Influence of rare earth metals on mechanisms of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel

Journal Pre-proof Influence of rare earth metals on mechanisms of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel Chao Liu (Conceptualization) (Methodology)Data acquisition) (Writing original draft), Zaihao Jiang (Validation) (Investigation), Jinbin Zhao (Resources) (Data curation), Xuequn Cheng (Project administration) (Supervision), Zhiyong Liu (Supervision), Dawei Zhang (Writing - review and editing) (Supervision), Xiaogang Li (Conceptualization) (Supervision)

PII:

S0010-938X(19)32400-X

DOI:

https://doi.org/10.1016/j.corsci.2020.108463

Reference:

CS 108463

To appear in:

Corrosion Science

Received Date:

9 November 2019

Revised Date:

30 December 2019

Accepted Date:

9 January 2020

Please cite this article as: Liu C, Jiang Z, Zhao J, Cheng X, Liu Z, Zhang D, Li X, Influence of rare earth metals on mechanisms of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel, Corrosion Science (2020), doi: https://doi.org/10.1016/j.corsci.2020.108463

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Influence of rare earth metals on mechanisms of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel Chao Liu a,b,1, Zaihao Jiang a,b,1, Jinbin Zhao a,b,c, Xuequn Cheng a,b, Zhiyong Liu a,b, Dawei Zhang a,b, Xiaogang Li a,b,* [email protected]

Institute of Advanced Materials & Technology, University of Science and Technology Beijing, Beijing, China.

b

National Materials Corrosion and Protection Data Center, University of Science and Technology Beijing, Beijing, China.

c

Jiangsu Key Laboratory for Premium Steel Material, Nanjing Iron and Steel Co., Ltd., Nanjing, China.

author: Xiaogang Li: Tel.: +86 010-62333931

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*Corresponding

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a

Highlights

Rare earth influence on pitting in Zr-Ti deoxidised steel in marine environment was studied.

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No galvanic couple formed between inclusion and Zr-Ti deoxidized steel

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Pitting initiated by microcrevices/lattice distortion surround ZrO2-Ti2O3-Al2O3

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inclusion. 

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before/after adding rare earth.

Chemical dissolution of ((RE)2O2S-(RE)xSy) in rare earth modified inclusion

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induced pits. 

Oxygen-concentration gradients and catalytic-occluded cells may accelerate pit growth.

Abstract The influence of rare earth addition on the mechanism of localised corrosion induced by inclusions in Zr-Ti deoxidised steel was investigated in a simulated marine environment. Inclusion size and composition changed after rare earth addition, profoundly affecting the localised corrosion. Current sensing atomic force microscopy results showed that no galvanic couple formed between inclusions and matrix owing to the insulating property of different inclusions. Matrix dissolution resulting from the microcrevices and lattice distortion surrounding ZrO2-Ti2O3-Al2O3 inclusions initiated localised corrosion in Zr-Ti deoxidised steel. After adding RE, localised corrosion

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was induced by dissolution of the (RE)2O2S-(RE)xSy part of the inclusion. Keywords: Low alloy steel (A); AFM (B); SEM (B); Atmospheric corrosion (C);

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Pitting corrosion (C)

1. Introduction

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Low alloy steel is widely used in different structures owing to its cost effectiveness and corrosion resistance. Different types of inclusions—such as Al2O3,

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MnS, ZrO2, Ti2O3, (Ce, La, Nd, Mn, Fe)-S, (Mn, Cr, RE)-O-S, CaO·MgO·Al2O3, and CaO·Al2O3—will form in the steel, depending on the deoxidiser and the modifying agent used in the steelmaking process [1-8]. Inclusions, undesirable but unavoidable

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by-product in steel [5, 6], can easily induce localised corrosion [1, 4, 9-11] and ultimately lead to equipment failure, resulting in heavy economic loss [12]. Our

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ability to develop new corrosion-resistant materials effectively is reliant on clarification of the mechanisms of induced localised corrosion, which vary with the composition of the inclusions.

