Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating

Accepted Manuscript Title: Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating Author: Qingfeng Ye Kai Feng Zhuguo Li Fenggui Lu Ruifeng Li Jian Huang Yixiong Wu PII: DOI: Reference:

S0169-4332(16)32619-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.11.176 APSUSC 34481

To appear in:

APSUSC

Received date: Revised date: Accepted date:

26-8-2016 21-11-2016 22-11-2016

Please cite this article as: Qingfeng Ye, Kai Feng, Zhuguo Li, Fenggui Lu, Ruifeng Li, Jian Huang, Yixiong Wu, Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.11.176 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating

Qingfeng Ye a,b), Kai Feng a,b) *, Zhuguo Li a,b) *, Fenggui Lu a,b), Ruifeng Li c), Jian Huang a,b), Yixiong Wu, a,b)

a)

Shanghai Key laboratory of Materials Laser Processing and Modification, School of

Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b)

Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration,

Shanghai, 200240, China c)

School of Materials Science and Engineering, Jiangsu University of Science and

Technology, Zhenjiang, Jiangsu, 212003, China *Corresponding Authors: Dr. Kai Feng Tel.: +86 21 54745878; E-mail: [email protected]

Prof. Zhuguo Li Tel.: +86 21 54745878; fax: +86 21 34203024 E-mail: [email protected]

Graphical abstract

Highlights

 Equimolar CrMnFeCoNi high entropy alloy coating are prepared by laser cladding.  The cladding layer forms a simple FCC phase solid solution with identical dendritic structure.  The cladding layer exhibits a noble corrosion resistance in both 3.5 wt. % NaCl and 0.5M sulfuric acid.  Element segregation makes Cr-depleted interdendrites the starting point of corrosion reaction.

Abstract

Equimolar CrMnFeCoNi high entropy alloy (HEA) is one of the most notable single phase multi-component alloys up-to-date with promising mechanical properties at cryogenic temperatures. However, the study on the corrosion behavior of CrMnFeCoNi HEA coating has still been lacking. In this paper, HEA coating with a nominal composition of CrMnFeCoNi is fabricated by laser surface alloying and studied in detail. Microstructure and chemical composition are determined by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are used to investigate the corrosion behavior. The coating forms a simple FCC phase with an identical dendritic structure composed of Fe/Co/Ni-rich dendrites and Mn/Ni-rich interdendrites. Both in 3.5 wt.% NaCl solution and 0.5 M sulfuric acid the coating exhibits nobler corrosion resistance than A36 steel substrate and even lower icorr than 304 stainless steel (304SS). EIS plots coupled with fitted parameters reveal that a spontaneous protective film is formed and developed during immersion in 0.5 M sulfuric acid. The fitted Rt value reaches its maximum at 24 h during a 48 hours’ immersion test, indicating the passive film starts to break down after that. EDS analysis conducted on a corroded surface immersed in 0.5 M H2SO4 reveals that corrosion starts from Cr-depleted interdendrites.

Keywords: High entropy alloy; Laser surface alloying; Microstructure; Element segregation; Corrosion behavior.

1. Introduction In the past, the development of traditional alloy systems, such as iron-, aluminum-, nickel-, magnesium-based alloy, has been based on one major element, of which concentration is remarkably higher than other minor alloying elements. Generally speaking, with the increase of numbers of alloying elements and concentration of minor elements, the alloy system tends to form more unstable and fragile intermetallic phase[1], which restricts the design concept of alloy systems to this one-principalelements mode for a long time. A novel concept of high entropy alloy (HEA), which was defined by Yeh et al. [2] as alloys composed of 5 or more alloying elements with concentration of each at 5 to 35 at.%, represents an innovative alloy design strategy. In Yeh’s statements, the high entropy effect restrained the formation of intermetallic compounds and stabilized solid solution structure in the center of phase diagram. Still, despite this, only several particular HEA systems have the ability to form a mono-phase solid solution structure and the equimolar CrMnFeCoNi system is a case in point [3-15]. CrMnFeCoNi is one of the most notable HEAs up-to-date and has attracted many researchers’ interest since first identified by Cantor in 2004 [16]. Although Cr (BCC), Fe (BCC), Ni (FCC), Co (HCP) and Mn (complex A12 structure) each has a different structure respectively, their equimolar system surprisingly forms a simple FCC monophase structure, which may be attributed to some unveiling mechanisms besides high entropy effect [17, 18]. According to a growing number of researches done in last few

