- Email: [email protected]
Effect of dissolved copper ions on erosion–corrosion synergy of X65 steel in simulated copper tailing slurry
Tribology International 114 (2017) 329–336
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of dissolved copper ions on erosion–corrosion synergy of X65 steel in simulated copper tailing slurry Javiera Aguirre, Magdalena Walczak
MARK
⁎
Department of Mechanical and Metallurgical Engieering, Escuela de Ingeniería, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago, Chile
A R T I C L E I N F O
A BS T RAC T
Keywords: Erosion-corrosion X65 steel Slurry pipeline Rotating cylinder electrode (RCE)
Erosion-corrosion is studied in the context of carbon steel pipelines transporting copper tailings with focus on the influence of dissolved copper ions present in the slurry. API 5L X65 steel is exposed to flow of slurry in rotating cylinder electrode (RCE) simulating flow conditions typical of a slurry pipe. Degradation is evaluated by means of electrochemical and gravimetric techniques, and the worn surfaces are analyzed by scanning electron microscope equipped with focused ion beam. It is found that 500 ppm of dissolved copper ions increases the total weight loss by at least 30% and the increase is attributed to the combined contributions of corrosion and the erosion-corrosion synergy, with the underlying mechanism being galvanic coupling of steel with the reduced copper ions deposited on steel surface.
1. Introduction Hydro-transport system is one of most energy and cost-efficient manner for transporting large volumes of raw and waste materials in the mining industry, for conveying slurries within the mining site and also to remote deposits [1]. During the processing of copper mineral, three types of slurries are generated: ore pulp (slurry of crushed and grinded mineral), copper concentrate and copper tailings (slurry of mineral waste once the copper has been extracted); among which, the concentrate and tailings slurries are most commonly transported using low-carbon steel pipelines. However, although this type of transport system is considered safe compared to other alternatives, corrosion and erosion have constituted an essential threat to the integrity of the slurry transport system. The phenomenon of erosion–corrosion (E-C) arises when the two phenomena, erosion and corrosion, affect each other. It can be described as wear caused by the combined action of the mechanical damage due to solid particle erosion and the electrochemical degradation due to corrosion [2]. The interaction between the two processes is complex in nature and function of several parameters that involve characteristics of the target, of the dispersed phase, and of the flow environment. The parameters of the target that have been identified as crucial include hardness, modulus of elasticity, toughness, chemical composition and microstructure [3–5]; whereas of the dispersed phase density, hardness, size, angularity and concentration of the slurry particle are the most relevant [6–8]. The slurry particles are suspended
⁎
Corresponding author. E-mail address: [email protected] (M. Walczak).
http://dx.doi.org/10.1016/j.triboint.2017.04.036 Received 30 January 2017; Received in revised form 13 April 2017; Accepted 23 April 2017 Available online 26 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.
in a flowing electrolyte, in case of which velocity, viscosity, pH, pressure and chemical composition have to be considered [9–11]. In addition, the processes of erosion and corrosion influence each other, generating material loss greater than that caused by each contribution individually resulting in accelerated wear loss of the material. The latter is often described as synergistic effect [12–16] and represented in terms of weight loss by Eq. (1) [17]:
TWL = E + C + S,
(1)
where TWL, E, C and S describe the total weight loss by erosion– corrosion, by pure mechanical erosion, by sheer electrochemical corrosion, and the weight loss attributed to the synergy, respectively; all of them considered for a representative period of time. In other words, the erosion-corrosion synergy corresponds to the difference between total weight loss determined in time and the sum of weight losses caused by erosion and corrosion independently, regardless the particular mechanism that causes it [17,18]. A valid challenge in the experimental study of E-C is the large number of involved variables making it difficult to describe the synergy in a generic system. In the case of copper tailing slurry, the presence of water and oxygen, combined with the presence of heavy metals ions, causes corrosion to the carbon steel pipe, which is accelerated by the flowing slurry and the high content of solid sand particles (ranging in the mining practice from 45 to 65 wt%). As of date, design of slurry pipelines relies on experience as there is no reliable method of predicting the rate of wear that would be based on laboratory data
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
prior to the system's deployment. With the objective of extending the comprehension of the phenomenon by the possible impact that dissolved metal ions may have, wear of low-alloy carbon steel exposed to copper tailing pipelines is considered. This situation is likely to occur in engineering practice, when chemical species are introduced to the slurry generating slightly acidic pH and releasing metal cations from the solid phase. The typical pipe steel, grade API 5L X65, is studied by means of a rotating cylinder electrode (RCE) system using a simulated copper tailing slurry. Although RCE has been shown suitable for studying erosion-corrosion in slurries by other authors [5,12,19–22] the specific aspect of how free metal ions interfere in erosion-corrosion has not yet been addressed in literature, neither by RCE nor any other experimental method. 2. Experimental 2.1. Steel samples The material for the study, API 5L X65 steel of nominal composition in C max. 0.12%, Si max. 0.45%, Mn max. 1.6%, P 0.025%, S max. 0.015%, and Fe the balance, hereafter referred to as X65, was cut from a pipe used previously in a water transporting system. Cylinders of 15 mm in diameter and 10 mm in length of were prepared by machining. Surface of each sample was polished using SiC paper starting from 320 down to 1200 grit using a lathe tool to provide a standardized surface roughness. The samples were rinsed and sonicated first in distilled water and then in acetone, dried by blowing hot air and stored in a desiccator for use in the experiment. Weight of each sample was measured three times using an analytical balance (Sartorius BP221S) with precision of 0.1 mg. 2.2. Erodent The erodent used in this study was a synthetic copper tailing (SCT) prepared by mixing determined quantities of quartz and alumina of selected size distribution. The quartz and alumina were sieved separately by mesh N°80 (mesh opening 177 µm) and then mixed in proportion of 4:1 in mass, respectively. The particle size was chosen to best match the size distribution found typically in Chilean copper tailings, i.e., mean particle diameter (D50) and diameter of 84% of the particles by weight (D84) of 52 µm and 174 µm, respectively [23]. Shape of the particles as seen under scanning electron microscope (SEM, model Hitachi TM3000) is shown in Fig. 1. Selected physical characteristics of the prepared SCT are presented in the Table 1. Density was determined by gravimetric method using a Gay-Lussac pycnometer. The circularity, also referred to as form or roundness factor, was obtained through analysis of SEM images employing open source image processing software, ImageJ [24], where circularity was calculated as:
Circularity = 4π ×
[Area] . [Perimeter]2
Fig. 1. SEM images of the sieved particles of: a) quartz, b) alumina and c) mixture (synthetic copper tailing).
(2)
The circularity indicator is 1 for perfect circle (rounded particles) and 0 for infinite elongated polygon (particles with sharp angular shapes). Finally, the particle size distribution and values of D50 and D84 were determined by laser scattering technique using Mastersizer 2000 (Malvern).
Table 1 Selected physical characteristics of the synthetic copper tailing (SCT). Density (g/cm3)
Circularity
D50 (µm)
D84 (µm)
2.77 ± 0.02
0.329 ± 0.04
44
152
2.3. Experimental rig of acrylic and contains four baffles mounted at 90° to each other for preventing sedimentation of the solid phase of the slurry. The rig does not allow for direct control of the angle by which the particles hit the surface. Two samples are mounted in the lower part of the shaft (15 mm in diameter), separated by two rubber washers to provide electrical
Experiments were carried out using an in-house build system of rotating cylinder electrode (RCE), capable of performing in-situ electrochemical measurements of small electrode surface exposed to tangential flow of slurry. Fig. 2 shows the configuration of the experimental system, consisting of a 2.2 kW three-phase motor with a maximum rotation speed of 24,000 rpm. The slurry container is made 330
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 2. Experimental set-up: a) Overview of the RCE when the pot is filled with slurry; b) Electrochemical assembly; the pot is filled with water to make the sample visible.
insulation between the samples. The shaft is immersed in a pot filled with slurry (maximum capacity of 300 mL) and the rotation speed is controlled by a frequency modulator. In-situ electrochemical measurements were completed by means of a three electrode set-up, with graphite rod as counter electrode, Ag/AgCl (KCl saturated) as reference and the X65 cylinder as working electrode. All electrodes are connected directly to a Gamry Reference 600 potentiostat for generation and collection of electrochemical data.
