Microstructure and corrosion properties of MnCrFeCoNi high entropy alloy-graphene oxide composite coatings

Materialia 5 (2019) 100249

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Microstructure and corrosion properties of MnCrFeCoNi high entropy alloy-graphene oxide composite coatings Ahmed Aliyu, Chandan Srivastava∗ Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

a r t i c l e Keywords: High entropy alloys Graphene oxide TEM Corrosion Microstructure Coating

i n f o

a b s t r a c t Microstructure and corrosion properties evaluation of MnCrFeCoNi high entropy alloys (HEA) composite coatings containing five different graphene oxide (GO) amounts was conducted. The HEA coatings were electrodeposited over mild steel substrate. The corrosion resistance properties of the HEA and HEA-GO composite coatings determined using potentiodynamic polarization and electrochemical impedance spectroscopy tests conducted in 3.5% NaCl solution revealed that with the addition of GO the corrosion resistance of the coating increased. Detailed microstructural characterization using the electron microscopy technique showed that GO enhanced the amount of Mn and Cr in the coatings leading to higher corrosion resistance.

1. Introduction High entropy alloys (HEAs) which contain five or more elements in nearly equiatomic ratio are presently receiving significant attention, especially for application in extreme service environments such as in the turbine, nuclear and aerospace industries, due to their exceptional properties which include high corrosion resistance [1–5], high fatigue resistance and fracture toughness [6,7], unique electrical and magnetic properties [8], resistance to heat softening [9], etc. Being different from the conventional alloys, HEAs usually adopt simple cubic structures without multiple intermetallic phases in order to achieve high configurational entropy for the system [1,9]. Choice of elemental combinations that are likely to yield HEAs solid solutions are based on Hume-Rothery rules (for formation of solid solution) that takes into consideration the atomic size differences (𝛿) and the enthalpy of mixing (ΔHmix ) [1,9,10]. The quantitative criterion for the formation of solid solutions is: −10 kJ/mol < ΔHmix < 5 kJ/mol, and 𝛿 < 4% [1,9,10]. To date, several techniques have been successfully tested and reported for producing HEA coatings. These techniques are magnetron sputtering [11–13], laser cladding [14,15], spraying [16], plasma transfer arc cladding [17], and electrochemical deposition [18]. Considerable research has been conducted on using the first four techniques to produce high quality HEA coatings; nevertheless, significant drawbacks of these techniques include high equipment costs, high operating temperatures with high energy consumption, and difficulty in microstructural engineering. In contrast, the electrochemical deposition methodology is non-equipment intensive, can be performed under ambient conditions and provides opportunities for easy tuning of the deposition parameters

∗

in order to perform microstructural engineering. However, despite these advantages, little information is available in the literature on electrodeposition of HEA coatings. This is mainly due to the complexity of the electrolytic bath with multiple metallic precursors. In the last few years, aqueous corrosion behavior of HEAs containing passivating elements (Al, Ni, Cr, Mo, Ti, etc.) has been investigated [19–23]. For instance, Lee et al. [24] have reported that the addition of Al to the Alx CrFe1.5 MnNi0.5 alloy reduces the resistance to pitting corrosion. They also observed that with high Al and low Cr additions, the alloy tends to form a porous oxide film which could not offer protective layers to the Cl− ion permeation. Hsu et al. [10] have demonstrated that the corrosion resistance of as-cast FeCoNiCrCux (x = 0, 0.5 and 1) alloys tend to decreases with increase of the Cu content due to its segregation in interdendrites. In a similar system, Qiu [25] reported that AlCrFeNiCoCu HEA show exceptional corrosion resistance in 1 M NaCl when compared to 304 stainless steel (304SS) by presenting a more noble potential and corrosion current density which was about two orders of magnitude smaller than stainless steel. These representative studies and similar other illustrate that the corrosion properties of HEAs are highly sensitive to their microstructure and elemental composition. Reports have illustrated that microstructural engineering conducted by the addition of foreign particle into the coating matrix to produce composite coatings improves the corrosion resistance performance. Few recent reports among them are on the addition of graphene in conventional protective coatings [26–28]. High chemical inertness, exceptional mechanical strength and impermeability to ion diffusion makes graphene a promising material for corrosion resistance [28,29]. Kumar et al. [26] and Rekha et al. [27] in separate studies have reported

Corresponding author. E-mail address: [email protected] (C. Srivastava).

https://doi.org/10.1016/j.mtla.2019.100249 Received 29 November 2018; Accepted 6 February 2019 Available online 11 February 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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Materialia 5 (2019) 100249

Table 1 Electrodepositing bath operating conditions and composition of the HEAs and HEAs-GO coatings. Sample

Bath composition

Concentration (gL−1 )

Condition

HEAs

MnCl2 •4H2 O CrCl3 •4H2 O FeCl2 •4H2 O CoCl2 •6H2 O NiCl2 •6H2 O MnCrFeCoNi with Graphene oxide

29.69 33.30 7.95 9.517 19.02 6.26 mg/100 ml 12.5 mg/100 ml 18.75 mg/100 ml 25.00 mg/100 ml 31.25 mg/100 ml

Current density (400 mA) Temperature (30 °C) Deposition Time (15 min) pH 1.5, under continuous stirring