years, the alloy shows unique and excellent properties differing from normal FCC metals, especially an excellent combination of strength, ductility and fracture toughness in cryogenic temperatures makes it a promising material to date [17, 19-27]. However, most existed published works have done in bulk material state. Benefiting its excellent damage tolerance, using it as a coating material will not only broaden its application fields but also lower the cost. Laser surface alloying and laser cladding technique with an extremely rapid solidification rate are outstanding approaches to fabricate HEA coating with following advantages: controllable dilution ratio, small thermal deformation, metallurgical bonding between coating and substrate, and relieved compositional segregation during non-equilibrium solidification [28-32]. Besides, detailed investigations have been carried out on mechanic properties in previous studies, while, more importantly as coating material, corrosion behavior still respectively remains unknown. In this work, HEA coating with a nominal composition of CrMnFeCoNi is fabricated by laser surface alloying and its microstructure and corrosion behavior in both 3.5 wt.% NaCl solution and 0.5 M sulfuric acid solution are investigated.

2. Experimental procedures 2.1 Preparation of HEA coating 10 mm thick A36 steel was chosen as substrate, and the chemical composition was presented in Table 1. The surface was polished to #400 to remove oxide and other

pollutions and then cleaned by acetone and dried. Raw materials, Cr, Mn, Co, Ni powders with high purity (≥ 99.5 %), were mixed by a high energy planetary ball milling machine with compositions in Table 1 to form an equimolar HEA coating. The chemical composition of the power was experimentally decided considering the dilution of substrate and evaporation during laser surface alloying process. The mixed powder was pre-placed on the substrate with a thickness of 2 mm. The coatings were fabricated by a 3.5 kW high power diode laser (Rofin DL-035Q) cladding system. The wavelength of the laser beam is in the range from 808 nm to 940 nm, and the spot size of the laser at the focal length (165 mm) is 2.5 mm × 6.5 mm. After a series of experiments, the parameters were optimized as 2 kW laser power with scan rate at 2 mm/s. High-purity argon gas was used as shielding gas to prevent oxidation through both a coaxial nozzle and a trail nozzle. 2.2 Structure analysis The specimens were sectioned into a suitable size, mounted, polished and etched by aqua regia for metallographic examination. The microstructure and morphology of the coatings were characterized using a Zeiss Axioplan 2 optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM JEOL JEM-7600F) equipped with Inca Energy energy-dispersive spectrometer (EDS). The phases of the coatings were determined by X-ray diffraction with an Ultima IV diffractometer. The radiation source was Cu Kα, operated at 40 kV and 30 mA with a scanning rate of 1°/min, from 10° to 100°.

2.3 Electrochemical measurements The corrosion specimens were sectioned, polished with SiC sandpaper from 80 grit to 2000 grit and mounted into the holder with an exposed facet in a size of 4 × 10 mm. The electrochemical tests were conducted in both 3.5 wt.% NaCl solution and 0.5 M sulfuric acid at 25 °C under atmospheric pressure, referred as salt and acid solution environment. Both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed in typical three electrode cell where a specimen was functioned as working electrode, a platinum sheet as counter electrodes and a saturated calomel electrode (SCE) with E=0.2415VSHE and a Hg/Hg2SO4 (saturated K2SO4) with E=0.658VSHE as reference electrode in NaCl solution and sulfuric acid solution respectively, to avoid anion contamination. In order to avoid ambiguity, all potential values are converted into VSHE in the following discussion. The potential and current were measured and recorded by a computer-controlled Thales Zennium potentiostat continuously. Before potentiodynamic polarization and EIS measurements, the open current potential (OCP) was recorded for 1 hour to ensure a steady-state potential. Potentiodynamic polarization curves were tested and plotted at a scan rate of 1 mV/s started from -0.5 VSHE to a final potential of 1 VSHE in NaCl solution and 2.5 VSHE in sulfuric acid versus the open current potential. After immersed in 0.5 M sulfuric acid for different time, the EIS were recorded in a range of frequencies from 10 kHz to 10 mHz. The amplitude of sinusoidal signals was 10 mV around the OCP. To study the morphology and chemical composition of the corroded surface, 5-day immersion tests

