Grain boundary engineering in Ni-carbon nanotube composite coatings and its effect on the corrosion behaviour of the coatings

Materialia 9 (2020) 100617

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Grain boundary engineering in Ni-carbon nanotube composite coatings and its effect on the corrosion behaviour of the coatings K. Sai Jyotheender, Abhay Gupta, Chandan Srivastava∗ Department of Materials Engineering, Indian Institute of Science, India

a r t i c l e

i n f o

Keywords: Carbon nanotubes Grain boundary engineering Coatings Corrosion Electron back scatter diffraction

a b s t r a c t Coatings have been traditionally used for protection against corrosion. One possible way to enhance the corrosion resistance performance of coatings is by grain boundary engineering (GBE). This study illustrates the role of optimum amount of carbon nanotubes (CNTs) in modifying the grain boundary constitution in nickel-CNT composite coatings thereby enhancing its corrosion resistance performance. Compact and crack free Ni-CNT composite coatings were electrodeposited on mild steel substrates using electrolyte bath with different concentrations of dispersed CNTs (4, 6, 8, 10, 20, 30, 40, 50, 70 and 100 mg/L). Corrosion performance of coatings exposed to 3.5 wt.% NaCl medium was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy techniques. Among the 10 different Ni-CNT composite coatings, relatively low corrosion rates were observed for two distinct CNTs concentrations. Microstructural characterization conducted using the electron back scatter diffraction method (EBSD) suggested that the grains in all coatings grew with ⟨110⟩ orientation. Ni-CNT3 coating (from 8 mg/l of CNT in electrolyte) which showed low corrosion rate (corrosion current density of 0.98 μA/cm2 ) primarily contained very high fraction of low angle grain boundaries. Ni-CNT8 coating (from 50 mg/l of CNT in electrolyte) which exhibited the lowest corrosion rate (corrosion current density of 0.66 μA/cm2 ) contained high fraction of Σ3 coherent twins. Ni-CNT10 coating (from 100 mg/l of CNT in electrolyte) which exhibited the highest corrosion rate (corrosion current density of 14.08 μA/cm2 ) contained random high energy high angle grain boundaries (HAGBs) along with high energy symmetrical and asymmetrical tilt boundaries.

1. Introduction Metal nano-composites coatings, containing oxide and carbide particles, have been developed due to their multifunctional properties and ability to withstand aggressive environments. Nickel, is one of the widely used material as metal matrix in composite coatings used in automobile and aerospace industries. Research reports suggests that inclusion of ceramic micro-particles such as SiC [1–3], Al2 O3 [4,5], CeO2 [6], MoO3 [7], SiO2 [8], ZrO2 [9], B [10], W [11], TiC [12], TiN [13], in Ni coatings have enhanced the tribological and corrosion properties of the coatings. In addition to the micro-sized particles, addition of second phase nanoparticles and other carbonaceous additives such as graphene, graphene oxide and carbon nanotubes (CNT) in nickel based composite coatings have also been explored. Low et al. [14] found that inclusion of nano-sized particles enhanced hardness to 750/100 g indenter (Ni-Al2 O3 ) and corrosion resistance to 7 gm2 /hr, modified the growth of metal deposit and also shifts the reduction potential of the metal ions. Nanoparticles get homogenously dispersed and provide improved reinforcement to the metal-matrix [15]. Graphene and graphene oxide

∗

incorporation in Ni composite coatings have been widely reported to enhance the corrosion resistance properties of the coatings. Praveen et al. [16] have investigated corrosion behaviour in pure Ni and Ni-graphene composite coatings and have observed grain refinement as well as a change in preferred orientation of Ni from ⟨220⟩ to ⟨200⟩. Qi et al. [17] have reported on electro-brush plating to achieve homogenous incorporation of graphene oxide into Ni, thereby enhancing the hardness to 8.65 ± 0.41, thermal stability and corrosion resistance to 39.3 × 10−3 mpy of the composite coatings. Wang et al. [18] have reported that texture coefficient, micro hardness and corrosion rate of Ni-graphene oxide composite coatings are highly sensitive to the coating current density. In their earlier work [19] the authors have also reported on the existence of an optimum with respect to the amount of graphene oxide (coating produced from 0.625 g/L of graphene oxide in electrolyte) in the coatings needed for achieving very high corrosion resistance (0.405 mpy) performance by the Ni-graphene oxide composite coatings. CNTs have attracted attention because of their unique chemical and physical properties which have applications in biosensors [20], composites [21], electronic devices [22], etc. Due to high elastic modulus up to 1 TPa, tensile strength about 60 GPa and large length/diameter

