The nature of the solid erodent particles present in corrosive petroleum fluid can cause transporting pipeline to experience severe erosion and corrosion damages. The effect of erosion on corrosion behavior of API X120 steel was investigated using aluminum oxide and silicon carbide particles with different sizes as erodent and 3.5 wt.% NaCl aqueous solution saturated with carbon dioxide as a corrosive medium. The effect of the erodent particle size on the corrosion behavior of the steel material at different particle speeds and impact angles was investigated using weight loss, potentiodynamic polarization and surface analysis techniques. The erosion results confirmed that the material damage increased with increasing particle speed. It was observed that in carbon dioxide-saturated saline solution, deposition of protective iron carbonate film occurred on the steel surface. It was found that the corrosion film can provide better protection at lower particle speed than at higher speed. The ratio of total erosion–corrosion (S)/effect of erosion on corrosion (T) analysis confirmed that at higher S/T ratio, the particle speed and material removal rate are low and vice versa at lower S/T ratio. Lower S/T values for the combined erosion and corrosion tests performed with erodent silicon carbide particle compared to erodent aluminum oxide particle showed that erosion enhancement of corrosion is more evident in the test performed using aluminum oxide particle than using silicon carbide particle. The result also suggests that when subjected to larger size erodent particle, the damage to pipeline due to effect of erosion on corrosion process can be more significant compared to smaller size erodent particle.
Pipeline has been used to transport products from one location to another. The presence of carbon dioxide in the petroleum products has shown to play a critical role in the corrosion of steels [1‐3]. It is accepted that the reaction of water with CO2 results in the formation of weak carbonic acid which is reduced to bicarbonate ions producing hydrogen gas. The corrosive medium produced in the presence of carbonic acid [4, 5] oxidizes metallic iron to ferrous (Fe2+) ions and induces the buildup of an iron carbonate layer that influences the corrosion rate [2]. Several researchers have reported that the iron carbonate layer formed on the steel surface during the corrosion process can be either protective or unprotective depending on the steel material [2, 6, 7]. CO2 reaction with steels in corrosive environment has been reported [8]. However, understanding the behavior of the highest steel grade (API X120 steel) pipeline in erosion–corrosion process may be necessary.
The presence of solid particles which can be of different sizes in the petroleum products can cause erosive mechanical impingement of the steel surface at different speeds and angles, resulting in the material degradation [9]. During transportation of the petroleum, the solid particles are directed toward the pipeline surface causing dramatic change in the particle trajectory [10]. The change in the particle speed, distribution and direction can cause significant degradation of the pipeline surface [11]. Extensive studies have been performed on the effects of particle speeds, sizes, and impact angle on the erosion and corrosive behaviors of various steel materials, and different results have been reported [11, 12]. Clark and Wong [13] investigated the impact of particle size on the steel material during slurry erosion. Their result revealed that higher erosion rate occurred at lower impact angle. On the other hand, Misra and Finnie [14] studied the effect of particle size on the erosive surface wear and material properties of ductile metals. Their result showed that hard surface layer was significantly abraded by smaller particle than the larger erodent particle. Other researchers have investigated the erosion behaviors of several steel grades at different particle sizes, speeds and impact angles and observed various erosion mechanisms ranging from plowing, plastic deformation and embedment of erodent particle on the steel surfaces [9, 15]. Naz et al. [16] performed aqueous erosion study of mild steel with two different erodent particle sizes. The result revealed significant increase in metal erosion that decreased with decreasing impact angle due to larger erodent particle compared to the smaller solid erodent particle. Ige and Umoru [17] investigated the effect of shear stress, solid particle shape and inhibitor on the erosion–corrosion of API X65 using mass loss and profilometry methods. The result revealed that rounded erodent particle caused less localized deformation and required more energy to remove debris from the eroded steel surface. Detailed study of erosion enhancement of corrosion and vice versa on API X70 pipeline steel showed correlation between erosion and corrosion [18]. It was concluded that while erosion enhances corrosion, corrosion increases erosion resulting in synergism effect. Matsumura et al. [19] showed that increase in the steel roughness caused by synergistic erosion–corrosion can influence the steel pipeline performance and material degradation by erosive particle and protection of the steel surface by corrosion film can vary depending on the steel material [20‐23].