in 0.5 M sulfuric acid were also conducted. 2.4 Surface morphology and chemical analysis Following the electrochemical measurements and immersion tests, the specimens were cleaned by ultra-sonic washing and dried in a drying cabinet. Immediately thereafter, the morphology of corroded surface of each specimen was examined by SEM. The chemical compositions were analyzed by EDS. 3. Results and discussion Fig. 1 shows the X-ray diffraction pattern of the CrMnFeCoNi coating. As can be seen, the coating formed a simple FCC structure with diffraction peaks at 43.68°， 50.70°，74.62°，90.32°，95.86 °. The lattice constant is calculated to be 3.596 Å by linear extrapolation method. As presented in Fig. 2, the coating forms a dendritic structure, without any voids or cracks. The dendrite growth direction is mainly affected by the heat flux in the melting pool. In the bottom area of the coating, the growth direction of column dendrite tends to be perpendicular to the surface of substrate because of the giant temperature gradient. As for the top area, the heat flux is controlled by the movement of laser beam largely, which makes the growth direction more parallel to the surface of substrate, respectively. As a result of the biggest temperature gradient (G) with a slow growth rate (V) at meantime, a planar crystal structure is formed in the joint area between the coating and substrate [8]. Chemical composition analysis was conducted by EDS. As presented in Fig. 3a, five

alloy elements are well distributed inside the coating and have formed a near equiatomic high entropy alloy coating uniformly. Concentration of major elements is presented in Table 2, from which element segregation can be found between dendrites and interdendrites. The result of line scanning across dendrites presented in Fig. 3b show the element segregation in a clearer way that Mn and Ni are enriched in interdendrites. The main reason may be the difference of melting point. Since melting point of Cr, Fe, Co are 1495 °C, 1857 °C and 1535 °C while 1244 °C and 1453 °C for Mn and Ni so that Mn and Ni with a relative lower melting point were enriched in interdendrites. This result was coordinated to results from studies on as-cast alloy by Cantor and Salishchev et al. [16, 33]. According to their results of EPMA and DSC measurements, Methilde et. al obtained a schematic phase diagram of the CrFeCo-MnNi system by analogy with binary alloy, giving an explanation that Cr/Fe/Co-rich dendrites are first formed before interdendrites enriched in Mn and Ni appear [4]. Potentiodynamic polarization curves of HEA coating in 3.5 wt.% NaCl and 0.5 M sulfuric acid are presented in Fig. 4a and Fig. 4b. As comparison, curves for 304 stainless steel and substrate (A36 steel) are also plotted together. Kinetic parameter can be calculated from the polarization curves through Butler-Volmer analysis using a linear fit method. In 3.5 wt.% NaCl solution, as we can see from the curves, HEA coating and 304SS show a significant nobler corrosion resistance than the substrate material. The polarization curves reveal that both 304SS and CrMnFeCoNi HEA coating have a

tendency of being “spontaneously passive” [34]. Calculated electrochemical parameters are listed in Table 3. The corrosion potential of HEA coating is slightly higher than that of 304SS, along with a similar corrosion current density. However, HEA coating shows a narrower window of passivity than 304SS when the potential continues to increase after Ecorr. Epit is the potential value where the current is suddenly increased, which means the protective passive film start to pit. The Epit of HEA coating and 304SS is 0.218 VSHE and 0.612 VSHE, respectively, proving that the passive film of CrMnFeCoNi HEA coating has worse corrosion resistance against attack of Cl- ion. After polarization measurement, the corroded surface of CrMnFeCoNi coating was ultrasonic cleaned and observed by SEM. SEM images (Fig. 5) prove that a typical pitting corrosion is occurred. Cl- ions attack the passive film by replacing O element and producing soluble product, resulting pits and holes on the passive film and consequently trigger the corrosion [35]. In 0.5 M sulfuric acid environment, all of those three samples are showing strong active-passive corrosion behavior as Fig. 4b shown. Electrochemical parameters calculated from electrochemical measurements are listed in Table 4. Due to a worse corrosion resistance, A36 steel substrate shows a more negative corrosion potential of -0.29 VSHE and a higher corrosion current density of 204 μA/cm2, which is larger than that of HEA and 304SS by more than one order of magnitude. As for HEA and 304SS, HEA coating exhibits a higher Ecorr and a lower icorr, though the icrit, the maximum anodic current density, of CrMnFeCoNi coating is much higher than that of 304SS.