C=
icorr WAt , zF
(3)
where C, icorr, W, A, t, F and z are the weight loss due to pure corrosion (g), corrosion current density (A/cm2), atomic weight (g/mol), surface area of the sample (cm2), time (s), Faraday constant, and charge number, respectively. Eq. (3) is considered for the following anodic reaction: (4)
Fe ↔ Fe2+ + 2ē.
Finally, the E-C synergy (S) was calculated using Eq. (1). Table 2 summarizes the experimental conditions employed in this study.
2.4. Experimental procedure Each test was run for 90 min to generate measurable weight loss without significant distortion of the geometry of the test sample or change in the characteristics of erodent (attrition phenomenon [25]). The experiments were performed using electrolyte of 0.1 M sodium sulfate (Na2SO4) with three levels of copper ions concentration: 0, 250, and 500 ppm, obtained by addition of copper sulfate (CuSO4). The particle concentration of the slurry was 55 wt% and the equivalent linear velocity at the tangential surface of the cylinder was 5 m/s (6300 rpm). Temperature of slurry was maintained at 33 ± 1 °C and pH of 5 was kept by adding drops 0.1 M H2SO4 when necessary. The total of about 5 mL was added during entire experiment. In order to determine the effect of copper ions (Cu2+) in the E-C synergy, four tests, each one in duplicate, were performed for each Cu2+ concentration. The first test employed a pair of metallic samples immersed in the slurry to simulate the damage by erosion-corrosion. After exposure, one sample was rinsed in an ultra-sonic bath of an inhibiting acid solution (Sulfex® in hydrochloric solution) to remove corrosion products and determine the total weight loss (TWL). The other sample was sonicated in acetone, dried by blowing hot air and stored in a desiccator for use in surface analysis (SEM-EDX, FIB). The second test was carried out using another pair of samples and applying −1.2 V (Ag/AgCl) with the objective of suppressing corrosion (cathodic protection, CP) and determining the weight loss attributable to pure erosion. After exposure, both samples were sonicated in acetone and dried by blowing hot air; one sample was used for determining weight loss and the other one for surface analysis (SEM). To determine weight loss by pure corrosion, two potentiodynamic polarization measurement (anodic and cathodic, separately) were carried out over one metallic sample in absence of the erodent, using a scan rate of 1 mV/s from −1.5 to +1.5 V vs. OCP for cathodic and anodic polarization, respectively. The amount of corrosion current density (icorr) was obtained via the Tafel extrapolation technique [26] and the weight loss due to pure corrosion was determined by Faraday's law:
2.5. Surface analysis Morphology of the worn surfaces was analyzed by SEM. To verify possible incorporation of slurry particles in case of samples exposed to Cu2+ containing slurries, SEM images were complemented by energy dispersive X-Ray spectroscopy (EDX) using Quanta FEG-250 (FEI) instrument coupled with SEM. Additionally, cross-sections of selected surfaces were inspected in another SEM system equipped with focused ion beam (FIB) dual beam station Helios NanoLab® (FEI), also coupled with an EDX detector. For the ion milling and in order to align the sample perpendicular to the ion beam column, samples were tilted by 52°. A liquid Ga source was used to produce Ga+ ions at the accelerating voltage of 30 kV. To facilitate the milling, a thin film of Pt was sputtered over the surface prior the process.
Table 2 Summary of sample codes and experimental conditions. Mechanism tested
Sample code
Copper ion concentration
Conditions
ErosionCorrosion
EC-0 EC-250 EC-500 E-0 E-250 E-500
0 ppm 250 ppm 500 ppm 0 ppm 250 ppm 500 ppm
SCT 55 wt% in 0.1 M Na2SO4; 5 m/s; pH 5
C-0 C-250 C-500
0 ppm 250 ppm 500 ppm
Erosion
Corrosion
331
SCT 55 wt% in 0.1 Na2SO4 M; 5 m/s; pH 5; CP:−1.2 V (Ag/AgCl) No SCT (Only 0.1 Na2SO4); 5 m/s; pH 5
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 3. Representative SEM images of unexposed reference and eroded samples of the EC-series. The arrows indicate direction of cylinder rotation, perpendicular to the rotation axis.
3. Results
Table 3 Elemental analysis (wt%) obtained by EDX. Elements not detected are indicated by “n. d.”.
3.1. Surface analysis Fig. 3 shows SEM micrographs of the external surface of the sample cylinders after completing the E-C tests at different Cu2+ concentrations. The polishing marks, perpendicular to the RCE rotation axis and parallel to the rotation direction, present at the samples prior exposure are not observed after the test. For each concentration, a considerable change in the surface morphology is observed along with a variable presence of erodent particles retained on the surface. Samples from slurry without copper ions retain more particles of different size and geometries, whereas presence of retained erodent decreases in size and number for slurries with 250 ppm and 500 ppm of Cu2+, respectively. Elemental composition obtained by EDX analysis of the EC samples is presented in Table 3, where the entire surface shown in individual images of Fig. 3 were used for the elemental analysis. The data reveal the presence of O, Al and Si on all worn surfaces, these elements are not detected in the base metal (reference sample). Concentration of these elements is highest and lowest for samples EC-0 and EC-500, respectively, confirming the visual observation of lesser retention of erodent for Cu2+ containing slurries. At the samples exposed to these slurries copper is also detected in positive correlation to the initial copper ion content. Fig. 4 shows SEM micrographs analogous to those of Fig. 3 but of samples tested at the condition of suppressed corrosion (samples of Eseries). A similar change in surface morphology after exposure is observed; however, no sample retains particles of the erodent. This observation is corroborated by EDX analysis (data not shown) in which no Si, Al or O elements were detected. The absence of retained particles allows observing narrow wear scars of length similar or less to that of particle size of the erodent. Unlike the polishing scars, the wear scars do not show any preferred orientation. Visual appearance of these wear
Sample
Area
O
Al
Si
Mn
Fe
Cu
S
Reference
Top surface (Fig. 3) Top surface (Fig. 3) Top surface (Fig. 3) Top surface (Fig. 3) Selection A (Fig. 6) Selection B (Fig. 6) Selection C (Fig. 6)
n.d.
n.d.
n.d
2.03
97.97
n.d
n.d
14.11
4.92
6.64
1.21
72.81
n.d
n.d
7.11
2.19
2.54
1.44
86.25
0.46
n.d
2.35
0.10
0.27
1.60
93.28
2.30
n.d
n.d
n.d
n.d
0.80
99.20
n.d
n.d
8.25
n.d
18.90
n.d
60.14
8.16
4.53
3.17
n.d
1.75
n.d
85.72
6.91
2.44
EC-0 EC-250 EC-500 EC-500 EC-500 EC-500
scars indicates strong plastic deformation, with size and apparent depth of the craters decreasing with increasing content of Cu2+ in the slurry. The seemingly loose fragments observed at the surface are interpreted as substrate material removed by cutting. Fig. 5 shows the FIB section of a sample after exposure to erosioncorrosion in a Cu-free slurry (EC-0 sample) along with corresponding elemental mapping of Pt, Si, and Al. The Pt layer corresponds with the film deposited for facilitating the FIB milling. Areas of increased concentration of Si and Al with respect to the background are observed. These areas correlate with position of particles observed in the SEM image. Fig. 6 shows transversal FIB section of X65 after erosion-corrosion by slurry of the highest concentration of copper ions (sample EC-500). The areas enclosed by dashed lines indicate the areas where chemical composition was analyzed and reported in Table 3. Predominant 332
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 4. Representative SEM images of reference and samples eroded under cathodic protection (E-series). The arrows indicate direction of cylinder rotation, perpendicular to the rotation axis.
presence of iron is observed in the area A corresponding with the steel substrate. A relatively high concentration of Si and O is observed in area B corresponding with a particulate deposit over the substrate. At
the interface between substrate and the Pt deposit of areas B and C an increased concentration of Cu and S is observed, although there is no distinctive morphology associated with these elements.
