Understanding the influences of pre-corrosion on the erosion-corrosion performance of pipeline steel

Journal Pre-proof Understanding the influences of pre-corrosion on the erosion-corrosion performance of pipeline steel Yunze Xu, Liang Liu, Qipiao Zhou, Xiaona Wang, Yi Huang PII:

S0043-1648(19)31274-8

DOI:

https://doi.org/10.1016/j.wear.2019.203151

Reference:

WEA 203151

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Wear

Received Date: 20 August 2019 Revised Date:

4 November 2019

Accepted Date: 4 December 2019

Please cite this article as: Y. Xu, L. Liu, Q. Zhou, X. Wang, Y. Huang, Understanding the influences of pre-corrosion on the erosion-corrosion performance of pipeline steel, Wear (2020), doi: https:// doi.org/10.1016/j.wear.2019.203151. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Understanding the influences of pre-corrosion on the erosion-corrosion performance of pipeline steel Yunze Xua, b, Liang Liua, Qipiao Zhoua, Xiaona Wangc, * and Yi Huanga, ** a School of Naval Architecture and Ocean Engineering, Dalian University of Technology, Dalian 116024, China; b School of Civil Engineering, Dalian University of Technology, Dalian 116024, China; c School of Physic and Optoelectronic Engineering, Dalian University of Technology, Dalian 116024, China; Corresponding author: E-mail address: [email protected] (Xiaona Wang) and [email protected] (Yi Huang) Postal address: School of Naval Architecture and Ocean Engineering, Dalian University of Technology, Linggong Road 2#, Ganjingzi District, Dalian 116024, Liaoning Province, China.

Abstract

The initiation and the dynamic progression of erosion-corrosion on a pre-corroded X65 pipeline steel surface was investigated using coupon electrodes and wire beam electrode (WBE). Results show that a concentrated erosion-corrosion region would appear on the steel surface after pre-corrosion which is significantly different from the scatter distributed impingement craters on a fine polished steel coupon. The initiation of localized erosion-corrosion on a pre-corroded steel surface is most probably induced by the instant interfacial chemical and electrochemical heterogeneity, and the instantaneous inhomogeneous sand impingements rather than the surface roughness increasing and the porous rust layer. The pre-corroded area having an initial intense sand impingements would propagate to a concentrated low-lying erosion-corrosion area and small cathodic regions would propagate to isolated islands in the low-lying area.

Keywords: Erosion-corrosion, pre-corrosion, pipeline steel, sand impingements 1. Introduction

Erosion-corrosion is a challenging issue for energy pipelines during transportation of natural resources containing corrosive slurry [1-3]. The coupling effect of mechanical abrasion and electrochemical corrosion sometimes could induce a higher materials loss than the summation of the materials loss due to separate pure corrosion and pure erosion [4-7]. The increased material loss induced by the synergy of erosion and corrosion is always defined as 1

erosion enhanced corrosion and corrosion enhanced erosion [1, 8, 9]. Various studies were conducted in past decades to figure out the synergy of erosion and corrosion which could lead to a quick thinning of the pipe wall [10-12]. For the steel having a passive film, it is found that the synergistic effect between erosion and corrosion is mainly caused by erosion enhanced corrosion which is associated with the breakdown of the passive film, leading to fresh metal be exposed to the electrolyte [10, 13, 14]. The wear of coatings on steel surfaces may also lead to erosion enhanced corrosion in corrosive slurries [15]. For steels under active corrosion, it is found that the surface hardness would have an obvious degradation due to the anodic dissolution [16]. The threshold flow velocity for erosion initiation would have an intense decrease along with the surface hardness degradation [11, 17]. Accordingly, corrosion enhanced erosion becomes the main synergy for steels under active corrosion [17-19]. The main impact factors for erosion-corrosion including hydrodynamic parameters, characteristics of solid particles, temperature, chemical components of the corrosive slurry and the mechanical properties of the target materials have been well studied in previous researches [20-26]. In most of these previous studies, fine polished steel samples with smooth and uniform surfaces were employed for erosion-corrosion test. However, during real pipeline installations, corrosive medium such as sea water or formation water might enter the pipe in advance of its operation [27, 28]. Moreover, pressure test and low flow rate test using seawater or tapwater were normally carried out to avoid the leakage and ensure the safety of the pipeline before they are used in high flow rate working conditions [29]. Therefore, pre-corrosion might occur on the pipeline internal surface due to the corrosive medium before the occurrence of erosion-corrosion. Unfortunately, nearly no researches have considered the influence of pre-corrosion on the subsequent erosion-corrosion process. Although the erosion-corrosion behaviours of some API pipeline steels were studied using cyclic erosion-corrosion method [30], trying to understand the interaction between erosion and corrosion, the individual pure corrosion process and pure erosion process in an erosion-corrosion cycle only can study the influences of pre-corrosion on pure erosion rather than the erosion-corrosion. Consequently, the impact of pre-corrosion on the subsequent erosion-corrosion process is still unclear.