With the increase of the potential, current density falls suddenly. Both HEA and 304SS have a wide range of passive zone in 0.5 M sulfuric acid solution. Though the range of passive window of HEA is still narrower than that of 304SS, a smaller ipass is obtained, denoting a better passive film protection restraining corrosion reactions in this zone. Different from the identical pitting behavior in NaCl solution, SEM images (Fig. 6) reveal a generous corroded surface with the absence of Cl-. Additionally, a strong secondary passivation can be observed from the polarization curve of HEA. Since the phenomenon was not observed on as-cast CrFe1.5MnNi0.5 according to studies of C. P. Lee et al.[36], the formation and dissolution of Co oxide film may be responsible for that [37].

To investigate the evolution of the corrosion resistance during immersion, electrochemical

impedance

spectroscopy

measurements

are

conducted

on

CrMnFeCoNi HEA samples that are immersed in 0.5 M H2SO4 solution for different periods. The evolution of Nyquist diagram is presented in Fig. 7. As illustrated in this figure, during the time of exposure in sulfuric solution, the plots presented a semicircular shape in the complex impedance plane, approximately. As the immersion time increased from 1 hour to 48 hours, the size of the semicircles varies with time. In the first 24 hours, the semicircle radius increased monotonously. After that, the semicircle began to shrink. Quantitative analysis is based on fitting the experimental data to equivalent circuit model RS(Q(Rt(RLL))) (Fig. 8), which has been used in simulating EIS of iron-acid interface previously [38]. In this model, RS is the impedance

of the solution between the sample and the counter electrode, which is around 2.5 ± 0.3 Ohm·cm2 in this case. Rt-Q can be attributed to charge transfer reaction. In the light of the nonideal capacity for double-layer in the real system, a constant phase element Q is used [39]. The impedance of Q is given by 𝑍𝑄 = 𝑌 −1 (𝑗𝜔)−𝑛

(1)

where Y is proportionality factor, j is imaginary unit, ω is angular frequency, and n is phase shift, ranging from 0-1. When n = 0, ZQ present a resistance R = Y-1; when n = 1, Q present a pure capacitor with C = Y. Considering the inductive behavior in the tail (Z’’< 0 at low frequencies), which is attributed to relaxation process obtained by adsorbed sulfate ions and protons [40, 41], RL-L in parallel is added into the model, of which RL represents the resistance that is associated with the inductive processes and L denotes the pseudo-inductance during the same process[42]. Fitting parameters are listed in Table 5 and some of them are plotted in Fig. 9 to show the evolution intuitively. As displayed, the variation of Y and Rt against to immersion time shows inverse relationship. According to the Helmholtz model[43]: 𝑌≈𝐶=

𝜀0 𝜀 𝛿

𝑆

(2)

where, δ is the thickness of the adsorptive layer, ε0 is the permittivity of a vacuum, ε is the dielectric constant of the medium, and S is the surface area of the electrode, the change of 1/Y denotes a change in the thickness of adsorptive layer proportionally [44]. 2