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Recently, Okonkwo et al. [24] studied the pure erosion behavior of API X100 steel at different particle speeds and angles. Various erosion mechanisms observed were attributed to the impact of particle speed and impingement angle on the target steel surface. Protective efficiency of an inhibitor on the corrosion behavior of API X120 steel has been reported [25]. Although the extensive studies were carried out on the erosion and corrosion processes, most of the studies have been on low carbon steels and other marine applications [26‐31]. More so, the effect of erosion on subsequent corrosion process has not been detailed. However, API X120 pipeline steel has been regarded as the highest quality pipeline steel available in the today market and its application in the petroleum industry is so far limited. It is pertinent to mention that material degradation and losses due to effect of erosion on corrosion of API X120 pipeline steel are not yet fully understood and thus need further investigation. More so, the comparative study of the erodent particle size with respect to effect of erosion on corrosion process has not been clearly reported in the literature. Understanding the effect of erosion on corrosion and the degradation mechanism of API X120 steel at different particle speeds and impact angles would be worthwhile to mitigate erosion–corrosion-induced failure to the petroleum industry.
This study, therefore, focuses on the effect of erosion on corrosion behavior of API X120 steel in 3.5 wt.% NaCl saturated with carbon dioxide at different particle speeds and impact angles. The erosion tests were conducted using aluminum oxide and silicon carbide as erodent particles. Potentiodynamic polarization was used to examine the electrochemical behavior. The eroded–corroded API X120 steel surfaces were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and profilometer to study their compositional, surface and structural morphologies. This conclusive study of the effect of erosion on the corrosion behavior of API X120 steels will be useful to the pipeline and corrosion engineers for the future design and erosion–corrosion analysis.
2 Experimental
2.1 Material
API X120 pipeline steel and erodent particles used in the current study were supplied by Southern Steel Supplies, Lonsdale and Burwell Technologies, Sydney, Australia, respectively. Table 1 summarizes the chemical composition of the steel. Each value was derived from the results of testing more than ten pipes.
Table 1
Chemical composition of API X120 steel used in test (wt.%)
C
Si
Mn
Others (Ni, Cr, Mo, Cu, V, Fe)
0.129 ± 0.004
0.101 ± 0.012
0.541 ± 0.005
Balance
Figure 1 shows the micrographs of API X120 pipeline steel, aluminum oxide and silicon carbide erodent particles used in the tests. The measured particle sizes of silicon carbide and alumina are (5 ± 2) µm and (70 ± 2) µm, respectively (Fig. 1). The selection of particles with widely different sizes is to differentiate their impacts on the target steel surface. Preparation of API X120 pipeline steel used in the study involved polishing the samples with 240, 320 and 600 grit silicon carbide papers before the samples were ultrasonically cleaned and dried for each test.
Fig. 1
Optical micrographs of API X120 steel (a) and SEM images of silicon carbide (b) and aluminum oxide particles (c)
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2.2 Experimental setup
The erosion test facility was designed following ASTM G76 guidelines to regulate the impingement of the test specimen by the erodent particles [32]. In the experimental design, a sand hopper capable of storing erodent particles for the tests was mounted directly above the sample holder. In order to enhance the free flow of the abrasive particles, a pneumatic vibrator was attached to the sand hopper as shown in Fig. 2.
Fig. 2
Schematic drawing of dry erosion setup used for API X120 study
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The airflow control system attached to the machine allows the desired erodent particle to bombard the target specimen at the required impact angles. The erodent particles were designed to strike the target specimen at pressurized air and standoff distance of 3 mm. The test parameters used in the tests are displayed in Table 2.