HEAs-GO1 HEAs-GO2 HEAs-GO3 HEAs-GO4 HEAs-GO5

a significant enhancement in the corrosion resistance of Ni-graphene and Cr-graphene composite coatings coated over mild steel substrate when compared to the corrosion resistance offered by pure Ni and Cr coating. However, studies on the use of graphene in metallic coatings for corrosion resistance are limited because of high cost of production and hydrophobicity of graphene [30]. With regard to the widely used electrodeposition method for producing coatings, hydrophobicity makes uniform dispersion of graphene in widely used aqueous electrolyte baths difficult. These issues have been addressed by replacing graphene with graphene oxide (GO) which is hydrophilic and easily produced in large quantities by chemical oxidation of low cost graphite [30–32]. Enhancment in the corrosion resistance property of HEA coatings by the addition of GO remains un-addressed in the current literature. In view of this, in this present study, bcc structured MnCrFeCoNi HEA coatings containing GO were electrodeposited on polished mild steel substrate. Electrochemical properties of this coating was then investigated as a function of the amount of GO in the coatings and correlated with the coating microstructure. 2. Experimental procedure 2.1. Graphene oxide (GO) preparation Graphite powder with 99% purity was used to prepare GO by the modified Hummer’s technique [33]. In this technique, a known quantity of graphite powder was added in conc. H2 SO4 and NaNO3 solution contained in a beaker. This solution was vigorously stirred for homogeneity at 0 °C, followed by slow addition of KMnO4 while the reaction temperature was maintained below 15 °C for 24 h until the mixture became viscous and brownish. Then after, the mixture was diluted with de-ionized H2 O under continued stirring for 20 min at room temperature. 20 ml of H2 O2 was then added dropwise into the mixture while the colour changed to bright yellow. Finally, the resulting mixture was centrifuged, washed with de-ionized H2 O and dried for further characterization. 2.2. Electrodeposition of MnCrFeCoNi HEAs with/without GO coatings Electrodeposition of MnCrFeCoNi HEA coatings with and without GO on a polished mild steel substrate was performed using plating bath composition and operating parameters provided in Table 1. The substrate was polished mechanically down to 2500 grit size using abrasive SiC papers. Prior to the coatings, the mild steel (20 × 20 × 0.53 mm) substrate were ultrasonically cleaned in acetone, activated in 10 wt.% HCl at room temperature for 5 s, washed in distilled water and then placed immediately in the electrodeposition bath as cathode. In the electrolyte, gelatin (0.7 g/100 ml), sodium dodecyl sulphate (0.25 g/100 ml), ammonium chloride (11.98 g/100 ml), ascorbic acid (0.5 g/100 ml), sulphanilic acid (0.52 g/100 ml), potassium chloride (12 g/100 ml), boric acid (4.94 g/100 ml) and formic acid (0.5 ml/100 ml) were used as additives to improve the morphology of the coatings. The pH of the elec-

trolyte was adjusted to 1.5 using NaOH. Electrodeposition was carried out with platinum foil (30 × 25 × 0.4 mm) as anode for 15 min. After the deposition, the coating surface was rinsed with distilled water and dried at ambient temperature. This experiment was repeated by adding different amount of GO in the electrodeposition bath and these coatings are referred as; HEA (0 mg GO/100 ml), HEA-GO1 (6.25 mg GO/100 ml), HEA-GO2 (12.5 mg GO/100 ml), HEA-GO3 (18.75 mg GO/100 ml), HEA-GO4 (25 mg GO/100 ml) and HEA-GO5 (31.25 mg GO/100 ml). It has been reported by researchers that in acidic electrolyte baths containing GO, metal cations can bind with GO in the electrolyte to form a metal-GO complex with positive charge. This complex, during electrodeposition, can move towards cathode (i.e. mild steel substrate) causing incorporate of GO into the coating [34,35]. GO incorporation into the HEA composite coating is possible by the same phenomenon as the electrodeposition was done under highly acidic conditions. To investigate the effect of GO impermeability, MnCrFeCoNi HEAGO-MnCrFeCoNi HEA multilayer coatings were produced by the electrodeposition of HEA coating over mild steel (MS) substrate followed by drop casting of GO over the HEA coating surface and then followed by electrodeposition of HEA coating over the drop casted GO. The electrodeposition conditions and bath chemistry used for the multilayer coating was same as the one used for the electrodeposition of MnCrFeCoNi HEA coating without GO. 2.3. Sample characterization The surface morphology and atomic percentage composition of the coatings were analyzed using scanning electron microscopy (SEM) coupled with energy dispersive X-Ray spectroscopy (EDS) operating at 20 kV (FEI Quanta SEM). Phase identification, Scherrer grain size and texture coefficient of the MnCrFeCoNi coatings with and without GO were derived from the analysis of the X-ray diffraction (XRD) patterns using the X-pert pro X-ray diffractometer employing a CuK𝛼 (𝜆 = 0.154 nm) source. While the coatings thickness was determined from the coatings cross-section samples prepared for the transmission electron microscopy (TEM) based analysis using SEM-focused ion beam (FEI Helios SEM-FIB) instrument. The TEM bright field images, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images, compositional mapping and compositional line profile were analyzed using field emission FEI TITAN TEM operating at 300 keV. While the as-prepared GO was characterized using the SEM (FEI Quanta), TEM (FEI TITAN), atomic force microscopy (JPK, NanoWizardR 3 AFM instrument), XRD (X-pert pro X-ray diffractometer employing a CuK𝛼 (𝜆 = 0.154 nm) source), Fourier transform infared spectroscopy (FTIR: Bruker Tensor 2 with Pt ATR module instrument) and UV-visible absorption spectroscopy (Perkin Elmer Lambda 35). 2.4. Electrochemical measurements Electrochemical measurements were carried out in 3.5% NaCl solution using CHI 604E set-up at room temperature with saturated Ag/AgCl