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The combination of electrochemical and gravimetric measurements is commonly used to separate the corrosion component and erosion component [11, 12, 18, 22], allowing the study of the general performance of erosion-corrosion in a corrosive slurry. However, due to the heterogeneous anodes and cathodes distribution on a corroded steel surface [31], the dynamic progression of erosion-corrosion on the local areas is hard to be probed by traditional methods, which limits the further understanding of the localized erosion-corrosion damage [26, 32]. In this work, normal steel coupon electrodes were employed together with an electrochemically integrated wire beam electrode (WBE) to study both the general performance and the dynamic progression of erosion-corrosion on a pre-corroded steel surface. The WBE technique has been adopted in previous studies which provided a high accuracy for probing the dynamic progression of erosion-corrosion [32, 33]. According to the variation of the current distribution maps, the influences of anodes and cathodes distribution and the surface roughness change on the subsequent localized erosion-corrosion process were analysed. 2. Methods

A rotation disc system shown in Fig. 1 is adopted to simulate a flowing hydrodynamic environment. The rotation disc is made of Teflon. The diameter and the thickness of the disc is 60 mm and 2 mm, respectively. A coupon electrode and a 10 × 10 WBE are arranged in a cylinder cell which diameter is 80 mm. The coupon electrode and the WBE used in the test were both made of X65 pipeline steel with the chemical components: (mass %): 0.04 C, 0.2 Si, 1.5 Mn, 0.011 P, 0.003 S, 0.02 Mo and Fe balance. The coupon electrodes which acted as the working electrode (WE) were machined in a rectangular shape with the size of 7 mm × 7 mm (0.49 cm2 working area exposed to the solution). The WBE was fabricated by 100 cylinder shape tiny electrodes which diameter was 1.8 mm. The interval between the centres of the neighbouring electrodes was 2 mm to ensure the electrochemical and chemical integrity of the WBE, aiming to mimic a one-piece electrode. The coupon electrodes and the WBE were successively grounded using 400, 800 and 1200 abrasive papers. Then, the steel surfaces were washed by aceton and distilled water before the pre-corrosion or erosion-corrosion test. The coupon electrode and the WBE were symmetrically placed in the test cell. The distance between the steel surface and the rotation disc was about 10 mm. The crevices between the 3

samples and the cell wall were sealed by hot melt glue to avoid leakage. A Pt electrode which worked as counter electrode (CE) and a saturated calomel electrode (SCE) which acted as reference electrode (RE) were closely fixed around the cell wall to minimize the interference of the flow. An electrochemical station (CS 350) was used to connect the WE, RE and CE which could establish a three electrode system for electrochemical impedance spectroscopy (EIS) measurements. An electrochemical instrument (CST 520) which contained a zero resistance ammeter (ZRA) and an auto-switcher array was used to scan the galvanic current distribution on the WBE.

Fig. 1 The test setup for erosion-corrosion using a WBE and a coupon electrode.

The slurry for erosion-corrosion test consists of 3.5% sodium chloride solution and 10% silica sand particles (by weight). It is seen from Fig. 2 that the diameter of the sand particles varies from 250 µm to 500 µm and the particles have irregular shapes with some sharp edges. The erosion-corrosion test was conducted using a 600 rpm rotation speed. The rotation disc was fixed at the middle level of the coupon electrode and the WBE. The Reynold Number (Re) in the cell could be roughly evaluated [34]: 4

Re =

(2)

where ω is the rotation speed, υ is the kinematic viscosity of the electrolyte, and r is the radius of the rotate disc. The calculated Re was 5.7 × 104 which indicates a turbulence flow condition for the 600 rpm rotation speed.

Fig. 2 The shape of the sand particles.

In order to study the impact of pre-corrosion on its erosion-corrosion behaviour of the pipeline steel, three groups of tests with different pre-treatments condition for both coupon electrodes and WBE were designed and conducted as presented in Table 1. In the first group of test, a fine polished coupon electrode and a WBE were directly immersed in the flowing slurry (600 rpm) for a 24 h of erosion-corrosion test. The coupon electrode and WBE in Group 2 were firstly pre-corroded in a 3.5% NaCl solution for 24 h with the rotation disc being kept static. After 24 h of pre-corrosion, the silica sand was added in the electrolyte and the rotation disc started to rotate at 600 rpm for another 24 h. In Group 3, the coupon electrodes and WBE were firstly pre-corroded in the static 3.5% NaCl solution for 24 h which was same with that in Group 2. Then, the samples were taken out to eliminate the rust layer through ultrasonic cleaning. After the pre-treatment, the coupon electrode and WBE were immersed in the flowing slurry (600 rpm) for a 24 h of subsequent erosion-corrosion test. 5

Three parallel coupon electrodes were adopted in each group for repeated tests. The three parallel coupon electrodes used in Group 1 were numbered as A1, A2 and A3. Likewise, the parallel coupon electrodes used in Group 2 and Group 3 were numbered as B1-B3 and C1-C3, respectively.