Fig. 5. SEM of transversal FIB section of sample EC-0 along with the corresponding elemental maps of platinum, silicon, and aluminum distribution.
333
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Table 4 Electrochemical parameters determined from the polarization diagrams shown in Fig. 7. Sample
Ecorr (mV vs Ag/AgCl)
icorr (mA/cm2)
C-0 C-250 C-500
−460 −521 −564
1.34 3.12 5.05
respectively. The contribution of corrosion to the total weight loss increases up to four times relative to weight loss in the copper-free slurry. Further, the weight loss attributed to erosion-corrosion synergy is in all cases greater than that attributed to pure corrosion but smaller than that attributed to pure erosion, and its amount as well as percentage increases with increasing concentration of copper ions reaching 27% in case of 500 ppm Cu2+. The ratio of weight loss attributable to corrosion and the E-C synergy is independent from Cu2+ content indicating that only the corrosion-enhanced erosion component of the synergy is affected. Fig. 6. SEM of transversal FIB section of sample EC-500. Chemical composition of the indicated areas is shown in Table 3.
4. Discussion Sample cylinders of X65 after exposure to erosion-corrosion by flow of SCT slurry retain some of the slurry particles at the surface, with less and smaller particles retained when copper ions are present in the slurry, until virtually no retention at the concentration of 500 ppm of Cu2+(Fig. 3). The EDX analysis corroborate that the retained particles correspond with the particles of slurry because of O, Al and Si, which are elements present in the erodent and not detected in the steel substrate. Although the erodent particles seem to be embedded in SEM images of the top surface, the FIB section reveals that they are adsorbed rather than embedded because no surrounding crater was found under the few particles that were tested. This apparent adsorption could be associated with the presence of paste-like corrosion products, such as iron hydroxides, likely to form at these conditions. This hypothesis is supported by the lack of erodent retention in case of samples eroded by pure erosion as cathodic polarization employed to suppress corrosion also prevents or at least slows down the formation of corrosion products. Independent of the exposure conditions, the worn surfaces show multiple superimposed lips and craters evidencing a strong plastic deformation of the substrate by subsequent impact of particles. Appearance of the crates is indicative to the predominance of the indentation and cutting I mechanism of erosion proposed by Hutchings [27], indicating high impact angles, i.e. almost normal to the surface. It should be noted that the elongated wear scars do not show any preferred orientation, indicating that the effective flow direction of slurry at the sample surface is not aligned with the nominal direction of angular velocity tangent to the surface. In other words, the effective flow is highly turbulent producing deviation of the impact angle from almost tangent expected for laminar flow to high impact angle as evidenced by the wear scars. This mechanism of indentation and cutting seems not to be affected by the presence of copper ions. The total weight loss (Fig. 8) tends to increase in the presence of Cu2+ showing a positive correlation to the studied concentrations. Wear by pure mechanical erosion is practically independent from Cu2+ concentration in slurry indicating that although erosion makes the biggest contribution to the total weight loss, it is not the controlling factor in erosion-corrosion by slurries containing copper ions. In this case, the controlling factor must be related with corrosion as weight losses attributed to both corrosion and E-C synergy increase with Cu2+ content. The limiting current observed in the cathodic branch in copper-free electrolyte (Fig. 7) is characteristic for oxygen diffusion and reduction on X65 steel surface [28–31]. As copper ions are added, the cathodic current increases which can be attributed to additional reaction of Cu2+
3.2. Polarization curves Fig. 7 shows polarization diagram recorded in the absence of erodent at the same rotation frequency (the sample C-series). In the absence of copper ions, a limiting current is observed at the cathodic branch spanning over about 400 mV, which increases in value by the factor of about 2 and 4 with in the presence of 250 and 500 ppm of Cu2+, respectively. No significant differences in the anodic branches is observed, although a shift in corrosion potential is observed toward negative values for samples C-250 and C-500. The electrochemical parameters determined from the diagrams are summarized in Table 4. 3.3. Weight loss and synergy Fig. 8 shows summary of all the weight loss data, both absolute and percentage, discriminating the contributions of erosion-corrosion, pure erosion (E), pure corrosion (C) and the resulting synergism (S). The addition of copper ions to the slurry system results in considerable increase in total weight loss amounting to 1.4 mg cm−2h−1 in case of slurry containing 500 ppm Cu2+. The contribution of pure mechanical erosion to the total weight loss at different copper ion concentrations remains relatively constant with about 0.7 mg cm−2h−1 corresponding to 76%, 65% and 55% of the total weight loss in case of 0, 250 and 500 ppm of Cu2+ content in the slurry,
Fig. 7. Polarization diagram recorded during exposure to erodent-free electrolyte of varying Cu2+ concentration.
334
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 8. Summary of weight loss data: a) total weight loss with discrimination of the contributing components, b) relative contribution of erosion (E), corrosion (C) and synergy (S).
Fig. 9. Schematic representation of the effect of copper ions in the system.
signalized in the proposed scheme.
reduction, such as:
Cu2+ + 2ē → Cu°
(5)
consuming additional electrons and thus increasing the total cathodic current as observed in the polarization curves (Fig. 7). The reaction described by Eq. (5) produces deposition of metallic copper at the steel's surface as evidenced by the presence of copper at the surface by the EDX analysis of the top surface as well as FIB sections (Table 3). As mentioned before, the contribution of E-C synergy to the total weight loss correlates positively with Cu2+ content and it is larger both in absolute and relative terms as compared to weight loss by corrosion alone, indicating that the electrochemical corrosion plays an important role in the enhancement of erosion-corrosion synergy. Considering the interference of copper ions in the electrochemical processes, this could be related to corrosion-induced degradation of mechanical properties due to more intensive currents through the interface and/or formation of Fe-Cu alloy of inferior mechanical properties [32]. Another possibility is the enhancement of surface's irregularity due to localization of anodic and cathodic reactions [33]. Reduction of copper ions (Eq. 5) produces metallic layer which is continuously attacked by impacting erodent, producing areas at which the steel surface is effectively exposed, at least at the time instances between plastic deformation of the target and deposition of new copper layer by reduction of Cu2+. This situation can be expected taking into account inferior mechanical properties of pure copper as compared with carbon steel. This process could rise the number of surface defects through increase of surface roughness associated with galvanic coupling of the cathodic and anodic sites. A graphical representation of this possible mechanism is provided in Fig. 9, where a crater formed by erodent particle is shown. The exposed steel area activates anodically and the released Fe2+/Fe3+ ions may react to form hydroxides. Retention of erodent particle in the hydroxides as well as formation of the metallic debris are also
5. Conclusion In this study, erosion-corrosion of API 5L X65 steel by slurry of simulated copper tailing was studied by means of RCE providing the following conclusions: – Total weight loss of the steel due to E-C increases with increasing concentration of Cu2+ and this accelerated wear is associated with increasing contribution of corrosion and erosion-corrosion synergy. – In the presence of copper ions slurry particles of the slurry tend to adhere less to the steel surface but in no case they become embedded into the steel. – Accelerated corrosion could be explained by reduction of copper ions at the surface, producing galvanic coupling between copper islands and steel surface exposed by the abrading particles. This work presents a contribution to the understanding of the erosion-corrosion synergy in wear of low-alloy steel by slurry of copper tailing; however, for full understanding of the involved mechanisms and for accurate prediction of the wear rate further studies are necessary.
