Erosion-corrosion of X-52 steel pipe under turbulent swirling impinging jets

Wear 376-377 (2017) 549–556

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Erosion-corrosion of X-52 steel pipe under turbulent swirling impinging jets C. Sedano-de la Rosa a,n, M. Vite-Torres a, J.G. Godínez-Salcedo b, E.A. Gallardo-Hernández a, R. Cuamatzi-Melendez c, L.I. Farfán-Cabrera a a

Instituto Politécnico Nacional, SEPI-ESIME-UZ, Grupo de Tribología Col. Lindavista, C.P. 07320 Ciudad de México, Mexico Instituto Politécnico Nacional, ESIQIE, C.P. 07738 Ciudad de México, Mexico c Instituto Mexicano del Petroleo, Col. San Bartolo Atepehuacan, CP 07730 Cd. de México, Mexico b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 September 2016 Received in revised form 11 December 2016 Accepted 21 December 2016

Erosion-corrosion tests were carried out at two regimes, non-swirling jets with swirl number (S ¼ 0), and weakly swirling jets up to reaching vortex breakdown (S ¼0.1, 0.2 and 0.3); the average flow velocity was adjusted at 3.2 ms  1, meanwhile the corrosive media consisted of 1 l of distilled water and 2 g/l NaCl purged with CO2 and abrasive particles of 20 μm of aluminium oxide ( Al2 O3 ) suspended into test solution with a content of 11 kg m−3. The impinging angle was 90 ° in the near-field. Electrochemical measurements were performed using the polarization resistance technique (Rp). Furthermore, to characterize the damaged surface, optical and scanning electron microscopy (SEM) images were taken, also the corrosion products were analyzed by means of X-ray diffraction (XRD) and EDS techniques to identify the material loss mechanism. The experimental results shows that the maximum corrosion rate was observed at high swirl numbers, moreover, this swirling regime is more severe than the non-swirling condition improving the pit formation. & 2017 Elsevier B.V. All rights reserved.

Keywords: Turbulent swirling impinging jet Swirl number Erosion-corrosion X-52 steel pipe Polarization resistance

1. Introduction The erosion-corrosion phenomenon causes wear when abrasive particles are present into a liquid solution for instance water; it caused by the relative movement of the solids with respect to the surface. Such wear is more prevalent where fluid is forced to change direction or where high shear stresses occur. Basically the particles must penetrate the laminar sublayer with enough force to remove the passive film on the alloy. For this reason, high shear stresses are often required. The above situation can cause significant damage in systems carrying saltwater and solids, for example, carbon steel carrying air plus particles [1]. In recent investigations, the erosion-corrosion behaviour has been characterized using different techniques, for instance the submerged jet impingement [2–5], the slurry pot erosion tester [6], the rotating disk electrode [7], as well as the close loop technique [8–11]. On the other hand, the API 5L X52 low carbon steel pipe, is typically used for the conduction of gas and fuel in large volumes over long distances in the petroleum industry, due to its high mechanical resistance and well response when is exposed to different aggressive environments [12,13]. Tests have been developed in different environments [13–17] such as Corrosion inhibitors, n

Corresponding author. E-mail address: [email protected] (C. Sedano-de la Rosa).

http://dx.doi.org/10.1016/j.wear.2016.12.063 0043-1648/& 2017 Elsevier B.V. All rights reserved.

H2 SO4 , neutral and high pH, and CO2-Saturated NaCl in brine media. In addition, swirling flows have achieved better technical applications, for example in cyclone separators, burners, propellers, dredges, excavators, etc. While, the rotating jet is a fluid stream forced by pressure of an opening or nozzle, where a tangential velocity is superimposed on the axial flow in a circular jet, both radial and axial pressure gradients are generated. These gradients may significantly influence the flow changing the geometry, the evolution and the interactions between the vortical structures. For swirling jets different flow regimes may be identified depending on the degree of swirl present in the jet [18]. There are different definitions of swirl number or swirl intensity, S, the most often used one is the proposed by Beer and Chigier [19] and used by Wu et al. [20], and is defined as the ratio of axial flux of angular momentum to the axial flux of the axial momentum.