n

As the immersion time increases, the Y value decreases from 1.23×104 S·cm ·s to

2

n

1.07×104 S·cm ·s and increases again after 36 hours’ immersion, denoting the change of the thickness of adsorptive layer. The increases of the Rt value in the first 24 hours, from 500 Ohm·cm2 (1 h) to 1.72×103 Ohm·cm2 (24 h), can be explained by the formation and development of protective oxide film. Thereafter, the Rt value decreases to 1.61×103 Ohm·cm2 (36 h) and then 1.40×103 Ohm·cm2 (48 h), implying electrochemical reactions re-activated and localize damage this passive film occurs. In order to study the long time immersion behavior of laser cladded HEA coating, sample immersed for 5 days is chosen for SEM examination. As seen in Fig. 10, the surface of CrMnFeCoNi coating has started to be corroded. Chemical composition analysis for different areas was conducted using EDS. Section A represents for lesscorroded area with a relative flat surface remained; section B is the area near the corroded holes, standing for the place where corrosion has begun. Each type of area was measured on more than three different spots and the results are listed in Table 6. According to the EDS results, section A can be inferred to be Cr-rich dendrites while section B with a relatively high concentration of Mn and Ni can be determined as interdendrites. In the light of a lower concentration of Cr, the protective film of interdendrites is weaker than that of dendrites, making interdendrites become anode against towards Cr-rich dendrites. As a result of coupling effect between interdendrites anode and dendrites cathode, section B become the start point where corrosion reactions begin. Similar phenomenon has also been found on other high entropy alloys such as AlCoCrFeNi [45].

4. Conclusions Crack-free CrMnFeCoNi high entropy alloy coating was fabricated using laser surface alloying technique with mixed powders preplaced on the substrate. The coating with a simple FCC phase structure is mainly composed of columnar dendrites and forms a good metallurgical bonding with the substrate. Element segregation is observed in the coating between dendrites and interdendrites: Mn and Ni with lower melting point are enriched in interdendrites. Potentiodynamic polarization tests in 3.5 wt.% NaCl and 0.5 M H2SO4 indicate that HEA coating can provide an excellent corrosion resistance with nobler Ecorr and a one-magnitude-smaller icorr value than A36 steel substrate. The corrosion resistance of HEA coating is similar to that of 304 stainless steel except for a narrower passive zone in both NaCl solution and sulfuric acid solution. EIS measurements on samples immersed in 0.5 M sulfuric acid for different periods show the evolution of corrosion behavior. A growing Rt is observed with the immersion time increased from 1 hour to 24 hours and begin to drop down after 36-hours exposure in sulfuric acid solution, which means the passive film starts to break down. Cr-depleted interdendrites are proved the starting point of corrosion by EDS analysis on the corroded surface of immersed samples.

Acknowledgements Financial support was jointly provided by the National Natural Science Foundation of China under grant number 51201106, “Chenguang” project (Grant Number 13CG07)

and “Shuguang” project (Grant Number 12SG15) supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation, “Chenxing” young scholar project of Shanghai Jiao Tong University (Grant Number 14X100010017).

References [1] A.L. Greer, Confusion by design, Nature, 366 (1993) 303-304. [2] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303. [3] E.W. Huang, D. Yu, J.-W. Yeh, C. Lee, K. An, S.-Y. Tu, A study of lattice elasticity from low entropy metals to medium and high entropy alloys, Scr. Mater., 101 (2015) 32-35. [4] M. Laurent-Brocq, A. Akhatova, L. Perrière, S. Chebini, X. Sauvage, E. Leroy, Y. Champion, Insights into the phase diagram of the CrMnFeCoNi high entropy alloy, Acta Mater. 88 (2015) 355-365. [5] C.C. Tasan, Y. Deng, K.G. Pradeep, M.J. Yao, H. Springer, D. Raabe, Composition Dependence of Phase Stability, Deformation Mechanisms, and Mechanical Properties of the CoCrFeMnNi High-Entropy Alloy System, JOM 66 (2014) 1993-2001. [6] G. Laplanche, U.F. Volkert, G. Eggeler, E.P. George, Oxidation Behavior of the CrMnFeCoNi High-Entropy Alloy, Oxid. Met. (2016) 1-17. [7] M.J. Yao, K.G. Pradeep, C.C. Tasan, D. Raabe, A novel, single phase, nonequiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility, Scr. Mater. 72-73 (2014) 5-8. [8] H. Zhang, Y. Pan, Y.-Z. He, J.-L. Wu, T.M. Yue, S. Guo, Application Prospects and Microstructural Features in Laser-Induced Rapidly Solidified High-Entropy Alloys,