Table 1 Pre-treatments of the coupon electrodes and WBE before erosion-corrosion Groups

Pre-treatments of coupon electrodes and WBE

Group 1

Fine polished to 1200 grit

Group 2

Pre-corroded in a static 3.5% NaCl solution for 24 h Pre-corroded in static 3.5% NaCl solution for 24 h and then, the rust

Group 3 layer on the steel surface was removed by ultrasonic cleaning

During the erosion-corrosion processes in the three group of tests, the general corrosion rates of the coupon electrodes in the flowing slurry were obtained from the EIS measurements. The EIS measurements were performed every 2 h with a 10 mV sinusoidal signal applied over the frequencies from 105 Hz to 10-2 Hz around the OCP. The EIS measurement results were fitted by Zview. After the 24 h of erosion-corrosion test, the corrosion product was cleaned using ASTM G1-03 acid washing solution and the hot melt glue remained at the edge of the sample was eliminated by heating the sample to 70 ℃. Thereafter, the residual mass of the coupon electrode was measured using a balance scale (SHIMADZU AUW 320) to calculate the total steel loss induced by erosion-corrosion in Group 1 and Group 3 and the total steel loss induced by both pre-corrosion and erosion-corrosion in Group 2. The coupon electrodes before and after erosion-corrosion were both photographed using a high definition digital camera and the erosion-corrosion morphologies were further observed using an Olympus infinite microscope. Each group of test was repeated three times using the three parallel coupon electrodes to ensure the repeatability of the test results. The galvanic current distribution on the steel surface was probed using a WBE to facilitate understanding of the dynamic erosion-corrosion process of the pipeline steel after different pre-treatments. The schematic diagram of the WBE measurement circuit is shown in 6

Fig. 1. The terminals of the 100 tiny electrodes were continually connected together to allow the electrons freely moving among the electrodes array. The galvanic currents of the 100 electrodes were measured through the switcher array which could automatically connect the ZRA between the chosen electrode and the other 99 electrodes. Thereafter, the current distribution map could be constructed through the measured individual galvanic current of each electrode. The progress of localized erosion-corrosion damage could be further analysed according to the dynamic change of the anodic and cathodic current distribution. The galvanic current maps were measured every 10 min, in the same way as that introduced in previous studies [33]. Besides the current distributions in the flowing slurry, the current distributions during the pre-corrosion process in the static electrolyte were measured using WBE as well. 3. Results

3.1 General erosion-corrosion behaviour of the coupon electrodes

The typical Nyquist plots of the coupon electrodes (A1, B1 and C1) in each test group are presented in Fig. 3a-1 to Fig. 3c-1, respectively. As X65 pipeline steel would undergo active dissolution in the 3.5% NaCl solution, the electrical circuit shown in Fig. 3a-1 is employed for EIS fitting. The corrosion current density (icorr) of the coupon electrodes at different test periods could be calculated from the fitted charge transfer resistance (Rt): =

B

(1)

where B is the Stern-Geary coefficient and it is adopted as 26 mV according to the previous studies [5, 33, 35]. Faraday law is further used to calculate the corrosion rate: = where

M

(2)

F

represents the corrosion rate (mm/y), M is the iron molecular weight, F is Faraday

constant, n is the number of the transferred electrons, and ρ is the mass density of the steel (7.8 g/cm3). The calculated average corrosion rates of the three parallel coupons in each test group are presented in Fig. 3b-1 to Fig. 3c-1. The error bar shows the standard deviation of the corrosion rates among the three parallel coupons in each group. As shown in Fig. 3, the corrosion rates of the coupon electrodes after different pre-treatments all show significant 7

increasing at the initial stage of the erosion-corrosion test. The average corrosion rate of the fine polished coupon electrodes (A1, A2 and A3) increased from 4.0 mm/y to 5.0 mm/y after 8 h of immersion and slightly reduced to the value around 4.6 mm/y. The average corrosion rate of the coupon electrodes (B1, B2 and B3) kept increasing from 3.5 mm/y to 4.4 mm/y during the initial 20 h of test and gradually became stable at the end of the test. For the pre-corroded coupon electrodes (C1, C2 and C3) with rust layer cleaned by ultrasonic cleaning in Group 3, the average corrosion rate dramatically increased from 3.8 mm/y to 4.4 mm/y after 6 h of immersion and kept relatively stable around 4.4 mm/y until the end of the test. On the basis of the EIS measurement results, it is found that different pre-treatments nearly had no influence on the general corrosion rate of the coupon electrodes.

Fig. 3 Typical Nyquist plots (a-1, b-1 and c-1) of the coupon electrodes (A1, B1 and C1) and the variation of the calculated average corrosion rate of the three parallel tests (a-2, b-2 and 8

c-2) in the corrosive slurry after different pre-treatments: (a) fine polished, (b) pre-corroded with rust layer and (c) pre-corroded with rust removed.