Acknowledgments This work has been funded by CONICYT through FONDECYT grant No. 1141107 and the doctoral scholarships CONICYT-PCHA/ Doctorado Nacional/2015–21150171. 335
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
G0119-09.2. [18] Barik RC, Wharton J a, Wood RJK, Stokes KR. Electro-mechanical interactions during erosion-corrosion. Wear 2009;267:1900–8. http://dx.doi.org/10.1016/ j.wear.2009.03.011. [19] He D, Jiang X, Li HG S. Erosion and erosion-corrosion behaviours of several stainless steels in dual phase flow. Corrosion 2002;58:276–82. [20] Guo HX, Lu BT, Luo JL. Interaction of mechanical and electrochemical factors in erosion-corrosion of carbon steel. Electrochim Acta 2005;51:315–23. http:// dx.doi.org/10.1016/j.electacta.2005.04.032. [21] Lu BT, Luo JL, Guo HX, Mao LC. Erosion-enhanced corrosion of carbon steel at passive state. Corros Sci 2011;53:432–40. http://dx.doi.org/10.1016/j.corsci.2010.09.054. [22] Stack MM, Zhou S, Newman RC. Identification of transitions in erosion-corrosion regimes in aqueous environments. Wear 1995;186–187:523–32. http:// dx.doi.org/10.1016/0043-1648(95)07175-X. [23] Innova Corfo JRI. Aplicación Industrial de la Tecnología de Espesamiento Extremo En Proyectos Mineros; 2010. [24] National Institutes of Health. ImageJ n.d. [25] ASTM international. ASTM-G75-15 Standard Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion Response of Materials (SAR Number) 1. Wear Erosion, Met Corros; 2013: p. 1–21. doi:http://dx.doi.org/10. 1520/G0075-07.2. [26] Stem M, Geary AL. Electrochemical polarization I. A Theoretical analysis of the shape of polarization curves. Electrochem Soc 1957;104:56–63. [27] Hutchings IM. Tribology: friction and wear of engineering materials. BH. UK; 1992. [28] Zhang G a, Cheng YF. Electrochemical corrosion of X65 pipe steel in oil/water emulsion. Corros Sci 2009;51:901–7. http://dx.doi.org/10.1016/j.corsci.2009.01.020. [29] Tian BR, Cheng YF. Electrochemical corrosion behavior of X-65 steel in the simulated oil sand slurry. I: effects of hydrodynamic condition. Corros Sci 2008;50:773–9. http://dx.doi.org/10.1016/j.corsci.2007.11.008. [30] Vera R, Vinciguerra F, Bagnara M. Comparative Study of the Behavior of API 5LX65 Grade Steel and ASTM A53-B Grade Steel against Corrosion in Seawater; 2015.10: p. 6187–98. [31] Sherif EM, Almajid AA, Khalil KA, Junaedi H, Latief FH. Electrochemical studies on the corrosion behavior of API X65 pipeline steel in chloride solutions. Int J Electrochem Sci 2013;8:9360–70. [32] Harvey TJ, Wharton JA, Wood RJK. Development of synergy model for erosion– corrosion of carbon steel in a slurry pot. Tribology 2007;1:33–47. http:// dx.doi.org/10.1179/175158407×181471. [33] Rajahram SS, Harvey TJ, Wood RJK. Evaluation of a semi-empirical model in predicting erosion-corrosion. Wear 2009;267:1883–93. http://dx.doi.org/ 10.1016/j.wear.2009.03.002.
References [1] Chadwick J. Hydrotransp Int Min 2011:57–65. [2] Heitz E. Mechanistically based prevention strategies of flow-induced corrosion. Electrochem Acta 1996;41:503–9. [3] Neville A, Hodgkiess T, Xu H. An electrochemical and microstructural assessment of erosion-corrosion of cast iron. Wear 1999;233–235:523–34. http://dx.doi.org/ 10.1002/1521-4176(200201)53:1 < 5::AID-MACO5 > 3.0.CO;2-R. [4] Zhang Y, Yin X, Wang J, Yan F. Influence of microstructure evolution on tribocorrosion of 304SS in artificial seawater. Corros Sci 2015;99:272–80. http:// dx.doi.org/10.1016/j.corsci.2014.07.062. [5] Lu BT, Luo JL. Correlation between surface-hardness degradation and erosion resistance of carbon steel—Effects of slurry chemistry. Tribol Int 2015;83:146–55. http://dx.doi.org/10.1016/j.triboint.2014.11.008. [6] Desale GR, Gandhi BK, Jain SC. Effect of erodent properties on erosion wear of ductile type materials. Wear 2006;261:914–21. http://dx.doi.org/10.1016/ j.wear.2006.01.035. [7] Arabnejad H, Shirazi SA, McLaury BS, Subramani H, Rhyne LD. The effect of erodent particle hardness on the erosion of stainless steel. Wear 2015;332– 333:1098–103. http://dx.doi.org/10.1016/0043-1648(91)90263-T. [8] Telfer CG, Stack MM, Jana BD. Particle concentration and size effects on the erosion-corrosion of pure metals in aqueous slurries. Tribol Int 2012;53:35–44. http://dx.doi.org/10.1016/j.triboint.2012.04.010. [9] Clark HM. Particle velocity and size effects in laboratory slurry erosion measurements OR… do you know what your particles are doing?. Tribol Int 2002;35:617–24. http://dx.doi.org/10.1016/S0301-679X(02)00052-X. [10] Lu B, Wang K, Wan X, Luo J. Effects of slurry pH on the surface mechanical properties and erosion-corrosion resistance. NACE Corros 2008:1–11. [11] Turenne S, Fiset M, Masounave J. The effect of sand concentration on the erosion of materials by a slurry jet. Wear 1989;133:95–106. http://dx.doi.org/10.1016/ 0043-1648(89)90116-6. [12] Zhou S, Stack MM, Newman RC. Characterization of synergistic effects between erosion and corrosion. Corros Sci 1996;52:934–46. [13] Watson SW, Friedersdorf FJ, Madsen BW, Cramer SD. Methods of measuring wear-corrosion synergism. Wear 1995;183–183:476–84. [14] Matsumura M. Erosion-corrosion of metallic materials in slurries. Corros Rev 1994;12:321–40. http://dx.doi.org/10.1515/CORRREV.1994.12.3-4.321. [15] Stack MM, Pungwiwat N. Erosion-corrosion mapping of Fe in aqueous slurries: some views on a new rationale for defining the erosion-corrosion interaction. Wear 2004;256:565–76. http://dx.doi.org/10.1016/S0043-1648(03)00566-0. [16] Meng H, Hu X, Neville A. A systematic erosion-corrosion study of two stainless steels in marine conditions via experimental design. Wear 2007;263:355–62. http://dx.doi.org/10.1016/j.wear.2006.12.007. [17] ASTM International. ASTM G119-09 Standard guide for determining amount of synergism between wear and corrosion; 1994. doi:http://dx.doi.org/10.1520/
336
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of dissolved copper ions on erosion–corrosion synergy of X65 steel in simulated copper tailing slurry Javiera Aguirre, Magdalena Walczak
MARK
⁎
Department of Mechanical and Metallurgical Engieering, Escuela de Ingeniería, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago, Chile
A R T I C L E I N F O
A BS T RAC T
Keywords: Erosion-corrosion X65 steel Slurry pipeline Rotating cylinder electrode (RCE)
Erosion-corrosion is studied in the context of carbon steel pipelines transporting copper tailings with focus on the influence of dissolved copper ions present in the slurry. API 5L X65 steel is exposed to flow of slurry in rotating cylinder electrode (RCE) simulating flow conditions typical of a slurry pipe. Degradation is evaluated by means of electrochemical and gravimetric techniques, and the worn surfaces are analyzed by scanning electron microscope equipped with focused ion beam. It is found that 500 ppm of dissolved copper ions increases the total weight loss by at least 30% and the increase is attributed to the combined contributions of corrosion and the erosion-corrosion synergy, with the underlying mechanism being galvanic coupling of steel with the reduced copper ions deposited on steel surface.