Table 2
Pure erosion and erosion–corrosion test parameters
Alumina spherical shape size
(70 ± 2) µm
Silicon carbide size
(5 ± 2) µm
Aluminum oxide erodent particle hardness
(1869 ± 90) HV
Silicon carbide particle hardness
(1131 ± 74) HV
Alumina erodent particle specific density
2.65 g cm−3
Silicon carbide particle specific density
3.2 g cm−3
Target material
API X120 steel (20 mm × 20 mm × 5 mm)
Impact angle
30°, 45°, 60°, 90°
Test duration
10 s
Erodent particle test speeds
30, 50, 70, 90 m s−1
Test temperature
Ambient temperature
Test solution
3.5 wt.% NaCl + CO2
Particle speed has a significant influence on the erosion behavior of target materials [9, 15]. Several researchers have reported the effectiveness of double-disk technique in measuring erodent particle speed [9, 33]. This method considers particle speed as a function of measured gas volume which is used in this study. In the test, API X120 pipeline steel specimens were exposed to two erodent particle sizes for 10 s at different particle speeds for pure erosion without corrosion. It was observed that significant erosion of the steel surface has occurred at 10 s for the lowest particle speed and was taken as benchmark for the experiment.
A three-electrode flat electrochemical cell in our recent study was used for the corrosion tests [34]. The electrochemical test setup consisted of graphite as counter electrodes, saturated calomel as reference electrodes and API X120 steel substrates as working electrode in contact with the test solution. Prior to each corrosion test, deaeration process was carried out using nitrogen gas for 2 h before bubbling CO2 gas into the solution for 1 h using a Bronkhorst mass flow controller to achieve a saturation pH value of 3.92 as shown in Fig. 3.
Fig. 3
Stability of CO2 in saline solution before each test
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Saturation level of CO2 in the solution was maintained throughout the tests, while avoiding turbulence or gas bubbles formation on the surface of the specimen. Potentiodynamic polarization test was performed at a scan rate of 0.165 mV s−1 resulting in the potentiodynamic polarization curves as discussed in Sect. 3.2 [35].
The effect of erosion on the corrosion behavior of the steel specimen was determined by first subjecting the steel specimen to erosion at different particle speeds for 10 s and then exposing the resulting eroded steel specimen further to corrosion for 2.5 h. This process of subjecting the eroded steel specimen to corrosion for 2.5 h is described as a cycle in this study. The weight loss of the eroded and corroded specimens was measured before and after each test using high-precision digital balance of accuracy up to fifth decimal place, and the average of five readings was reported. The erosion and corrosion rates are then calculated using Eq. (1):
$$E_{0} /C_{0} = \frac{\Delta m}{ADt}$$
(1)
where E0 and C0 stand for the pure erosion and corrosion rates in the absence of corrosion and erosion, respectively, (mm/year); \({\Delta m}\) represents the weight loss in the test, g; A is the sample area, mm; and Dt is the test time, year.
The effect of erosion on corrosion (T) mechanism of API X120 steel was derived using Eq. (2) [34]:
where \(C_{\text{e}}\) is the total corrosion component and \(E_{\text{c}}\) is the total erosion component.
3 Results and discussion
3.1 Erosion result
Figure 4 shows the distinction of the normalized erosion rates as a function of impact angles for different particle speeds. The erosion rates were derived by dividing the weight loss of steel specimens by the mass of erodent particles that bombard the surface. The normalized erosion rate of API X120 steel decreased with increasing impact angles as shown in Fig. 4. As expected, higher particle speed resulted in higher material removal from the target API X120 steel surface as reported in other erosion studies [16, 36]. This behavior is due to association of higher particle speed with more erodent particles bombarding the steel surface at a given time resulting in more material removal.
Fig. 4
Normalized erosion rate as a function of impact angles for aluminum particle (a) and silicon carbide (b) with different particle speeds
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Figure 4 shows that the erosion rate is maximum at an impact angle of 30° for both erodent particles and increased with particle speed up to 90 m s−1. The maximum erosion rate for aluminum particle is approximately two times higher than that for silicon carbide particle due to erodent particle size difference [37]. The result shows consistency as the particle speed is increased, which is well in agreement with the previous studies [38, 39].