The weight losses of the coupon electrodes induced by the 24 h of erosion-corrosion in Group 1 and Group 3 could be accurately calculated from the measured weight difference before and after erosion-corrosion. However, the pre-corroded coupon electrodes in Group 2 could not be taken out from the electrolyte for weight loss measurement after pre-corrosion. Accordingly, the measured weight losses of Coupons B1-B3 were caused by both pre-corrosion and erosion-corrosion. Since the corrosion rate of the steel in the static electrolyte is about 0.2 mm/y which is far below the corrosion rate in the flowing slurry, the steel loss induced by pre-corrosion could be neglected in comparison with that induced by erosion-corrosion. Thus, the general total steel loss rate (mm/y) of all the coupon electrodes in the flowing slurry could be calculated as: =

3.65 × 10 ∆" (3) #

where ∆m is the measured weight loss and A is the surface area of the coupon electrode. Thereafter, the erosion rate could be further calculated according to the ASTM standard [36]: $

=

−

(4)

The averages of the total steel loss rate, erosion rate and corrosion rate of the coupon electrodes with various pre-treatments are plotted in Fig. 4. The differences among the general erosion rates of the coupon electrodes are less than 0.4 mm/y, indicating that the general erosion performances of the coupons with different pre-treatments were also similar. It seems that pre-corrosion would not influence the general erosion-corrosion performance on the basis of the calculated general erosion component and corrosion component using traditional electrochemical methods in conjunction with gravimetric measurements.

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Fig. 4 The averages of total steel loss rate, corrosion rate and erosion rate of the coupon electrodes with various pre-treatments in the 24 h of erosion-corrosion tests. 3.2 The surface morphologies of the coupon electrodes after erosion-corrosion

Two of the coupon electrodes were selected from the three parallel tests of each group for surface morphology observation. The photos and the morphologies of the coupon electrodes (A1, A2, B1, B2, C1 and C2) are presented in Fig. 5 to Fig. 7, respectively. Fig. 5a shows the surface morphologies of the coupon electrodes (A1 and A2) in Group 1 before the erosion-corrosion test. Prior to the erosion-corrosion test, the surfaces of the coupon electrodes were smooth and uniform. As shown in Fig. 5b, impingement craters generate and discretely distribute on the whole steel surface of the coupon electrodes after the erosion-corrosion test, indicating a typical erosion-corrosion morphology of the pipeline steel under active corrosion [26, 33, 35, 37]. Although the sand particles in the corrosive slurry could impinge the whole steel surface, impingement craters only could form at the local areas under anodic dissolution due to the local surface hardness degradation, which is reported in the references [11, 33]. Since the anodic sites would randomly appear on a fine polished steel surface at the beginning of the test, the impingement craters show a relatively scattered distribution (separated by the cathodic regions) on the steel surfaces as shown in Fig. 5b and 5c. It is seen from Fig. 5b that the quantity of the impingement craters gradually increase from the left to the right of the coupon. The local 3D profiles on the coupon electrodes as framed in Fig. 5b are plotted in Fig. 5c. It is seen that the diameters and depths of the 10

impingement craters on the left side of the coupon is smaller and shallower than those of the craters appeared at the right side of the coupon, suggesting the erosion-corrosion would become more serious at the downstream of the flow direction.

Fig. 5 The photos of the coupon electrodes (A1 and A2) in Group 1 before erosion-corrosion test (a), the photos of the coupon electrodes after erosion-corrosion test (b) and the local 3D profiles on the coupon electrodes as framed in Fig. 5b (c).

The surface status and morphologies of two pre-corroded coupon electrodes (B1 and B2) before and after 24 h of erosion-corrosion test are shown in Fig. 6. It is seen from Fig. 6a that after 24 h of pre-corrosion in the static electrolyte, a brown rust layer covers the top half and left half of the two coupons, respectively, suggesting non-uniform corrosion patterns in the 11

static electrolyte. Unlike the scattered impingement craters shown in Fig. 5b, it is seen from Fig. 6b and 6c that erosion-corrosion damages all concentrate at the pre-corroded area, leading to the formation of a concentrated low-lying erosion-corrosion area (the topography of the erosion-corrosion damage area is much lower than the cathodic area) on the steel surface. Nearly no erosion-corrosion occurred at the original cathodic regions where almost having no rust layer. As shown in Fig. 6c, obvious impingement craters only exist at the edge of the low-lying erosion-corrosion area, and some small cathodic sites which presented as isolated islands [38] appear in the low-lying area. Although the general corrosion rate and the erosion rate of the pre-corroded coupon electrodes were nearly the same with those of the fine polished coupon electrode, obvious difference of the localized erosion-corrosion performances could be observed from the comparison of the surface morphologies shown in Fig. 5 and Fig. 6, respectively. The localized erosion-corrosion area nearly all located in the pre-corroded area. Nevertheless, the area having no corrosion in the static electrolyte would only suffer slight pure erosion in the flowing slurry.