1. Introduction Hydro-transport system is one of most energy and cost-efficient manner for transporting large volumes of raw and waste materials in the mining industry, for conveying slurries within the mining site and also to remote deposits [1]. During the processing of copper mineral, three types of slurries are generated: ore pulp (slurry of crushed and grinded mineral), copper concentrate and copper tailings (slurry of mineral waste once the copper has been extracted); among which, the concentrate and tailings slurries are most commonly transported using low-carbon steel pipelines. However, although this type of transport system is considered safe compared to other alternatives, corrosion and erosion have constituted an essential threat to the integrity of the slurry transport system. The phenomenon of erosion–corrosion (E-C) arises when the two phenomena, erosion and corrosion, affect each other. It can be described as wear caused by the combined action of the mechanical damage due to solid particle erosion and the electrochemical degradation due to corrosion [2]. The interaction between the two processes is complex in nature and function of several parameters that involve characteristics of the target, of the dispersed phase, and of the flow environment. The parameters of the target that have been identified as crucial include hardness, modulus of elasticity, toughness, chemical composition and microstructure [3–5]; whereas of the dispersed phase density, hardness, size, angularity and concentration of the slurry particle are the most relevant [6–8]. The slurry particles are suspended
⁎
Corresponding author. E-mail address: [email protected] (M. Walczak).
http://dx.doi.org/10.1016/j.triboint.2017.04.036 Received 30 January 2017; Received in revised form 13 April 2017; Accepted 23 April 2017 Available online 26 April 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.
in a flowing electrolyte, in case of which velocity, viscosity, pH, pressure and chemical composition have to be considered [9–11]. In addition, the processes of erosion and corrosion influence each other, generating material loss greater than that caused by each contribution individually resulting in accelerated wear loss of the material. The latter is often described as synergistic effect [12–16] and represented in terms of weight loss by Eq. (1) [17]:
TWL = E + C + S,
(1)
where TWL, E, C and S describe the total weight loss by erosion– corrosion, by pure mechanical erosion, by sheer electrochemical corrosion, and the weight loss attributed to the synergy, respectively; all of them considered for a representative period of time. In other words, the erosion-corrosion synergy corresponds to the difference between total weight loss determined in time and the sum of weight losses caused by erosion and corrosion independently, regardless the particular mechanism that causes it [17,18]. A valid challenge in the experimental study of E-C is the large number of involved variables making it difficult to describe the synergy in a generic system. In the case of copper tailing slurry, the presence of water and oxygen, combined with the presence of heavy metals ions, causes corrosion to the carbon steel pipe, which is accelerated by the flowing slurry and the high content of solid sand particles (ranging in the mining practice from 45 to 65 wt%). As of date, design of slurry pipelines relies on experience as there is no reliable method of predicting the rate of wear that would be based on laboratory data
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
prior to the system's deployment. With the objective of extending the comprehension of the phenomenon by the possible impact that dissolved metal ions may have, wear of low-alloy carbon steel exposed to copper tailing pipelines is considered. This situation is likely to occur in engineering practice, when chemical species are introduced to the slurry generating slightly acidic pH and releasing metal cations from the solid phase. The typical pipe steel, grade API 5L X65, is studied by means of a rotating cylinder electrode (RCE) system using a simulated copper tailing slurry. Although RCE has been shown suitable for studying erosion-corrosion in slurries by other authors [5,12,19–22] the specific aspect of how free metal ions interfere in erosion-corrosion has not yet been addressed in literature, neither by RCE nor any other experimental method. 2. Experimental 2.1. Steel samples The material for the study, API 5L X65 steel of nominal composition in C max. 0.12%, Si max. 0.45%, Mn max. 1.6%, P 0.025%, S max. 0.015%, and Fe the balance, hereafter referred to as X65, was cut from a pipe used previously in a water transporting system. Cylinders of 15 mm in diameter and 10 mm in length of were prepared by machining. Surface of each sample was polished using SiC paper starting from 320 down to 1200 grit using a lathe tool to provide a standardized surface roughness. The samples were rinsed and sonicated first in distilled water and then in acetone, dried by blowing hot air and stored in a desiccator for use in the experiment. Weight of each sample was measured three times using an analytical balance (Sartorius BP221S) with precision of 0.1 mg. 2.2. Erodent The erodent used in this study was a synthetic copper tailing (SCT) prepared by mixing determined quantities of quartz and alumina of selected size distribution. The quartz and alumina were sieved separately by mesh N°80 (mesh opening 177 µm) and then mixed in proportion of 4:1 in mass, respectively. The particle size was chosen to best match the size distribution found typically in Chilean copper tailings, i.e., mean particle diameter (D50) and diameter of 84% of the particles by weight (D84) of 52 µm and 174 µm, respectively [23]. Shape of the particles as seen under scanning electron microscope (SEM, model Hitachi TM3000) is shown in Fig. 1. Selected physical characteristics of the prepared SCT are presented in the Table 1. Density was determined by gravimetric method using a Gay-Lussac pycnometer. The circularity, also referred to as form or roundness factor, was obtained through analysis of SEM images employing open source image processing software, ImageJ [24], where circularity was calculated as:
Circularity = 4π ×
[Area] . [Perimeter]2
Fig. 1. SEM images of the sieved particles of: a) quartz, b) alumina and c) mixture (synthetic copper tailing).
(2)
The circularity indicator is 1 for perfect circle (rounded particles) and 0 for infinite elongated polygon (particles with sharp angular shapes). Finally, the particle size distribution and values of D50 and D84 were determined by laser scattering technique using Mastersizer 2000 (Malvern).
Table 1 Selected physical characteristics of the synthetic copper tailing (SCT). Density (g/cm3)
Circularity
D50 (µm)
D84 (µm)
2.77 ± 0.02
0.329 ± 0.04
44
152
2.3. Experimental rig of acrylic and contains four baffles mounted at 90° to each other for preventing sedimentation of the solid phase of the slurry. The rig does not allow for direct control of the angle by which the particles hit the surface. Two samples are mounted in the lower part of the shaft (15 mm in diameter), separated by two rubber washers to provide electrical
Experiments were carried out using an in-house build system of rotating cylinder electrode (RCE), capable of performing in-situ electrochemical measurements of small electrode surface exposed to tangential flow of slurry. Fig. 2 shows the configuration of the experimental system, consisting of a 2.2 kW three-phase motor with a maximum rotation speed of 24,000 rpm. The slurry container is made 330
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 2. Experimental set-up: a) Overview of the RCE when the pot is filled with slurry; b) Electrochemical assembly; the pot is filled with water to make the sample visible.
insulation between the samples. The shaft is immersed in a pot filled with slurry (maximum capacity of 300 mL) and the rotation speed is controlled by a frequency modulator. In-situ electrochemical measurements were completed by means of a three electrode set-up, with graphite rod as counter electrode, Ag/AgCl (KCl saturated) as reference and the X65 cylinder as working electrode. All electrodes are connected directly to a Gamry Reference 600 potentiostat for generation and collection of electrochemical data.