Corresponding author. Department of Materials Engineering, Indian Institute of Science, Bangalore, 560012, India. E-mail address: [email protected] (C. Srivastava).

https://doi.org/10.1016/j.mtla.2020.100617 Received 9 October 2019; Accepted 4 February 2020 Available online 5 February 2020 2589-1529/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

K.S. Jyotheender, A. Gupta and C. Srivastava

ratio enhancement of wear properties in CNT containing metal matrix composite coatings have been widely reported [23–25]. Reports on the effect of CNT to enhancement the corrosive properties are also present in the literature [26]. With respect to the Ni based composite coating, CNT incorporation in Ni coatings are concerned mainly with improvement in the wear resistance properties. Few reports are also available on the corrosion properties of Ni-CNT composite coatings. Guo et al. [25] reported on the development of homogenous surface Ni-CNT coatings produced by the pulse-reverse electrodeposition method. Their report indicates an increases then decreases in corrosion resistance of the coatings by a corresponding increase in the pulse frequency and reverse ratio, therefore suggesting combination of 30% reverse ratio and 100 Hz pulse frequency as ideal conditions. Chen et al. [27] have reported that CNT addition improves the resistance to pitting in Ni-CNT composite coatings. Carpenter et al. [28] reported that Ni-CNT coatings produced using functionalised CNTs show significant homogeneity and hardness compared to coatings developed from as-synthesised CNTs. Their report suggested that CNTs enhance the wear resistance of nickel coatings by forming a thin adherent oxide layer. In crystalline solids, nature and relative abundance of different types of grain boundaries determine the physical, chemical, mechanical properties and corrosive behaviour of the material. Grain boundary energy varies as a function of all five macroscopic crystallographic parameters, in which lattice misorientation corresponds to three and grain boundary plane orientation corresponds to the remaining two [29–31]. Only few reports in the literature provide details on correlation between corrosion properties and grain boundary character distribution. An et al. [32] reported on the intergranular corrosion behaviour of stainless steel by correlating it with five parameter grain boundary character distribution. Their report suggests that high angle grain boundaries (HAGBs) and low coincidence site lattice (CSL) do not show a significant difference with respect to intergranular corrosion, while for the low angle grain boundaries (LAGBs), the corrosion behaviour is very sensitive to the misorientation angles. Grain boundary engineering (GBE) is an effective way to modify grain boundary structure in order to obtain desired properties. GBE typically involves increasing the percentage of special boundaries like CSLs rather than random HAGBs via thermo-mechanical processing (TMP) [33]. TMP involves uniaxial tension/compression or cold rolling followed by annealing. Few researchers have reported on the impact of GBE and the role of CSLs on the corrosion behaviour of materials. Deepak et al. [34] reported hot corrosion and depletion of alloying elements in the specimen with continuous network of HAGBs in comparison with GBE specimen. Telang et al. [35] reported enhanced corrosion and stress corrosion cracking resistance in alloy 600 after GBE via TMP. According to the reports on corrosion behaviour-microstructure correlations, GBE is typically conducted by TMP (annealing). In the present paper it is illustrates that aside form TMP, inclusion of nanoscale secondary phase particles such as CNTs can also help in GBE. Five parameter grain boundary character distribution was used to correlate the corrosion behaviour of Ni-CNT coatings with its microstructure. 2. Experiment 2.1. CNT synthesis A quartz tube furnace was used for the CNT synthesis. The furnace was heated to 800 °C at a rate of 5 °C/min. Argon gas was continuously passed at a constant rate of 30 sccm into the furnace. Precursor consisting of 1 g ferrocene (99% Alfa Aesar) mixed with 100 ml toluene was used for the CNT synthesis. Precursor solution was poured at a rate of 30 ml/hour into the quartz tube maintained at 800 °C. After 1 hour of pouring, heating is stopped and quartz tube was allowed to cool down in argon atmosphere. CNT was later collected from the walls of the quartz tube. In order to remove excess of iron (due to ferrocene) and amorphous carbon, as-synthesised CNTs were washed with 10 wt.% HCl solution

Materialia 9 (2020) 100617

Table 1 Composition of electrolyte bath and deposition parameter used for Ni and NiCNT composite coatings. Sample Ni only