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The pitting corrosion mechanism of several types of non-metallic inclusions,

such as MnS, Al2O3, rare earth (RE)-modified inclusions, CaS·CaO·Al2O3·MgO, and (Ti, Nb)N, have been previously reported [2, 3, 9, 13]. The depletion of chromium around MnS [9], an iron sulphide enriched ‘halo’ surrounding MnS [14], a nano-scale precipitated MnCr2O4 phase around MnS [15], and the galvanic couple between MnS and steel [16, 17] have been assumed as the main reasons for localised corrosion induced by MnS inclusions in stainless steels and low alloy steels. Because of the

insulating properties of Al2O3, localised corrosion is initiated by dissolution of microcrevices and lattice distortions in the matrix rather than by a galvanic couple between Al2O3 inclusion and the matrix [1, 2]. To reduce the lattice effects of the high hardness of Al2O3 inclusion, calcium (Ca) and RE have been used to modify the inclusions. After modification, the inclusions are selectively dissolved, forming pits. Owing to their lower surface potential, (Mg, Al, Ca)-oxide inclusions are preferentially dissolved over the steel matrix and MgO·Al2O3 crystal inclusions [18]. The Ca-Al-O part of (Mg, Al, Ca)-oxide inclusions dissolves owing to its lower localised electrochemical potential [19], and because it is thermodynamically unstable

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in aqueous solution at room temperature [4]. The RE-modified inclusion dissolves easily in the aggressive environment because of its lower chemical stability [20]. Hence, the composition of inclusions affects the localised corrosion mechanism.

To improve the weldability and toughness of steel, Zr-Ti has been used as a

complex deoxidiser [21-23]. Many studies have shown that the proportion of acicular

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ferrite can be increased by fine inclusions in the weld zone [21, 24]. Zr-Ti complex oxides are uniformly dispersed, and provide more sites for grain nucleation during

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solidification [25]. ZrO2 and Ti2O3 can improve the toughness of the heat-affected zone by promoting the formation of effective inclusions and further increase the

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proportion of AF efficiently in the high-heat-input welding process [23, 26-29]. However, the mechanism of pit corrosion induced by inclusions including ZrO2-Ti2O3 in the Zr-Ti deoxidised steel is still unclear.

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The size of most inclusions in Zr-Ti deoxidised steel has been reported as larger than 1 μm [8, 21]. Inclusions larger than 1 μm present a high risk of being transformed into steadily growing pits originating from pit nuclei [30, 31]. Hence, an

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effective method to reduce localised corrosion resistance is to control the size of the inclusions in steel. Rare earth metals are considered to be effective for both improving

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mechanical properties [32] and decreasing the size of inclusions [1, 20]. They can soften the high hardness inclusions by forming (RE)2O2S and (RE)AlO3 and reduce the lattice distortion induced by inclusions [33-35]. Moreover, RE as an deoxidiser has been reported as improving the pitting resistance of steel [36]. With the modifying effect of RE, the pitting corrosion mechanism may totally differ. The matrix that surrounds Al2O3 inclusions in steel is selectively dissolved and forms pits [1, 37], whereas after modification with RE, the modified inclusion is preferentially dissolved and forms pits [20]. Adding RE into Zr-Ti deoxidised steel may promote the

mechanical property of the steel, but the mechanism of localised corrosion in REmodified Zr-Ti deoxidised steel is also still unclear. In this work, the influence of RE on mechanisms of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel was investigated in marine environments. Field emission-scanning electron microscopy-energy dispersive spectrometry (FE-SEM-EDS) analyses, focused ion beam-scanning electron microscopy (FIB-SEM), scanning Kelvin probe force microscopy (SKPFM), current sensing atomic force microscopy (CSAFM), electron backscattered diffraction (EBSD), inclusion automatic analyser, and immersion tests were conducted to

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investigate the mechanism of pitting corrosion. 2. Experimental 2.1 Materials and methods

The chemical composition of two experimental steels are given in Table 1. Steel

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#1 was complex deoxidised with Zr-Ti, and steel #2 was complex deoxidised with ZrTi and modified with RE elements (Ce and La). These two types of steel were purchased from Wuhan Iron and Steel Group Company. Specimens of size 10 mm ×

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10 mm × 6 mm were mechanically ground with silicon-carbide paper to 3000 grit and polished with 0.5 μm diamond paste to eliminate the effect of surface roughness on

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the localised corrosion. The specimens were cleaned by ultrasonic rinsing in ethanol. For the corrosion experiments, ion thinning samples of these two steels were prepared to avoid possible influence from the mechanical polishing process. These specimens

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were manually ground to 30 μm with silicon-carbide papers, and then polished using a

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precision ion polishing system to produce the thin zone.

2.2 Characterisation of microstructure and corrosion morphology

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Sample microstructures were characterised using a Carl Zeiss Axio Scope A1 optical microscope and an FE-SEM (Zeiss Merlin, Germany) after etching with 4% Nital solution. The morphology and the chemical composition of the inclusions were characterised by FE-SEM-EDS. An accelerating voltage of 15 kV with a 10 nA probe current and a working distance of 8 mm were used to obtain the secondary electron images and conduct the EDS analysis. An integrated dual-beam FIB-SEM system (QUANTA 200 FEG, FEI Company, USA) was used to acquire three-dimensional images of the inclusions in the specimens. The system was operated with an

accelerating voltage of 10 kV and I-beam current of 200 –1000 pA. Samples were tilted to an inclination of 54° such that the focused Ga+ ion beam was at normal incidence. The apparent size distribution and density of the inclusions in the two steel specimens were measured using an Aspex Explorer instrument (FEI Company, USA) in automated feature analysis mode, operated at an acceleration voltage of 10 kV, 41 pct spot size, and an approximate pixel size of 0.1 μm. The analysed area of each specimen was approximately 40 mm2. Lattice distortions induced by the inclusions in the iron thinning specimens, and the influence of mechanical polishing were both characterised by FE-SEM combined with EDS and EBSD. Corrosion morphologies

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were also observed by FE-SEM. 2.3 Characterisation of localised electrochemical properties

The electrochemical nature of different inclusions in the specimens was

investigated by the SKPFM and CSAFM; in particular, it was determined by the Volta potential and the electrical conductivity with respect to the matrix. SKPFM

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measurements were conducted with a commercial atomic force microscope (Bruker

Icon AFM, Germany). Silicon tips on conductive silicon nitride cantilevers (PFQNE-

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AL) were employed, and comprised a nominal resonant frequency of about 300 kHz and a nominal spring constant of about 0.8 N/m. Further, a single-pass methodology

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was employed in which the topography and the corresponding contact potential difference between the probe and the sample were simultaneously measured. CSAFM measurement was also conducted by the Icon AFM instrument, operating in the

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current sensing mode, which measures the surface topography in the contact mode along with a voltage applied between the tip and the sample; this led to the simultaneous provision of topography and current data. An electrically conductive

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Multi75E-G (Budget Sensors, tip radius < 25 nm, force constant ~3.0 N/m, resonant frequency ~75 kHz) tip was used. Both SKPFM and the CSAFM measurements were

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performed in air at a relative humidity of approximately 25% and a room temperature. The surface was scanned at a frequency of 0.5 Hz. 2.4 Immersion test Immersion test is a reliable and simple method to analyse the localised corrosion mechanism in the early stage of the corrosion process. A series of immersion tests (specifically, 5 min, 30 min, and 72 h) were carried out to investigate the pitting corrosion mechanism induced by different types of inclusions at a room temperature.

For the immersion tests, an aqueous solution (pH = 4.9) containing 0.1 wt % NaCl, 0.05 wt % Na2SO4, and 0.05 wt % CaCl2 was used to simulate the thin liquid films formed under a humid atmosphere at Xisha Island in the South China Sea. Prior to examination of the corrosion morphology, a solution containing HCl and hexamethylenetetramine was used to remove the rust, and then the specimens were washed with alcohol and blow-dried with pressurised air. 3. Results 3.1 Microstructure Corrosion resistance is always related to the microstructures in low alloy steel.

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The microstructures of the two specimens prior to corrosion testing are shown in Figs. 1a and b. The two specimens were mainly composed of ferrite and a small amount of pearlite. The secondary electron microscopy image showed large amounts of grain

boundary carbides scattered over grain boundaries in steel #1 (Fig. 1c), which formed

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in the cooling process [38]. Conversely, the grain boundaries in steel #2 (Fig. 1d)

were relatively clean, with little evidence of carbide distribution. RE can not only modify the inclusions [5, 20, 39] but is also usually distributed at the grain boundaries

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[40] owing to its larger atomic size than Fe. This can reduce the segregation and concentration of harmful impurity elements at the grain boundary [41].

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3.2 Physical properties of inclusions

Inclusions in different specimens were characterised by means of FE-SEM in combination with EDS on the polished specimens. Figs. 2a–f show the SEM images

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and elemental maps of typical inclusions found in steel #1. Fig. 2a shows that the inclusions in steel #1 appeared as single particles or clusters. Figs. 2c and d show the

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inclusions at higher magnification. Figs. 2e and f are the compositions of the inclusions in Figs. 2c and d, respectively. The EDS result shows that the complex

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inclusion is mainly ZrO2-Ti2O3. This is the common non-metallic inclusion in the ZrTi deoxidised steel [8, 21]. Around the inclusions, some Al2O3 was formed, which may have resulted from contamination of molten steel by Al-containing refractories. Therefore, the complex inclusion in steel #1 was ZrO2-Ti2O3-Al2O3. Figs. 2c and d show that Ti2O3 (marked with yellow arrows) tends to distribute in the boundary of the complex inclusion. This results from the more negative Gibbs free energy and higher melting point of ZrO2 relative to the other oxides; it provides a large number of nucleation cores for Ti2O3 and Al2O3 [8, 21, 42]. The morphology and composition of

the inclusions of steel #2 modified by RE are shown in Figs. 2g–k. The RE-modified inclusions have an elongated or a spherical shape (Fig. 2i). Figs. 2j and k show the composition of the inclusions marked as I and II in Fig. 2i. The EDS result illustrates that the complex inclusions consisted mainly of two regions—a sulphur-containing region and sulphur-free region. As shown in the secondary electron microscopy image (Fig. 2i), the sulphur-containing region, marked with yellow arrows, appears whiter than the sulphur-free region. RE has a strong affinity for sulphur and oxygen, hence (RE)2O2S-(RE)xSy are easily formed owing to their more negative standard free energy than the other oxides [20, 43-46]. The sulphur-free region consists mainly of

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(RE, Zr, Ti)Ox. Some (RE)AlO3 was formed on the periphery of the inclusion [20, 43, 47], resulting from Al contamination of molten steel, as mentioned above. Hence, the

complex inclusion in the steel #2 was composed of (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox(RE)AlO3.

To investigate the effect of RE on the size and quantity of inclusions, a statistical

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analysis was conducted using an FEI Aspex Explorer instrument. After adding the RE to the steel, the inclusions density increased from 13.2 per mm2 (steel #1) to 20.4 per

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mm2 (steel #2). This may have resulted from the strong affinity for sulphur and oxygen [48, 49]. Fig. 2m shows the inclusion size distributions in steel #1 and #2. In

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the Zr-Ti deoxidised steel #1, 82.3% of the inclusions are in the size range 0–2 μm. This demonstrates that Zr-Ti deoxidation could increase the fraction of inclusions with smaller size, as previously reported [21, 28]. However, ≥90% of the RE-

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modified inclusions in steel #2 were larger than 1 μm, and distributed relatively uniformly over the different size intervals. This indicates that the average inclusion size increased after adding RE.

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As reported in our prior work [1], the pristine steel microstructural morphology might be hidden by the debris produced by the mechanical polishing process. A

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precision ion polished sample was prepared to avoid this negative effect. Fig. 3 shows the top and cross-section morphology of the microstructure around the inclusions in the two specimens without mechanical polishing. Fig. 3a-d shows the morphology and the EDS of inclusions in 1# steel. Microcrevices were present at the interface between the matrix and ZrO2-Ti2O3-Al2O3, and at the interface between ZrO2 and Ti2O3-Al2O3 (Fig. 3a,b). Fig. 3d is the section morphology of the ZrO2-Ti2O3-Al2O3 inclusion prepared by FIB. Microcrevices are visible at the inclusion/matrix interface and within the complex inclusion. Figs. 3e–h show the original morphology of the

microstructures around an RE-modified inclusion in steel #2. Microcrevices were present inside the inclusion and at the inclusion/matrix interface (Figs. 3e and f). The cross-section morphology of inclusions (Fig. 3h) shows larger-sized microcrevices. These are easily formed between inclusions and the strip matrix in the rolling process, resulting from a difference in strain values [50, 51] and coefficients of thermal expansion [52-54]. The differing elasticity modulus (lower deformability of the hard inclusions) between the inclusions and matrix could result in the magnitude of stresses building up to a significant stress concentration around inclusions [55], and this will induce the formation of microcrevices at the matrix/inclusion interface in the hot

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rolling or cold rolling process. Such microcrevices can serve as sites for localised corrosion in stainless steel [56] or low alloy steel [1, 4, 57, 58]. Microcrevices also

formed within the complex inclusion shown in Figs. 3e and h due to the difference in the thermal expansion coefficients between different phases.

EBSD mapping was performed to investigate lattice distortions around the

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inclusions in the specimens. The FE-SEM image, kernel average misorientation

(KAM), map and the EDS result of inclusions and the adjacent matrix in the two

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samples are shown in Fig. 4. KAM maps the distribution of local misorientation [1, 59, 60], which is regarded as an important indicator of dislocation density. As

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reported, the KAM value for each pixel is calculated as the average misorientation between the central pixel in the kernel and its neighbours [61, 62]. Hence, the KAM map represents the degree of local lattice rotation and provides a quantitative measure

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of the local plastic deformation. The inclusions are incoherent with the steel matrix owing to the differences in the mechanical characteristics referred to above, and the intensity of the lattice deformation that occurs near the inclusions [63]. The KAM

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maps in Figs. 4a and b indicate that local plastic deformation of the steel matrix occurred around the less-deformable inclusions. The density of the local plastic

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deformation of steel #1 around the ZrO2-Ti2O3-Al2O3 inclusion (Fig. 4a) is much higher than that of steel #2 around the (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 inclusion (Fig. 4b). This is consistent with RE addition softening the inclusions by forming (RE)2O2S-(RE)xSy and (RE)AlO3 [33-35]. The strain and modulus of elasticity of the inclusion are reduced and the lattice distortion decreased. According to the mechanoelectrochemistry theory illustrated by Gutman [64], mechanical deformation could result in a redistribution of electrochemical heterogeneities and area ratio for the cathodic/anodic reaction. An applied stress and/or strain, particularly

when accompanied by crack formation, has been reported to enhance the corrosion of steel remarkably [65-67]. The results suggest that the activity of the lattice distortion region increases under these conditions [68]. 3.3 Localised electrochemical properties The localised electrochemical inhomogeneity between inclusions and steel matrix is one of the main causes of localised corrosion initiation [1, 4, 10, 20, 37]. Previous reports [19] have suggested that the galvanic effect between inclusion and steel may be the main mechanism involved in pitting initiation. However, our research has demonstrated that a galvanic couple between Al2O3 inclusions and matrix is not

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possible [1]. Hence, SKPFM and CSAFM tests were conducted to investigate the electrochemical characteristics associated with local surface inhomogeneities, and the pitting mechanism around inclusions, in the Zr-Ti dioxide and RE-modified Zr-Ti dioxide steels. Both tests were performed by AFM, using different probes and

after multiple trials (more than 10 times).

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scanning modes. The following representative results were based on the selection

The AFM and FE-SEM-EDS results are both shown in Fig. 5; FE-SEM-EDS in

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Figs. 5a and b; AFM topography maps in Figs. 5c(1) and d(1); Volta potential maps in Figs. 5c(2) and d(2); and current sensing maps in Figs. 5c(3) and d(3), obtained with a tip

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bias of -8 V in the selected area of each inclusion. AFM height/current/Volta potential line profiles are provided in Figs. 5c(4) and d(4). The EDS result in Fig. 5a shows that the composition of the selected inclusion is ZrO2-Ti2O3-Al2O3. According to the

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corresponding Volta potential map (Fig. 5c(2)) and line profile analysis (Fig. 5c(4)), this inclusion has a higher Volta potential than the matrix. Hence, ZrO2-Ti2O3-Al2O3 is likely to be higher in electrochemical stability relative to the matrix, such that the

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inclusion is unlikely to act as an electron acceptor [18, 37]. The corresponding CSAFM current map (Fig. 5c(3)) shows a current of 12 nA (approximately) at -8.0 V.

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However, the absolute current obtained from the inclusion area was less than 2 pA (Fig. 5c(4)). The insulator properties of ZrO2-Ti2O3-Al2O3 result from the energy gaps in the Fermi levels [28, 69, 70]. Consequently, a galvanic couple is not possible between the ZrO2-Ti2O3-Al2O3 inclusion and the steel #1 matrix owing to the poor conductivity of the inclusion.

Fig. 5b shows that the inclusion selected in steel #2 is (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox; with two areas visibly distinguishable. The large sulphur-free region (RE, Zr, Ti)Ox exhibits a higher Volta potential than the matrix (Fig. 5d(2)). The sulphurcontaining region (RE)2O2S-(RE)xSy exhibits a lower Volta potential than the matrix (Figs. 5d(2) and d(4)), in accordance with our previous work [20]. This indicates that the sulphur-containing region has higher activity in the corrosion process. CSAFM (5d(3)) indicates that (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox inclusion had poor conductivity, consistent with the negligible current recorded in (5d(4)). These results indicate that the RE-modified inclusion is an insulator. Hence, addition of the RE

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metals does not increase the possibility of corrosion by galvanic coupling. The ZrO2Ti2O3-Al2O3 inclusion (Fig. 5c(1)) and the sulphur-free area of the (RE)2O2S-(RE)xSy(RE, Zr, Ti)Ox-(RE)AlO3 inclusion (Fig. 5d(1)) protruded slightly after polishing, being harder than adjacent materials [71]. However, the sulphur-containing area

formed a slight depression after polishing, which may have resulted from a small

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amount of decomposition due to poor stability of the RE sulphides [72], which is consistent with previous work [20].

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3.4 Localised corrosion morphology at the early stage

A series of immersion tests were conducted to investigate the localised corrosion

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induced by different inclusions in steel #1 and steel #2. For each kind of steel, immersion tests were performed on three parallel samples simultaneously and then observed. Fig. 6 shows the corrosion morphologies of the Zr-Ti deoxidised steel #1

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after three different immersion times in the simulated salt solution (Section 2.4). After 5 min, no dissolution was observed on the matrix far from the inclusions (Fig. 6a(1)), however, the matrix around the inclusions had dissolved slightly (Fig. 6a(2),a(3)). This

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is consistent with SKPFM results; i.e. detection of a lower Volta potential. Moreover, micro-pits tend to initiate at the corners or the protuberant boundary of the inclusions,

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which may result from the higher lattice distortion density in these areas as shown in Fig. 4a [1]. After 30 min, matrix dissolution had still mainly occurred in close proximity to inclusions. Micro-pits surrounding inclusions had expanded into deeper cavities indicating faster propagation in the vertical direction (Fig. 6b). The matrix dissolved almost all around the inclusions, which differs from the microcrevices occasionally observed at the inclusion/matrix interface. Corrosion ions could be enriched in the deeper cavities and accelerate the development of localised corrosion

[73]. After a prolonged immersion time of 72 h, the matrix surface had dissolved (Fig. 6c), the polishing line disappeared, and the grain and grain boundaries became evident. The micro-pit surrounding the inclusions propagated deeply in the vertical direction owing to dissolution of the matrix and formed interconnected deep cavities (Figs. 6c(1) and c(3)). The presence of an inclusion cluster in the steel may result in the formation of a large micro-pit by coalescence of the smaller micro-pits formed around the independent inclusions (Fig. 6c(2) and c(3)). This could reduce the localised corrosion resistance of the materials [1]. Fig. 7 shows the corrosion morphologies of the RE-modified inclusions in steel

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#2 after different immersion times. FE-SEM images of the micro-pit morphologies induced by RE-modified inclusions after 5 min immersion are presented in Fig. 7a.

Several smaller micro-pits induced by inclusions (marked by red arrows in Fig. 7a(1))

are apparent. Figs. 7a(2) and a(3) are higher resolution images of this region. Micro-pits have formed by partial dissolution of the RE-modified inclusions. The EDS results

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from Fig. 7a(3) show that dissolution mainly occurred in the sulphur-containing area of the (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 inclusion. This may be due to lower

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Volta potential than that of the matrix (Fig 5d(4)). However, the sulphur-free area ((RE, Zr, Ti)Ox-(RE)AlO3) had not dissolved, which may be due to a higher Volta

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potential than that of the matrix. After 30 mins, further dissolution had occurred within the inclusions (Fig. 7b(1),(2),(3)). The EDS result from Fig. 7b(3) indicates that dissolution was still mainly taking place in the sulphur-containing part of the

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inclusions. Over a 72 h immersion period, the inclusions dissolved continuously, as indicated by Fig. 7c. As corrosion proceeds, the concentration of aggressive ions is expected to increase. Subsequently, the metallic ions will hydrolyse to produce

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H+ ions, lowering the pH of the micro-pit solution [1, 74], which may lead to dissolution of the sulphur-free region of the inclusion (Fig. 7c(2)). With total

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dissolution of the inclusion cluster, several deep cavities were left on the surface of the steel (Fig. 7c(3)). 4. Discussion

On the basis of the preceding analysis, the effect of ZrO2-Ti2O3-Al2O3 and (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 inclusions on the initiation and propagation of pitting corrosion in the experimental steels are schematically presented in Fig. 8. The two types of inclusion different apparently in their behaviour.

According to the CSAFM results (Figs. 5c and d) both exhibit insulator performance. Therefore, neither can form electrochemical galvanic couples with the matrix. The ZrO2-Ti2O3-Al2O3 inclusion tends to induce dissolution of the matrix. Aggressive ions (e.g., Cl- and SO42-) could be enriched in the microcrevices (Fig. 3a) around a ZrO2-Ti2O3-Al2O3 inclusion and contact with the bare steel directly [73]. This would result in a crevice corrosion initially occurring at the inclusion/matrix interface (Fig. 8a(1)) [4, 75]. The EBSD KAM map (Fig. 4a) shows that a high-density lattice distortion [53, 61] surrounds the inclusions in steel #1. According to Gutman’s mechanoelectrochemistry theory [36], these distortion regions have enhanced activity

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and may dissolve easily. The anodic reaction may be promoted, leading to dissolution of the matrix around the inclusion, and generation of an interstice between the

inclusion and matrix [68]. With further development and expansion of the localised corrosion in the microcrevices, the high distortion region at the bottom of the

inclusion would be exposed to the aggressive corrosion environment. This high-

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energy region is susceptible to attack and would further promote the micro-pit

corrosion process by its own dissolution (Fig. 8a(2)). Similar experimental phenomena

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involving induction of matrix dissolution at the matrix-inclusion interface by nonmetallic inclusions have been reported previously [1, 4, 7, 18, 76, 77]. In the micro-pit

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evolution process, formation of a rust layer covering the micro-pits leads to blocking of oxygen transfer. An oxygen-concentration cell will be formed in the micro-pits owing to the resulting oxygen-concentration gradient (Fig. 8a(2)). This would

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accelerate further dissolution of the matrix [78]. With aggressive ions (e.g., Cl- ) migrating to, and enriching in the micro-pits, a catalytic-occluded cell with the micropits acting as anodes would be formed [79]. The matrix would keep dissolving

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continually in order to maintain an electroneutral environment (Fig. 8a(2)) [80, 81]. Under these conditions, electrochemical activity differences exist between the steel

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matrix in the vicinity of and remote from the inclusions. A galvanic couple could then be formed between these spatially separated regions of steel in different electrochemical environments. Electrons could transfer through the matrix in the corrosion process. The micro-pits will continue to develop and evolve under the effect of the oxygen-concentration and catalytic-occluded cells (Fig. 8a(3)). In the case of the (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 in steel #2, the localised corrosion initiation and propagation processes (as shown in Fig. 8b) are different from those in steel #1. Microcrevices exist not only at the interface between

the inclusion and the matrix, but also at the interface between the sulphur-containing and sulphur-free regions of the (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 inclusion (Figs. 3c and f). Hence, both fresh steel, through microcrevices, and the sulphurcontaining area of the inclusion will be exposed to the aggressive environment (Fig. 8b(1)). The sulphur-containing region (RE)2O2S-(RE)xSy could dissolve first owing to its lower Volta potential relative to the matrix (Fig. 5d(2)). In the inclusion dissolve process, the chemical reaction may have occurred as follows[20]. (R E ) 2 O 2 S -(R E ) x S y  H 2 O  R E (R E ) 2 O 2 S -(R E ) x S y  H

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 RE

3

3

2

 SO 4  H 



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 H S  H 2O  e

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(1)

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(2)

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With the formation of H+ and HS-, a cavity would form inside the inclusion, thus creating a local, aggressively acidic environment consisting of H2SO4, HCl, and H2S after the (RE)2O2S-(RE)xSy-(RE, Zr, Ti)Ox-(RE)AlO3 inclusion dissolved (Fig. 8b(2)) [20, 82, 83]. Then the sulphur-region of the inclusion ((RE, Zr, Ti)Ox-(RE)AlO3)

-p

would start to dissolve (Fig. 8b(3)). After the RE-modified inclusion dissolved completely, matrix in the micro-pits would exposed directly in the aggressive

re

environment. As the corrosion process evolves, rust will accumulate on the micro-pits and block the transfer of oxygen. The oxygen concentration will decrease sharply

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with consumption in the oxidation reaction. An oxygen-concentration cell will form in the micro-pits, which can accelerate the corrosion process. Analogous to the micro-pit environment in steel #1, the aggressive solution in the steel #2 micro-pits will

na

promote dissolution (as formula (3) and (4)) of matrix in order to maintain an electroneutral environment[84], with formation of catalytic-occluded cells (Fig. 8b(3)) [57,58]. After the inclusion dissolves completely, the micro-pits will develop under

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the synergistic effects of oxygen-concentration gradient and catalytic-occluded cells. Fe  Fe

2

 e



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FeC l2  2 H 2O  Fe  O H

(3) 2

 2H



 2C l



(4)

5. Conclusions The effect of the rare earth (RE) metals Ce and La on the initiation and

propagation of localised corrosion induced by inclusions in Zr-Ti deoxidised low alloy steel was investigated. The results are summarised as follows. (1) Microstructures in both the Zr-Ti deoxidised and the RE-modified Zr-Ti deoxidised low alloy steels are made of ferrite and pearlite. A significant amount of

carbide was distributed over the grain boundaries in steel #1. The inclusions in the ZrTi deoxidised steel were composed of ZrO2-Ti2O3-Al2O3. After modification by RE to produce steel #2, the inclusions were composed of (RE)2O2S- (RE)xSy-(RE, Zr, Ti)Ox(RE)AlO3. (2) Microcrevices were formed at the matrix/inclusion interface and inside the inclusions, owing to differences between the strain values and coefficients of thermal expansion. A high proportion of lattice distortions was observed surrounding ZrO2Ti2O3-Al2O3 inclusions. After adding RE, the inclusions were softened and the occurrence of lattice distortions decreased dramatically.

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(3) Both the ZrO2-Ti2O3-Al2O3 inclusion in steel #1 and the sulphur-free ((RE, Zr, Ti)Ox-(RE)AlO3) regions of steel #2 inclusions exhibit higher electrochemical

stability than the matrix owing to higher Volta potentials. In contrast, the sulphur-

containing ((RE)2O2S-(RE)xSy) region of the inclusions has lower Volta potential,

resulting in decreased electrochemical stability. CSAFM results indicate that both

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types of inclusions are electrical insulators. Hence, no galvanic couple could be formed between either of the inclusions and the matrix.

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(4) Localised corrosion of Zr-Ti deoxidised steel was initiated at the microcrevices surrounding the ZrO2-Ti2O3-Al2O3 inclusions. Following enrichment of

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aggressive ions, the region of high dislocation occurrence dissolved and promoted expansion of localised corrosion. With the formation of oxygen-concentration cells and catalytic-occluded cells, the propagation of the micro-pits further accelerated.

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(5) The mechanism of the localised corrosion initiation and propagation was entirely different after adding RE to the Zr-Ti deoxidised steel. Localised corrosion was initiated by the dissolution of (RE)2O2S-(RE)xSy. With the development of

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localised corrosion, an acidic aggressive environment was formed in the micro-pits, and the (RE, Zr, Ti)Ox-(RE)AlO3 part of the inclusion started to dissolve. After the

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inclusion dissolved completely, micro-pits developed under the synergistic effect of oxygen-concentration and catalytic-occluded cells.

Author Statement Chao Liu：Conceptualization, Methodology, Data acquisition, Writing-Original draft preparation

Zaihao Jiang: Validation, Investigation Jinbin Zhao: Resources, Data Curation Xuequn Cheng: Project administration, Supervision Zhiyong Liu: Writing-Reviewing, Supervision Dawei Zhang: Writing - Review & Editing, Supervision Xiaogang Li: Conceptualization, Supervision Funding This work was supported by the China Postdoctoral Science Foundation [grant

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number 2019M650485]; the National Nature Science Foundation of China [grant

number 51871024]; and the Fundamental Research Funds for the Central Universities (FRF-TP-19-030A1).

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Declaration of interests

The authors declare that they have no known competing financial interests or personal

Acknowledgement

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relationships that could have appeared to influence the work reported in this paper.

The authors would like to thank Mr. Rungang Wang at University of Science and

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Technology Beijing for his assistance with the experiments.

References

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Fig. 1. Metallography images (a, b) and secondary electron microscopy images (c, d) of the microstructures in the experimental specimens etched with 4% Nital solution. (a, c) Steel #1, (b, d) steel #2.

Fig. 2. Morphology and composition of the inclusions in steel #1 (a, b, c, d, e, f), and steel #1 (g, f, g, h, i, j, k). (m) Inclusion size distributions in steel #1 and #2.

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Fig. 3. FE-SEM images of inclusions in the surfaces of (a-d) steel #1, and (e-h) steel #2 prepared using precision ion polishing. (a, b, e, f) Morphology image of inclusions, (c, g) EDS result of inclusions, (d, h) cross-section morphology of the inclusions in (c) and (e).

Fig. 4. FE-SEM image, KAM map, and EDS analysis of inclusions and adjacent matrix in (a) steel

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#1, and (b) steel #2.

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Fig. 5. FE-SEM images and EDS results (a, b); AFM topography (c(1), d(1)); SKPFM maps (c(2),

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d(2)); CSAFM maps (c(3), d(3)); and AFM line profile analyses (c(4), d(4)) for the selected inclusion

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areas: (a,c) steel #1, (b,d) steel #2.

Fig. 6. Corrosion morphology of the micro-pits induced by inclusions in steel #1 after different immersion times: (a) 5 min. (b) 30 min. (c) 72 h.

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Fig. 7. Corrosion morphology of the micro-pits induced by inclusions in steel #2 after different

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immersion times: (a) 5 min, (b) 30 min, and (c) 72 h.

Fig.8. Schematic representation of the micro-pit initiation and propagation process induced by

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different types of inclusions: (a) Steel #1, (2) steel #2.

Table 1. Chemical composition of the experimental low alloy steel (wt. %). Si

Mn

P

S

Cu

Cr

Ni

Ti

Zr

#1

0.045

0.24

0.12

0.0068

0.0020

0.43

1.22

0.32

0.0074

0.0063

#2

0.046

0.26

0.11

0.0062

0.0022

0.40

1.24

0.31

0.0087

0.0070

La/Ce

0.036

Al

Fe

<0.001

Bal.

<0.001

Bal.

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C