C=
icorr WAt , zF
(3)
where C, icorr, W, A, t, F and z are the weight loss due to pure corrosion (g), corrosion current density (A/cm2), atomic weight (g/mol), surface area of the sample (cm2), time (s), Faraday constant, and charge number, respectively. Eq. (3) is considered for the following anodic reaction: (4)
Fe ↔ Fe2+ + 2ē.
Finally, the E-C synergy (S) was calculated using Eq. (1). Table 2 summarizes the experimental conditions employed in this study.
2.4. Experimental procedure Each test was run for 90 min to generate measurable weight loss without significant distortion of the geometry of the test sample or change in the characteristics of erodent (attrition phenomenon [25]). The experiments were performed using electrolyte of 0.1 M sodium sulfate (Na2SO4) with three levels of copper ions concentration: 0, 250, and 500 ppm, obtained by addition of copper sulfate (CuSO4). The particle concentration of the slurry was 55 wt% and the equivalent linear velocity at the tangential surface of the cylinder was 5 m/s (6300 rpm). Temperature of slurry was maintained at 33 ± 1 °C and pH of 5 was kept by adding drops 0.1 M H2SO4 when necessary. The total of about 5 mL was added during entire experiment. In order to determine the effect of copper ions (Cu2+) in the E-C synergy, four tests, each one in duplicate, were performed for each Cu2+ concentration. The first test employed a pair of metallic samples immersed in the slurry to simulate the damage by erosion-corrosion. After exposure, one sample was rinsed in an ultra-sonic bath of an inhibiting acid solution (Sulfex® in hydrochloric solution) to remove corrosion products and determine the total weight loss (TWL). The other sample was sonicated in acetone, dried by blowing hot air and stored in a desiccator for use in surface analysis (SEM-EDX, FIB). The second test was carried out using another pair of samples and applying −1.2 V (Ag/AgCl) with the objective of suppressing corrosion (cathodic protection, CP) and determining the weight loss attributable to pure erosion. After exposure, both samples were sonicated in acetone and dried by blowing hot air; one sample was used for determining weight loss and the other one for surface analysis (SEM). To determine weight loss by pure corrosion, two potentiodynamic polarization measurement (anodic and cathodic, separately) were carried out over one metallic sample in absence of the erodent, using a scan rate of 1 mV/s from −1.5 to +1.5 V vs. OCP for cathodic and anodic polarization, respectively. The amount of corrosion current density (icorr) was obtained via the Tafel extrapolation technique [26] and the weight loss due to pure corrosion was determined by Faraday's law:
2.5. Surface analysis Morphology of the worn surfaces was analyzed by SEM. To verify possible incorporation of slurry particles in case of samples exposed to Cu2+ containing slurries, SEM images were complemented by energy dispersive X-Ray spectroscopy (EDX) using Quanta FEG-250 (FEI) instrument coupled with SEM. Additionally, cross-sections of selected surfaces were inspected in another SEM system equipped with focused ion beam (FIB) dual beam station Helios NanoLab® (FEI), also coupled with an EDX detector. For the ion milling and in order to align the sample perpendicular to the ion beam column, samples were tilted by 52°. A liquid Ga source was used to produce Ga+ ions at the accelerating voltage of 30 kV. To facilitate the milling, a thin film of Pt was sputtered over the surface prior the process.
Table 2 Summary of sample codes and experimental conditions. Mechanism tested
Sample code
Copper ion concentration
Conditions
ErosionCorrosion
EC-0 EC-250 EC-500 E-0 E-250 E-500
0 ppm 250 ppm 500 ppm 0 ppm 250 ppm 500 ppm
SCT 55 wt% in 0.1 M Na2SO4; 5 m/s; pH 5
C-0 C-250 C-500
0 ppm 250 ppm 500 ppm
Erosion
Corrosion
331
SCT 55 wt% in 0.1 Na2SO4 M; 5 m/s; pH 5; CP:−1.2 V (Ag/AgCl) No SCT (Only 0.1 Na2SO4); 5 m/s; pH 5
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 3. Representative SEM images of unexposed reference and eroded samples of the EC-series. The arrows indicate direction of cylinder rotation, perpendicular to the rotation axis.
3. Results
Table 3 Elemental analysis (wt%) obtained by EDX. Elements not detected are indicated by “n. d.”.
3.1. Surface analysis Fig. 3 shows SEM micrographs of the external surface of the sample cylinders after completing the E-C tests at different Cu2+ concentrations. The polishing marks, perpendicular to the RCE rotation axis and parallel to the rotation direction, present at the samples prior exposure are not observed after the test. For each concentration, a considerable change in the surface morphology is observed along with a variable presence of erodent particles retained on the surface. Samples from slurry without copper ions retain more particles of different size and geometries, whereas presence of retained erodent decreases in size and number for slurries with 250 ppm and 500 ppm of Cu2+, respectively. Elemental composition obtained by EDX analysis of the EC samples is presented in Table 3, where the entire surface shown in individual images of Fig. 3 were used for the elemental analysis. The data reveal the presence of O, Al and Si on all worn surfaces, these elements are not detected in the base metal (reference sample). Concentration of these elements is highest and lowest for samples EC-0 and EC-500, respectively, confirming the visual observation of lesser retention of erodent for Cu2+ containing slurries. At the samples exposed to these slurries copper is also detected in positive correlation to the initial copper ion content. Fig. 4 shows SEM micrographs analogous to those of Fig. 3 but of samples tested at the condition of suppressed corrosion (samples of Eseries). A similar change in surface morphology after exposure is observed; however, no sample retains particles of the erodent. This observation is corroborated by EDX analysis (data not shown) in which no Si, Al or O elements were detected. The absence of retained particles allows observing narrow wear scars of length similar or less to that of particle size of the erodent. Unlike the polishing scars, the wear scars do not show any preferred orientation. Visual appearance of these wear
Sample
Area
O
Al
Si
Mn
Fe
Cu
S
Reference
Top surface (Fig. 3) Top surface (Fig. 3) Top surface (Fig. 3) Top surface (Fig. 3) Selection A (Fig. 6) Selection B (Fig. 6) Selection C (Fig. 6)
n.d.
n.d.
n.d
2.03
97.97
n.d
n.d
14.11
4.92
6.64
1.21
72.81
n.d
n.d
7.11
2.19
2.54
1.44
86.25
0.46
n.d
2.35
0.10
0.27
1.60
93.28
2.30
n.d
n.d
n.d
n.d
0.80
99.20
n.d
n.d
8.25
n.d
18.90
n.d
60.14
8.16
4.53
3.17
n.d
1.75
n.d
85.72
6.91
2.44
EC-0 EC-250 EC-500 EC-500 EC-500 EC-500
scars indicates strong plastic deformation, with size and apparent depth of the craters decreasing with increasing content of Cu2+ in the slurry. The seemingly loose fragments observed at the surface are interpreted as substrate material removed by cutting. Fig. 5 shows the FIB section of a sample after exposure to erosioncorrosion in a Cu-free slurry (EC-0 sample) along with corresponding elemental mapping of Pt, Si, and Al. The Pt layer corresponds with the film deposited for facilitating the FIB milling. Areas of increased concentration of Si and Al with respect to the background are observed. These areas correlate with position of particles observed in the SEM image. Fig. 6 shows transversal FIB section of X65 after erosion-corrosion by slurry of the highest concentration of copper ions (sample EC-500). The areas enclosed by dashed lines indicate the areas where chemical composition was analyzed and reported in Table 3. Predominant 332
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 4. Representative SEM images of reference and samples eroded under cathodic protection (E-series). The arrows indicate direction of cylinder rotation, perpendicular to the rotation axis.
presence of iron is observed in the area A corresponding with the steel substrate. A relatively high concentration of Si and O is observed in area B corresponding with a particulate deposit over the substrate. At
the interface between substrate and the Pt deposit of areas B and C an increased concentration of Cu and S is observed, although there is no distinctive morphology associated with these elements.