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Table 2 The carbon content in HEAs and HEAs-GO coatings using SEM-EDS. Element/System

HEAs (wt.%)

HEAs-GO1 (wt.%)

HEAs-GO2 (wt.%)

HEAs-GO3 (wt.%)

HEAs-GO4 (wt.%)

HEAs-GO5 (wt.%)

C

4.21 ± 0.35

6.64 ± 0.26

7.39 ± 0.33

8.50 ± 0.34

9.85 ± 0.46

10.98 ± 0.33

Table 3 The atomic percent compositional data of the HEAs and HEAs-GO coatings using SEM-EDS. Elements/System

HEAs(at.%)

HEAs-GO1(at.%)

HEAs-GO2(at.%)

HEAs-GO3(at.%)

HEAs-GO4(at.%)

HEAs-GO5(at.%)

Mn Cr Fe Co Ni

25.92 ± 4.27 27.49 ± 2.55 21.00 ± 1.20 12.06 ± 1.74 13.53 ± 0.98

24.99 ± 3.26 23.17 ± 1.27 23.70 ± 1.32 13.59 ± 2.19 14.85 ± 2.20

25. 44 ± 0.96 24.46 ± 4.27 21.96 ± 3.24 13.09 ± 1.64 14.99 ± 1.07

26.94 ± 1.59 24.51 ± 4.28 20.39 ± 3.41 12.94 ± 1.27 15.23 ± 2.40

27.97 ± 0.49 26.18 ± 2.66 20.11 ± 2.39 12.25 ± 1.23 13.49 ± 0.87

26.25 ± 2.23 21.47 ± 0,71 20.95 ± 1.53 15.57 ± 2.57 15.77 ± 1.40

electrode and platinum foil (25 × 25 × 0.53 mm) as the reference electrode and counter electrode respectively. Coated mild steel samples with 1 cm2 exposed area were used as the working electrode for the corrosion studies in 3.5% NaCl solution. The electrochemical impedance spectroscopy (EIS) and Tafel polarization curves measurement were conducted on a three-electrode CHI 604E electrochemical work-station (CH Instruments Ltd., USA). Before the EIS tests, the open circuit potential (OCP) was performed for 3600 s and recorded. The EIS behaviour of the coatings was analyzed at the respective OCP values with applied AC amplitude of 5 mV and frequency range from 106 Hz to 0.01 Hz, while the Tafel polarization curves were measured at a scanning rate of 1 mV/m. The recorded EIS data for each of the coated sample were interpreted using ZSimpWin 3.21 software and equivalent electrical circuits (EEC). The EIS experiments for each coating was repeated five times under the same condition to ensure reproducibility of the results. 3. Results and discussion 3.1. Characterisation of GO Representative TEM bright field image showing the sheet morphology with few numbers of layers of as-prepared GO is presented in Fig. 1(a). While, Fig. 1(b) shows the AFM topographical image of asprepared GO sheets with average thickness of 3.2 ± 1.3 nm obtained from the analysis of Z-height profile of several GO sheet in AFM images. Fig. 1(c) presents X-ray diffraction pattern of as-prepared GO with a diffraction peak (001) at the diffraction angle of 10.3° with interplanar spacing of 0.855 nm, which is the same for a typical GO as provided in the literature [35,36]. The FT-IR spectrum of as-prepared GO is shown in Fig. 1(d). The recorded FT-IR spectrum shows the presence of oxygen containing functional groups. The absorption peak at ∼3292 cm−1 is attributed to the O–H stretch vibrations [37–39]. The absorption peak at 1717 cm−1 and 1618 cm−1 can be attributed to C=O stretch of the carboxyl group and C=C stretch from un-oxidized graphite domain respectively [39], while the 1163 cm−1 peak corresponds to C–OH stretch of alcohol group [39] and 1034 cm−1 corresponds to C–O stretching vibrations of C–O–C [17]. Fig. 1(e) shows the UV-visible absorption spectrum of as-prepared GO. The UV-visible spectrum show a major absorption peak at 230 nm attributed to the 𝜋-𝜋 ∗ transitions of C–C bond and a shoulder peak at 300 nm which is due to the n-𝜋 ∗ transition of the carbonyl groups (C=O) bonds [37]. 3.2. Surface morphology and phase constitution of HEA coatings with and without GO Fig. 2(a) shows the X-ray diffraction patterns of the HEA coatings with and without GO. All the coatings exhibit a simple body centered cubic (BCC) structure. The average crystallite size calculated using the Scherrer formula [45] was 47, 34, 31, 30, 25 and 26 nm for HEA, HEAGO1, HEA-GO2, HEA-GO3, HEA-GO4 and HEA-GO5 coatings respec-