Ni-CNT1 Ni-CNT2 Ni-CNT3 Ni-CNT4 Ni-CNT5 Ni-CNT6 Ni-CNT7 Ni-CNT8 Ni-CNT9 Ni-CNT10

Electrolyte composition

Concentration

Operating parameters

NiSO4 •6H2 O

0.1 M

NiCl2 •6H2 O Na2 SO4 Boric acid (H3 BO3 ) SLS Ni only + specific concentrations of CNT

0.1 M 0.3 M 0.3 M 1 g/L 4 mg/L 6 mg/L 8 mg/L 10 mg/L 20 mg/L 30 mg/L 40 mg/L 50 mg/L 70 mg/L 100 mg/L

pH = 2.5 Time = 60 min Stirring speed = 100 RPM Temperature = 45 ± 2 °C Current density = 3 mA/cm2

several times till the yellow colour (iron chloride) of the washing solution vanished. CNTs were then stirred in the solution of H2 SO4 and HNO3 (volumetric ratio of 1:2) at 80 °C for 3 h to functionalise them. The functionalised CNTs were then thoroughly washed with distilled water to remove acid contents and later dried. Functionalization of CNTs is important to make them hydrophilic for uniform dispersion in the aqueous based electrolytes.

2.2. Electrodeposition of Ni and Ni-CNT composite coatings Electrodeposition was done on polished mild steel coupons of 2 cm × 2 cm × 0.05 cm dimensions. Polishing was systematically conducted using 200–2500 grit emery papers. Platinum plate with 2.5 cm × 2 cm × 0.05 cm dimensions was used as anode. Two terminal D.C power at a constant current mode was supplied for electrodeposition. Electrolyte bath used for Ni electrodeposition consisted of nickel sulphate and nickel chloride as sources of nickel ions. Chemicals such as boric acid, sodium lauryl sulphate (SLS) and sodium sulphate were also added into the electrolyte bath respectively as buffering agent, surfactant and to enhance the deposition rate. CNTs of proposed weight as shown in Table 1 were dispersed into the electrolyte bath to develop Ni-CNT composite coatings containing different volume fractions of CNTs. In order to obtain uniform dispersion of CNT, the electrolyte was subjected to probe sonication for about 15 min while maintaining the bath temperature at 45 °C. Table 1 provides details of the bath composition and deposition parameters.

2.3. Corrosion test Tafel polarisation & EIS measurements were conducted to study the corrosion properties. Polarization studies were carried out at OCP values obtained after exposing the working electrode (coating) in 3.5 wt.% NaCl solution for 1 hour. The investigations were done on 1 cm2 working electrode within the potential range of ±400 mV. Parameters like linear polarization resistance (Rp ), cathodic and anodic Tafel constants (𝛽 c , 𝛽 a ) and corrosion potential (Ecorr ) were determined from the Tafel curves and are shown in Table 2. The corrosion current density (icorr ) of coatings was evaluated from corrosion kinetic parameters using the Stern-Geary equation [19]. 𝑅𝑝 =

𝑖corr

(

1 1 𝛽𝑎

+

1 𝛽𝑐

)

(1) ln 10

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Table 2 Corrosion potential (Ecorr ), anodic and cathodic Tafel slopes (𝛽 a , 𝛽 c ) and linear polarisation resistance (Rp ) values obtained from potentiodynamic polarisation test (Tafel polarisation). Sample

Ni Ni-CNT1 Ni-CNT2 Ni-CNT3 Ni- CNT 4 Ni- CNT 5 Ni- CNT 6 Ni- CNT 7 Ni- CNT 8 Ni- CNT 9 Ni-CNT10

voltage and 3.2 nA current and step size of 70 nm. TSL-OIM Analysis 8 was used to determine grain size, Grain Boundary Character Distribution (GBCD) and Grain Boundary Plane Distribution (GBPD) of OIM scans.

Parameters Ecorr (V)

1/𝛽 c (1/V)

1/𝛽 a (1/V)

Rp (ohm)

icorr (μA/cm2 )

−0.469 −0.392 −0.356 −0.330 −0.400 −0.413 −0.400 −0.350 −0.303 −0.358 −0.466

4.597 6.413 7.200 7.286 5.916 5.017 4.914 5.511 7.804 4.871 5.260

4.51 3.585 3.070 4.072 5.164 5.240 5.250 3.945 2.954 6.020 4.987

4496.5 22,774.4 28,978.1 42,152.6 22,733.4 12,981.2 12,984.0 20,838.7 61,411.1 15,738.7 3014.7