S = Mangular /RMaxial =

∫0

R

Uaxial Utangential r 2dr /R

∫0

R

2 Uaxial rdr

(1)

Where Utangential is the characteristic tangential velocity, Uaxial is axial velocity, Mangular is axial flux of angular momentum, Maxial is axial flux of axial momentum, and R is the inlet radius. In some publications [21–27], this type of flow has been studied particularly with applications on cavitation reactors, heat transfer distribution, flame jets, gas turbine blades, acceleration and

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Fig. 1. Schematic diagram of the erosion-corrosion rig developed. Fig. 3. Bright field optical image of API 5L X-52 steel microstructure, etched with Nital 2%.

Table 2 Operating conditions for erosion-corrosion of API 5L X-52 steel. Nozzle diameter Standoff distance Test media

6.35 mm 6.35 mm Distilled waterþ2 g/l of NaClþ 11 kgm  3 of 20 μm Al2 O3 Test duration 1 h, 2 h, 3 h and 4 h Test temperature Room temperature Angle of incidence 90° Swirl number (S) 0, 0.1, 0.2, and 0.3 Average flow velocity 3.2 m s  1

2. Experimental Fig. 2. (a) Specimens for electrochemical measurements; (b) Specimens for surface damage and corrosion products characterization.

penetration of micro-particles, excavation and pipeline flows. In addition, some researchers [28] have conducted erosion-corrosion experiments at submerged conditions; nevertheless, an impinging jet can be classified as a) Submerged Jet or b) Free Jet; if the fluid issuing from the nozzle is of the same density and nature as that of the surrounding fluid, then the jet is called a Submerged Jet. On the contrary if it has different density than the surrounding fluid then is called a Free Jet. When the jet is submerged, the turbulence induced due to shear layer is carried toward the centre of the jet. On the opposite, if the jet is free, this effect is not so prominent [29]. The range of materials used on the tester, could be metals and its alloys susceptible to erosion-corrosion damage, for example carbon steel, stainless steel, aluminum, lead, copper, among others [30]. The aim of this work is to gain knowledge of erosion-corrosion wear behaviour on API 5L X-52 carbon steel, under swirling and non-submerged jet conditions not yet implemented in tribological tests.

2.1. Experimental set-up The erosion-corrosion tester was designed to control and adjust nozzle rotation frequency, flow velocity, specimen distance and orientation relative to the impinging stream. Fig. 1, shows the schematic diagram of the developed rig, manufactured by IPNSEPI-ESIME-Zacatenco, Tribology Group. This equipment was designed considering a few aspects on wear corrosion phenomenon mentioned in the G119 ASTM Standard [31,32]. The nozzle has an inner diameter of 6.35 mm, turned by means of an AC motor through a pulley and belt system. The impinging angle employed was 90° in the near field, that is the standoff distance was 6.35 mm, equal to the nozzle inner diameter. A 20 mm high honeycomb section with grid diameter of 2.38 mm, is fitted inside the straight nozzle tube to produce the rotation movement of the solid-body [33]. The electrolyte consists of 1 l of distilled water with 2 g/l of NaCl and saturated with CO2 to reach a pH of 3.86 before the experiments, additionally abrasive particles of 20 μm of Al2 O3 were suspended with a content of 11 kgm  3 to generate the erosion-corrosion wear. Besides, two different configurations of specimens were used; a probe arrangement with three-electrode concentric rings was used for electrochemical

Table 1 Physical and chemical properties of API 5L X-52 steel. Yield strength Density Equivalent weight Hardness Composition (%) Fe 98.3

C 0.075

429 MPa 7.82 g cm  3 18.62 g 197 HV0.1

Si 0.194

Mn 0.675

P 0.0754

S 0.047

Cr 0.0118

Nb 0.0354

Ti 0.0075

V 0.003

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Fig. 4. Wear scar after 15 min of erosion-corrosion at 90° of incidence and swirling condition S¼ 0.1, (b) 400X magnification of wear scar at 15 min of erosion-corrosion at S ¼0.1, (c) Bright-field image of surrounding area of incident region after 4 hrs of erosion-corrosion at swirling condition of S¼ 0.3, (d) Dark-field image of surrounding area of incident region after 4 hrs of erosion-corrosion at S ¼ 0.3. Table 3 Evaluation of pitting density. Density (pits/mm2)