JOM 66 (2014) 2057-2066. [9] K.Y. Tsai, M.H. Tsai, J.W. Yeh, Sluggish diffusion in Co-Cr-Fe-Mn-Ni high-entropy alloys, Acta Mater. 61 (2013) 4887-4897. [10] P.P. Bhattacharjee, G.D. Sathiaraj, M. Zaid, J.R. Gatti, C. Lee, C.-W. Tsai, J.-W. Yeh, Microstructure and texture evolution during annealing of equiatomic CoCrFeMnNi high-entropy alloy, J. Alloys Compd. 587 (2014) 544-552. [11] W.H. Liu, Y. Wu, J.Y. He, T.G. Nieh, Z.P. Lu, Grain growth and the Hall-Petch relationship in a high-entropy FeCrNiCoMn alloy, Scr. Mater., 68 (2013) 526-529. [12] B. Gludovatz, E.P. George, R.O. Ritchie, Processing, Microstructure and Mechanical Properties of the CrMnFeCoNi High-Entropy Alloy, JOM 67 (2015) 22622270. [13] W. Shaoqing, Atomic Structure Modeling of Multi-Principal-Element Alloys by the Principle of Maximum Entropy, Entropy 15 (2012) 5536-5548. [14] M.C. Gao, D.E. Alman, Searching for Next Single-Phase High-Entropy Alloy Compositions, Entropy 15 (2013) 4504-4519. [15] N. Stepanov, M. Tikhonovsky, N. Yurchenko, D. Zyabkin, M. Klimova, S. Zherebtsov, A. Efimov, G. Salishchev, Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy, Intermetallics, 59 (2015) 8-17. [16] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375-377 (2004) 213-218. [17] F. Otto, Y. Yang, H. Bei, E.P. George, Relative effects of enthalpy and entropy on

the phase stability of equiatomic high-entropy alloys, Acta Mater. 61 (2013) 2628-2638. [18] Z. Wang, Y. Huang, Y. Yang, J. Wang, C.T. Liu, Atomic-size effect and solid solubility of multicomponent alloys, Scr. Mater. 94 (2015) 28-31. [19] G. Laplanche, P. Gadaud, O. Horst, F. Otto, G. Eggeler, E.P. George, Temperature dependencies of the elastic moduli and thermal expansion coefficient of an equiatomic, single-phase CoCrFeMnNi high-entropy alloy, J. Alloys Compd. 623 (2015) 348-353. [20] A. Haglund, M. Koehler, D. Catoor, E.P. George, V. Keppens, Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures, Intermetallics 58 (2015) 62-64. [21] Y.Z. Xia, H. Bei, Y.F. Gao, D. Catoor, E.P. George, Synthesis, characterization, and nanoindentation response of single crystal Fe-Cr-Ni alloys with FCC and BCC structures, Mater. Sci. Eng. A 611 (2014) 177-187. [22] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428-441. [23] Z. Wu, H. Bei, F. Otto, G.M. Pharr, E.P. George, Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys, Intermetallics 46 (2014) 131-140. [24] F. Otto, N.L. Hanold, E.P. George, Microstructural evolution after thermomechanical processing in an equiatomic, single-phase CoCrFeMnNi highentropy alloy with special focus on twin boundaries, Intermetallics 54 (2014) 39-48.

[25] B. Gludovatz, A. Hohenwarter, D. Catoor, E.H. Chang, E.P. George, R.O. Ritchie, A fracture-resistant high-entropy alloy for cryogenic applications, Science 345 (2014) 1153-1158. [26] F. Otto, A. Dlouhy, C. Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi highentropy alloy, Acta Mater. 61 (2013) 5743-5755. [27] A. Gali, E.P. George, Tensile properties of high- and medium-entropy alloys, Intermetallics 39 (2013) 74-78. [28] H. Zhang, Y. Pan, Y.-Z. He, Synthesis and characterization of FeCoNiCrCu highentropy alloy coating by laser cladding, Mater. Des. 32 (2011) 1910-1915. [29] H. Zhang, Y. He, Y. Pan, Enhanced hardness and fracture toughness of the lasersolidified FeCoNiCrCuTiMoAlSiB0.5 high-entropy alloy by martensite strengthening, Scr. Mater. 69 (2013) 342-345. [30] X.-W. Qiu, Y.-P. Zhang, L. He, C.-g. Liu, Microstructure and corrosion resistance of AlCrFeCuCo high entropy alloy, J. Alloys Compd. 549 (2013) 195-199. [31] X. Ye, M. Ma, W. Liu, L. Li, M. Zhong, Y. Liu, Q. Wu, Synthesis and Characterization of High-Entropy Alloy AlXFeCoNiCuCr by Laser Cladding, Adv. Mater. Sci. Eng. (2011). [32] C. Huang, Y. Zhang, R. Vilar, J. Shen, Dry sliding wear behavior of laser clad TiVCrAlSi high entropy alloy coatings on Ti-6Al-4V substrate, Mater. Des. 41 (2012) 338-343.