10.62 1.91 1.46 0.98 1.73 3.26 3.29 2.20 0.66 2.53 14.08

2.4. Characterisation Fourier transform infrared (FTIR) spectroscopy was conducted on functionalised CNT using Bruker Tensor 2 (with Pt ATR module). ESEM QUANTA 200 scanning electron microscope (SEM) operating at 25 kV coupled with energy dispersive X-ray spectroscopy (EDS) were used to image surface morphology and to determine the elemental composition of the coatings. CHI electrochemical workstation was used to conduct corrosion studies. Aerated 3.5 wt.% NaCl solution was used as corrosive medium. Raman spectroscopy was performed on Lab RAM HR (UV) system consisting of a diode pumped solid state laser operating at 532 nm wavelength with a charge coupled detector. A three electrode apparatus with coating surface of 1 cm2 as working electrode, standard Ag-AgCl electrode and platinum foil (2.5 cm × 2 cm × 0.05 cm dimensions) as reference and auxiliary electrodes respectively were used. Electron back scatter diffraction (EBSD) analysis was conducted on the coating cross-section samples prepared using the FEI Helios focussed ion beam (FIB) system. EBSD measurements were taken at 52° tilt 30 kV operating

3. Results Representative TEM micrograph of as-synthesised CNT provided in Fig. 1(a) clearly reveals the formation of nanotubes with average diameter of 30–40 nm. FTIR spectrum obtained from as-synthesised CNTs is provided in Fig. 1(b). The FTIR spectrum reveals the presence of various functional groups: O–H stretching vibrations around 3230 cm−1 , C–H vibrations at 2910 cm−1 and 2845 cm−1 , C = O vibrations at 1690 cm−1 and C–O vibrations around 1070 cm−1 . Fig. 1 collectively confirmed the formation of CNTs. Representative SEM micrographs showing surface morphology of Ni and Ni-CNT coatings are provided in Fig. 2(a). Compact and crack free morphology can be observed for all the coatings. Variation of carbon content (wt.% carbon due to CNT addition) of coatings was as follows: Ni-CNT1 (0.26 ± 0.04), Ni-CNT2 (0.47 ± 0.04), Ni-CNT3 (0.9 ± 0.06), Ni-CNT4 (1.59 ± 0.15), Ni-CNT5 (1.61 ± 0.1), Ni-CNT6 (1.63 ± 0.12), Ni-CNT7 (1.76 ± 0.10), Ni-CNT8 (1.84 ± 0.21), Ni-CNT9 (1.856 ± 0.36), Ni-CNT10 (2.01 ± 0.38). It can be observed that the carbon percentage increases distinctly from Ni-CNT1 to Ni-CNT4, and thereafter it exhibits a gradual increase. Note that the error bars which represent the spread of composition data increases with increase in the CNT amount. This strongly indicated towards agglomeration of CNTs in the composite coatings with higher CNT amounts. The representative SEM morphological images of the Ni-CNT3, Ni-CNT8 and Ni-CNT10 coatings showing partially embedded CNTs is provided in Fig. 2(b). To further examine the presence of CNTs in Ni-CNT composite coatings, Raman spectroscopy was performed. Fig. 3 shows representative Raman spectrum obtained from Ni-CNT3 and Ni-CNT8 composite coatings which showed very low corrosion rate and Ni-CNT10 composite coating which showed the highest corrosion rate. In all the Raman spectrums the typical peaks corresponding to CNT were observed. These Fig. 1. (a) TEM micrographs of as-synthesised CNT, (b) FTIR spectrum of as-synthesised CNT.

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 2. (a) SEM micrographs of Ni and Ni-CNT composite coatings, (b) High magnification SEM micrographs of Ni-CNT3, Ni-CNT8 and Ni-CNT10 coatings.

growth happened along the (220) plane. This is again confirmed from the EBSD analysis described later. 3.1. Corrosion studies

Fig. 3. Raman spectrum of Ni-CNT3, Ni-CNT8 and Ni-CNT10 coatings.

peaks are d-peak at ~1350 cm−1 , G-peaks at ~1587 cm−1 , D’-peaks at ~1625 cm−1 and G’(2D) peak at ~2695 cm−1 . Raman spectrum confirmed the presence of CNT in the Ni-CNT composite coatings [36]. XRD curves obtained from the as-deposited coatings is provided in Fig. 4. The observed peaks in the XRD profile correspond to face centred cubic Ni phase. The XRD profiles also show that for all the coatings the