tively. The observed decrease in the average crystallite size can be attributed to the increase in the heterogenous sites for nucleation and growth of grains as a results of GO addition. Texture co-efficient (TCo-eff ) values for the coatings were calculated [30,41] using the relation: ∑ 𝐼O(ℎ𝑘𝑙) 𝐼(ℎ𝑘𝑙) 𝑇Co−ef f (ℎ𝑘𝑙) = ∑ × (1) 𝐼O(ℎ𝑘𝑙) 𝐼(ℎ𝑘𝑙) where I(hkl) is the peak intensity of the (hkl) plane of the coating and IO(hkl) is from the standard reference pattern. The calculated texture coefficient were plotted against the crystallographic planes and presented in Fig. 2(b). From Fig. 2(b), it can be observed that the addition of GO has no significant effect on the growth texture of all the coatings. Presented in Fig. 3(a) and (b) are the SEM micrographs of as-coated HEA and HEA-GO composite coatings at low and high magnification respectively. The micrographs reveal that all the coatings possess similar granular morphology and with the addition of GO, the compactness and smoothness of the coating increases. Table 2 provides the values of the carbon content in the coatings determined from the EDS compositional analysis. Compared with the HEA without GO, the carbon content in the coatings increases from HEA to HEA-GO5. This indicated towards increase in the GO content of the coatings produced from the electrolyte bath with increasing GO concentration. While, the elemental atomic percent composition of the coatings (excluding carbon) is provided in Table 3. Normally, for high entropy alloys, the principal elements are expected to be between 5 at.% and 35 at.% [1,9]. Therefore, from Table 3, it can be seen that all the coatings contain all the five component elements in near equal atomic percent (at.%). It should be noted that the total carbon content in the coating also has contribution from the other chemicals that were used during the electrodeposition process. As others carbon containing chemicals were kept constant and only the graphene oxide was varied between the different electrochemical baths, therefore trend of increase in the carbon wt.% in Table 1 should be considered rather than the absolute values. The increasing trend clearly indicates towards increase in the GO content of the coatings. To investigate the presence and further the distribution of GO in coatings, SEM-EDS compositional line scan analysis was conducted for HEA and HEA-GO composite coatings. Representative results obtained for HEA, HEA-GO2 and HEA-GO4 coatings are presented in Fig. 4 which shows the SEM image of the region of interest and the corresponding carbon composition profile obtained along the indicated line. In Fig. 4, the carbon content profile from HEA coating defines the “base-line” composition which is due to the carbon from the carbon containing chemicals in the electrolyte bath. With the incorporation of GO into the coatings (for HEA-GO2 and HEA-GO4), it can be observed that the composition profile is always above the baseline limit defined by HEA coating thus confirming the presence of GO in the composite coatings. The fluctuation in the composition profiles, however, clearly indicate towards a non-uniformity in the distribution of GO within the coatings. Furthermore, it can be observed that the non-uniformity in distribution of GO is greater for HEA-GO4 coating

A. Aliyu and C. Srivastava

Materialia 5 (2019) 100249

Fig. 1. (a) TEM bright field image (b) AFM topographical image (c) X-ray diffraction image (d) FTIR spectrum (e) UV-visible absorption spectrum of as-synthesized graphene oxide (GO).

which indicates towards higher agglomeration of GO in the HEA-GO4 electrolyte bath with higher GO concentration. Furthermore, to investigate the growth mechanism of HEA-GO coatings, specifically with regard to the incorporation of GO into the coating, a new set of coatings were conducted using the bath constitution and electrodeposition parameters that was used for the HEA-GO5 coating but instead of doing one single coating, in the new experiment, three different coatings

were deposited each for 3, 6 and 12 min by using fresh electrolyte bath each time. Coating morphology and composition was analyzed using the SEM-EDS techniques. From the analysis of the SEM images it was observed that till 3 min only the deposition of MnCrFeCoNi happened and no trace of GO was observed. GO sheet over the coating surface was however clearly observed after 6 min deposition. With increase in deposition time to 12 min, the GO was again not observed on the coating surface.

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Materialia 5 (2019) 100249

Fig. 2. (a) X-ray diffraction pattern for 2𝜃 angle range of 35°−110°, and (b) Texture orientation of the HEA and HEA-GO coatings.

Fig. 3. (a) Low magnification (b) High magnification SEM micrograph of the HEA with/without GO composite coatings.

Fig. 4. SEM line scan profile showing distribution of carbon inside the HEA, HEA-GO2 and HEA-GO4 composite coatings.

Representative SEM micrograph of the coatings after 3, 6 and 12 min deposition are provided in Fig. 5. These observations clearly indicated that the GO deposition/incorporation did not happen throughout the electrodeposition process but most likely it occurred within a certain time period during the deposition process thus creating a composite coating in which GO is mostly distributed in the middle of the coating cross-section. SEM-EDS line scan was conducted along the GO sheet in Fig. 5(b) (after 6 min of deposition) and the compositional profiles obtained are presented in Fig. 6. The line scan profile shows high carbon contents from the GO sheet along with traceable amount of all other elements which indicated that the nucleation and growth of coating happened over the GO surface during the electrodeposition process leading to embedding and eventual incorporation of the GO into the coating matrix.

To investigate the change in the chemical state and de-oxidation of the GO due to cathodic polarization, FTIR spectra was recorded in the wavelength range of 650–4000 cm−1 for the HEA-GO5 electrolyte before and after the electrodeposition process. The FTIR analysis result shown in Fig. 7 reveal that the intensity of the characteristic peaks of GO decreased after the electrodeposition, which indicates that the GO got reduced during the electrodeposition process. 3.3. Electrochemical studies 3.3.1. Potentiodynamic polarization measurements The open circuit potential (OCP) of the HEA coatings with and without GO in 3.5% NaCl solution is plotted against time of immersion and the results are presented in Fig. 8(a). For each measurement,

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Fig. 5. SEM images of HEA-GO5 at (a) 3 mins (b) 6 mins and (c) 12 mins electrodeposition time using fresh electrolyte.

the coatings samples were allowed to equilibrate and the OCP was determined until stable potential was reached. Fig. 8(a) shows that, the potential steadily decreases with time of immersion and get slow down towards the end. Over the entire 3600 s, the OCP moves to a more