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Fig. 6 The photos of the coupon electrodes (B1 and B2) in Group 2 before erosion-corrosion test (a), the photos of the coupon electrodes after erosion-corrosion test (b) and the local 3D profiles on the coupon electrodes as framed in Fig. 6b (c).

The third group of test was conducted to study whether the concentrated localized erosion-corrosion on the pre-corroded coupons is induced by the surface roughness increase at the pre-corroded areas. Fig. 7a presents the surface status of the coupon electrodes (C1 and C2) before the erosion-corrosion test with the pre-corroded areas changing rough. As shown in Fig. 7b, scattered impingement craters appear on the coupon surfaces after 24 h of erosion-corrosion, indicating a similar erosion-corrosion performance of the coupon electrodes in Group 1. It is seen from Fig. 7b and Fig. 7c that the quantity and depths of the impingement craters at the pre-corroded area are slightly higher than those of the 13

non-corroded areas in the static electrolyte, suggesting that erosion-corrosion prefers to happen on rough areas caused by pre-corrosion. However, as the impingement craters also generated on the areas having no corrosion in the static electrolyte with rust layer removed, indicating that the influence of the surface roughness would not be an important factor for the modification of the localized erosion-corrosion performance. The localized erosion-corrosion behaviour in Group 2 might be preferably controlled by instant surface states involving the remained rust layer and the interfacial electrochemical and chemical status.

Fig. 7 The photos of the coupon electrodes (C1 and C2) in Group 3 before erosion-corrosion test (a), the photos of the coupon electrodes after erosion-corrosion test (b) and the local 3D profiles on the coupon electrodes as framed in Fig. 7b (c).

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3.3 The galvanic current distribution maps measured by WBE

Through the coupon electrodes test, it is found that the progress of erosion-corrosion damage on the steel surface with rust layer and without rust layer were of significant difference. The erosion-corrosion morphology would transfer from scattered impingement craters to a concentrated low-lying erosion-corrosion area if the coupon surface has suffered pre-corrosion. However, the dynamic progression of erosion-corrosion on the steel surface could not be captured from the test using coupon electrodes. Therefore, the in-situ transitions of the current distribution on the steel surface in the static electrolyte and corrosive slurry were probed using WBE technique to further understand how pre-corrosion would influence the dynamic progression of localized erosion-corrosion. The variation of the galvanic current distribution on the WBE during the 24 h of erosion-corrosion in Group 1 is plotted in Fig. 8. It is seen that randomly distributed anodic sites show up on the WBE after 10 min of immersion. The magnitude of the current on these major anodes kept growing in the first 4 h of immersion. Slight expansion of the initial anodic sites along the flow direction could be observed through the current distribution changes from 10 min to 1 h. The major anodic areas gradually moved to the WBE right side and some new anodes (not appeared at 10 min of immersion) could be seen at the right edge of the WBE after 24 h of erosion-corrosion. The dynamic change of the current distribution shown in Fig. 8 effectively reveals the progress of erosion-corrosion on a fine polished steel surface. The randomly appeared anodic sites would gradually grow and the new anodic sites would prefer to generate at the downstream of the initial anodic sites, leading to a relative serious erosion-corrosion performance at the right side of the sample, which is well corresponding to the surface morphologies of the coupon electrodes (A1 and A2) in Group 1.

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Fig. 8 Time dependence of the galvanic current distribution on the WBE having a fine polished surface (Group 1) during the 24 h of erosion-corrosion test.

The galvanic current distribution on the WBE during 24 h of pre-corrosion in Group 2 is plotted in Fig. 9a. It is seen that a large anodic region appears at the upper left of the WBE in the static electrolyte, suggesting that pre-corrosion mainly occurred at this area. After the corrosive slurry began to rotate, it is clearly seen from Fig. 9b that the major anodic sites would still concentrate on the original anodic areas as shown in Fig. 9a. Along with the increasing of the anodic current on some local anodic sites, some cathodic sites gradually became significant in the concentrated anodic area, leading to an obvious anode shrinkage (the area of the main anodes began to decrease) after 8 h of immersion in the flowing slurry. From comparing the galvanic current maps shown in Fig. 7 and Fig. 9b, it is found that the distribution of local anodes would significantly change once the steel surface has suffered pre-corrosion. Scattered anodic sites would appear on a fine polished steel surface, leading to the formation of obvious scattered impingement craters. However, erosion-corrosion would occur at the original anodic areas formed in the static electrolyte, and the formation of the isolated islands in the large anodic areas is most probably induced by the appearance of the cathodic sites in the large anodic area. 16

Fig. 9 The galvanic current distribution on the WBE in Group 2 during 24 h of pre-corrosion (a) and variation of the galvanic current distribution during the 24 h of erosion-corrosion after pre-corrosion without removing the rust layer (b).