[33] G.A. Salishchev, M.A. Tikhonovsky, D.G. Shaysultanov, N.D. Stepanov, A.V. Kuznetsov, I.V. Kolodiy, A.S. Tortika, O.N. Senkov, Effect of Mn and V on structure and mechanical properties of high-entropy alloys based on CoCrFeNi system, J. Alloys Compd. 591 (2014) 11-21. [34] Y. Qiu, M.A. Gibson, H.L. Fraser, N. Birbilis, Corrosion characteristics of high entropy alloys, Mater. Sci. Technol. 31 (2015) 1235-1243. [35] T. Poornima, N. Jagannatha, A. Nityananda Shetty, Studies on corrosion of annealed and aged 18 Ni 250 grade maraging steel in sulphuric acid medium, Portugaliae Electrochim. Acta 28 (2010) 173-188. [36] C.P. Lee, Y.Y. Chen, C.Y. Hsu, J.W. Yeh, H.C. Shih, Enhancing pitting corrosion resistance of AlxCrFe1.5MnNi0.5 high-entropy alloys by anodic treatment in sulfuric acid, Thin Solid Films 517 (2008) 1301-1305. [37] Y.L. Chou, J.W. Yeh, H.C. Shih, The effect of molybdenum on the corrosion behaviour of the high-entropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments, Corros. Sci. 52 (2010) 2571-2581. [38] E. Poorqasemi, O. Abootalebi, M. Peikari, F. Haqdar, Investigating accuracy of the Tafel extrapolation method in HCl solutions, Corros. Sci. 51 (2009) 1043-1054. [39] B.A. Boukamamp, Solid State Ionics 20 (1980). [40] M.A. Amin, S.S. Abd El-Rehim, E.E.F. El-Sherbini, R.S. Bayoumi, The inhibition of low carbon steel corrosion in hydrochloric acid solutions by succinic acid. Part I. Weight loss, polarization, EIS, PZC, EDX and SEM studies, Electrochim. Acta 52

(2007) 3588-3600. [41] M.A. Veloz, I. González, Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H2S, Electrochim. Acta 48 (2002) 135-144. [42] V. de Freitas Cunha Lins, G.F. de Andrade Reis, C.R. de Araujo, T. Matencio, Electrochemical impedance spectroscopy and linear polarization applied to evaluation of porosity of phosphate conversion coatings on electrogalvanized steels, Appl Surf Sci 253 (2006) 2875-2884. [43] M. Kissi, M. Bouklah, B. Hammouti, M. Benkaddour, Establishment of equivalent circuits from electrochemical impedance spectroscopy study of corrosion inhibition of steel by pyrazine in sulphuric acidic solution, Appl Surf Sci 252 (2006) 4190-4197. [44] K.M. Ismail, A.M. Fathi, W.A. Badawy, Effect of nickel content on the corrosion and passivation of copper-nickel alloys in sodium sulfate solutions, Corrosion 60 (2004) 795-803. [45] Q.H. Li, T.M. Yue, Z.N. Guo, X. Lin, Microstructure and Corrosion Properties of AlCoCrFeNi High Entropy Alloy Coatings Deposited on AISI 1045 Steel by the Electrospark Process, Metall. Mater. Trans. A 44 (2012) 1767-1778.

