mm/a in the CO₂ environment, but increased significantly when there was 0.2 MPa O₂ in the environment, reaching about 4.3 mm/a. Compared with the change from 0 MPa to 0.2 MPa in the partial pressure of O₂, the erosion-corrosion rate of 3Cr steel increased slowly when the O₂ partial pressure increased from 0.2 MPa to 0.6 MPa.

The macrostructure of the erosion-corrosion scales and the substrate after the removal of the erosion-corrosion products are presented in Figure 5. As shown, the erosion-corrosion products were dense, and the substrate was flat and smooth under the condition of CO₂ (Figure 5a,b). In the CO₂–O₂ environment, the erosion-corrosion product was loose and reddish-brown (Figure 5c,e,g); the surface of the substrate after their removal was quite rough, with several pits of different sizes (Figure 5d,f,h). A certain amount of erosion-corrosion product fell off the surface of the specimens when the pressure of O₂ was 0.6 MPa in the environment.

To further explore the microstructure and components of the erosion-corrosion products in CO₂ and CO₂–O₂ environments, SEM and EDS analysis were performed. The micro image and elemental composition are set out in Figure 6 and

Table 3. Quantitative EDS results of erosion-corrosion products of the tested material under different conditions.

Conditions	Elements Concentration, at.%
	C	O	S	Cl	Cr	Si	Fe
PCO2: 2.5 MPa	50.23	37.79	-	-	0.65	0.29	11.04
PCO2: 2.5 MPa PO2: 0.6 MPa	28.54	43.96	0.10	0.90	0.13	-	26.36

. From Figure 6, we can see that grains of different sizes are stacked tightly on the sample surface in the CO₂ environment, while there are no obvious grains on the surface of the specimen; the shape of the erosion-corrosion products is irregular in the CO₂–O₂ environment. The components of the erosion-corrosion scales were mainly elemental C, O, and Fe in the CO₂ and CO₂–O₂ environments. The atomic ratio of elemental O and Fe was about 3 under the condition of CO₂, while it was about 1.7 in the CO₂–O₂ environment. The elemental Cr content in the erosion-corrosion products was obviously lower than that in the substrate of 3Cr steel under the conditions of CO₂ and CO₂–O₂.

The composition of the erosion-corrosion products under different conditions was also analyzed by XRD, as set out in Figure 7. The data obtained from Figure 7 indicates that the erosion-corrosion scale was generally made up of FeCO₃ in the CO₂ environment, which is consistent with the atomic ratio of elemental O and Fe in Table 3. In addition to FeCO₃, there were also Fe₂O₃ and Fe₃O₄ in the erosion-corrosion products under the condition of CO₂–O₂. There was also a certain amount of amorphous erosion-corrosion product in the CO₂ and CO₂–O₂ environments. Kermani et al. demonstrated that doping steel with Cr causes the erosion-corrosion product film to transform from a crystalline to an amorphous state, the composition of which is mainly Cr(OH)₃, Cr₂O₃ and/or some FeCO₃ [16]. Comparing with the CO₂ environment, there were fewer amorphous erosion-corrosion products in the CO₂–O₂ environment. A semiquantitative calculation of the crystalline erosion-corrosion scales of the tested steels is presented in

Table 4. XRD semiquantitative calculation data of erosion-corrosion scales of tested steels in different conditions.

Tested Conditions	XRD Results, wt %
	FeCO3	Fe3O4	Fe2O3
PCO2: 2.5 MPa	100	--	--
PCO2: 2.5 MPa, PO2: 0.2 MPa	33.2	19.0	47.8
PCO2: 2.5 MPa, PO2: 0.4 MPa	24.0	26.2	49.8
PCO2: 2.5 MPa, PO2: 0.6 MPa	34.5	33.4	32.1

. The data shows that the quantity of FeCO₃ decreases, and the amount of Fe₂O₃ and Fe₃O₄ increases when the O₂ pressure rises from 0 MPa to 0.4 MPa. However, there was some increase for the amount of FeCO₃, while the amount of Fe₂O₃ and Fe₃O₄ decreased when the O₂ pressure reached 0.6 MPa.

Figure 8 displays the cross-section morphologies and the concentration of Cr in the erosion-corrosion products under CO₂ and CO₂–O₂ environments. As shown in Figure 8, the interface between the erosion-corrosion products and the substrate was flat in the CO₂ environment. The concentration of Cr increases and then decreases gradually with the erosion-corrosion product from inside to outside. Comparing with the Cr element concentration in the steel substrate, it was obviously higher in the erosion-corrosion product near the substrate. The erosion-corrosion products may be divided into two distinct layers, and the interface between erosion-corrosion products and the substrate is relatively rough in the CO₂–O₂ environment. The concentration of Cr in the inner erosion-corrosion product was similar to that of the substrate. However, there was only a small amount of Cr in the outer erosion-corrosion product under the condition of CO₂–O₂.