The current distribution maps in both static electrolyte and flowing slurry of Group 3 are plotted in Fig. 10. It is observed that with the rust layer removed using ultrasonic cleaning, the anodic sites on the WBE would be significantly different from that in the static electrolyte 17

after 30 min of erosion-corrosion. It is seen from Fig. 10b that new anodes could form at the original cathodic areas in the static electrolyte, suggesting that only the change of the surface roughness could not determine the initiation sites of the local anodes on the steel surface. The major anodes gradually transferred from the left side to the right side of the WBE during the 24 h of test in the flowing slurry, which is similar with the dynamic progression of erosion-corrosion presented in Group 1. As the dynamic variation of the galvanic current distribution in Group 3 was more like that presented in Group 1, it indicates that the surface roughness increasing caused by pre-corrosion would not significantly influence the initiation sites of the localized erosion-corrosion.

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Fig. 10 The galvanic current distribution on the WBE in Group 3 during 24 h of pre-corrosion (a) and the variation of the galvanic current distribution during the 24 h of erosion-corrosion after pre-corrosion with rust removed by ultrasonic cleaning (b).

The statistic of the maximum anodic current on the WBE in the flowing slurry are plotted in Fig. 11. It is found that the maximum anodic current of the three groups of test kept increasing at the initial test period. The increasing of the maximum anodic current is associated with the propagation of the impingement craters and the sequence sand impacts which is introduced in the previous study [33]. It is seen that the peak value of the maximum anodic current in Group 2 and Group 3 nearly reached 10-4 A (4 mA/cm2, corresponding to a localized corrosion rate of 47 mm/y), which was about 1.7 times higher than that in Group 1, suggesting pre-corrosion would facilitate the initiation and propagation of localized erosion-corrosion on the steel surface. In comparison with the 3D profiles shown in Figs. 5-7, it is observed that the depths of the impingement craters in Group 2 and Group 3 are about several micrometres deeper than those in Group 1, which is well corresponding to the WBE measurement results.

Fig. 11 Time dependence of the maximum anodic current on the WBE in the flowing slurry after different pre-treatments (Group 1 – Group 3).

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4. Discussions

In order to understand how pre-corrosion could influence the initiation and the dynamic progression of erosion-corrosion, the erosion-corrosion process on a fine polished steel surface should be firstly discussed. It is reported that the progress of erosion-corrosion damage on the steel surface would be determined by the local anode and cathode distribution, local turbulences, sequence sand impingements and the change of local chemical environment [26, 32, 33, 39]. According to the coupon electrode test and the WBE test, the progress of erosion-corrosion damage on a fine polished steel surface could be basically understood and the schematic diagram of the erosion-corrosion process is plotted in Fig. 12. It is seen from Fig. 12 that randomly distributed anodic sites would appear on the steel surface in the corrosive slurry at the initial stage. Accompanied by the sand impingements at the initial anodic sites, craters would generate on the steel surface due to the surface hardness degradation under active corrosion [16]. The deep impingement craters would become the nucleation sites for pitting corrosion, allowing the ferrous ions and hydrogen ions to accumulate in the deep craters. It is seen from Fig. 2 that the shapes of the sand particles are irregular with sharp edges, suggesting the impingement craters would also present as irregular shapes. However, the impingement craters shown in Fig. 5 and Fig. 7 all present as hemispheres, suggesting that the impingement craters would propagate to stable pits [40]. Local micro-turbulences would generate around the craters, which could enhance the transportation of the analyte from the initial deep crater travelling to the downstream along the flow direction [33, 39]. The rim and the vicinity area of the crater downstream would become the new anodic site which corresponds to the slight anode expansion shown in Fig. 8. Thereafter, new craters would form once sand particles impact at the downstream of the initial craters, resulting in new major anodic areas gradually moving to the right side of the steel surface. Consequently, the local morphology of the coupon surface would present as continuous impingement craters which are observable in Figs. 5-7.

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Fig. 12 Schematic diagram of the progress of erosion-corrosion damage on a fine polished steel surface.

It is seen from the experimental results that the progress of erosion-corrosion damage on a pre-corroded steel surface would concentrate on the original corroded area formed in the static solution, which suggests more influence factors of erosion-corrosion should be considered for a pre-corroded steel surface. It should be noted that in real pipeline working conditions, it is always thought that serious localized erosion-corrosion would occur at the areas having a high wall shear stress, intense local turbulences or high sand concentrations [12, 41, 42]. However, it is investigated that once a local area has already become the anode during pre-corrosion, most serious erosion-corrosion would prefer to occur at this area when high flow rate slurry passing through. The influences of the hydrodynamic parameters seem not to be the main contributors for the initiation of erosion-corrosion on the pre-corroded 21