S¼0 S ¼ 0.1 S ¼ 0.2 S ¼ 0.3

1h

2h

3h

4h

3.3 3.0 1.0 1.3

3.8 3.0 1.3 1.3

3.9 3.1 2.2 1.5

3.9 3.4 2.4 1.8

measurements, as shown in Fig. 2(a), the rings were made of API 5L X-52 steel, the inner ring serves as the working electrode with a diameter of 11.28 mm and 3 mm thick; electrical connections were

made via wires soldered to the rear to measure the electrochemical signals. The rings were then encapsulated into polyester resin with an exposed surface of 1 cm2 to the electrolyte. Prior to experiments, the probe surface was polished with 120, 180, 240, 320, 400, and 600 grit silicon carbide paper. Afterwards the electrodes were rinsed with distilled water and placed in an ultrasonic bath with ethyl alcohol for a few minutes, and air dried. Specimens for surface damage and corrosion products characterization were square sections of 25 mm  25 mm  3 mm thick, see Fig. 2(b). Finally, the electrochemical measurements were made by using a EG& G potentiostat/galvanostat model 263 A, and the electrochemical techniques used were Polarization Resistance (Rp), and the Potentiodynamic Polarization techniques [34].

Fig. 5. Pitting growth behaviour of: (a) Pit opening along the time of exposure; (b) Pit depth growth at different swirling conditions tested.

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Fig. 6. XRD and EDS analysis of corrosion product layer at 12 days of simple immersion.

Fig. 7. Micrograph of: (a) Pitting, and (b) CPL on pitting.

2.2. Materials The metallic specimens were made of API 5L X-52 pipeline steel. The microhardness obtained was 197 HV0.1, with a load of 100 gf, measured by means of a LECO microhardness tester model LM 700. The chemical analyses and physical properties of the tested material are given in Table 1. Fig. 3, shows the microstructure which reveals a mixture of 99.47 % ferrite (white color) and 0.53 % of pearlite microconstituent (black color), as expected in this type of steel. 2.3. Electrochemical measurements In order to evaluate the corrosive effect on the X-52 specimens,

the electrochemical measurements were performed using the Polarization Resistance Technique (Rp), which it may be related to the overall corrosion rate for metals at near their corrosion potential (Ecorr). Polarization resistance measurements are an accurate and rapid way to measure the general corrosion rate [34]. Due to the experiments were carried out at not submerged conditions, then it was necessary to employ a three-electrode concentric ring probe arrangement, used by other researchers in corrosion tests [35,36]. Open circuit potential and polarization resistance tests were performed to measure the corrosion potential (Ecorr) and corrosion rate at different swirling conditions. The corrosion rates were calculated via Faraday's Law from the icorr data [34] and expressed in mils per year (mpy). Potentiodynamic polarization measurements were conducted to observe the tendency of the

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Fig. 8. Micrograph of: (a) Analysis of removed material from pitting, (b) Displaced material from pitting hole, and (c) Pitting surrounding area.

corrosion rate of steel into the aggressive media, like shown in Fig. 11(a). In summary, the operating conditions for erosion-corrosion tests under turbulent swirling jets are shown in Table 2.

3. Results and discussion 3.1. Effect of tangential velocity on wear scar Fig. 4(a) displays after 15 min of testing the wear scar obtained at swirling condition S¼ 0.1, exhibiting a hole-shaped metal loss into the impinging region. Fig. 4(b) shows a localized attack on the metallic surface associated to pitting corrosion [37]. In Fig. 4(c) it can be seen, the traces of the material detached from the impinging region towards outside. The evaluation of pitting corrosion obtained was performed according to G46-94 ASTM Standard [38], and includes: size, shape, density, uniformity of distribution and depth. These parameters were measured along the experiments at different swirl numbers, the Table 3 shows the evaluation of pitting density into the impinging region. Maximum value was obtained at nonswirling condition, while the minimum value was reached with the maximum swirl number, it is probably that the tangential velocity imposed by the nozzle affects the pit nucleation on the metal surface, promoting a scattered pitting distribution. Fig. 5