3.2 Potentiodynamic polarization
The potentiodynamic polarization test was performed to study the effect of erosion on corrosion behavior of API X120 steel. Each corrosion test was completed at ambient temperature for 2.5 h and was taken from the specimen eroded at 30° impact angle. Figure 5 displays the typical polarization curves of eroded–corroded API X120 steel specimen using the two erodent particles in a 3.5 wt.% NaCl solution saturated with carbon dioxide starting from cathodic to the anodic direction at a scan rate of 0.165 mV s−1.
Fig. 5
Potentiodynamic polarization curves of eroded–corroded API X120 steel using aluminum oxide (a) and erodent particles (b) as silicon carbide. E Potential; I current
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The scan rate was selected based on the test standard and previous studies in understanding the protective performance of the formed corrosion product on the steel surface [40, 41]. The result shows that corrosion rate increased as the particle speed was increased for the two erodent particles (Fig. 5). This behavior is expected because increasing the erodent particle speed enhances the number of solid particles striking the target steel surface at a given time. Continuous bombardment of the target steel surface by the erodent particle resulted in degradation and increase in the steel surface roughness. Under this condition, the steel surface is exposed to more corrosion attack [17]. Eliyan et al. [42] reported that the reaction of CO2 with steels can affect the cathodic and anodic process as observed in this study. Table 3 shows Tafel parameters extracted from Tafel measurements shown in Fig. 5.
Table 3
Potentiodynamic polarization parameters for impinged API X120 steel at different particle speeds and impact angle of 30°
It can be seen that increasing the erodent particle speed enhanced Icorr, which is attributed to increased corrosion rate [34]. Comparatively, higher Icorr value for the test performed with aluminum oxide particle than with silicon carbide suggests greater corrosion rate for the steel specimen eroded with aluminum particle than with silicon carbide and may be attributed to the difference in the particle sizes [43].
3.3 Surface characterization of erosion–corrosion surfaces
Scanning electron microscopy and optical profilometry techniques were employed for understanding the morphology of the eroded–corroded steel surface. Profilometry result of API X120 steel surface eroded with silicon carbide showed eroded scar depth of approximately 80 µm, while the test performed with aluminum oxide particle impinging the steel surface, creating erosion scar depth of approximately 100 µm as shown in Fig. 6.
Fig. 6
Optical profilometry image of API X120 steel eroded with silicon carbide (a) and aluminum oxide particle (b) at 90 m s−1 and eroded–corroded with aluminum oxide (c) at 30° impact angle before exposure to 3.5 wt.% NaCl saturated with CO2
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Figure 6 shows that the depth of the erosion scar increased with the erodent particle size. This result is in good agreement with the report of Nguyen et al. [43], which revealed that the erosion scar becomes deeper as the particle size coarseness increases, suggesting that larger particle can destroy components faster than smaller particles. Furthermore, the impact force of smaller erodent particle can be smaller than that of the bigger erodent particle, resulting in smaller erosion rate for smaller particle compared to larger particle [10, 43].
Figure 7 shows SEM micrographs of API X120 steel surfaces eroded with two different erodent particles at various particle speeds and impact angles of 30°. When bombarding the target material at low angles, the hard erodent particles can embed on the target steel surface as well as penetrate the surfaces of the specimens as evident in EDX result (Fig. 7). The higher the particle speed, the deeper the erodent particles penetrate the matrix as evident in Fig. 7a–c.
Fig. 7
SEM micrograph of API X120 steel purely eroded with silicon carbide (a, b, c) and aluminum oxide (d, e, f) at different particle speeds and impact angle of 30° and corresponding EDX results (g, h)
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Continuous bombardment of the steel surface can cause material removal by micro-cutting [15]. It was reported that the bombardment can create work-hardened layer which led to cutting and low-angle metal cutting mechanism [12, 36, 44]. For the silicon carbide, increasing the particle speed showed deeper embedment of the erodent particle on the steel surface. Continuous bombardment of the target steel material surface by solid erodent aluminum oxide particles at higher particle speed led to metal cutting of the steel surface as shown in Fig. 7d–f. It has been reported that increasing the particle speed can result in the incoming particles striking the embedded particle at different angles, causing reflection and deflection of the incoming erodent particle stream [45]. This process can cause the solid aluminum oxide particle to cut the target steel surface as observed in Fig. 7f.