region. As a result, the pipeline areas where are undergoing active corrosion during the installation process or pressure test should be paid more attention to avoid the preferential perforation of these areas when the flow rate is adjusted to a high value. According to the test results of Group 3, it is seen that surface roughness increasing induced by pre-corrosion would aggravate the localized erosion-corrosion, however, the formation of the concentrated erosion-corrosion area is not induced by the surface roughness increasing. Erosion-corrosion could also occur on the non-corroded area with the rust layer totally removed. By comparing the three groups of test, it seems that the rust layer might play an important role on the initiation of erosion-corrosion. Therefore, an additional test was conducted to observe the change of the rust layer after 10 min of immersion in the flowing slurry. Fig. 13a shows the surface changes of a coupon electrode before and after 10 min of erosion-corrosion, respectively. It is observed from Fig. 13a that after 10 min of erosion-corrosion test, the porous brown rust layer is nearly all washed away. Although some dense black rust layer remains on the steel surface, no erosion-corrosion could be seen beneath the black rust after acid washing. The quick disappearance of the rust layer indicates that the rust layer could not fix the localized erosion-corrosion on the pre-corroded area. More details of the local surface morphologies are presented in the scanning electron microscopy (SEM) images as shown in Fig. 13b. It is seen that dense impingement craters appear on part of the pre-corroded areas (Area I) with ridges forming between the vicinity craters. However, part of the pre-corroded area (Area II) only suffers micro-cutting which is similar to the non-corroded area (Area III). Through the local SEM image of area IV shown in Fig. 13c, it is seen that after 24 h of erosion-corrosion, closely connected impingement craters appear in the concentrated low-lying erosion-corrosion area. The ridges would propagate to the isolated islands which represent the cathodic sites in the low-lying erosion-corrosion area. Serious micro-cutting would occur on the cathodic areas as shown on Area V, indicating a surface damage induced by continually pure erosion.

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Fig. 13 Photos of the coupon electrodes before and after erosion-corrosion for 10 min (a), the local SEM images of the areas I, II, III as framed in Fig. 13a (b), and the photo of a coupon electrode in Group 2 and local SEM images of areas IV and V (c).

It could be further concluded that both the surface roughness increasing and the remained brown rust layer are not the main contributors for the transition of the localized erosion-corrosion behaviour. It is investigated from Fig. 13b that the local erosion-corrosion region could form in a very short period with impingement craters concentrating at a local area. The generation of the craters would lead these areas become anodes and further propagate to a low-lying morphology shown in Fig. 13c. However, the newly formed cathodic regions in the original pre-corroded area would only suffer pure erosion induced by micro-cutting. As a result, it could be assumed that the concentration of the erosion-corrosion on the local pre-corroded area would be most probably induced by the instant interfacial electrochemical and chemical status, and the instantaneous sand impingements when the 23

slurry begins to flow. Both the chemical and electrochemical heterogeneity, and the inhomogeneous sand impingements determine the initiation sites of erosion-corrosion. The schematic diagram of the initiation and dynamic progression of erosion-corrosion on a pre-corroded steel surface could be illustrated by Fig. 14. Along with the slurry beginning to flow, the porous rust layer on the steel surface would be soon washed away and sand particles would immediately impinge the steel surface. However, at the right beginning of the slurry starts to flow, the distribution of the sand particles would be chaotic in the test cell, indicating inhomogeneous sand impingements would occur on the steel surface. Unlike the fine polished steel surface which anodes and cathodes would randomly generate on the steel surface, the cathodes and anodes are already separately distributed on the steel interface which is observable from the WBE mapping. Due to the viscosity of the fluid, a thin viscous layer could form on the steel interface when the slurry begins to flow [39]. The interfacial aggressive agents could remain in the viscous layer, leading to the original anodic sites be kept at the initial test stage. Deep craters would form in the original anodic area when sand particles impinge at the anodic sites. As the sand impingements are not uniform, the original anodic area having an intense sand impingements would form a large quantity of impingement craters which present as Area I shown in Fig. 13b. However, only several scattered craters could form at the original anodic area where suffers rare sand impingements at the initial test stage. The original anodic area without initial sand impingements would gradually transfer from anodic sites to new cathodic sites. The anodic region would be more concentrated in the original pre-corroded area, leading to a shrinkage performance of the anodic areas. As the local hydroxyl remained in the viscous layer could not immediately disperse to the bulk solution as well, the original cathodic regions could be maintained, leading to only plastic deformation induced by pure erosion occur on these areas. The intensive impingement craters at the highly impinged original anodic areas would propagate and merge together to form a concentrated low-lying erosion-corrosion area. In contrast, some original anodic areas where are not eroded by sand particles at the initial stage would become the ridges among the impingement craters. Since the adjacent impingement craters perform as main anodic sites, the ridges would become the new cathodic sites due to the continually transferring of electrons from the adjacent deep craters. These new cathodic regions remained 24

among the density craters would further propagate to isolated islands along with the formation of the large and deep low-lying area. Finally, with the forming of the stable separated cathodic area and the anodic area, the erosion-corrosion would continually occur in the low-lying area, resulting in a finial surface morphology shown in Fig. 13c (area IV). As the major anodic area formed on the pre-corroded steel surface is a concentrated low-lying area, the local micro-turbulences might be more moderate at this area than that around the small individual craters on a fine polished sample, resulting in a weakening of analyte transportation. Moreover, it is seen from Fig. 13a that the dense black rust layer could remain at the edge of the original anodic areas after 10 min of erosion-corrosion. The remained black rust might work as a physical barrier for the analyte transportation, which is helpful for the formation of the localized erosion-corrosion area in the original anodic area.