Figures and tables captions Fig. 1 XRD patterns of CrMnFeCoNi high entropy alloy coating. Fig. 2 OM (optical microscope) images of CrMnFeCoNi coating: a. longitudinal section of the coating shows the coating, the heat affected zone (HAZ) and the substrate; b. bottom area of the coating; c. topside area of the coating; d. bonding area between the coating and the substrate Fig. 3 Chemical analysis of CrMnFeCoNi coating by EDS: a. longitudinal distribution of alloy elements; b. linear scan crossing columnar dendrites. Fig. 4 Polarization curves in3.5 wt.% NaCl solution (a) and 0.5 M H2SO4 (b). Fig. 5 SEM images of corroded surface after polarization test in 3.5 wt.% NaCl solution. Fig. 6 SEM images of corroded surface after polarization test in 0.5 M sulfuric acid solution. Fig. 7 Evolution of Nyquist plots for HEA coating samples immersed in 0.5 M sulfuric acid in 48 hours. Fig. 8 Equivalent circuit used to analyze the EIS results. Fig. 9 Evolution of equivalent circuit fitting parameters against immersion time. Fig. 10 Micrograph and EDS analysis for CrMnFeCoNi coating sample immersed in 0.5 M sulfuric acid for 5 days. Table 1 Chemical composition (wt.%) of substrate material and mixed powder. Table 2 EDS results of CrMnFeCoNi high entropy alloy coating. Table 3 Electrochemical parameters extracted and calculated from anodic polarization

curves in 3.5 wt.% NaCl solution. Table 4 Electrochemical parameters extracted and calculated from anodic polarization curves in 0.5 M H2SO4 solution. Table 5 Equivalent circuit fitting parameters for EIS measurements. Table 6 Chemical analysis on HEA corroded surface after immersed in 0.5 M sulfuric acid for 5 days.

Fig. 1

Fig. 2a, Fig.2b

Fig.2c, Fig.2d

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Table 1 Fe

Co

Ni

Cr

Mn

Si

Cu

Mo

C

Substrate (wt.%)

Bal.

-

≤0.4

≤0.2

0.9-1.6

≤0.5

≤0.35

≤0.08

≤0.18

Mixed powder (wt.%)

0

29.7

24.7

18.2

27.4

-

-

-

-

Table 2 Cr

Mn

Fe

Co

Ni

Dendrites (at.%)

20.7

18.8

22.1

21.8

16.6

Interdendrites (at.%)

18.4

24.5

19.5

16.6

21.0

Table 3 2

*

Ecorr (VSHE)

icorr (A/cm )

Epit (VSHE)

HEA

-0.0995

1.05E-07

0.218

304SS

-0.107

1.06E-07

0.612

Substrate

-0.586

2.92E-06

-0.493

Epit*: pitting potential

Table 4 Ecorr

icorr

(VSHE) HEA

*

*

*

(VSHE)

(VSHE)

7.75E-04

5.33E-06

1.32

1.11

1.132

-0.0860

3.11E-05

1.13E-05

-

1.12

1.204

0.932

0.186

3.90E-04

1.70

1.55

0.618

(A/cm )

-0.127

8.96E-06

-0.0250

304SS

-0.145

1.16E-05

Substrate

-0.295

2.04E-04

ipass 2

2

Epp*: primary passivation potential icrit*: critical current density, maximum anodic current density in active region

ΔE*: Eb-Epp, length of passive zone

*

(VSHE)

(VSHE)

Eb*: breakdown potential

ΔE

(A/cm )

(A/cm )

Esp*: secondary passivation potential

*

Eb

icrit

2

*

Esp

Epp

Table 5

Immersion time (h)

4

Rs

RL

Rt

10 Y

L

n 2

2

n

2

2

(Ohm·cm )

(Ohm·cm )

(H·cm2)

0.860

500

19.4

9.78

1.26

0.863

656

18.2

27.3

2.51

1.13

0.870

1.10E3

69.5

141

24

2.48

1.07

0.885

1.72E3

99.9

453

36

2.49

1.10

0.882

1.61E3

115

715

48

2.48

1.48

0.888

1.40E3

82.0

286

(Ohm·cm )

(S·cm ·s )

1

2.53

1.23

6

2.49

12

Table 6 Cr

Mn

Fe

Co

Ni

Section A (at.%)

20.7

18.4

23.1

22.1

15.6

Section B (at.%)

18.9

25.0

21.9

15.6

18.9