Figure 8 shows SEM of pure corroded steel specimen at lower magnification. Fine feature of approximately 20 µm in grain size has developed on corroded steel surface (Fig. 8b).
Fig. 8
SEM micrograph of API X120 steel after exposure to pure corrosion for 2.5 h. a Corroded API X120 steel specimen; b high-magnification images of corroded steel specimen; c EDX micrograph of corroded steel surface
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Higher magnification of the corroded steel surface revealed a flaky plate-like microstructure (Fig. 8b), which is rich in carbon as identified by EDX analysis (Fig. 8c). When steels are exposed to carbon dioxide environment, iron carbonates are often formed [18]. Subsequently, the iron carbonate grows, forming flaky passive layer, which precipitates on the steel surface [18]. The formed layer can cover and protect the degraded steel surface depending on the nature of the formed layer and the steel surface roughness.
Figure 9 shows SEM micrographs of API X120 steel after the steel was exposed to three successive erosion–corrosion cycles of 30-s erosion and 7.5-h corrosion tests. Figure 9a, d shows evidence of embedded aluminum oxide, silicon carbide and corrosion film on the eroded–corroded steel surface, which is in good correlation with the erosion and corrosion results (Figs. 6, 7). It is evident that the embedded erodent particles have partly been covered by the corrosion film as the silicon carbide particle speed is increased (Fig. 8).
Fig. 9
SEM micrograph of eroded-corroded X120 steel with silicon carbide (a, b, c) and aluminum oxide (d, e, f) at different particle speeds and impact angle of 30°
×
As observed in individual pure erosion results (Fig. 7), the embedment of aluminum oxide which occurred at lower speed on the steel surface during the erosion appears to be evident in this result. Furthermore, metal and low-angle cutting present on the steel surface have partly been covered by formed corrosion film (Fig. 9). Evidence of corrosion film found on the same eroded–corroded steel surface may be due to the presence of iron carbonate in the corrosive medium. More so, the effect of erosion on corrosion mechanisms observed on the corroded steel surface at different sizes particle speeds is in good agreement with the individual erosion and corrosion mechanisms (Figs. 7, 8). It appeared that some of the degradation created on the steel surface has been covered by corrosion films [46, 47].
3.4 Effect of erosion on corroded steel surface
Pure erosion, corrosion and effect of erosion on corrosion plots of the two erodent particle sizes calculated from Eqs. (1) and (2) are shown in Fig. 10. Each plot was determined from the average of five cycle tests.
Fig. 10
Change in corrosion due to erosion of API X120 steel surface. Erosion effect on corrosion plots caused by impinging steel surface with aluminum oxide particle is plotted in black while that of silicon carbide is plotted in red colors
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It can be seen that the erosion rate increased with increasing particle speeds for the two erodent particles, which correlates with Tafel result (Fig. 4). This is expected because an increase in the particle speed increases the surface roughness as well as degradation of the exposed API X120 steel [48]. This behavior is more evident for larger erodent steel particle (aluminum oxide) compared to smaller erodent one (silicon carbide) as shown in Fig. 11. It is believed that the erosion process increases the plastic deformation, thereby providing active sites for the corrosion initiation and enhancing the corrosion rate [49]. Eroding the steel surface with high-speed particles may result in degradation of the steel surfaces to the extent that the deposited corrosion film may not be able to cover the plastically deformed steel surface as observed in the study (Fig. 7). The variation in the plots shown in Fig. 10a, which increases with increased erodent particle speed and size, demonstrates the effects of erodent particle size and erosion on the corrosion behavior of API X120 steel. The phenomenon can be attributed to inability of the formed corrosion film to protect the deformed steel surface as the particle speed is increased.