Fig. 5. SEM of transversal FIB section of sample EC-0 along with the corresponding elemental maps of platinum, silicon, and aluminum distribution.
333
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Table 4 Electrochemical parameters determined from the polarization diagrams shown in Fig. 7. Sample
Ecorr (mV vs Ag/AgCl)
icorr (mA/cm2)
C-0 C-250 C-500
−460 −521 −564
1.34 3.12 5.05
respectively. The contribution of corrosion to the total weight loss increases up to four times relative to weight loss in the copper-free slurry. Further, the weight loss attributed to erosion-corrosion synergy is in all cases greater than that attributed to pure corrosion but smaller than that attributed to pure erosion, and its amount as well as percentage increases with increasing concentration of copper ions reaching 27% in case of 500 ppm Cu2+. The ratio of weight loss attributable to corrosion and the E-C synergy is independent from Cu2+ content indicating that only the corrosion-enhanced erosion component of the synergy is affected. Fig. 6. SEM of transversal FIB section of sample EC-500. Chemical composition of the indicated areas is shown in Table 3.
4. Discussion Sample cylinders of X65 after exposure to erosion-corrosion by flow of SCT slurry retain some of the slurry particles at the surface, with less and smaller particles retained when copper ions are present in the slurry, until virtually no retention at the concentration of 500 ppm of Cu2+(Fig. 3). The EDX analysis corroborate that the retained particles correspond with the particles of slurry because of O, Al and Si, which are elements present in the erodent and not detected in the steel substrate. Although the erodent particles seem to be embedded in SEM images of the top surface, the FIB section reveals that they are adsorbed rather than embedded because no surrounding crater was found under the few particles that were tested. This apparent adsorption could be associated with the presence of paste-like corrosion products, such as iron hydroxides, likely to form at these conditions. This hypothesis is supported by the lack of erodent retention in case of samples eroded by pure erosion as cathodic polarization employed to suppress corrosion also prevents or at least slows down the formation of corrosion products. Independent of the exposure conditions, the worn surfaces show multiple superimposed lips and craters evidencing a strong plastic deformation of the substrate by subsequent impact of particles. Appearance of the crates is indicative to the predominance of the indentation and cutting I mechanism of erosion proposed by Hutchings [27], indicating high impact angles, i.e. almost normal to the surface. It should be noted that the elongated wear scars do not show any preferred orientation, indicating that the effective flow direction of slurry at the sample surface is not aligned with the nominal direction of angular velocity tangent to the surface. In other words, the effective flow is highly turbulent producing deviation of the impact angle from almost tangent expected for laminar flow to high impact angle as evidenced by the wear scars. This mechanism of indentation and cutting seems not to be affected by the presence of copper ions. The total weight loss (Fig. 8) tends to increase in the presence of Cu2+ showing a positive correlation to the studied concentrations. Wear by pure mechanical erosion is practically independent from Cu2+ concentration in slurry indicating that although erosion makes the biggest contribution to the total weight loss, it is not the controlling factor in erosion-corrosion by slurries containing copper ions. In this case, the controlling factor must be related with corrosion as weight losses attributed to both corrosion and E-C synergy increase with Cu2+ content. The limiting current observed in the cathodic branch in copper-free electrolyte (Fig. 7) is characteristic for oxygen diffusion and reduction on X65 steel surface [28–31]. As copper ions are added, the cathodic current increases which can be attributed to additional reaction of Cu2+
3.2. Polarization curves Fig. 7 shows polarization diagram recorded in the absence of erodent at the same rotation frequency (the sample C-series). In the absence of copper ions, a limiting current is observed at the cathodic branch spanning over about 400 mV, which increases in value by the factor of about 2 and 4 with in the presence of 250 and 500 ppm of Cu2+, respectively. No significant differences in the anodic branches is observed, although a shift in corrosion potential is observed toward negative values for samples C-250 and C-500. The electrochemical parameters determined from the diagrams are summarized in Table 4. 3.3. Weight loss and synergy Fig. 8 shows summary of all the weight loss data, both absolute and percentage, discriminating the contributions of erosion-corrosion, pure erosion (E), pure corrosion (C) and the resulting synergism (S). The addition of copper ions to the slurry system results in considerable increase in total weight loss amounting to 1.4 mg cm−2h−1 in case of slurry containing 500 ppm Cu2+. The contribution of pure mechanical erosion to the total weight loss at different copper ion concentrations remains relatively constant with about 0.7 mg cm−2h−1 corresponding to 76%, 65% and 55% of the total weight loss in case of 0, 250 and 500 ppm of Cu2+ content in the slurry,
Fig. 7. Polarization diagram recorded during exposure to erodent-free electrolyte of varying Cu2+ concentration.
334
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
Fig. 8. Summary of weight loss data: a) total weight loss with discrimination of the contributing components, b) relative contribution of erosion (E), corrosion (C) and synergy (S).
Fig. 9. Schematic representation of the effect of copper ions in the system.
signalized in the proposed scheme.
reduction, such as:
Cu2+ + 2ē → Cu°
(5)
consuming additional electrons and thus increasing the total cathodic current as observed in the polarization curves (Fig. 7). The reaction described by Eq. (5) produces deposition of metallic copper at the steel's surface as evidenced by the presence of copper at the surface by the EDX analysis of the top surface as well as FIB sections (Table 3). As mentioned before, the contribution of E-C synergy to the total weight loss correlates positively with Cu2+ content and it is larger both in absolute and relative terms as compared to weight loss by corrosion alone, indicating that the electrochemical corrosion plays an important role in the enhancement of erosion-corrosion synergy. Considering the interference of copper ions in the electrochemical processes, this could be related to corrosion-induced degradation of mechanical properties due to more intensive currents through the interface and/or formation of Fe-Cu alloy of inferior mechanical properties [32]. Another possibility is the enhancement of surface's irregularity due to localization of anodic and cathodic reactions [33]. Reduction of copper ions (Eq. 5) produces metallic layer which is continuously attacked by impacting erodent, producing areas at which the steel surface is effectively exposed, at least at the time instances between plastic deformation of the target and deposition of new copper layer by reduction of Cu2+. This situation can be expected taking into account inferior mechanical properties of pure copper as compared with carbon steel. This process could rise the number of surface defects through increase of surface roughness associated with galvanic coupling of the cathodic and anodic sites. A graphical representation of this possible mechanism is provided in Fig. 9, where a crater formed by erodent particle is shown. The exposed steel area activates anodically and the released Fe2+/Fe3+ ions may react to form hydroxides. Retention of erodent particle in the hydroxides as well as formation of the metallic debris are also
5. Conclusion In this study, erosion-corrosion of API 5L X65 steel by slurry of simulated copper tailing was studied by means of RCE providing the following conclusions: – Total weight loss of the steel due to E-C increases with increasing concentration of Cu2+ and this accelerated wear is associated with increasing contribution of corrosion and erosion-corrosion synergy. – In the presence of copper ions slurry particles of the slurry tend to adhere less to the steel surface but in no case they become embedded into the steel. – Accelerated corrosion could be explained by reduction of copper ions at the surface, producing galvanic coupling between copper islands and steel surface exposed by the abrading particles. This work presents a contribution to the understanding of the erosion-corrosion synergy in wear of low-alloy steel by slurry of copper tailing; however, for full understanding of the involved mechanisms and for accurate prediction of the wear rate further studies are necessary.
Acknowledgments This work has been funded by CONICYT through FONDECYT grant No. 1141107 and the doctoral scholarships CONICYT-PCHA/ Doctorado Nacional/2015–21150171. 335
Tribology International 114 (2017) 329–336
J. Aguirre, M. Walczak
G0119-09.2. [18] Barik RC, Wharton J a, Wood RJK, Stokes KR. Electro-mechanical interactions during erosion-corrosion. Wear 2009;267:1900–8. http://dx.doi.org/10.1016/ j.wear.2009.03.011. [19] He D, Jiang X, Li HG S. Erosion and erosion-corrosion behaviours of several stainless steels in dual phase flow. Corrosion 2002;58:276–82. [20] Guo HX, Lu BT, Luo JL. Interaction of mechanical and electrochemical factors in erosion-corrosion of carbon steel. Electrochim Acta 2005;51:315–23. http:// dx.doi.org/10.1016/j.electacta.2005.04.032. [21] Lu BT, Luo JL, Guo HX, Mao LC. Erosion-enhanced corrosion of carbon steel at passive state. Corros Sci 2011;53:432–40. http://dx.doi.org/10.1016/j.corsci.2010.09.054. [22] Stack MM, Zhou S, Newman RC. Identification of transitions in erosion-corrosion regimes in aqueous environments. Wear 1995;186–187:523–32. http:// dx.doi.org/10.1016/0043-1648(95)07175-X. [23] Innova Corfo JRI. Aplicación Industrial de la Tecnología de Espesamiento Extremo En Proyectos Mineros; 2010. [24] National Institutes of Health. ImageJ n.d. [25] ASTM international. ASTM-G75-15 Standard Test Method for Determination of Slurry Abrasivity (Miller Number) and Slurry Abrasion Response of Materials (SAR Number) 1. Wear Erosion, Met Corros; 2013: p. 1–21. doi:http://dx.doi.org/10. 1520/G0075-07.2. [26] Stem M, Geary AL. Electrochemical polarization I. A Theoretical analysis of the shape of polarization curves. Electrochem Soc 1957;104:56–63. [27] Hutchings IM. Tribology: friction and wear of engineering materials. BH. UK; 1992. [28] Zhang G a, Cheng YF. Electrochemical corrosion of X65 pipe steel in oil/water emulsion. Corros Sci 2009;51:901–7. http://dx.doi.org/10.1016/j.corsci.2009.01.020. [29] Tian BR, Cheng YF. Electrochemical corrosion behavior of X-65 steel in the simulated oil sand slurry. I: effects of hydrodynamic condition. Corros Sci 2008;50:773–9. http://dx.doi.org/10.1016/j.corsci.2007.11.008. [30] Vera R, Vinciguerra F, Bagnara M. Comparative Study of the Behavior of API 5LX65 Grade Steel and ASTM A53-B Grade Steel against Corrosion in Seawater; 2015.10: p. 6187–98. [31] Sherif EM, Almajid AA, Khalil KA, Junaedi H, Latief FH. Electrochemical studies on the corrosion behavior of API X65 pipeline steel in chloride solutions. Int J Electrochem Sci 2013;8:9360–70. [32] Harvey TJ, Wharton JA, Wood RJK. Development of synergy model for erosion– corrosion of carbon steel in a slurry pot. Tribology 2007;1:33–47. http:// dx.doi.org/10.1179/175158407×181471. [33] Rajahram SS, Harvey TJ, Wood RJK. Evaluation of a semi-empirical model in predicting erosion-corrosion. Wear 2009;267:1883–93. http://dx.doi.org/ 10.1016/j.wear.2009.03.002.
References [1] Chadwick J. Hydrotransp Int Min 2011:57–65. [2] Heitz E. Mechanistically based prevention strategies of flow-induced corrosion. Electrochem Acta 1996;41:503–9. [3] Neville A, Hodgkiess T, Xu H. An electrochemical and microstructural assessment of erosion-corrosion of cast iron. Wear 1999;233–235:523–34. http://dx.doi.org/ 10.1002/1521-4176(200201)53:1 < 5::AID-MACO5 > 3.0.CO;2-R. [4] Zhang Y, Yin X, Wang J, Yan F. Influence of microstructure evolution on tribocorrosion of 304SS in artificial seawater. Corros Sci 2015;99:272–80. http:// dx.doi.org/10.1016/j.corsci.2014.07.062. [5] Lu BT, Luo JL. Correlation between surface-hardness degradation and erosion resistance of carbon steel—Effects of slurry chemistry. Tribol Int 2015;83:146–55. http://dx.doi.org/10.1016/j.triboint.2014.11.008. [6] Desale GR, Gandhi BK, Jain SC. Effect of erodent properties on erosion wear of ductile type materials. Wear 2006;261:914–21. http://dx.doi.org/10.1016/ j.wear.2006.01.035. [7] Arabnejad H, Shirazi SA, McLaury BS, Subramani H, Rhyne LD. The effect of erodent particle hardness on the erosion of stainless steel. Wear 2015;332– 333:1098–103. http://dx.doi.org/10.1016/0043-1648(91)90263-T. [8] Telfer CG, Stack MM, Jana BD. Particle concentration and size effects on the erosion-corrosion of pure metals in aqueous slurries. Tribol Int 2012;53:35–44. http://dx.doi.org/10.1016/j.triboint.2012.04.010. [9] Clark HM. Particle velocity and size effects in laboratory slurry erosion measurements OR… do you know what your particles are doing?. Tribol Int 2002;35:617–24. http://dx.doi.org/10.1016/S0301-679X(02)00052-X. [10] Lu B, Wang K, Wan X, Luo J. Effects of slurry pH on the surface mechanical properties and erosion-corrosion resistance. NACE Corros 2008:1–11. [11] Turenne S, Fiset M, Masounave J. The effect of sand concentration on the erosion of materials by a slurry jet. Wear 1989;133:95–106. http://dx.doi.org/10.1016/ 0043-1648(89)90116-6. [12] Zhou S, Stack MM, Newman RC. Characterization of synergistic effects between erosion and corrosion. Corros Sci 1996;52:934–46. [13] Watson SW, Friedersdorf FJ, Madsen BW, Cramer SD. Methods of measuring wear-corrosion synergism. Wear 1995;183–183:476–84. [14] Matsumura M. Erosion-corrosion of metallic materials in slurries. Corros Rev 1994;12:321–40. http://dx.doi.org/10.1515/CORRREV.1994.12.3-4.321. [15] Stack MM, Pungwiwat N. Erosion-corrosion mapping of Fe in aqueous slurries: some views on a new rationale for defining the erosion-corrosion interaction. Wear 2004;256:565–76. http://dx.doi.org/10.1016/S0043-1648(03)00566-0. [16] Meng H, Hu X, Neville A. A systematic erosion-corrosion study of two stainless steels in marine conditions via experimental design. Wear 2007;263:355–62. http://dx.doi.org/10.1016/j.wear.2006.12.007. [17] ASTM International. ASTM G119-09 Standard guide for determining amount of synergism between wear and corrosion; 1994. doi:http://dx.doi.org/10.1520/
336