(a) shows the results obtained after the measurements performed three times of the pit opening (pit diameter), where it can be seen a substantial rise in pits diameter when increase the exposure time. Moreover when swirl numbers of S ¼0.2 and 0.3 hole pits reach maximum opening. Fig. 5(b) displays the pit growth versus time exposure plot, for all conditions on early stage, where pits begin with approximately the same depth, however the deepest pits were observed with S ¼0.2. 3.2. Effect of tangential velocity on corrosion products To obtain a corrosion product layer (CPL) of 20 microns, was necessary 12 days of immersion into the corrosive media, as can be seen in Fig. 6(a). The XRD diffractograms were obtained by means of PANalytical X-ray diffractometer model X'Pert PRO, using a Cobalt tube and grazing incidence technique, revealing a CPL in amorphous phase, where only the peaks corresponding to Fe index were observed as shown in Fig. 6(b). The impinging surfaces was evaluated using Scanning Electronic Microscopy (SEM/EDS) through a JEOL microscope model JSM 5600-LV, to obtain information about the element distribution on CPL at different swirling conditions, the EDS analysis shows a content of oxygen and chlorine plus the steel elements, see Fig. 6(c). Additionally, Fig. 7(a), exhibits the micrograph of corrosion products on pitting cluster at S¼ 0.2 and 30 minutes of erosion-corrosion exposure;

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Fig. 9. Element mapping of corrosion products at non-swirling and swirling conditions.

where it can be seen the detached metal path trajectory of corrosion products formed. Fig. 7(b), shows a micrograph of pitting full of corrosion products, the corrosion product fracture suggest that is prone to large volume changes, shrinking when the water content was reduced, forming cracks in drier atmosphere. Meanwhile, the Fig. 8(a), shows the EDS analysis at the site where the material removal occurs by pitting, it can be seen the presence of Fe, C, Mn and Si, dissolved in reduction-oxidation process. Fig. 8(b), displays the displaced material from pitting hole, it was observed an increase of Na, Al, Si and Cl; a depletion of O and Fe, and a Mn disappearance. Moreover, analysis of the nondamaged surface surrounding the pitting shows a reduced amount of oxygen, preserving the entire surface without loss of material, as illustrated in Fig. 8(c). On the other hand, an element mapping was performed to observe the corrosion products distribution at different swirling numbers. From Fig. 9(a), it can be seen the CPL formation at accelerated corrosion and simple immersion condition, displaying an uniform CPL with irregular shape, Fig. 9(b), exhibits a less uniform layer with a funnel shape corrosion formation also observed by Sapre et al. [39], in their observations reported that at high turbulence, the CPL is unlikely to stick to the surface, displaying an intermittent funnel shaped growths, which promotes a localized attack wherein the reactive species are able to reach the metal surface more easily through these funnels. Fig. 9(c), shows a similar corrosion formation than the non-swirling condition, but involve a major surface where oxides presence was very plenty, it can be seen in Fig. 9(e) and (f). The different corrosion products distribution it was due to different swirling velocities in the erosion-corrosion tests.

3.3. Effect of swirl number on electrochemical measurements Fig. 10(a) shows the erosion-corrosion of X-52 steel open circuit potential (OCP) and Fig. 10(b) shows the polarization resistance (Rp) curve obtained at swirling condition of S ¼0.1. The system at non-swirling condition presents a Rp average value of 857.67 Ω. On the contrary the minimum value was obtained at maximum swirling condition of S¼ 0.3 (Rp ¼641.70 Ω). Fig. 11(a) exhibits a potentiodynamic polarization curve where it can be seen that the curve showed no tendency to decrease the corrosion rate of steel surface into the aggressive media, making evident the porousness of the CPL. The corrosion rates at both swirling and non-swirling conditions were obtained and plotted in Fig. 11(b), in which it can be seen an average corrosion rate at non-swirling condition of 7.77 mpy, and an increase of corrosion rate related to swirling conditions, reaching a steady state between swirl numbers 0.2 and 0.3.

4. Conclusions Pitting corrosion was the dominant damage mechanism encountered at all conditions tested. The swirling flow condition enhances the pit formation, promoting the dissolution of the underlying metal. Overall the corrosion wear was the predominant wear mechanism over the erosion wear. The corrosion product layer obtained showed poor adhesion which facilitated its detachment and breaking. Finally, the swirling regime is more severe than the non-

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Fig. 10. Erosion-corrosion electrochemical measurements of: (a) Open circuit potential; (b) Polarization resistance, at swirling condition, S ¼0.1.

Fig. 11. (a) Potentiodynamic polarization curve at non-swirling condition; (b) Corrosion rate at non-swirling and swirling conditions for API X-52 steel during erosioncorrosion process.

swirling condition, and the maximum corrosion rate was observed at high swirl numbers.

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