Materialia 5 (2019) 100249

positive side with addition of GO into the HEA coatings and acquires stable potential over time and this is in accordance with literature [40, 42], for a material to achieve better corrosion resistance performance, the potential should shift towards a more positive value. These results show that, with the addition of GO, the corrosion resistance of the HEA improved in 3.5% NaCl solution. The potentiodynamic/Tafel polarization curves of the HEA coatings with and without GO in 3.5% NaCl solution are presented in Fig. 8(b). The anodic polarization curves in Fig. 8(b) for both HEA coatings with and without GO show a similar corrosion process. Table 4 provides the values of corrosion potential (Ecorr ), corrosion current density (icorr ), cathodic (𝛽 cathodic ) and anodic (𝛽 anodic ) Tafel slopes evaluated from the curves. According to the literature, the shift of the potentiodynamic polarization curves toward lower corrosion current densities is due to decrease in the corrosion rate [43–46]. The value of Ecorr for HEA coating without GO increases from −0.845 ± 0.004 V to −0.842 ± 0.001 V, −0.792 ± 0.002 V, −0.785 ± 0.003 V, −0.767 ± 0.003 V, and −0.74 ± 0.002 V for HEA-GO1, HEA-GO2, HEA-GO3, HEA-GO4 and HEA-GO5 respectively. This increase in Ecorr with the addition of GO revealed that the potential of the HEA-GO for the release of an electron are more compared to HEA without GO. Generally, the icorr which is determined by the intersection of the linear portions of the cathodic and anodic curves have been established to be a substantial parameter compared to the Ecorr in evaluating the corrosion resistance [47]. As provided in Table 4, HEA-GO5 coating showed the maximum reduction in icorr (10.89 ± 0.22 μA/cm2 ), indicating enhancement in the substrate corrosion protection. Also, the 𝛽 cathodic and 𝛽 anodic values significantly increase with the addition of GO, which showed that the HEA-GO coat-

Fig. 6. SEM line scan showing the distribution of elements across a GO sheet in HEA-GO5 coating obtained after 6 mins of electrodeposition.

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Table 4 Corrosion parameters in 3.5% NaCl determined from the Tafel plots for HEAs and HEAs-GO coatings under OCP condition. System

HEAs

HEAs-GO1

HEAs-GO2

HEAs-GO3

HEAs-GO4

HEAs-GO5

Ecorr (V) icorr (μA/cm2 ) 𝛽 cathodic (V/dec) 𝛽 anodic (V/dec)

−0.845 ± 0.004 43.44 ± 0.11 21.063 ± 0.32 12.522 ± 0.18

−0.842 ± 0.001 38.94 ± 0.29 21.920 ± 0.20 14.334 ± 0.15

−0.792 ± 0.002 32.82 ± 0.28 23.511 ± 0.24 20.06 ± 0.12

−0.785 ± 0.003 24.35 ± 0.18 26.880 ± 0.13 22.449 ± 0.10

−0.767 ± 0.003 16.89 ± 0.31 28.339 ± 0.19 23.51 ± 0.07

−0.74 ± 0.002 10.89 ± 0.22 32.225 ± 0.28 27.869 ± 0.02

Fig. 7. FTIR spectra of HEA-GO5 electrolyte before and after electrodeposition showing GO reduction.

ings retards both the cathodic and anodic reactions. The sequence of corrosion resistance was HEA-GO5 > HEA-GO4 > HEA-GO3 > HEAGO2 > HEA-GO1 > HEA, which basically follow the trend of amount of GO in the coating. 3.3.2. Electrochemical impedance spectroscopy (EIS) To understand the corrosion mechanism of the HEA coating with and without GO, the coated samples were further analyzed using the electrochemical impedance spectroscopy (EIS) measurement at their respective OCP values. Presented in Fig. 9(a) is the Nyquist plots of the HEA coatings with and without GO in 3.5% NaCl solution under the OCP condition. As illustrated in this figure, two capacitive loops were observed i.e. the smaller and larger one at higher and lower frequency respectively for both HEA coating with and without GO. Increase in the capacitive loop indicates an increase in the ability to resist corrosion and vice versa [43–48]. Also, the radius of the impedance semi-capacitive loop represents the interface resistance of the charge transfer which corresponds to the corrosion resistance [48,49]. Thus, it is obvious from Fig. 9(a) that the diameter of the semi-capacitive loop increases with the addition of

GO, which suggested an enhancement in the corrosion resistance properties in the sequence of HEA, HEA-GO1, HEA-GO2, HEA-GO3, HEA-GO4, and HEA-GO5 respectively. Fig. 9(b) is the Bode plots of the impedance spectrum present in Fig. 9(a). The shift in the Bode impedance modulus and Bode phase angle plots can better explain the frequency specific impedance behavior which reflects the corrosion resistance of an alloy in the solution [49,50]. Therefore, it can be seen from Fig. 9(b) that the high frequency impedance modulus is very close to each other with only very little difference in their corrosion resistances. But, at the low frequency region, the low frequency impedance modulus plots shifted upward and far apart from each other, which implies that the corrosion resistance increases with increase in the GO content. Also, in the phase angle, there was an increased and broadening phase angle in the phase maximum with the increase in GO content which also suggests maximum corrosion resistance property for the HEA-GO5 coating [50,51]. The equivalent electrical circuit models (EEC) RS (CPE1 (RC (CPE2 Rct ))) shown in Fig. 10 was used to fit the impedance spectra data under OCP condition using ZsimpWin 3.21 software. In this circuit, the impedance model (Fig. 10) was used to fit the experimental impedance data obtained for HEA, HEA-GO1, HEA-GO2, HEA-GO3, HEA-GO4 and HEA-GO5. The very small chi-square values of the order of ∼10−3 suggest good fitting of the models. In these circuits, RS represent the electrolyte resistance; RC and Rct represent the coatings and charge transfer resistance, respectively; CPE represent the constant phase element (CPE) which is employed to replace the pure capacitance component to obtain better fittings [52]. The impedance of the CPE is given by; 𝑍CPE =