Fig. 14 Schematic diagram of the initiation and dynamic progression of erosion-corrosion on a pre-corroded steel surface.

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The assumption that the formation of the new major anodic sites on the original pre-corroded areas is influenced by the inhomogeneous sand impingements during the starting of the rotation disc is hard to be verified in this work since the instantaneous sand distribution on the steel surface cannot be probed. More test could be designed and conducted in future to further study this impact factor. However, the instantaneous electrochemical and chemical status i.e. the distribution of the anodes and cathodes and the distribution of the chemical components at different local areas could be well verified to be a certain influence factor, which determines the initiation and progression of erosion-corrosion on a pre-corroded steel surface. After the elimination of the chemical and electrochemical difference on the steel surface in Group 3, the erosion-corrosion behaviour would change back to scatter distributed craters which is the most significant evidence to support this assumption. As the area ratio between the corroded area and the non-corroded area was almost 1 in the static electrolyte, the concentrated erosion-corrosion at the pre-corroded area only had a slight aggravation on the localized erosion-corrosion. However, once the pre-corroded area is much smaller than the non-corroded area, more serious localized erosion-corrosion would occur at the small pre-corroded region due to the intense macro-cell current. The concentrated erosion-corrosion damage might lead to an unexpected quick perforation of the pipe wall. Therefore, it is better to physically clean the pipeline internal surface before it steps into working under a high flow rate condition, which could prevent serious concentrated erosion-corrosion occurring at the pre-corroded area. 5. Conclusions

Based on the erosion-corrosion performances of the pipeline steel with different pre-treatments using coupon electrodes and WBE technique, the following conclusions can be drawn: 1. Although general erosion rates and corrosion rates of the X65 pipeline steel with and without pre-corrosion are similar, the erosion-corrosion morphologies would be significantly different on the steel surfaces with different pre-treatments. Scattered impingement craters separated by cathodic regions would appear on the fine polished steel surface and the pre-corroded steel surface with rust layer removed. However, a concentrated low-lying 26

erosion-corrosion area where consists closely connected impingement craters would appear on the pre-corroded steel surface without removing the rust layer. 2. The effects of the hydrodynamic parameters on the progress of erosion-corrosion damage in the pipeline might become weak once the steel surface has already suffered pre-corrosion with anode and cathode separately distributing on the steel surface. The surface roughness increasing induced by pre-corrosion would facilitate the initiation of serious localized corrosion. However, the surface roughness increasing would not induce the formation of the concentrated erosion-corrosion area on the original pre-corroded area. 3. The initiation of localized erosion-corrosion on a fine polished pipeline steel surface is induced by the sand impingements at the randomly generated local anodic sites, leading to the formation of scattered impingement craters. The impingement craters would propagate to stable pits with the accumulation of the analyte in the deep crater. The local turbulences around the craters would facilitate the analyte transportation to the downstream of the craters, resulting in the moving of the major anodic areas along the flow direction and finally forming local continuous impingement craters. 4. The initiation of the localized erosion-corrosion on a pre-corroded steel surface would be most probably induced by the instant interfacial chemical and electrochemical heterogeneity, and the instantaneous inhomogeneous sand impingements at the initial stage of erosion-corrosion. The local intense sand impingements at the original anodic areas would result in the formation of dense craters. The dense craters would propagate and gradually merge together to form the low-lying erosion-corrosion area. Along with the propagation of the local erosion-corrosion areas, the original anodic areas where only suffer slight sand impingements would transfer to new cathodic sites, leading to the shrinkage of the main anodes. The ridges among the dense impingement craters at the initial stage would become the isolated islands along with the formation of the large and deep low-lying erosion-corrosion area. 5. Once the area ratio between the corroded area and the non-corroded area is small during pre-corrosion, serious erosion-corrosion might occur on the pre-corroded area due to the intense macro-cell current. To avoid the initiation of serious concentrated erosion-corrosion on the pipeline internal surface, it is better to thoroughly wash the pipeline 27

internal surface to eliminate both electrochemical and chemical inhomogeneity on the steel interface before the pipeline starts to work under an erosion-corrosion condition.

Acknowledgements

This research was sponsored by Key Projects in the National Science & Technology Pillar Program during the Thirteenth Five-year Plan Period of China (No. 2016ZX05057) and China Postdoctoral Science Foundation (No. 2019TQ0049). References

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1. The localized erosion-corrosion performance of the pre-corroded steel was investigated. 2. The dynamic progression of erosion-corrosion on the pre-corroded was probed. 3. The formation of a concentrated erosion-corrosion area on the pre-corroded area is explained. 4. The influences of the chemical and electrochemical status, and sand impingements on the progress of erosion-corrosion was analyzed.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: