domain: chim spi

text:
Dr Hiba Azzeddine
email: hiba.azzeddine@univ-msila.dz

Abdelkader Hanna

Achour Dakhouche

Lyacine Rabahi

Nico Scharnagl

Milan Dopita

François Brisset

Anne-Laure Helbert

Thierry Baudin

Impact of rare-earth elements on the corrosion performance of binary magnesium alloys

Keywords: Corrosion resistance, Chloride ion, Magnesium alloy, Rare earth element

The corrosion behaviour of Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La (wt.%) alloys in 3.5 wt.% NaCl solution was investigated using electrochemical tests. The ascast microstructures of the Mg-RE alloys were characterized by the presence of second phases (MgxCe, Mg41Dy5, Mg12Gd, Mg12Nd, Mg41Nd5, Mg24Nd and Mg12La) with different volume fraction and distribution. Results show that the corrosion mechanism was altered from uniform to localized corrosion mechanism depending on the specific RE alloying elements.

The corrosion resistance of the Mg-RE alloys is increasing in the following order: Mg-1.43La, Mg-1.44Nd, Mg-0.3Ce, Mg-0.63Gd and Mg-0.41Dy. Accordingly, the corrosion morphology in the best resistant Mg-0.41Dy alloy and the worst Mg-1.43La alloy were observed and compared after 2h and 24 h of immersion using SEM-EDS, XPS and XRD analysis. The formation of the Dy2O3 oxide prevents the Mg-0.41Dy alloy from pitting corrosion and lead to an excellent corrosion surface even after 24 h of immersion. Meanwhile, the presence of a high fraction of the Mg12La phase along the grains boundaries in the Mg-1.43La alloy causes severe pitting corrosion by acting as anodic phase.

Introduction

During the last decades, the use of Magnesium (Mg) based alloys as structure materials in automotive and aerospace industries have been increased due to their low density, high specific strength, high damping capacity, corrosion resistance and easy recyclability [START_REF] Hirsch | Superior light metals by texture engineering: Optimized aluminum and magnesium alloys for automotive applications[END_REF][START_REF] Esmaily | Fundamentals and advances in magnesium alloy corrosion[END_REF][START_REF] Atrens | Viewpoint -Understanding Mg corrosion in the body for biodegradable medical implants[END_REF].

However, Mg-based alloys generally suffer from relatively poor corrosion resistance in the high chloride environment of physiological systems due to their low standard electrochemical potential (-2.37 V), as compared to the SHE (Standard Hydrogen Electrode) [START_REF] Udhayan | On the corrosion behaviour of magnesium and its alloys using electrochemical techniques[END_REF] which therefore restricts their use in a variety of applications [START_REF] Atrens | Review of Recent Developments in the Field of Magnesium Corrosion[END_REF][START_REF] Song | Understanding Magnesium Corrosion-A Framework for Improved Alloy Performance[END_REF][START_REF] Abbott | Magnesium: Industrial and Research Developments Over the Last 15 Years[END_REF]. The poor corrosion resistance of Mg-based alloys was attributed to the internal galvanic corrosion by inhomogeneous distribution of second phases or the presence of impurities elements such Fe, Ni, Cu and Co and the instability of the hydroxide film (MgOH)2 that formed on the Mg surface [START_REF] Atrens | Review of Recent Developments in the Field of Magnesium Corrosion[END_REF][START_REF] Song | Understanding Magnesium Corrosion-A Framework for Improved Alloy Performance[END_REF].

Many strategies have been proposed to improve the corrosion resistance of Mg-based alloys.

Surface treatment by anodic coating is the most employed technique for improving the corrosion behaviour of Mg-based alloys [START_REF] Tekin | Electrochemical behavior of plasma electrolytic oxide coatings on rare earth element containing Mg alloys[END_REF][START_REF] Zhang | A composite anodizing coating containing superfine Al2O3 particles on AZ31 magnesium alloy[END_REF][START_REF] Jafari | In-vitro biodegradation and corrosion-assisted cracking of a coated magnesium alloy in modified-simulated body fluid, Materials science & engineering. resistance of magnesium alloys[END_REF][11]. Among the existing surface treatment methods, the plasma electrolytic oxidation (PEO) coating technique shows a remarkable increase in the corrosion resistance of Mg alloys by forming a relatively thick and crystalline oxide layer on the surface [START_REF] Tekin | Electrochemical behavior of plasma electrolytic oxide coatings on rare earth element containing Mg alloys[END_REF][START_REF] Zhang | Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys[END_REF][START_REF] Liang | Electrochemical corrosion behaviour of plasma electrolytic oxidation coatings on AM50 magnesium alloy formed in silicate and phosphate based electrolytes[END_REF][START_REF] Blawert | 8 -Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys[END_REF].

Fabricate ultrafine-grained materials using severe plastic deformation (SPD) such as equal channel angular pressing (ECAP), high pressure torsion (HPT) and groove pressing (GP) have proven their effectiveness to improve the homogeneity of the microstructure of Mg alloys as well as their corrosion resistance by dissolution of impurities and accelerating passivity of Mg-based alloys [START_REF] Gao | Homogeneous corrosion of high pressure torsion treated Mg-Zn-Ca alloy in simulated body fluid[END_REF][START_REF] Sunil | Role of biomineralization on the degradation of fine grained AZ31 magnesium alloy processed by groove pressing[END_REF][START_REF] Torbati-Sarraf | Electrochemical behavior of a magnesium ZK60 alloy processed by high-pressure torsion[END_REF][START_REF] Ahmadkhaniha | Corrosion behavior of severely plastic deformed magnesium based alloys: A review[END_REF][START_REF] Wang | The Effect of Pre-Processing and Grain Structure on the Bio-Corrosion and Fatigue Resistance of Magnesium Alloy AZ31[END_REF]. However, the existing literature on the impact of the microstructure and the deformation processing on the corrosion behaviour is often contradictory [START_REF] Ahmadkhaniha | Corrosion behavior of severely plastic deformed magnesium based alloys: A review[END_REF]. The effect of the second phase distribution on corrosion behaviour, induced by mechanical processing, was found depending on the role of the second phase as anode or cathode [START_REF] Ahmadkhaniha | Corrosion behavior of severely plastic deformed magnesium based alloys: A review[END_REF].

For example, the presence of the net-like Mg41Sm5 phase in the as-cast Mg-Sm-Zn-Zr alloy was found to act as a cathodic phase and thereby cause the galvanic corrosion [START_REF] Feng | Unexpected cathodic role of Mg41Sm5 phase in mitigating localized corrosion of extruded Mg-Sm-Zn-Zr alloy in NaCl solution[END_REF]. While the decrease in the size of the same phase and its homogeneous distribution caused by the extrusion processing leads to more uniform corrosion [START_REF] Feng | Unexpected cathodic role of Mg41Sm5 phase in mitigating localized corrosion of extruded Mg-Sm-Zn-Zr alloy in NaCl solution[END_REF].

Alloying elements are another effective strategy to enhance the mechanical properties and improve the corrosion resistance of Mg-based alloys. Recently, it was reported that rare earth (RE) elements have significant effects on the high temperature strength and creep resistance of Mg-based and can improve considerably their corrosion resistance [START_REF] Meng | 2 -Corrosion performance of magnesium (Mg) alloys containing rare-earth (RE) elements[END_REF][START_REF] Huang | Role of multi-microalloying by rare earth elements in ductilization of magnesium alloys[END_REF][START_REF] Boby | Mechanical characterization and corrosion behavior of newly designed Sn and Y added AZ91 alloy[END_REF]. Moreover, Mg-RE alloys are receiving increasing attention in biomedical applications as potential biodegradable implant materials due to their biodegradability and suitable mechanical properties [START_REF] Liu | Mechanical properties, corrosion resistance and biocompatibilities of degradable Mg-RE alloys: A review[END_REF]. However, some recent reports indicated that some RE elements such as La and Ce elements should be used carefully because of the toxicity issues compared to the other RE elements [START_REF] Willbold | Effect of the addition of low rare earth elements (lanthanum, neodymium, cerium) on the biodegradation and biocompatibility of magnesium[END_REF].

Several suppositions have been given to explain the benefic effect of RE element on the corrosion behaviour of Mg-based alloys. Among of them: (i) RE elements have the ability to reduce the effect of impurity by forming intermetallic compounds and purify the melt [START_REF] Liu | Effect of rare earth element Ce and La on corrosion behavior of AM60 magnesium alloy[END_REF],

(ii) RE elements can reduce the amount of second phases and grain size, resulting in a much homogeneous microstructure which will decrease the probability of occurring the microgalvanic corrosion [START_REF] Shi | Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg-RE alloys: RE=Ce[END_REF], (iii) RE elements can reduce the hydration of the corrosion product film of Mg-based alloys and then improve the protective performance of the corrosion product [START_REF] Chang | Investigation of the corrosion for Mg-xGd-3Y-0.4Zr (x=6,8,10,12wt%) alloys in a peak-aged condition[END_REF], and (iv) RE elements can decrease the Mg dissolution of the deformed alloys by decreasing the volume fraction of twins and dislocations density [START_REF] Zhang | Corrosion of hot extrusion AZ91 magnesium alloy. Part II: Effect of rare earth element neodymium (Nd) on the corrosion behavior of extruded alloy[END_REF].

The results of electrochemical tests showed that the E21 and WE43 alloys exhibit higher corrosion resistance than the conventional AZ31(Mg-3Al-1Zn, wt.%) alloy [START_REF] Tekin | Electrochemical behavior of plasma electrolytic oxide coatings on rare earth element containing Mg alloys[END_REF]. However, it was reported that the corrosion rate of binary Mg-RE (RE = Ce, La, Nd, Y and Gd) was greater than that of pure Mg and it was attributed to the presence of second phase [START_REF] Shi | Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg-RE alloys: RE=Ce[END_REF]. A binary high-pressure die-cast Mg-Ce alloy was reported to exhibit a lower corrosion rate than binary Mg-Nd and Mg-La alloys in 0.1 M NaCl (~0.6 wt.% NaCl) solution [START_REF] Birbilis | On the corrosion of binary magnesium-rare earth alloys[END_REF]. Such difference in the corrosion rate was attributed to the effect of the specific RE element contained in the second phase [START_REF] Birbilis | On the corrosion of binary magnesium-rare earth alloys[END_REF].

Up to now, the effect of different RE addition elements on the properties of second phases and their impact on the corrosion behaviour of Mg-based alloys are not fully understood and is more complicated than expected. Therefore, the objective of the present work is to elucidate the effect of a small content of RE elements such as Ce, Dy, Gd, Nd and La on the corrosion mechanism of binary Mg-RE alloy in 3.5% NaCl solution.

Experimental procedure

Materials

The as-cast Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La (wt.%) alloys were kindly supplied by the Institute für Metallkunde und Metallphysik, Aachen, Germany.

Accordingly, the alloys were produced by induction melting and casting under a protective gas atmosphere of Ar/CO2 using a preheated copper mould. The as-cast alloys have undergone heat treatment at 420 °C for 20 h and then quenched in water.

Electrochemical tests

The immersion test was conducted in 3.5% (% wt.) NaCl which was prepared at room temperature (25 °C) by dissolving the NaCl in distilled water.

Electrochemical tests were conducted using an AUTOLAB PGSTAT302N electrochemical work station and a standard three-electrode cell, wherein the sample is the working electrode, a platinum plate as the counter electrode, and a calomel electrode (SCE)(0.24 vs. SHE) is the reference electrode. The samples were embedded in an epoxy resin to isolate a 0.5 cm 2 area from the non-studied surfaces and were then immersed in 3.5% NaCl solution at room temperature. The open-circuit potential (OCP) was measured for at least 2 h for all samples.

The polarization curves were recorded in the range of -2.6 to 0 V vs. SCE with a scan rate of 2 mV.s -1 . The impedance measurements were performed after the open circuit potential with a sinusoidal potential signal with an amplitude of 10 mV in the frequency range from 10 -2 Hz to 10 5 Hz. Experimental curves were fitted using ZView software. Three replicated tests were performed for each alloy to ensure reproducibility.

Immersion test

Cylindric samples with the 10 mm of diameter and 3 mm of thickness were exposed to 3.5% NaCl solution for 2 h and 24 h at room temperature. The ratio of sample surface area (cm 2 ) to the volume of 3.5% NaCl solution (mL) was set to 1:100. The pH value of the solution was measured using a calibrated pH meter.

Microstructure characterization

The initial microstructures were investigated by scanning electron microscopy (SEM, FEG-SEM ZEISS Gemini) equipped with energy dispersive spectrometry (EDS) system in backscattering (BSE) mode operated at 15 kV. While, the morphology of the corrosion product was examined by SEM (TESCAN Vega 3 SB) using a range of 8-10 kV as acceleration voltages.

The surface preparation consisted of grinding with progressively finer SiC paper followed by mechanical polishing using a diamond solution with particle sizes ranging between 3 and 1 μm, followed by electropolishing with 5:3 solution of ethanol (C2H5OH) and phosphoric acid (H3PO4) for 30 min at 3V. The grain structure was revealed by subsequent etching at room temperature in an acetic-nital solution (5% HNO3, 15% acetic acid, 20% distilled water and 60% ethanol) for 3 s.

The cross-sections of the corroded samples after 24 h of immersion were also performed with SEM (FEG SEM SIGMA HD) by vertically mounting the samples in epoxy resin and fixation of the corroded surface by Silver. Element mapping was carried out with EDS (SAMx IDFix) operated at 15 kV to determine the distribution of elements in the corrosion product.

X-ray photoelectron spectroscopy measurement

X-ray photoelectron spectroscopy (XPS) measurements were performed using a KRATOS AXIS Ultra DLD (Kratos Analytical, Manchester, United Kingdom) equipped with a monochromatic Al Kα anode working at 15 kV (225W). For the survey spectra pass energy of 160 eV was used. The investigated area was a region of 700 by 300 µm for Mg-0.41Dy alloy while for the Mg-1.43La alloy a spot of 110 µm diameter was used. Ar etching was performed for 120 sec with an etching rate of 10 nm / min related to Ta2O5 (acceleration voltage 3.8 kV with an extraction current of 160 μA). For all of the samples, charge neutralization was necessary. The evaluation and validation of the data were carried out with the software CASA-XPS version 2.3.19. Calibration of the spectra was done by adjusting the C1s signal to 284.8 eV.

X-ray diffraction analysis

The X-ray diffraction (XRD) measurements were performed on Rigaku SmartLab diffractometer equipped with 9 kW copper rotating anode X-ray source (CuKα radiation λ = 0.15418 nm), parabolic multilayer mirror in the primary beam, a set of axial divergence eliminating soller slits in both incident and diffracted beam (acceptance 5°), a parallel beam soller slit collimator (acceptance 0.5°) and HighPix-3000 2D hybrid pixel single photon counting detector in the diffracted beam. Constant incidence angle of the primary beam ω = 0.6° was used for the measurement.

Results

Initial microstructures

Figure 1 shows the SEM micrographs in the backscattering mode with low and high resolution of the as-cast Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys, respectively. Table 1 summarized the EDS analysis (in wt.%) in several positions (points 1-12) in the microstructures of the as-cast Mg-RE alloys. The as-cast microstructure of Mg-0.3Ce alloy exhibits a bimodal grain size distribution with an average grain size of 70 μm. The higher magnitude of the SEM micrograph (Figure 1b) demonstrated that the Mg-0.3Ce contains very small circular particles located within the grain and along the grain boundaries. Table 1 shows that these particles (point 2) contain around 9% of Ce and their volume fraction was estimated around 8.5% by measuring the area fraction of the particles in the SEM micrographs.

The Mg-0.41Dy and Mg-0.63Gd alloys exhibit almost similar an elongated granular microstructure with an approximately large grain size of 400 μm and the presence of second phases. Both alloys contain twins as shown by arrows. The presence of twins is mostly caused by the mechanical preparation of the samples. The second phases were identified as the Mg41Dy5 (point 4) with a small volume fraction of 0.4% and the Mg12Gd (point 6) with a volume fraction of 3%, respectively. The size of the Mg41Dy5 phase seems to be smaller than Mg12Gd phase and homogeneously distributed in the microstructure of Mg-0.41Dy alloy. In contrast, the microstructure of Mg-1.44Nd alloy shows the presence of three different types of second phases. First, circular dark particles identified as Mg12Nd (point 8) and the second one are small white particles identified as Mg41Nd5 (point 9). The third types of particles have elongated rod shape and contain around ~20% of Nd element (point 10) which could be identified as Mg24Nd. The later second phase was not reported in the literature and may be formed during the as-cast processing. The volume fraction of these phases was calculated to be around 6%. The Mg-1.43La alloy exhibits a totally different microstructure compared to the other Mg-RE alloys where it is characterised by a dendritic morphology. As shown in Figure 1i, the second phase identified as Mg12La phase (point 12) is located at grain boundaries and triple junctions. Previously, based on the XRD measurement this second phase was identified as Mg17La2 phase [START_REF] Elfiad | Investigation of texture, microstructure, and mechanical properties of a magnesium-lanthanum alloy after thermomechanical processing[END_REF]. However, the EDS analysis (point 12) shows clearly that this second phase belongs to the Mg12La phase. Obviously, the Mg-1.43La alloy shows the highest fraction of the second phase with a value of 40%.

3.2.Electrochemical tests

Figure 2 shows the potentiodynamic polarization curves after 2 h of immersion in a 3.5% NaCl solution for the Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys, respectively. As can be seen, all alloys exhibit similar trends where the anodic and cathodic branches are not symmetrical. Usually, the anodic branches in the polarization curve represent the anodic dissolution of Mg metal, while the cathodic branches represent the cathodic hydrogen evolution reaction [START_REF] Chang | Effect of heat treatment on corrosion and electrochemical behaviour of Mg-3Nd-0.2Zn-0.4Zr (wt.%) alloy[END_REF]. The anodic part of the polarization curve coincides for all alloys. In the cathodic region, the Mg-1.43La alloy seems to have the highest current density followed by Mg-1.44Nd alloy, while, the Mg-0.3Ce, Mg-0.63Gd and Mg-0.41Dy alloys seem to have a similar current density. These results agree well with the fact that the corrosion process in Mg-based alloys is catholically controlled [START_REF] Curioni | Correlation between electrochemical impedance measurements and corrosion rate of magnesium investigated by real-time hydrogen measurement and optical imaging[END_REF]. The corrosion potential, (Ecorr) and the corrosion current density (Icorr) values obtained from the curve fitting are listed in Table 2.

Moreover, the polarization resistance (Rp) of different Mg-RE alloys represented in Table 2 was calculated from Icorr according to the Stern-Geary equation [START_REF] Lin | Development of a novel microtextured surface using duplex surface modification for biomedical Mg alloy applications[END_REF] :

𝑅 𝑝 = 𝛽 𝑎 |𝛽 𝑐 | 2.303(𝛽 𝑎 +|𝛽 𝑐 |)𝐼 𝑐𝑜𝑟𝑟 (1)
where βa and βc are the anodic and cathodic slopes, respectively.

As can be revealed from Table 2, the corrosion potential Ecorr of Mg-RE alloys is shifted towards negative value in the order of Mg-0.41Dy <Mg-0.63Gd <Mg-0.3Ce <Mg-1.44Nd < Mg-1.43La. The corrosion current density, Icorr of Mg-RE alloys increase in the same order than the Ecorr.

The Nyquist plots obtained from the electrochemical impedance spectroscopy (EIS) for the Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys in a 3.5% NaCl solution are shown in Figure 3. For more visibility, the Nyquist plot of Mg-1.43La alloy is inserted at the upper right with a small scale. The Nyquist plots exhibit three different shapes as a function of the RE alloying elements indicating a different corrosion mechanism in each Mg-RE alloys. First, the Nyquist plot of Mg-0.41Dy alloy shows two capacitive loops: one capacitive loop at the high-frequency region and the second loop located at the medium frequency region. Second, the Mg-0.3Ce, Mg-0.63Gd and Mg-1.44Nd alloys have a similar shape with two capacitive loops at the high and medium frequency region, respectively and an inductive loop in the low-frequency region. Third, the Nyquist plot of Mg-1.43La alloy exhibits one capacitive loop at the high-frequency region and two inductive loops in the low and medium frequency regions. However, it seems that the Mg-1.43La alloy may exhibit a very small loop almost indistinguishable at the high-frequency region. Such small capacitive loop has been already reported for several Mg-based alloys [START_REF] Cao | Corrosion behavior of magnesium and its alloy in NaCl solution[END_REF][START_REF] Cheng | Comparison of corrosion behaviors of AZ31, AZ91, AM60 and ZK60 magnesium alloys[END_REF]. The disappearance of the capacitive loop may be attributed to the high corrosion rate of the Mg-1.43La alloy with small corrosion products on the surface of the sample. Accordingly, the capacitive loop observed at the high-frequency region resulted from both a charge transfer reaction between the metal interface and the solution and also the formation of a double layer [START_REF] Baril | The corrosion of pure magnesium in aerated and deaerated sodium sulphate solutions[END_REF]. The capacitive loop observed at the medium-frequency was attributed to the growth of the corrosion product layer and its resistance. It has been suggested that the medium-frequency capacitive loop represents the combination of pseudo resistance and capacitance of the film formation and dissolution process [START_REF] Song | Crystal orientation and electrochemical corrosion of polycrystalline Mg[END_REF]. The inductive loop suggests the occurrence of pitting corrosion, dissolution of the protective film, the occurrence of relaxation processes of adsorbed species such as Mg(OH) +ads and the existence of metastable Mg + ion during the dissolution of Mg-based alloy [START_REF] Song | Biodegradable behaviors of AZ31 magnesium alloy in simulated body fluid[END_REF]. In fact, the presence of the inductive loop is frankly observed in the Nyquist plots of various Mg-based alloys [START_REF] King | Accurate Electrochemical Measurement of Magnesium Corrosion Rates; a Combined Impedance, Mass-Loss and Hydrogen Collection Study[END_REF].

As can be seen from Figure 3, the dimension of both capacitive loops in Mg-0.41Dy alloy was higher in comparison to the other Mg-RE alloys indicating a better corrosion resistance of Mg-0.41Dy alloy. It can also be seen that the dimension of medium capacitive loops shrinks with RE elements as Mg-0.3Ce, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys reflecting the heterogeneity of the corrosion products and their less protective effect.

The corresponding equivalent electrical circuit and the fitted data of the Nyquist curves are presented in Figure 4 and Table 4, respectively. Using a simulated equivalent circuit can usually help to explain the general corrosion process. Rs, Rct, Rf and RL are the solution resistance, charge transfer resistance, film resistance and inductance resistance, respectively.

CPEdl and CPEf are constant phase elements representing the capacitance in the high and medium frequencies region, respectively. The CPE is defined by two values, Y and n and usually employed instead of the capacitance in case of the non-homogeneity of the sample surface [START_REF] Mohajernia | Modified nanostructured hydroxyapatite coating to control the degradation of magnesium alloy AZ31 in simulated body fluid[END_REF]. If n = 1, CPE will be identical to a capacitor.

All the Mg-RE alloys have almost the same value of Rs demonstrating that RE alloying elements have similar behaviour in the contact with the 3.5% NaCl solution. Nevertheless, the Rs value is more sensitive to the concentration of Cl -ions [START_REF] Gao | The corrosion of a novel Mg-11Li-3Al-0.5RE alloy in alkaline NaCl solution[END_REF].

The double-layer capacitance Ydl value is generally found in the range of 50-100 μF cm -2 [START_REF] Leleu | Corrosion rate determination of rare-earth Mg alloys in a Na2SO4 solution by electrochemical measurements and inductive coupled plasmaoptical emission spectroscopy[END_REF]. It is interesting to note that the Ydl values of the present Mg-RE alloys are lower compared to the literature and vary slightly as a function of RE alloying elements. Usually, double-layer capacitance increases with the decrease of the surface of the protective film [START_REF] Liu | Corrosion behavior of AZ31 magnesium alloy in simulated acid rain solution[END_REF].

Moreover, the value of ndl is close to being similar, indicating that the capacitive loop is welldefined, except in case of the Mg-1.43La alloy where ndl = 0.79.

The resistance value Rct indicates the dissolution rate of Mg metal. The smaller corrosion rate is obtained the higher Rct is. Accordally, the Rct increases as Mg-1.43La < Mg-1.44Nd < Mg-0.63Gd < Mg-0.3Ce < Mg-0.41Dy, which indicates that the corrosion rate is lower in Mg-0.41Dy alloy and higher in Mg-1.43La alloy.

The Yf values of the corrosion product capacity were found much higher than those of Ydl (double layer capacity), except for Mg-1.43La alloy which was in the same order of magnitude (Ydl = 5.16×10 -5 vs. Yf = 2.38 ×10 -5 F.cm -2 ). This is in good agreement with the assumption that the surface covered by the corrosion products is large compared to the free area of the surface [START_REF] Leleu | Corrosion rate determination of rare-earth Mg alloys in a Na2SO4 solution by electrochemical measurements and inductive coupled plasmaoptical emission spectroscopy[END_REF].

The inductance in the equivalent circuit of Mg-0.3Ce, Mg-0.63Gd and Mg-1.44Nd alloys shows high values (in range of 5586 to 15876 H.cm -2 ) while the value in the case of the Mg-1.43La alloy decrease drastically (76.76 H.cm -2 ). It was reported that a high value of the inductance indicates relative uniform corrosion in the surface of the sample [START_REF] Shi | Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg-RE alloys: RE=Ce[END_REF].

Corrosion morphology

Based on the electrochemical tests, the Mg-0.41Dy and Mg-1.43La alloys show the best and worst corrosion resistance, respectively. Following, the characterization of the corrosion morphology for those alloys will be taken into account.

Figure 5 shows the photographs of the surface of Mg-0.41Dy and Mg-1.43La alloys after 2 h and 24 h of immersion in 3.5% NaCl solution, respectively. As can be seen, the surface of Mg-0.41Dy alloy was slightly affected by the extension of immersion time. Small grey spots appeared on the surface of the Mg-0.41Dy alloy after 24 h of immersion. Meanwhile, the surface of the Mg-1.43La exhibits severe corrosion after 24 h of immersion in 3.5% NaCl solution. At the beginning (after 2 h of immersion) the corrosion started from one edge of the sample (zone with grey colour) by the presence of small white spots on the surface of the Mg-1.43La sample and with ongoing time a dense rough surface formed by a white-grey layer grows until 24 h of immersion.

Figure 6 shows the surfaces in the cross-section of the Mg-0.41Dy and Mg-1.43La alloys obtained by optical microscopy after immersion for 24h in 3.5% NaCl solution, respectively.

It can be seen that the corrosion layer morphology of the Mg-0.41Dy alloy is thin and uniform. In contrast, the corroded surface of the Mg-1.43La alloy is very thick and exhibits several localized corrosion sites as shown by arrows.

The SEM micrographs by backscattering mode presented in Figure 7 and 8 with low and high resolution show the microscopic corrosion morphologies of the Mg-0.41Dy and Mg-1.44La alloys after 2 h and 24 h immersion in 3.5% NaCl solution, respectively.

Figure 7a shows that the morphology of the surface of the Mg-0.41Dy alloy after 2h of immersion was uniform and smooth and some cracks can be noticed on the surface of the alloy after 24h of immersion (Figure 7c). The presence of cracks is caused by the dehydration of the corrosion product. Increasing the resolution of the SEM images of the corroded surfaces as shown in Figure 7b and 7d indicate that the alloy exhibits typical corrosion morphology of needle-like particles. No significant difference in the morphology of the surface of Mg-0.41Dy alloy after 24 h of immersion, except that the needle got thinker. It has been suggested that the small and dense needle-like particles are more protective [START_REF] Song | Study of the corrosion product films formed on the surface of Mg-xZn alloys in NaCl solution[END_REF].

The corrosion morphology of the Mg-1.43La alloy after immersion for 2h is relatively similar to those of the Mg-0.41Dy alloy. In can see from Figure 8a that the Mg12La phase is visible along the grain boundaries (shown by arrows) mostly due to the attack effect from the 3.5%NaCl solution. This observation may indicate that the Mg-1.43La alloy is corroded by an intergranular corrosion type. Besides, the volume fraction of the Mg12La phase seems to decrease after immersion for 2 h. In the higher resolution, the Mg12La phase seems to be covered by the same needle-like particles as the Mg matrix (dashed line in Figure 8b). The needle-like particles are larger and less compact than to those on the Mg matrix. The surface of the Mg-1.43La alloy after immersion for 24 h is covered by a thick and irregular film of corrosion products which is completely different from the Mg-0.41Dy alloy.

Figure 7 and 8 show also the presence of few mushrooms like shaped precipitations formed on the surface of both alloys expect for Mg-1.43La after 24 h of immersion. The absence of these agglomerations in the immersed Mg-1.43La alloy up 24 h is attributed the severely corroded surface. It can be seen that these agglomerations are bigger in the Mg-1.43La alloy.

A close up is presented in Figure 9 showing the micrograph of these mushrooms like shaped precipitations on the surface of the Mg-0.41Dy alloy after immersion for 24 h. The corresponding EDS analysis of point 1, 2 and 3 areas are presented in Table 4. Obviously, these particles (point 1) and around them (point 2) contain a high fraction of Cl. The surface of the point 3 revealed the presence of the magnesium hydroxide Mg(OH)2 compound which is a typical corrosion product largely reported for Mg-based alloys in aqueous solutions [START_REF] Song | Corrosion Mechanisms of Magnesium Alloys[END_REF].

The evolution of pH value as a function of immersion time of both alloys shown in the upper right side of Figures 7 (a,c) and Figures 8(a,c) indicate clearly that the corrosion processing in the Mg-0.41Dy and Mg-1.43La alloys is totally different. It is to be noted that the initial pH value of the 3.5% NaCl solution was measured to be 6.55. The pH value of the Mg-0.41Dy alloy increases slightly after immersion for 2 h and then reach a value of 8.4 after 24 h of immersion. In contrast, the pH value for the Mg-1.43La alloy increases rapidly to 10.3 after immersion for 24 h.

To get more detailed information of the corrosion product composition, XPS and XRD analysis were performed for the Mg-0.41Dy and Mg-1.43La alloys after 2 h and 24 h of immersion in 3.5% NaCl solution (Figures 10 and11). The XPS survey spectra for all samples revealed the presence of Mg, O, C and Cl at the very surface. So, the outer corrosion layer consists mainly from the MgCO3 and MgO/Mg(OH)2 with some enrichment of Cl that is caused either by the 3.5% NaCl solution (remaining Cl) or by the MgCl2 as a reaction product. The concentrations of the different elements seem to be stable with increasing immersion time for both alloys as shown in Table 5. In contrast, neither Dy nor La can be found on the outer layer of the corroded surfaces. First, this is can be attributed to the low concentration of the alloying element in the Mg matrix (0.41% Dy and 1.43% La).

Furthermore, it might be that corrosion product consisting of Dy of La will not be present in the very first 3-5 nm, which is the investigation limit of the XPS method while XRD penetrates more the bulk material.

Figure 11 shows the XRD patterns of as-cast material as well as after immersion for 2h and 24 h in 3.5% NaCl of Mg-0.41Dy and Mg-1.43La alloys, respectively. The XRD patterns of both as-cast alloys confirm the presence of the Mg24Dy5 and Mg12La phases as already demonstrated by the SEM-EDS analysis in Figure 1. Immersion in 3.5% NaCl for 2h leads to the development of news peaks identified as the corrosion product Mg(OH)2 in both alloys.

Contrary to the Mg-1.43La alloy, the second phase was not detected in the Mg-0.41Dy alloy. Such difference could be attributed to the total covering of the surface by the corrosion product in Mg-0.41Dy alloy. An interesting feature can be seen after immersion of 24 h.

Besides the presence of the corrosion product Mg(OH)2 a new oxide Dy2O3 is formed on the Mg-0.41Dy surface. In the meantime, the XRD pattern of Mg-1.43La shows the disappearance of the Mg matrix peaks and the appearance of small peaks belonging to the MgCl2 phase.

Discussion

The kinetics of the corrosion process in various Mg-based alloys in aqueous solution are generally expressed by the following equations [START_REF] Song | Corrosion Mechanisms of Magnesium Alloys[END_REF]:

Anodic reaction: Mg  Mg 2+ + 2e - (1)
Cathodic reaction: H2O + 2e -2OH -+ H2

The corrosion reaction leads to the formation of Mg(OH)2 accompanied by hydrogen evolution:

The total reaction: Mg + H2O Mg(OH)2+ H2

It was expected that Mg alloys containing RE elements will have similar corrosion behaviour; however, the electrochemical measurements plotted in Figures 2 and3 show clearly that the present Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys exhibit relative different rate response of the cathodic kinetics and different corrosion mechanism as well. The corrosion rate of binary Mg-Y and Mg-Nd alloys in a similar 0.1 M NaCl solution (pH ∼6.1) was also found totally different [START_REF] Birbilis | On the corrosion of binary magnesium-rare earth alloys[END_REF][START_REF] Sudholz | graingraing[END_REF]. The present results indicate that the Mg-0.41Dy alloy has the better corrosion resistance (5.58 Ohm.cm 2 ) and the Mg-1.43La alloy the lower one (2.89 Ohm.cm 2 ). It is interesting to note that for the Mg-0.41Dy alloy a more noble (i.e. Ecorr shift toward 0 value) was found compared with several Mg-based alloys such as Mg-Al-Mn [START_REF] Yang | Effect of Ca addition on the corrosion behavior of Mg-Al-Mn alloy[END_REF], AZ31 [START_REF] Zhang | A composite anodizing coating containing superfine Al2O3 particles on AZ31 magnesium alloy[END_REF][START_REF] Huo | Simultaneously enhanced strength and corrosion resistance of Mg-3Al-1Zn alloy sheets with nano-grained surface layer produced by sliding friction treatment[END_REF], AZ91[11] and pure Mg [START_REF] Singh | A comparative corrosion behavior of Mg, AZ31 and AZ91 alloys in 3.5% NaCl solution[END_REF] immersed in 3.5% NaCl solution.

The electrochemical investigations demonstrated that the present Mg-RE alloys exhibit three different corrosion mechanisms. 1) The presence of only two capacitive loops in the Mg-0.41Dy alloy corresponds to uniform corrosion mechanism, i.e. the alloy surface was completely covered by the corrosion product film. This is confirmed by the very good corrosion surface of the Mg-0.41Dy alloy even after immersion for 24 h in 3.5% NaCl solution (Figures 5 and6), except maybe the presence of few mushrooms like shaped precipitations (Figure 8). However, these agglomerations do not cause pitting corrosion since no induction loop was seen in the Nyquist plot of the alloy.

2) The second corrosion mechanism is characterized by the addition of the inductive loop in the Nyquist plot in the case of Mg-0.3Ce, Mg-0.63Gd and Mg-1.44Nd alloys. It indicates a heterogeneous corrosion mechanism [START_REF] Shi | Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg-RE alloys: RE=Ce[END_REF]. The similarity in the shapes of the Nyquist plots for Mg-0.3Ce, Mg-0.63Gd and Mg-1.44Nd alloys suggest that the Ce, Gd and Nd elements have no effect on the mechanism of the formation but affect considerably the properties of corrosion products. As shown in Table 4, the corrosion resistance of the protective product Rf was estimated at 2749 Ω.cm 2 in the Mg-0.3Ce alloy and decrease to 1993 and 769.9 Ω.cm 2 in Mg-0.63Gd and Mg-1.44Nd alloys, respectively. In addition, the decrease of the inductance value with respect to Ce (15876 H.cm -2 ), Gd (10228 H.cm -2 ) and Nd (5586 H.cm -2 ) indicated that Nd suffer more from the pitting corrosion than Ce and Gd.

3) The third corrosion mechanism is shown in the Mg-1.43La alloys where the alloy exhibits two inductive loops indicating severe corrosion attacks (as shown by the morphology in the Figures 5, 6 and8) or micro-galvanic corrosion.

The presence of tow inductive loops was already reported for Mg-based alloys [START_REF] Zhang | Corrosion of hot extrusion AZ91 magnesium alloy. Part II: Effect of rare earth element neodymium (Nd) on the corrosion behavior of extruded alloy[END_REF][START_REF] Li | Effect of heat treatment on corrosion behavior of AZ63 magnesium alloy in 3.5wt.% sodium chloride solution[END_REF]. The presence of the second inductive loop is caused by the extra Mg dissolution due to the reaction of Mg + with H2O in the non-protective area of the surface following the non-Faraday process [START_REF] Zhang | Corrosion of hot extrusion AZ91 magnesium alloy. Part II: Effect of rare earth element neodymium (Nd) on the corrosion behavior of extruded alloy[END_REF][START_REF] Li | Effect of heat treatment on corrosion behavior of AZ63 magnesium alloy in 3.5wt.% sodium chloride solution[END_REF]:

2Mg + + 2H2O 2Mg 2+ + 2OH -+ H2 (4)
The release of hydroxide ions (OH -) leads to the highest increase in pH value of the Mg-1.43La alloy compared to the Mg-0.41Dy as reported for Figures 7 and8.

The result of EIS is in good agreement with the polarization curve analyses. It can be seen that the Mg-0.41Dy alloy exhibits the higher Rf value (3568 Ω.cm 2 ) and Mg-1.43La alloy the lower value (429.1 Ω.cm 2 ). The Rf values of different Mg-RE alloys shown in Table 4 that the corrosion product formed at the surface of Mg-0.41Dy alloy has a better capacity to protect the alloy against metal dissolution. It was found that among the RE (Y, Nd Gd and Dy) metals, the Dy metal exhibits the best corrosion resistance in 0.1 mol/L NaCl solution [START_REF] Zhao | A Comparison of Corrosion Behavior in Saline Environment: Rare Earth Metals (Y, Nd, Gd, Dy) for Alloying of Biodegradable Magnesium Alloys[END_REF].

The difference in the corrosion mechanism in the present Mg-RE alloys is strongly related to two major factors. The first factor is the effect of the specific RE elements added in the Mg matrix. The second one is the specific microstructure of the alloys, i.e. the presence of second phase, grain size.

Effect of specific RE alloying element

The rare earth elements are usually divided into two subgroups: the cerium subgroup which includes La, Ce, Nd, Pm, Sm and Eu and the yttrium subgroup which contain the remaining RE elements such as Gd and Dy [START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF]. This subdivision is based on the differences and similarities in the properties of the different RE elements. The change in the physicochemical properties between the different RE elements is observed with a successive increase of the RE metal atomic number and it is more related to the electron configuration of each specific element [START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF]. Table 6 summarized some specific features of the RE elements used in the present study. It was reported that the RE elements could show similar chemical properties when the energy levels of the 4f and 5d in the atoms are close [START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF]. Based on that, Table 5 shows that the Ce, Dy, Gd, Nd elements exhibit different properties. For example, among the used RE elements, La is considered the only transition metal as demonstrated by the configuration of the outer electronic shells (5d 1 6s 2 ) and the Dy element is the most stable as shown by the complete filled 4f shell. The electronegativity values of the RE elements were close to the electronegativity of Mg (1.2) [START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF]. However, La presents the highest electronegativity difference which suggests that the formed compounds between Mg and La are more stable than the other RE elements.

The density of RE elements is an important physical characteristic for the formed Mg-RE alloys. Table 6 shows that the density of the present RE elements increases with the increasing atomic number. Interestingly the corrosion resistance was found to increase with increasing atomic number of RE elements, expect for Ce element showing higher corrosion resistance than Gd and Nd elements. This anomaly could be attributed to the difference in the microstructures between the alloys as shown in Figure 1, which leads us to discuss the second factor.

Effect of the microstructure on the corrosion behavior

Contrary to the other binary alloys, the Mg-0.3Ce alloy has the smallest grain size (~ 70 μm).

It was reported that the grain boundaries can act as barriers for corrosion in Mg-based alloys [START_REF] Aung | Effect of grain size and twins on corrosion behaviour of AZ31B magnesium alloy[END_REF]. Theoretically, a microstructure with small grain size involves a high number of grain boundaries which result in more improvement of the corrosion resistance in case of the Mg-0.3Ce alloy. On the other hand, the diversity of the second phase (Mg12Nd, Mg41Nd5 and Mg24Nd) is the reason of the poor corrosion of the Mg-1.44Nd alloy compare to the Mg-0.3Ce and Mg-0.63Gd (the group with the same corrosion mechanism) by providing more cathodic phases and causing more dissolution of the Mg matrix.

Dy and Gd are more distributed into the Mg matrix since their solubility in the Mg matrix, 25.23% and 23.49%, respectively are much higher than that of Ce (0.23%), Nd (3.6%) or La (0.74%) [START_REF] Gusieva | Corrosion of magnesium alloys: the role of alloying[END_REF]. Therefore it is expected that the potential difference between the Mg matrix and the second phase will be small which will reduce the microgalvanic corrosion.

It is important to note, that the present Mg-RE alloys were used in the as-cast condition where the resulted microstructures were rather irregular (Figure 1). The Mg41Dy2 phase is considered metastable since the Mg-Dy phase diagram [START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals[END_REF] shows that the Mg-0.41Dy alloys is (0.41% of Dy) is a solid solution. It is expected that the metastable phase will have less effect against corrosion than the stable one such as Mg12La phase in case of the Mg-1.43La alloy. Besides, the results of the micro-capillary electrochemical cell method on Mg12Ce, Mg3Nd and Mg12La second phases show that the Mg12La phase is the less inert compared to the other second phases [START_REF] Birbilis | On the corrosion of binary magnesium-rare earth alloys[END_REF].

The amounts of second phases in the microstructure have a great influence on corrosion behaviour. Figure 1 demonstrated that Mg-1.43La alloy exhibits a high fraction of second phase (40%) and the Mg-0.41Dy alloy the lower one (0.4%). It was reported that the corrosion resistance decreases with the increasing amount of Y in binary Mg-Y alloy due to the increase of volume fraction of the intermetallic Mg24Y5 phase [START_REF] Sudholz | graingraing[END_REF]. Also, the XRD patterns (Figure 10) and SEM micrograph (Figure 7a) show the presence of the Mg12La phase in Mg-1.43La alloy even after 2h of immersion. Obviously, the immersion for 2 h causes the dissolution of the Mg12La phase as shown by the decrease of their volume fraction which suggests that this phase is preferentially corroded than the Mg matrix by acting as the anodic phase.

It is well established that the second phases are nobler than the Mg matrix and act as a cathodic phase in the corrosion process of traditional Mg-based alloys such as AZ91 (Mg-9Al-1Zn, wt%) alloy which explains the acceleration of the dissolution of the Mg matrix and the corrosion rate of Mg-based alloys [START_REF] Cheng | Comparison of corrosion behaviors of AZ31, AZ91, AM60 and ZK60 magnesium alloys[END_REF]. However, because RE elements are more active than Mg, it was demonstrated recently that the second phases in Mg-RE based alloys preferentially dissolved at the initial corrosion stage by acting as micro-anodes, which is completely different from the role of second phases in traditional Mg-based alloys [START_REF] Liu | The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy[END_REF][START_REF] Song | Pitting corrosion of a Rare Earth Mg alloy GW93[END_REF][START_REF] Cai | Effect of anodic T phase on surface micro-galvanic corrosion of biodegradable Mg-Zn-Zr-Nd alloys[END_REF].

For more comparison, the morphology of the corrosion product in the cross-sections of Mg-0.41Dy and Mg-1.43La alloys after 24h of immersion in 3.5 % NaCl solution are presented by the SEM images and chemical element mapping in Figure 12 and 13, respectively. The Mg-0.41Dy exhibits very good corrosion protection with a uniform and flat surface. No pitting and located corrosion were detected in the cross-section of the sample. Meanwhile, some crack could be noticed on the corrosion product. The depth of corrosion product in Mg-0.41Dy alloy was measured to be around 7 μm. The elemental maps on the cross-section of the Mg-0.41Dy alloy shown in Figure 8 demonstrated that the chemical composition of the corrosion layer consists mostly of both hydroxides Mg(OH)2 and/or magnesium oxides MgO.

In contrast, it can be observed in Figure 13 that two distinguish corrosion layers are formed on the surface of the Mg-1.43La alloy which is consistent with the higher corrosion rate found by The actual result is in very good agreement with the recent proposed pitting corrosion mechanism in Mg-RE alloys [START_REF] Song | Pitting corrosion of a Rare Earth Mg alloy GW93[END_REF]. It was reported that corrosion pitting involved three stages.

Firstly, the anodic phase dissolved followed by the corrosion of the Mg matrix and then the pitting corrosion appeared along with the depth of the dissolved phases [START_REF] Song | Pitting corrosion of a Rare Earth Mg alloy GW93[END_REF].

Furthermore, the corrosion products formed on the surfaces of the Mg-0.41Dy and Mg-1.43 alloy are quite different. The XRD patterns demonstrated that the surface of Mg-0.41Dy is covered by the Dy2O3 oxide; contrary to the Mg-1.43alloy (Figure 11). The enthalpy of formation of oxide Dy2O3 , ∆Hf = -1869 kJ/mole, suggest that this oxide is chemically more stable than the La2O3 oxide with the value of ∆Hf = -1791 kJ/mole [START_REF] Konings | The Thermodynamic Properties of the f-Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides[END_REF]. Instead of that, the surface of Mg-1.43La shows the presence of MgCl2 phase. The presence of Cl ions in the solution accelerates the dissolution of the protective film by penetrating in the micro-pores and transforming the Mg(OH)2 into more soluble MgCl2 [START_REF] Altun | Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy[END_REF] which cause accumulation of massive corrosion as demonstrated by the Photography of the Mg-1.43La alloy (Figure 5c)

and the occurrence of second inductance loop in the Nyquist plot (Figure 3).  The formation of the Dy2O3 oxide prevents the Mg-0.41Dy alloy from pitting corrosion and lead to an excellent corrosion surface even after 24 h of immersion.

Meanwhile, the high volume fraction of the Mg12La phase and the size of its dendriticshaped are responsible for the severe corrosion of Mg-1.43La alloy.

 The Mg12La second phase acts as micro-anode during the corrosion of the Mg-

  electrochemical tests. The outer corrosion layer (~38 μm of thickness) is non-uniform and porous with high amounts of O, Mg, and C elements. Meanwhile, the chemical element mapping demonstrated that besides the presence of Mg and O elements the compact inner corrosion layer (with 20 μm of thickness) contain also Cl elements. It can be noticed that the inner corrosion layer exhibits small cracks compared the outer corrosion layer and localized corrosion around Mg12La phase. It can be observed that the front of the corrosion attack is consuming the lamellar Mg12La phase. The occurrence of localized corrosion and the presence of Cl in the corrosion product facilitate the penetration of the solution into the underneath of the Mg metal and then cause the propagation of the pitting corrosion.

Conclusions

  Taking into account that the penetration depth of X-ray in SEM is around 20 to 40 μm with a huge accelerated volume (depending on the acceleration voltage) while the XPS investigations (shown in Figure11) which will only give information of the very first 3-5 nm of the surface, information from both methods might be different. This leads to the conclusion that the Dy2O3 phase should be formed in the inner layer of the corrosion product since the outer layer (3-5 nm) for both alloys was composed mainly of MgCO3 and MgO/Mg(OH)2. It is to be noted that etching of the sample surface in XPS by Ar ions for 120 sec removed only about 12-16 nm from the surface of the alloy. However, the XPS measurement of both alloys without etching (not shown here) present similar concentration of Mg, O and Cl, expect an increase of C concentration before etching due to the surface contamination by the CO2 adsorption from the environment.Hence, the present results would offer some useful information for a better understanding of the corrosion behaviour of Mg-RE alloys concerning the RE alloying addition to develop a new generation of Mg-based alloys with excellent corrosion resistance and therefore extend their use in a variety of applications. The corrosion behaviour of Mg-0.41Dy, Mg-0.3Ce, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys in 3.5% NaCl solution was evaluated by electrochemical tests and morphological observations.  The corrosion resistance of Mg-RE alloys increases with RE alloying elements in the following order: Mg-1.43La, Mg-1.44Nd, Mg-0.3Ce, Mg-0.63Gd and Mg-0.41Dy.  A complete protective layer covers the Mg-0.41Dy alloy surface which indicates uniform corrosion mechanism. A partially protective film covers the surface of Mg-0.3Ce; Mg-1.44Nd and Mg.0.63Gd alloys caused a non-uniform corrosion mechanism. The Mg-1.43La alloy exhibits micro-galvanic corrosion mechanism caused by the pitting corrosion.

Figure captions Figure 1 :

 captions1 Figure captionsFigure 1: SEM images of the initial microstructure with low and high resolution of: (a, b) Mg-0.3Ce, (c, d) Mg-0.41Dy, (e, f) Mg-0.63Gd, (g, h) Mg-1.44Nd and (i, j) Mg-1.43La alloys.

Figure 2 :

 2 Figure 2: Potentiodynamic polarization curves in 3.5% NaCl solution for the Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys.

Figure 3 :

 3 Figure 3: Nyquist plots for the Mg-0.3Ce, Mg-0.41Dy, Mg-0.63Gd, Mg-1.44Nd and Mg-1.43La alloys after 2 h in 3.5% NaCl solution.

Figure 4 :

 4 Figure 4: Equivalent electric circuit of Mg-RE alloys immersed in 3.5% NaCl solution: a) Mg-0.41Dy, b) Mg-0.3Ce, Mg-0.63Gd and Mg-1.44Nd and c) Mg-1.43La alloys.

Figure 5 :

 5 Figure 5: Photographs of the Mg-0.41Dy and Mg-1.43La alloys surface as a function of immersion time in 3.5% NaCl solution: (a, c) 2h and (b, d) 24 h.

Figure 6 :

 6 Figure 6: Cross-section images obtained by optical microscopy of the alloys after immersion for 24 h in 3.5% NaCl solution: (a) Mg-0.41Dy alloy and (b) Mg-1.43La alloy.

Figure 7 :

 7 Figure 7: SEM micrographs showing corrosion morphologies of Mg-0.41Dy alloy after immersion in 3.5% NaCl solution for: (a, b) 2 h and (c, d) 24 h.

Figure 8 :

 8 Figure 8: SEM micrographs showing corrosion morphologies of Mg-1.43La alloy after immersion in a 3.5% NaCl solution for: (a, b) 2 h and (c, d) 24 h.

Figure 9 :

 9 Figure 9: SEM micrograph showing the mushrooms like shaped precipitation on the surface of the Mg-0.41Dy alloy after immersion for 24 h in 3.5% NaCl solution.

Figure 10 :

 10 Figure 10: XPS survey spectra after 120 s Ar etching for Mg-0.41Dy (a, b) and Mg-1.43La (c, d) alloy after immersion in 3.5% NaCl solution for 2 and 24 h, respectively.

Figure 11 :

 11 Figure 11: XRD patterns of as-cast and immersed samples of: a) Mg-0.41Dy and b) Mg-1.43La alloys.

Figure 12 :

 12 Figure 12: SEM images and chemical element mapping of the cross-sections of Mg-0.41Dy alloy after 24h of immersion in 3.5 % NaCl solution.

Figure 13 :

 13 Figure 13: SEM images and chemical element mapping of the cross-sections of Mg-1.43La alloy after 24h of immersion in 3.5 % NaCl solution.

Figure 2 Figure 3

 23 Figure 2

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Figure 8 Figure 9 Figure 10

 8910 Figure 8

Figure 11

 11 Figure 11

Table captions Table 1 :

 captions1 EDS analysis in weight percentage in several positions in the microstructures of ascast Mg-RE alloys (from Fig.1).

Table 2 .

 2 Fitting parameters of the potentiodynamic polarization curves of Mg-RE alloy in 3.5% NaCl solution.

Table 3 .

 3 Electrochemical parameters obtained from the fits of the experimental EIS data of Mg-RE alloys in 3.5% NaCl solution.

Table 4 .

 4 EDS analysis in weight percentage (wt. %) in several positions in the microstructure of immersed Mg-0.41Dy alloy for 24 h in 3.5% NaCl solution (from Fig.9).

Table 5 .

 5 Elemental composition (in atomic percentage) of the corrosion product in Mg-0.41 and Mg-1.43La alloys after immersion for 2 h and 24 h in 3.5% NaCl solution, respectively.

Table 6 .

 6 Specific physicochemical properties of the RE elements used in the present work[START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF].

Table 4 .

 4 EDS analysis in weight percentage (wt. %) in several positions in the microstructure of immersed Mg-0.41Dy alloy for 24 h in 3.5% NaCl solution (from Fig.9).

	Position Number O (wt. %) Mg (wt. %) Cl (wt. %)
	1	48.13	35.10	16.77
	2	45.21	40.59	14.20
	3	47.01	50.03	2.95

Table 5 .

 5 Elemental composition (in atomic percentage) of the corrosion product in Mg-0.41 and Mg-1.43La alloys after immersion for 2 h and 24 h in 3.5% NaCl solution, respectively.

	Sample	Mg (at.%) O (at.%) C (at.%) Cl (at.%)
	Mg-0.41Dy
	2h of immersion 22.16	65.35	12.11	0.38
	24h of immersion 21.46	65.08	11.00	1.14
	Mg-1.43La
	2h of immersion 20.36	65.80	13.57	-
	24h of immersion 23.62	65.74	9.78	0.86

Table 6 .

 6 Specific physicochemical properties of the RE elements used in the present work[START_REF] Rokhlin | Magnesium Alloys Containing Rare Earth Metals: Structure and Properties[END_REF].

	RE	Atomic	Atomic mass	Electron	Electronegativity	Density
	element	number	(g/mol)	configuration		(g/cm 3 )
	La	57	138.9	[Xe]5d 1 6s 2	1.17	6.14
	Ce	58	140.12	[Xe]4f 1 5d 1 6s 2	1.21	7
	Nd	60	144.24	[Xe]4f 4 6s 2	1.19	7
	Gd	64	157.25	[Xe]4f 7 5d 1 6s 2	1.2	7.9
	Dy	66	162.93	[Xe]4f 10 6s 2	1.21	8.55

Acknowledgements

The authors gratefully acknowledge Dr. Talal Al-Samman; Institute für Metallkunde und Metallphysik (IMM-RWTH), Aachen, Germany, for supplying the Mg-RE alloys. This work was supported by the PRFU national project under Grant Agreement No. B00L02UN280120180005. M.D. acknowledges the financial support from the project NanoCent -Nanomaterials Centre for Advanced Applications, Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.

Funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.


RECONSTRUCTION OF OCEAN CIRCULATION BASED ON NEODYMIUM ISOTOPIC COMPOSITION Potential Limitations and Application to the Mid-Pleistocene Transition

By Kazuyo Tachikawa, William Rapuc, Quentin Dubois-Dauphin, Abel Guihou, and Charlotte Skonieczny SPECIAL ISSUE ON PALEOCEANOGRAPHY: LESSONS FOR A CHANGING WORLD increased carbon storage in the ocean via a more active biological pump in the Southern Ocean and/or more efficient carbon storage in the strongly stratified deep ocean [START_REF] Hain | Carbon dioxide effects of Antarctic stratification, North Atlantic Intermediate Water formation, and subantarctic nutrient drawdown during the last ice age: Diagnosis and synthesis in a geochemical box model[END_REF][START_REF] Adkins | The role of deep ocean circulation in setting glacial climates[END_REF]. During the deglaciation, dissipation of old deep water that was rich in respired carbon in the Southern Ocean contributed to CO 2 release to the atmosphere by the upwelling branch of the Atlantic overturning circulation [START_REF] Skinner | Ventilation of the deep Southern Ocean and deglacial CO 2 rise[END_REF].

Although proxy-based reconstructions have provided important insights into past ocean circulation, they do not always tell consistent stories because each proxy has its own bias. Recent advances in analytical instruments and coordinated international programs such as GEOTRACES have provided abundant high-quality data and contributed to a better understanding of proxy behavior in the modern ocean. Furthermore, the spatial coverage of marine sediment cores has greatly improved over recent decades, thanks to international programs such as the Ocean Drilling Program (ODP)/ International Ocean Discovery Program (IODP) and the International MArine Global changE Study (IMAGES). Lastly, tremendous progress in climate modeling has revealed the physical and biogeochemical processes involved in climate variability. In particular, proxy-enabled models make it possible to simulate proxy fields, which in turn can be compared to proxy reconstructions [START_REF] Arsouze | Reconstructing the Nd oceanic cycle using a coupled dynamical' biogeochemical model[END_REF][START_REF] Menviel | Simulating atmospheric CO 2 , 13 C and the marine carbon cycle during the last glacial-interglacial cycle: Possible role for a deepening of the mean remineralization depth and an increase in the oceanic nutrient inventory[END_REF][START_REF] Rempfer | On the relationship between Nd isotopic composition and ocean overturning circulation in idealized freshwater discharge events[END_REF][START_REF] Roche | δ 18 O water isotope in the iLOVECLIM model (version 1.0) -Part 2: Evaluation of model results against observed δ 18 O in water samples[END_REF][START_REF] Friedrich | Ocean circulation reconstructions from εNd: A model-based feasibility study[END_REF], allowing evaluation of the capacity of climate models to predict deepwater circulation changes.

The objective of this paper is to revisit the behavior of neodymium (Nd) isotopic composition of seawater as a tracer of water mass provenance and to apply this tracer to a major climate transition, the Mid-Pleistocene transition (MPT, about one million years). In the modern ocean, the Nd isotope composition of seawater follows the global thermohaline circulation (Figure 1), but factors other than water mass mixing influence its distribu-

INTRODUCTION

The ocean contains approximately 60 times more carbon than the atmosphere. About 20%-35% of the carbon dioxide (CO 2 ) emitted by burning fossil fuels has been absorbed by the ocean [START_REF] Khatiwala | Reconstruction of the history of anthropogenic CO 2 concentrations in the ocean[END_REF]. Changes in the ocean state, such as those that occurred during the last glacial maximum (LGM) and subsequent deglaciation, could significantly affect the carbon cycle by releasing absorbed carbon to the atmosphere in the future. The glacial atmospheric CO 2 concentration was lower than the pre-industrial value mainly because of ABSTRACT. As the ocean is Earth's largest reservoir of carbon, its circulation strongly influences the global carbon cycle. The neodymium (Nd) isotopic composition ( 143 Nd/ 144 Nd or ε Nd ) of seawater has been used as a tracer for ocean circulation. We revisit the capacity of this tracer using compiled modern seawater data sets and recent data (≤10,000 years, 10 kyr) extracted from the sedimentary record. Empirical equations that predict seawater ε Nd values from hydrography parameters can be used to evaluate possible biases in Nd isotopic ratios. The good overall agreement between measured seawater and predicted ε Nd values confirms the usefulness of Nd isotopic composition as a tracer of large-scale deepwater circulation in many parts of the modern ocean. Offsets observed between the sedimentary record and predicted values in certain oceanic regions can be partly explained by the contribution of porewaterderived Nd to sedimentary authigenic fractions. We use Nd isotopic composition to study a major climate transition in the middle Pleistocene called the "900 ka event, " which is characterized by a major perturbation in ocean carbon chemistry. All available reconstructed seawater ε Nd data indicate an increase in isotopic composition at the 900 ka event relative to the present value in the eastern Atlantic Ocean. This shift cannot be explained solely by more active formation of southern-sourced water that has a higher ε Nd value than the northern-sourced water. We suggest that a reduction in the Atlantic meridional overturning circulation and/or changes in Nd sources to the North Atlantic were the main cause(s) of the change in ε Nd observed during the evolution of the Northern Hemisphere cryosphere. where CHUR is a chondritic uniform reservoir with a 143 Nd/ 144 Nd ratio of 0.512638 [START_REF] Jacobsen | Sm-Nd isotopic evolution of chondrites[END_REF] or of 0.512630 [START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF]. The advantages of the Nd isotopic composi-tion of seawater, as compared to the carbon isotopic composition ( 13 C/ 12 C or δ 13 C) of seawater, another frequently used water mass tracer, is that the former experiences negligible biological fractionation and is not influenced by air-sea exchange.

In modern deep waters (≥1,500 m), the lowest ε Nd values of about -15 are found in an area surrounded by an old craton in the Labrador Sea and area. In contrast, the highest values close to 0 are observed in tropical and North Pacific waters as a result of abundant young volcanogenic material found there [START_REF] Jeandel | Isotopic Nd compositions and concentrations of the lithogenic inputs into the ocean: A compilation, with an emphasis on the margins[END_REF] FIGURE 1. Distribution of deepwater Nd isotopic composition in the modern ocean [START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF] and schematized global thermohaline circulation [START_REF] Broecker | The great ocean conveyor[END_REF]. [START_REF] Schlitzer | Ocean data view[END_REF].

haline circulation (Figure 1). The spatial heterogeneity of deepwater ε Nd values is also coherent with the estimated residence time of Nd in the ocean of 360 to 700 years [START_REF] Tachikawa | Neodymium budget in the modern ocean and paleo-oceanographic implications[END_REF][START_REF] Siddall | Towards explaining the Nd paradox using reversible scavenging in an ocean general circulation model[END_REF][START_REF] Rempfer | Modelling Nd-isotopes with a coarse resolution ocean circulation model: Sensitivities to model parameters and source/ sink distributions[END_REF], less than the mean mixing time of the deep ocean, which is about 1,500 years in the modern ocean [START_REF] Broecker | Tracers in the Sea[END_REF].

Net Nd sources to the ocean include partial dissolution of atmospheric inputs and of river sediments as well as river water after Nd removal in estuaries [START_REF] Tachikawa | Neodymium budget in the modern ocean and paleo-oceanographic implications[END_REF]; Figure 2). In reconciling both Nd concentration and isotopic distribution, water-sediment interaction along continental margins, called "boundary exchange, " has been considered the most important Nd source to the ocean [START_REF] Lacan | Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent-ocean interface[END_REF][START_REF] Arsouze | Reconstructing the Nd oceanic cycle using a coupled dynamical' biogeochemical model[END_REF][START_REF] Rempfer | On the relationship between Nd isotopic composition and ocean overturning circulation in idealized freshwater discharge events[END_REF].

The precise mechanism of boundary exchange, however, has not yet been fully identified, and it may include submarine ground water discharge [START_REF] Johannesson | Balancing the global oceanic neodymium budget: Evaluating the role of groundwater[END_REF] as well as benthic flux (Abbott et al., 2019, and references therein). In the open ocean, away from continental inputs, reverse scavenging transports Nd isotopic signatures vertically from shallower toward deeper water masses by dissolved/particulate interaction [START_REF] Bertram | The geochemical balance of the rare earth elements and Nd isotopes in the oceans[END_REF]; Figure 2). Even if this process is essential for producing observed depth profiles of seawater Nd concentration, the main control on deepwater ε Nd values in the open ocean is estimated to be the mixing of water masses characterized by distinct isotopic signatures [START_REF] Von Blanckenburg | Tracing past ocean circulation?[END_REF][START_REF] Frank | Radiogenic isotopes: Tracers of past ocean circulation and erosional input[END_REF][START_REF] Van De Flierdt | Neodymium in the oceans: A global database, a regional comparison and implications for palaeoceanographic research[END_REF][START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF].

IS Nd ISOTOPIC COMPOSITION A RELIABLE TRACER OF OCEAN CIRCULATION?

In order to identify oceanic regions significantly affected by local/regional lithogenic inputs that may mask water mass mixing effects, a compiled deepwater ε Nd database was compared with the values derived from empirical equations that predict seawater ε Nd from hydrography parameters (temperature, salinity, concentration of silicate, phosphate, and oxygen; [START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF]. Assuming that the offset from the predicted values indicates the local/regional lithogenic influence on deepwater ε Nd values, regions with strong local/regional effects were compared with the open ocean (Figure 3a,b). The most striking global feature is the positive offset in the tropical Pacific (Figure 3a,b) that is due to the strong influence of boundary exchange [START_REF] Lacan | Tracing Papua New Guinea imprint on the central equatorial Pacific Ocean using neodymium isotopic compositions and Rare Earth Element concentrations[END_REF][START_REF] Grenier | From the subtropics to the central equatorial Pacific Ocean: Neodymium isotopic composition and Rare Earth Element concentration variations[END_REF]. In this region, abundant labile volcanogenic matter releases radiogenic Nd to deep water. Moreover, the deep water in this area is poorly ventilated, increasing the importance of local Nd sources relative to water mass mixing effects.

Predicted ε Nd seawater signatures can be compared to marine archives of fish teeth/debris, deep-sea corals, and authigenic fractions associated with foraminiferal tests or bulk sediments [START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF]; Figure 3c). The archival ε Nd values from the major part of the Atlantic Ocean (except for the northern North Atlantic; [START_REF] Elmore | Testing the extraction of past seawater Nd isotopic composition from North Atlantic deep sea sediments and foraminifera[END_REF] show general agreement with the predicted values, suggesting that this proxy is reliable. In contrast, some areas of the Pacific Ocean are characterized by positive offsets (Figure 3c). One possible reason for the offsets is the contribution of porewater Nd [START_REF] Du | Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction[END_REF][START_REF] Skinner | Rare Earth Elements in early-diagenetic foraminifer 'coatings': Pore-water controls and potential palaeoceanographic applications[END_REF] that is more evident in the Pacific because the Nd isotopic composition of porewater could be significantly higher than that of the bottom water (Figure 3d). The detrital fraction can contain exchangeable Nd such as volcanogenic matter, Fe-Mn coating, and clay minerals that release Nd to porewater during early diagenesis [START_REF] Abbott | Are clay minerals the primary control on the oceanic Rare Earth Element budget[END_REF][START_REF] Du | Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction[END_REF].

It is possible that the predicted ε Nd values do not perfectly represent bottom water isotopic composition because the equations are based on empirical relationships established using limited seawater data. Nevertheless, a possible porewater Nd contribution should be taken into account in the interpretation of Nd isotopic composition, in particular, at sites where recent marine sediment values significantly differ from the seawater values. Additional process studies and further modeling efforts are required to better understand the possible contribution of local Nd inputs to the ocean. For paleoceanography studies, a multiproxy approach is recommended to obtain robust reconstructions. If proxy response diverges from predicted values, a better understanding of proxy behavior will provide new insights into overlooked processes.

CLIMATE TRANSITION DURING THE MID-PLEISTOCENE AND POSSIBLE OCEAN CIRCULATION CHANGES

In this final section, we reconstruct seawater ε Nd by studying cores from the eastern Atlantic in order to obtain insight into the role of ocean circulation changes during the MPT (1,200-800 kyr; Figure 4). The MPT is marked by an amplification of the glacial/interglacial cycle with the development of longer and more severe glacial periods and a shift from a 41 kyr to a 100 kyr cycle [START_REF] Clark | The middle Pleistocene transition: Characteristics, mechanisms, and implications for long-term changes in atmospheric pCO 2[END_REF] (Figure 4a). Because insolation forcing does not explain such changes (Figure 4b), internal feedbacks are of primary importance.

Among possible mechanisms explaining the MPT, one hypothesis is regolith removal that allowed the formation of thicker and more stable ice sheets by increasing basal stability [START_REF] Clark | The middle Pleistocene transition: Characteristics, mechanisms, and implications for long-term changes in atmospheric pCO 2[END_REF] and subsequent atmospheric circulation changes. A recent study suggests that changes in ice sheet dynamics alone could not explain the MPT and that the combination of regolith removal and reduced atmospheric CO 2 concentration by an efficient biological carbon [START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF] as an indicator of glacial/interglacial cycles. (b) Summer insolation at 65°N over the last 2 million years [START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. (c) Reconstructed atmospheric CO 2 concentration [START_REF] Hönisch | Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition[END_REF][START_REF] Bereiter | Revision of the EPICA Dome C CO 2 record from 800 to 600 kyr before present[END_REF][START_REF] Higgins | Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica[END_REF][START_REF] Chalk | Causes of ice age intensification across the Mid-Pleistocene Transition[END_REF][START_REF] Dyez | Early Pleistocene obliquity-scale pCO 2 variability at ~1.5 million years ago[END_REF]. (d) Benthic foraminiferal carbon isotopic composition (δ 13 C) record of core from Deep Sea Drilling Project (DSDP) site 552 from the northern Northeast Atlantic (56°N, 23°W, 2,300 m water depth) over the past 1.3 million years [START_REF] Raymo | Evolution of Atlantic-Pacific δ 13 C gradients over the last 2.5 m.y[END_REF] (2016); [START_REF] Hu | Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr[END_REF][START_REF] Pena | Thermohaline circulation crisis and impacts during the mid-Pleistocene transition[END_REF]. The green arrow along the y-axis indicates the present-day seawater ε Nd value estimated by the empirical equation using temperature and dissolved oxygen concentration [START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF]. The yellow band indicates the "900 ka event."

pump was necessary [START_REF] Chalk | Causes of ice age intensification across the Mid-Pleistocene Transition[END_REF].

In addition to the biological pump, the development of Southern Ocean stratification during glacial periods between 900 ka and 600 ka was suggested as a physical mechanism that permitted the storage of more carbon in the deep ocean and resulted in colder and longer glacials [START_REF] Hasenfratz | The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle[END_REF]. To date, a precise evaluation of the carbon cycle at this transition is not possible because of the scarcity of atmospheric CO 2 reconstruction for the MPT [START_REF] Hönisch | Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition[END_REF][START_REF] Bereiter | Revision of the EPICA Dome C CO 2 record from 800 to 600 kyr before present[END_REF][START_REF] Higgins | Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica[END_REF][START_REF] Chalk | Causes of ice age intensification across the Mid-Pleistocene Transition[END_REF][START_REF] Dyez | Early Pleistocene obliquity-scale pCO 2 variability at ~1.5 million years ago[END_REF]; Figure 4c). Regarding ocean circulation, benthic foraminiferal δ 13 C records show a prominent decrease centered at 900 ka, called the "900 ka event" on the global scale [START_REF] Raymo | Evolution of Atlantic-Pacific δ 13 C gradients over the last 2.5 m.y[END_REF][START_REF] Clark | The middle Pleistocene transition: Characteristics, mechanisms, and implications for long-term changes in atmospheric pCO 2[END_REF]; Figure 4d). This change is considered to be a major perturbation of oceanic carbon chemistry.

In order to investigate the ocean circulation changes at the 900 ka event, seawater ε Nd values were reconstructed at ODP sites 1267 (28°S, 4,400 m water depth), 1088 (41°S, 2,080 m water depth), and 1090 (43°S, 3,700 m water depth) [START_REF] Pena | Thermohaline circulation crisis and impacts during the mid-Pleistocene transition[END_REF][START_REF] Farmer | Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles[END_REF] in the southeastern Atlantic (Figures 4e and5a,b). We combine these ε Nd records with other reported data at ODP sites 1088 [START_REF] Hu | Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr[END_REF][START_REF] Dausmann | Glacial reduction of AMOC strength and long-term transition in weathering inputs into the Southern Ocean since the mid-Miocene: Evidence from radiogenic Nd and Hf isotopes[END_REF] and 1090 [START_REF] Howe | North Atlantic Deep Water production during the Last Glacial Maximum[END_REF] to extract the main features. ODP site 1267 is located north of Walvis Ridge where a bathymetric barrier minimizes the southern-sourced deepwater contribution to the basin. Thus, it is mostly influenced by North Atlantic Deep Water (NADW; [START_REF] Farmer | Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles[END_REF]. In contrast, ODP sites 1088 and 1090 are currently bathed by the southern-sourced water masses, Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW), with a possible contribution of Antarctic Bottom Water (AABW) at 1090 (Figure 5b). Reflecting these water masses, the present-day ε Nd value at ODP site 1267 is lower than those at sites 1088 and 1090, and the difference has been maintained for the past 1,200 ka (Figure 4e). In general, glacial ε Nd values are higher than interglacial values, and smaller glacial/interglacial ε Nd amplitude is observed at the three sites before the 900 ka event compared with the period after the MPT. At the 900 ka event, the reconstructed Nd isotopic composition is as high as in more recent glacial values. We define the high ε Nd values at the 900 ka event relative to the modern isotopic ratio as ε Nd anomalies.

We extended the spatial coverage of ε Nd anomalies at the 900 ka event by adding two new cores, MD03-2705 (18°N, 3,090 m water depth) and ODP 1085 (29°S, 1,730 m water depth) from the Northeast and Southeast Atlantic (Figure 5a,b). In the modern ocean, the location of core MD03-2705 is occupied by NADW, whereas the core from ODP site 1085 is located at the boundary between NADW and UCDW, with a higher ε Nd value (Figure 5b). It is worth mentioning that ε Nd values reconstructed from eastern Atlantic Holocene sediment archives generally agree with predicted seawater values (Figure 3c). Our new results and previous studies all indicate positive ε Nd anomalies at the 900 ka event. The details of new records are combined with epibenthic foraminiferal stable isotope records and modeling experiments in recent work of author Tachikawa and colleagues.

If we assume that the observed ε Nd shift is produced by circulation changes alone, a possible explanation is either a reduction in the formation of northernsourced water or enhanced formation of southern-sourced water. To evaluate these possibilities, we compare the distribution of the reconstructed ε Nd anomalies at the 900 ka event with simulated zonally averaged modeled Nd isotope anomalies using an Earth system model of intermediate complexity LOVECLIM [START_REF] Friedrich | Ocean circulation reconstructions from εNd: A model-based feasibility study[END_REF]; Figure 5c,d). The enhanced southern-sourced water formation and the reduction in northern-sourced water formation were simulated by removal of fresh water from the Southern Ocean and input of excess fresh water to the North Atlantic [START_REF] Friedrich | Ocean circulation reconstructions from εNd: A model-based feasibility study[END_REF]. The simulation indicates that enhanced deepwater formation in the Southern Ocean has little impact on the ε Nd values of intermediate and deep waters in the Atlantic Ocean (Figure 5c), whereas reduced NADW formation increases the seawater Nd isotopic signature in the major part of the Atlantic basin deeper than 1,000 m (Figure 5d). Comparison of the seawater ε Nd distribution pattern between the simulated and the reconstructed anomalies clearly demonstrates that the reduction in northern-sourced water formation better explains the change in seawater ε Nd . A source for excess freshwater is still to be elucidated. Perturbation of surface hydrology associated with the evolution of the Northern Hemisphere cryosphere could be a key process.

Alternatively, the observed increase in Nd isotopic composition at the 900 ka event can be explained by changes in Nd sources to the North Atlantic that have higher ε Nd values. The northern-sourced water mass with higher ε Nd values may have been transported toward the South Atlantic if the Atlantic meridional overturning circulation was not completely shut down. We may infer this possibility by comparing the 900 ka event with the LGM state. The 900 ka event is the first time over the past 1,200 ka when seawater ε Nd values were as high as the isotopic ratio of more recent glacial periods including the LGM (Figure 4e), and it is when the first drop in sea level comparable to that of the post-MPT glacial levels occurred (Figure 4a). The ε Nd value of northern-sourced water has been assumed to be constant during the late Pleistocene glacial/interglacial cycles, but several modeling studies suggest the possibility of a more radiogenic glacial Nd isotopic ratio for northern-sourced water [START_REF] Arsouze | A modeling sensitivity study of the influence of the Atlantic meridional overturning circulation on neodymium isotopic composition at the Last Glacial Maximum[END_REF][START_REF] Menviel | Enhanced mid-depth southward transport in the northeast Atlantic at the Last Glacial Maximum despite a weaker AMOC[END_REF]. For instance, the contribution of Labrador Sea water with a very low ε Nd value (Figure 1) to the northern-sourced water might have declined during the LGM because of sea ice coverage that prevented deep convection in the Labrador Sea, leading to a higher ε Nd value in northern-sourced water [START_REF] Arsouze | A modeling sensitivity study of the influence of the Atlantic meridional overturning circulation on neodymium isotopic composition at the Last Glacial Maximum[END_REF]. If such a modification of northernsourced water occurred, seawater ε Nd becomes generally more radiogenic in both reduced NADW and enhanced Southern Ocean deepwater formation cases (recent work of author Tachikawa " The increase in high-quality data concerning seawater Nd isotopic composition in the modern ocean confirms the capacity of this proxy to trace large-scale ocean circulation patterns.

"

. and colleagues). This hypothesis should be further examined with direct evidence for changes in the deepwater convection zone in the North Atlantic at the 900 ka event and reconstruction of sea ice configuration at that period. The evolution of the Northern Hemisphere cryosphere is estimated to have been involved in the mechanism of the 900 ka event either via reduced deepwater formation or changes in Nd sources to the North Atlantic. Better spatial coverage of records including data from the different ocean basins and the combination of Nd isotopic composition with other proxies will further clarify the processes involved.

CONCLUSIONS

The increase in high-quality data concerning seawater Nd isotopic composition in the modern ocean confirms the capacity of this proxy to trace large-scale ocean circulation patterns. The tropical Pacific Ocean, which is potentially affected by regional/local lithogenic Nd inputs, is characterized by the presence of labile volcanogenic matter and less than vigorous bottom water circulation. The ε Nd offsets between the sedimentary record and predicted values can be explained at least partly by a porewater Nd contribution of diagenetic origin that varies spatially.

At the 900 ka event, the eastern Atlantic records indicate a positive shift in the reconstructed seawater ε Nd values relative to the modern values in both the northern and the southern basins. This shift can be explained either by a reduction in North Atlantic Deep Water formation and/or by changes in the Nd source to the North Atlantic with higher ε Nd values relative to the evolution of the Northern Hemisphere cryosphere. More active southern-sourced water formation alone cannot account for the observed changes in Nd isotopic composition.

  Figure 1). The Southern Ocean and Indian Ocean present deepwater ε Nd values intermediate between the North Atlantic and the tropical and North Pacific Oceans, which is consistent with the pattern of global thermo-

FIGURE 2 .

 2 FIGURE 2. Neodymium sources and processes affecting oceanic Nd concentration and isotopic composition as well as possible marine sedimentary records of Nd isotopic composition. Figure based on data from GEOTRACES (http://www. geotraces.org/) modified to summarize the oceanic Nd cycle.

FIGURE 3 .

 3 FIGURE 3. Comparison of observed and predicted values for Nd isotopic composition. (a) Spatial distribution of ε Nd offsets between modern deepwater (≥ 1,500 m depth) and predicted values using empirical equations (Tachikawa et al., 2017). (b) Relationship between measured deepwater Nd isotopic composition and predicted values. (c) Spatial distribution of ε Nd offsets between archived Holocene (≤10 kyr) sediments and predicted seawater values. (d) Schematic presentation showing the porewater Nd contribution that creates ε Nd offsets between archival and predicted values. The white dashed ellipse in (a) and the black solid ellipse in (b) indicate the observed ε Nd offsets in the tropical Pacific. (a), (b), and (c) are adapted from Tachikawa et al. (2017) and (d) is adapted from[START_REF] Du | Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction[END_REF]

FIGURE 4 .

 4 FIGURE 4. Variability across the Mid-Pleistocene transition (MPT). (a) Stack of benthic foraminiferal oxygen isotopic composition (δ 18 O) over the last 2 million years[START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF] as an indicator of glacial/interglacial cycles. (b) Summer insolation at 65°N over the last 2 million years[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. (c) Reconstructed atmospheric CO 2 concentration[START_REF] Hönisch | Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition[END_REF][START_REF] Bereiter | Revision of the EPICA Dome C CO 2 record from 800 to 600 kyr before present[END_REF][START_REF] Higgins | Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica[END_REF][START_REF] Chalk | Causes of ice age intensification across the Mid-Pleistocene Transition[END_REF][START_REF] Dyez | Early Pleistocene obliquity-scale pCO 2 variability at ~1.5 million years ago[END_REF]. (d) Benthic foraminiferal carbon isotopic composition (δ 13 C) record of core from Deep Sea Drilling Project (DSDP) site 552 from the northern Northeast Atlantic (56°N, 23°W, 2,300 m water depth) over the past 1.3 million years[START_REF] Raymo | Evolution of Atlantic-Pacific δ 13 C gradients over the last 2.5 m.y[END_REF] as an example of oceanic δ 13 C changes. (e) Previously published seawater ε Nd records across the Mid-Pleistocene transition at three sites in the eastern Atlantic: Ocean Drilling Program (ODP) sites 1088 (41°S, 13°E, 2,080 m) and 1090 (43°S, 9°E, 3,700 m), and ODP site 1267 (28°S, 2°E, 4,400 m). Data are from[START_REF] Dausmann | Glacial reduction of AMOC strength and long-term transition in weathering inputs into the Southern Ocean since the mid-Miocene: Evidence from radiogenic Nd and Hf isotopes[END_REF];[START_REF] Farmer | Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles[END_REF];[START_REF] Howe | North Atlantic Deep Water production during the Last Glacial Maximum[END_REF];[START_REF] Hu | Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr[END_REF][START_REF] Pena | Thermohaline circulation crisis and impacts during the mid-Pleistocene transition[END_REF]. The green arrow along the y-axis indicates the present-day seawater ε Nd value estimated by the empirical equation using temperature and dissolved oxygen concentration[START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF]. The yellow band indicates the "900 ka event."

  FIGURE 4. Variability across the Mid-Pleistocene transition (MPT). (a) Stack of benthic foraminiferal oxygen isotopic composition (δ 18 O) over the last 2 million years[START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF] as an indicator of glacial/interglacial cycles. (b) Summer insolation at 65°N over the last 2 million years[START_REF] Laskar | A long-term numerical solution for the insolation quantities of the Earth[END_REF]. (c) Reconstructed atmospheric CO 2 concentration[START_REF] Hönisch | Atmospheric carbon dioxide concentration across the Mid-Pleistocene Transition[END_REF][START_REF] Bereiter | Revision of the EPICA Dome C CO 2 record from 800 to 600 kyr before present[END_REF][START_REF] Higgins | Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica[END_REF][START_REF] Chalk | Causes of ice age intensification across the Mid-Pleistocene Transition[END_REF][START_REF] Dyez | Early Pleistocene obliquity-scale pCO 2 variability at ~1.5 million years ago[END_REF]. (d) Benthic foraminiferal carbon isotopic composition (δ 13 C) record of core from Deep Sea Drilling Project (DSDP) site 552 from the northern Northeast Atlantic (56°N, 23°W, 2,300 m water depth) over the past 1.3 million years[START_REF] Raymo | Evolution of Atlantic-Pacific δ 13 C gradients over the last 2.5 m.y[END_REF] as an example of oceanic δ 13 C changes. (e) Previously published seawater ε Nd records across the Mid-Pleistocene transition at three sites in the eastern Atlantic: Ocean Drilling Program (ODP) sites 1088 (41°S, 13°E, 2,080 m) and 1090 (43°S, 9°E, 3,700 m), and ODP site 1267 (28°S, 2°E, 4,400 m). Data are from[START_REF] Dausmann | Glacial reduction of AMOC strength and long-term transition in weathering inputs into the Southern Ocean since the mid-Miocene: Evidence from radiogenic Nd and Hf isotopes[END_REF];[START_REF] Farmer | Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles[END_REF];[START_REF] Howe | North Atlantic Deep Water production during the Last Glacial Maximum[END_REF];[START_REF] Hu | Neodymium isotopic evidence for linked changes in Southeast Atlantic and Southwest Pacific circulation over the last 200 kyr[END_REF][START_REF] Pena | Thermohaline circulation crisis and impacts during the mid-Pleistocene transition[END_REF]. The green arrow along the y-axis indicates the present-day seawater ε Nd value estimated by the empirical equation using temperature and dissolved oxygen concentration[START_REF] Tachikawa | The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data[END_REF]. The yellow band indicates the "900 ka event."

FIGURE 5 .

 5 FIGURE 5. Reconstructed seawater ε Nd records from the eastern Atlantic. (a) Core location map in the Atlantic Ocean. The two red stars indicate cores from MD03-2705 (18°N, 21°W, 3,085 m water depth) and ODP site 1085 (29°S, 14°E, 1,725 m) studied here. The three white stars indicate the sites of previous studies: ODP sites 1088, 1090, and 1267. The black x shows DSDP site 552. (b) Latitudinal transect of present-day seawater ε Nd values in the Atlantic Ocean (Howe et al., 2016) with the main water masses: NADW = North Atlantic Deep Water; UCDW and LCDW = Upper and Lower Circumpolar Deep Water, AABW = Antarctic Bottom Water, AAIW = Antarctic Intermediate Water. More seawater ε Nd data are available GEOTRACES IDP2017 (https://www.bodc.ac.uk/geotraces/data/idp2017/), (c) Comparison between reconstructed and zonally averaged simulated seawater ε Nd anomalies in the case of enhanced southern-sourced water formation. The orange stars mark the sites of reconstructed seawater ε Nd with positive anomalies at the 900 ka event. Orange and blue shading corresponds to oceanic zones characterized by positive and negative simulated ε Nd anomalies. (d) The same as (c) in the case of reduced northern-sourced water formation. The simulation results are from Friedrich et al., 2014.

ACKNOWLEDGMENTS

KT's contribution to this work was supported by two French National LEFE projects, RICH (IMAGO) and NEOSYNMPA (CYBER). Samples were provided by the International Marine Global Change Study (IMAGES) program and the Ocean Drilling Program (ODP). The reviews by two anonymous reviewers were very helpful. The EQUIPEX ASTER-CEREGE project (Pierre Deschamps, one of the PIs) is acknowledged for Nd isotopic measurements. Hélène Mariot and Marta Garcia are thanked for technical support. The joint workshop PAGES-GEOTRACES provided a great opportunity to discuss the behavior of Nd isotopes in modern and past oceans.

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