1 (𝑗𝜔)−𝑛 𝑌o

(2)

where YO is the admittance magnitude of CPE, 𝜔 = 2𝜋𝑓 is the angular frequency, f is the frequency (Hz), j is the imaginary number and n is the dispersion coefficient related to surface homogeneity. From the perspective of dimension, the unit of CPE (Ω−1 cm−2 S−n ) is not the same as that of the capacitance (F cm−2 or Ω−1 cm−2 S−n ), so, unit correction was carried out as described in literature [52] and the corresponding EEC parameters are provided in Table 5. It is important to also note that, the interpretation of CPE depend on the value of n, i.e. resistance for n = 0, Warburg impedance for n = 0.5 and capacitance for n = 1 [47,52]. From

Fig. 8. (a) The open circuit potential (OPC) and (b) Tafel polarization curves of HEA and HEA-GO coatings in 3.5% NaCl solution. Insert shows the same curves in the potential range of −0.86 to −0.72 V.

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Fig. 9. Electrochemical impedance spectra (EIS) of the HEA and HEA-GO coatings in 3.5% NaCl solution (a) Nyquist plots and (b) Bode plots. Table 5 Equivalent circuit parameters obtained by fitting the experimental EIS results of the HEA and HEA-GO coatings in 3.5% NaCl solution under OCP condition. System

HEAs

HEAs-GO1

HEAs-GO2

HEAs-GO3

HEAs-GO4

HEAs-GO5

RS (Ω cm2 ) RC (Ω cm2 ) CPE1 (Ω−1 cm−2 S−n ) n- CPE1 Rct (Ω cm2 ) CPE2 (Ω−1 cm−2 S−n ) n- CPE2 Polarization resistance (RP ) (Ω cm2 ) 𝜒 2 × 10−3

6.30 15.78 3.875 × 10−2 1 405 0.01196 0.6963 420.78

5.34 16.24 4.633 × 10−2 1 408 0.01156 0.7159 424.24

7.03 14.99 3.115 × 10−2 1 589 0.01029 0.6531 603.99

5.22 17.11 4.635 × 10−2 1 885 0.00550 0.6766 902.11

5.13 16.75 4.396 × 10−2 1 928 0.00998 0.6284 944.75

5.77 14.85 4.018 × 10−2 1 1206 0.01062 0.7173 1220.85

1.25

1.33

4.10

2.95

1.00

3.05

Fig. 10. Equivalent electrical circuit models (EEC) RS (CPE1 (RC (CPE2 (RCt )))) used to fit the EIS data of the HEA and HEA-GO coatings.

Table 5, the values of n-CPE1 are all equal to 1, while n-CPE2 values are close to 1 (n > 0.6) for all the coatings in this research; suggesting a near capacitive behavior. To further buttress the fitted data for the proposed EEC, the polarization resistance (RP = Rc + Rct ) were calculated and the results revealed an optimal barrier in HEA-GO5 (1221 Ω cm2 ) coating, indicating its better ability to prevent the penetration of corrosive medium because of the impermeability property of the GO and this is in agreement with the result from the Tafel polarization tests.

Fig. 11. SEM-FIB Cross-section lamellar micrographs of pure HEA and HEAGO5 coatings.

3.4. TEM microstructure of HEA with/without GO For understanding the reason behind the observed enhancement in the corrosion resistance of HEA coating due to the addition of GO, coating cross-section samples were prepared using SEM-FIB technique (See Fig. 11) and studied using TEM for the microstructure of the HEA coating without GO and HEA-GO5 composite coating. Fig. 12(a) shows the bright-field TEM image of cross-section of HEA coating without GO. To determine the distribution of the constituent elements in the coating microstructure, STEM-EDS compositional mapping and compositional line profile analysis were carried out. Fig. 13 provides the STEM-HAADF image of the region of interest and elemental compositional maps for the 5 elements. It is apparent from the compositional mapping anal-

Fig. 12. TEM bright field images of cross-section of (a) HEA and (b) HEA-GO5 coating.

ysis that all the five elements (Mn, Cr, Fe, Co and Ni) are fairly uniformly distributed in the coating microstructure. Also, the results for the compositional line profile analysis along line marked X-Y presented in Fig. 14 confirms the uniform distribution of all the elements throughout the microstructure during the electrodeposition.

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Materialia 5 (2019) 100249

Fig. 13. STEM-HAADF image and compositional mapping of the HEA coatings.

Fig. 14. TEM bright field image and composition line profile of cross section of HEA coatings, measured along line XY.

Fig. 15. STEM-HAADF image and compositional mapping of the HEA-GO5 coatings.

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Materialia 5 (2019) 100249

Fig. 16. TEM bright field micrograph and composition line profile of cross section of HEA-GO5 coatings, measured along line XY.

Fig. 17. Impermeability test Illustration and SEM micrograph of the coatings.

Bright-field TEM image of the cross-section of the HEA-GO5 composite coating is presented in Fig. 12(b). To determine the distribution of elements, STEM-EDS compositional mapping and compositional line profile analysis were again carried out and the result are presented in Figs. 15 and 16 respectively. Fig. 15 shows STEM-HAADF image of the region of interest and elemental compositional maps for all the five elements. It can be observed from the figure (Fig. 15) that the coating microstructure contains Mn and Cr-rich matrix (R1) containing Fe-CoNi rich phase (R2). The substrate coating interface however contains all the five elements (R3). The compositional line profile analysis along line marked X-Y presented in Fig. 16 revealed that R1 region contains ∼30–40 at.% Mn with ∼30–35 at.% Cr and minor amounts of Fe, Co and Ni. R2 region contains ∼30–40 at.% Fe, ∼20–30 at.% Co, and ∼30–40 at.% Ni, with negligible amounts of Mn and Cr. The R3 region exhibit random distribution of all the five component atoms. Therefore, the enhancement observed in the corrosion resistance of the HEA coatings with GO can be correlated with the coating microstructure. Unlike the HEA coating without GO which has homogeneous mi-

crostructure, the addition of GO encourages phase separation and inhomogeneous microstructure in which highly Mn-Cr-rich phase form the coating matrix. As this phase can form stable oxide corrosion product so the corrosion resistance increased with the increase in the GO content of the coating. Furthermore, presence of GO layer will also impart impermeability to the penetration of the corrosive media and increase the corrosion resistance [53,54]. 3.5. Multi-layer coatings for the impermeability impact of GO To further explore the effect of impermeability of GO towards inhibiting the penetration of the corrosive media, HEA-GO-HEA multilayer coatings were produced as illustrated in Fig. 17 with their corresponding SEM images placed by side. The different multilayer samples are identified as ML1: 3 min HEA-GO-12 min HEA; ML2: 6 min HEA-GO-9 min HEA; ML3: 9 min HEA-GO-6 min HEA; ML4: 12 min HEA-GO-3 min HEA. To make the multilayer geometry GO was drop dried over the HEA coatings. It can be observed (Fig. 17) that the

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Materialia 5 (2019) 100249

Table 6 Corrosion parameters in 3.5% NaCl determined from the Tafel plots for the multi-layer deposition time coatings under OCP condition. System

ML1 (3 min HEA-GO-12 min HEA)

ML2 (6 min HEA-GO-9 min HEA)

ML3 (9 min HEA-GO-6 min HEA)

ML4 (12 min HEA-GO-3 min HEA)

Ecorr (V) icorr (μA/cm2 ) 𝛽 cathodic (V/dec) 𝛽 anodic (V/dec)

−0.819 ± 0.006 60.73 ± 0.19 17.915 ± 0.22 12.679 ± 0.20

−0.788 ± 0.004 35.42 ± 0.21 19.403 ± 0.26 13.673 ± 0.18

−0.748 ± 0.003 18.89 ± 0.31 18.864 ± ± ± 0.31 14.592 ± 0.19

−0.735 ± 0.004 18.05 ± 0.11 20.790 ± 0.23 15.465 ± 0.17

Table 7 Equivalent circuit parameters obtained by fitting the experimental EIS results of the multi-layer deposition time coatings in 3.5% NaCl solution under OCP condition. System RS (Ω cm2 ) RC (Ω cm2 ) CPE1 (Ω−1 cm−2 S−n ) n- CPE1 Rct (Ω cm2 ) CPE2 (Ω−1 cm−2 S−n ) n- CPE2 Polarization resistance (RP ) (Ω cm2 ) 𝜒 2 × 10−4

ML1 (3 min HEA-GO-12 min HEA)

ML2 (6 min HEA-GO-9 min HEA)

ML3 (9 min HEA-GO-6 min HEA)

ML4 (12 min HEA-GO-3 min HEA)

6.42 15.41 3.777 × 10−2 1 259 0.05879 0.5741 274.41

6.41 15.41 4.111 × 10−2 1 916 0.03572 0.6198 931.41

5.96 16.29 4.896 × 10−2 1 1002 0.0250 0.6957 1018.29

6.06 16.18 4.937 × 10−2 1 1504 0.02045 0.7315 1520.18

5.207

4.339

4.574

5.075

Fig. 18. The Tafel polarization curves of the impermeability test coatings in 3.5% NaCl solution. Insert shows the same curves in the potential range of −0.85 to −0.7 V.

coating morphology became finer and relatively more compact as the GO layer moved towards the top surface. The potentiodynamic polarization curves of the multilayer coatings in 3.5% NaCl solution are presented in Fig. 18 and the corre-

sponding corrosion properties are summarized in Table 6. As shown in Fig. 18 and Table 6, the Ecorr shifts positively from −0.819 ± 0.006 V to –0.735 ± 0.004 V, while the icorr decreases significantly form 60.73 ± 0.19 μA/cm2 to 18.05 ± 0.11 μA/cm2 as GO moves towards the surface form ML1 to ML4 coating. This is an evidence of the impermeability of GO towards the corrosive media and its role in enhancing the corrosion resistance of the HEA-GO composite coatings. Presented in Fig. 19(a) is the Nyquist plots of the multilayer coatings in 3.5% NaCl solution. It can be observed from Fig. 19(a) that the diameter of the larger semi-capacitive loop increases as the GO layers in the multilayer coating moves towards the surface, suggesting enhancement in the corrosion resistance in the sequence of ML1, ML2, ML3, and ML4. Furthermore, the Bode plot in Fig. 19(b) show an upward shift in the impedance modulus plot at low frequency with corresponding increased and broadening phase angle in the phase maximum movement of GO towards the surface, which also confirmed maximum corrosion resistance properties for ML4 coating. The equivalent electrical circuit models (EEC) RS (CPE1 (RC (CPE2 Rct ))) earlier presented in Fig. 10 was used to fit the multi-layer coatings impedance spectra data and the EEC parameters are summarized in Table 7. From Table 7, the Rct and RP value increases significantly for ML4 coating. The Rct value of the ML4 coating is 5.8 times greater than that of the ML1 coating. More so, the decrease in the CPE2 revealed a more smooth and protective nature of the ML4 coating. Therefore, it is reasonable to conclude that as the layer of the GO moves upward of the substrate, the coatings gains enhanced the corrosion properties.