Fig. 11
Schematic diagram illustrating effect of erosion on corrosion mechanism at smaller silicon carbide particle (a) and larger aluminum oxide particle (b)
×
As the particle speed increases during the erosion process, the material removal rate increases, exposing API X120 steel surface and the corroded steel surface to more degradation as described in the schematic diagram shown in Fig. 11. During this process, the solid erodent particles can easily embed or cut the exposed steel surface as observed in the SEM micrographs (Fig. 7) and schematic diagram (Fig. 11). The larger the erodent particle size (aluminum oxide particle), the larger the steel surface or formed corroded film is cut and removed from the pipeline steel surface (Fig. 11b).
The role of erosion on the corrosion material degradation can be understood by considering the ratio of total erosion–corrosion (S) to the effect of erosion on corrosion (T) calculated using weight loss technique tabulated in Table 4.
Table 4
Erosion–corrosion parameter of API X120 steel in saturated 3.5 wt.% NaCl impinged with aluminum oxide and silicon carbide particles at different particle speeds
Particle speed/(m s−1)
30
30
60
60
90
90
Erodent particle used in tests
Al2O3
SiC
Al2O3
SiC
Al2O3
SiC
Total material loss/(mm year−1)
48.72 × 103
32.15 × 103
153.1 × 103
118.70 × 103
296.28 × 103
224.15 × 103
E0/(mm year−1)
18.31 × 103
11.62 × 103
104.22 × 103
84.38 × 103
237.12 × 103
181.44 × 103
C0/(mm year−1)
0.46 × 103
0.46 × 103
0.46 × 103
0.46 × 103
0.46 × 103
0.46 × 103
Total erosion component/(mm year−1)
48.72 × 103
32.15 × 103
153.1 × 103
118.70 × 103
296.28 × 103
224.15 × 103
Ce/(mm year−1)
2.53 × 103
2.14 × 103
2.82 × 103
2.35 × 103
3.11 × 103
2.76 × 103
Change in erosion rate due to corrosion
30.41
20.53
48.88
34.32
59.16
42.71
Change in corrosion rate due to erosion
2.07
1.68
2.36
1.89
2.65
2.30
S/T ratio
0.63
0.62
0.32
0.28
0.19
0.18
Table 4 shows that S/T ratio decreases as the particle speed increases for the two erodent particles during the erosion and corrosion processes. Decrease in S/T with increasing particle speed indicates that less material is removed at lower particle speed. Deposition of corrosion film on the eroded steel surfaces occurred for all the steel specimens. However, coverage of the eroded steel surfaces with corrosion film decreases due to increasing particle speed that has increased the specimen surface roughness resulting in decrease in S/T values (Table 4). At lower S/T ratio, the material removal rate and particle speed are high, causing an increase in the erosion rate. However, at higher S/T ratio, the particle speed and material removal rate are low (Table 4). The lower S/T value for the total erosion and corrosion tests performed with erodent silicon carbide particle compared to erodent aluminum oxide particle for each particle speed indicates that erosion enhancement of corrosion is more dominant in the test performed using aluminum oxide particle than silicon carbide particle. It also suggests that the damage caused to pipeline component due to the erosion–corrosion can be more significant when subjected to bigger size erodent particle compared to smaller size erodent particles.
4 Conclusion
To understand the role of erodent particle size in the total erosion and corrosion processes, sets of pure erosion, pure corrosion and combined erosion–corrosion tests were performed on newly fabricated highest available pipeline steel (API X120 steel). Material damage on the steel surface was more noticeable as the particle speed was increased. However, formation of iron carbonate enhanced the coverage of the degraded eroded steel surface at lower particle speed compared to higher particle speed tests. S/T ratio analysis confirmed that at higher S/T ratio, the particle speed and material removal rate are low, whereas at a lower S/T ratio, the particle velocity and material removal rate are high. The lower S/T values observed for the combined erosion and corrosion tests performed with the two erodent particles showed that erosion enhancement of corrosion is more prevailing in the test performed using aluminum oxide particle than silicon carbide particle. It is concluded that the damage caused to pipeline due to the combined erosion and corrosion processes can be more significant when subjected to bigger erodent particle compared to smaller erodent particle, and this knowledge can assist engineers for the future erosion–corrosion analysis.
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