In the CO₂ environment, CO₂ can react with H₂O to form H₂CO₃, and some H₂CO₃ will be further dissociated to H⁺, HCO₃^-, and CO₃²⁻ in the solution (Equation (2)–(4)). Therefore, the solution is slightly acidic. A lot of micro-corrosion cells can be formed at the interface between the 3Cr steel and the solution. The H⁺ is oxidized to hydrogen (H₂), and Fe and Cr are reduced to Fe²⁺ and Cr³⁺ (Equation (5)–(7)). The solution on the steel surface gradually becomes slightly alkaline as the reaction progresses. The OH⁻ can react with Cr³⁺ to form Cr(OH)₃ near the 3Cr steel surface (Equation (9)). Because the solubility product (K_sp) of Cr(OH)₃ is quite small, i.e., 6.3 × 10⁻³¹, some Cr(OH)₃ may further react to form Cr₂O₃, according to Equation (10). Fe²⁺ can react with CO₃²⁻ to form FeCO₃ (Equation (11)), and the FeCO₃ will be deposited on the surface of metal to form an erosion-corrosion product film. However, the generation of FeCO₃ occurs later than that of Cr(OH)₃, because the K_sp of FeCO₃ (3.2 × 10⁻¹¹) is larger than that of Cr(OH)₃. That may be the reason why there is Cr enrichment in the inner erosion-corrosion scale in Figure 8b. Cr(OH)₃ and Cr₂O₃ are amorphous, so they are not recognized by XRD in Figure 7. The erosion-corrosion scale is dense and stable in terms of preventing the corrosive medium from reaching the surface of the substrate effectively [17]. Therefore, the erosion-corrosion rate of 3Cr steel is quite low under the condition of CO₂ (Figure 4).

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\to {\mathrm{H}}_2{\mathrm{CO}}_3$$

(2)

$${\mathrm{H}}_2{\mathrm{CO}}_3\to {\mathrm{H}}^{+}+{\mathrm{HCO}}_3^{-}$$

(3)

$${\mathrm{HCO}}_3^{-}\to {\mathrm{H}}^{+}+{\mathrm{CO}}_3^{2-}$$

(4)

$$2{\mathrm{H}}^{+}+2\mathrm{e}\to {\mathrm{H}}_2$$

(5)

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

(6)

$$\mathrm{Cr}-3\mathrm{e}\to {\mathrm{Cr}}^{3+}$$

(7)

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{OH}}^{-}$$

(8)

$${\mathrm{Cr}}^{3+}+3{\mathrm{OH}}^{-}\to \mathrm{Cr}{(\mathrm{OH})}_3$$

(9)

$$2\mathrm{Cr}{(\mathrm{OH})}_3\to {\mathrm{Cr}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(10)

$${\mathrm{Fe}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{FeCO}}_3$$

(11)

O₂ may react with H₂O and form OH^- near the 3Cr steel surface (Equation (12)). The solution will become alkaline. Fe²⁺ can be oxidized to Fe³⁺ by O₂ (Equation (13)). Fe²⁺ and Fe³⁺ react with OH^- to form Fe(OH)₂ and Fe(OH)₃ (Equation (14), (15)). Some Fe(OH)₂ will be further oxidized to Fe(OH)₃ by O₂ (Equation (16)). Fe₂O₃ and Fe₃O₄ can be formed through Equation (17) and (18). Some FeCO₃ may be oxidized to Fe₂O₃ according to Equation (19). Cr(OH)₃, Fe₂O₃ and/or Fe₃O₄ may deposit on the surface of 3Cr steel simultaneously, which is not conducive to the formation of a dense and protective erosion-corrosion scale film. The erosion-corrosion scale will be loose and porous, so that a corrosive medium cannot be prevented from reaching the substrate of the steel under the condition of CO₂–O₂. Thus, the erosion-corrosion rate increases significantly when O₂ is present in the environment (Figure 4). There will be more dissolved O₂ in the solution and at the interface of 3Cr steel and the solution, with the partial pressure of O₂ increasing. This is conducive to the formation of Fe₂O₃ and/or Fe₃O₄ according to Equation (12)–(19) (Table 4), which will accelerate the erosion-corrosion of 3Cr steel. The outer erosion-corrosion product is mainly loose Fe₂O₃ and/or Fe₃O₄, which easily falls off the surface of 3Cr steel under the action of scouring. More Fe₂O₃ and/or Fe₃O₄ falls off the erosion-corrosion scales when the partial pressure of O₂ reaches 0.6 MPa, resulting in a lower quantity of Fe₂O₃ and Fe₃O₄ (Table 4).

$$2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\to 4{\mathrm{OH}}^{-}$$

(12)

$$4{\mathrm{Fe}}^{2+}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{Fe}}^{3+}+4{\mathrm{OH}}^{-}$$

(13)

$${\mathrm{Fe}}^{2+}+2{\mathrm{OH}}^{-}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2$$

(14)

$${\mathrm{Fe}}^{3+}+3{\mathrm{OH}}^{-}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3$$

(15)

$$4\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$$

(16)

$$2\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\to {\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(17)

$$2\mathrm{Fe}{\left(\mathrm{OH}\right)}_3+\mathrm{Fe}{\left(\mathrm{OH}\right)}_2\to {\mathrm{Fe}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2\mathrm{O}$$

(18)

$$4{\mathrm{FeCO}}_3+{\mathrm{O}}_2+4{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}}_2{\mathrm{O}}_3+4{\mathrm{H}}_2{\mathrm{CO}}_3$$

(19)

4. Conclusions

The erosion-corrosion rate of 3Cr steel apparent increases when there is O₂ in the CO₂ environment; this rate correlates with the partial pressure of O₂.

The erosion-corrosion product is uniform with obvious Cr enrichment in a CO₂ environment, composed of large FeCO₃ particles and some amorphous erosion–corrosion products. However, the erosion-corrosion scale is loose and porous with little Cr enrichment in CO₂–O₂ environments. It is made up of FeCO₃, Fe₂O₃, Fe₃O₄, and a bit of amorphous erosion-corrosion products.

3Cr steel exhibits excellent erosion-corrosion performance under the condition of CO₂. However, mixing with O₂ may destroy the dense and protective FeCO₃ and Cr enrichment in the erosion-corrosion product. The erosion-corrosion products become loose and porous, which is favorable for the corrosive medium to diffuse into the steel surface and the erosion-corrosion product. Thus, the erosion-corrosion rate increases significantly, and pitting corrosion may occur under the condition of CO₂–O₂.