Fig. 19. Electrochemical impedance spectra (EIS) of the impermeability test coatings in 3.5% NaCl solution (a) Nyquist plots and (b) Bode plots.

A. Aliyu and C. Srivastava

4. Conclusion High corrosion resistance property of high entropy alloys (HEAs) makes them a candidate protective coating material. This study investigated the microstructural changes that occur in electrodeposited MnCrFeCoNi HEA coating with the addition of GO and its resultant effect on the corrosion behavior of the composite coating. Graphene oxide (GO) content of the HEA coating was varied by changing the amount of GO in the electrolyte bath. With the addition of GO, corrosion current density and corrosion rate reduced, while the corrosion potential and polarization resistance increased, indicating enhancement in the corrosion resistance properties of the HEA coatings with increased in the GO content. Microstructural characterization of the coatings samples revealed that the addition of GO resulted in distinct microstructural changes; with the addition of GO, the microstructure transformed from a nearly homogenous one to one containing FeCoNi rich regions embedded in a Mn-Cr rich matrix. Formation of strongly oxidizing matrix along with the impermeability imparted by the GO was accounted for the observed enhancement in the corrosion resistance of the HEA-GO composite coatings as compared to only HEA coating. Acknowledgement Authors acknowledge the research grant (EMR/2017/000913) received from SERB Govt. of India. Electron Microscopy facilities in the AFMM, IISc are also acknowledged. ‘Declaration of interest’ statement Authors declare that there are no competing interests. References [1] Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. [2] 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. [3] D.B. Miracle, O.N. Senkov, A critical review of high entropy alloys and related concepts, Acta Mater. 122 (2017) 448–511. [4] M.H. Tsai, J.W. Yeh, High-entropy alloys: a critical review, Mater. Res. Lett. 2 (2014) 107–123. [5] Y.J. Hsu, W.C. Chiang, J.K. Wu, Corrosion behavior of FeCoNiCrCux high-entropy alloys in 3.5% sodium chloride solution, Mater. Chem. Phys. 92 (2005) 112–117. [6] S. Katakam, S.S. Joshi, S. Mridha, S. Mukherjee, N.B. Dahotre, Laser assisted high entropy alloy coating on aluminum: microstructural evolution, J. Appl. Phys. 116 (2014) 104906. [7] Z. Tang, T. Yuan, C.W. Tsai, J-W. Yen, C.D. Lundin, P.K. Liaw, Fatigue behavior of a wrought Al0.5CoCrCuFeNi two-phase high-entropy alloy, Acta Mater 99 (2015) 247–258. [8] E.J. Pickering, N.G. Jones, High-entropy alloys: a critical assessment of their founding principles and future prospects, Int. Mater. Rev. 61 (2016) 183–202. [9] J.W. Yeh, Recent progress in high-entropy alloys, Ann. Chim. Sci. Mater. 31 (6) (2006) 633–648. [10] Y.-J. Hsu, W.-C. Chiang, J.-K. Wu, Corrosion behavior of FeCoNiCrCux high-entropy alloys in 3.5% sodium chloride solution, Mater. Chem. Phys. 92 (2005) 112–117. [11] Z.F. Wu, X.D. Wang, Q.P. Cao, G.H. Zhao, J.X. Li, D.X. Zhang, J.J. Zhu, J.Z. Jiang, Microstructure characterization of AlxCo1Cr1Cu1Fe1Ni1 (x = 0 and 2.5) high entropy alloy films, J. Alloys Compd 609 (2014) 137–142. [12] V. Dolique, A.L. Thomann, P. Brault, Y. Tessier, P. Gillon, Complex structure/composition relationship in thin films of AlCoCrCuFeNi high entropy alloy, Mater. Chem. Phys. 117 (2009) 142–147. [13] Z. An, H. Jia, Y. Wu, P.D. Rack, A.D. Patchen, Y. Liu, Y. Ren, N. Li, P.K. Liaw, Solid– solution CrCoCuFeNi high-entropy alloy thin films synthesized by sputter deposition, Mater Res Lett 3 (2015) 203–209. [14] X. Ji, H. Duan, H. Zhang, J. Ma, Slurry erosion resistance of laser clad NiCoCrFeAl3 high-entropy alloy coatings, Tribol T. 58 (2015) 1119–1123. [15] H. Zhang, W.F. Wu, Y.Z. He, M.X. Li, S. Guo, Formation of core–shell structure in high entropy alloy coating by laser cladding, Appl. Surf. Sci. 363 (2016) 543–547. [16] T. Yue, H. Xie, X. Lin, H. Yang, G. Meng, Microstructure of laser Re-melted AlCoCrCuFeNi high entropy alloy coatings produced by plasma spraying, Entropy 15 (2013) 2833–2845. [17] D. Liu, J.B. Cheng, H. Ling, Electrochemical behaviours of (NiCoFeCrCu)95 B5 high entropy alloy coatings, Mater. Sci. Tech. 31 (2015) 1159–1164. [18] C-Z. Yao, P. Zhang, M. Liu, G-R. Li, J-Q. Ye, P. Liu, Y-X. Tong, Electrochemical preparation and magnetic study of Bi–Fe–Co–Ni–Mn high entropy alloy, Electrochim. Acta. 53 (2008) 8359–8365.

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