Fig. 5 shows the Tafel polarization curves of Ni and Ni-CNT coatings. Higher (more positive) values of corrosion potential (Ecorr ) and low values of corrosion current density (icorr ) signify anti-corrosive nature. In the Ni-CNT coatings, with the addition of CNT the corrosion potential moved to positive potentials till Ni-CNT3 concentrations and then showed opposite trend. With continued CNT addition, the potential then again shifted back to higher positive values till Ni-CNT8 concentration. With very high concentrations of CNT in coatings i.e. for Ni-CNT10 coating, the corrosion potential dropped to values below the one observed for pristine Ni coating. Similarly, pure Ni and Ni-CNT10 coatings showed highest icorr values whereas lowest icorr values were observed for Ni-CNT3 and Ni-CNT8 coatings. The icorr values for Ni, Ni-CNT3, Ni-CNT8 and Ni-CNT10 coatings respectively were 10.62 μA/cm2 , 0.98 μA/cm2 , 0.66 μA/cm2 and 14.08 μA/cm2 . In comparison to pure nickel coating, Ni-CNT3 and Ni-CNT8 coatings exhibited approximately 91% and 94% drop in corrosion rate respectively and Ni-CNT10 coating showed 32.67% higher corrosion rate than pristine Ni coating thereby suggesting degradation of corrosion properties. A sinusoidal amplitude of 5 mV with frequency range of 10−2 –106 Hz was used to conduct EIS measurements. Fig. 6(a and b) shows Nyquist and Bode plots depicting the impedance data of Ni and Ni-CNT coatings. The Nyquist plot displays impedance curves as imaginary impedance verses real impedance which generates data as semicircle capacitive loops. Larger the capacitive loop diameter, higher the impedance value which signifies higher anti-corrosive property. Large diameter capacitive loops can be observed for Ni-CNT3 and Ni-CNT8 coatings and for Ni-CNT10 coating the capacitive loop diameter is smaller than pure Ni coating. At certain critical concentrations CNT, the composite coating

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 4. X-ray diffraction patterns of Ni and Ni-CNT composite coatings.

is the solution resistance, Rcoat represents passive film resistance, Rct is the charge transfer resistance i.e. resistance offered to the metal ion diffusion. The overall resistance offered to the corrosion reaction is denoted by polarization resistance (Rp ) and is the sum of Rcoat and Rct (Rp = Rcoat + Rct ). By taking surface roughness and inhomogeneity on the coating surface into account, the ideal capacitance element in EEC is replaced with constant phase element (CPE) and defined as Q. The impedance of CPE is defined as: 𝑍CPE =

Fig. 5. Tafel polarisation curves of Ni and Ni-CNT composite coatings.

offers very high corrosion resistance, but for higher CNT concentrations, the corrosion resistance property decreases below pure Ni coating. Bode plots corresponding to EIS measurements are shown in Fig. 6(b). The |Z| values in the high frequency range are close together in all the coatings, whereas in low frequency range they move apart. An upward shift in |Z| plot with CNT addition indicates improved corrosion resistance. Besides, coatings with broadened phase angle and maximum phase suggests anti-corrosive properties. Ni-CNT3 and Ni-CNT8 coatings exhibit higher |Z| values as well as maximum phase with broad phase angle in the bode plot. But |Z| and phase maximum of pure Ni coating was higher than Ni-CNT10 showing coatings with higher CNT amount show degraded corrosion properties. The data points of Nyquist and Bode plots obtained after EIS measurements were fitted to electrical equivalent circuit (EEC) shown in Fig. 6(c) and magnitude of circuit parameters are tabulated in Table. 3. The EEC has 2 RC circuits, where Rcoat and Ccoat are high frequency elements corresponding to the passive behaviour and low frequency elements Rct and Qdl corresponding to the corrosion activity [37]. Rs

1 𝑄(𝑗𝜔)𝑛

(2)

where Q is the magnitude of CPE, j is an imaginary value, 𝜔 is angular frequency (𝜔 = 2𝜋f, f is frequency), n is CPE exponent (-1 ≤ n ≤ 1). Q corresponds to ideal capacitor for n = 1, an ideal resistor for n = 0, an ideal inductor for n = −1. Qdl is the double layer capacitance corresponding to the electrochemical gradient at the conjunction of electrolyte and coating surface due to corrosion products. Coatings with lowest Qdl value and highest Rct value typically shows high corrosion resistance. From the Table. 3 it can be noted that Ni-CNT3 and Ni-CNT8 coatings exhibit the lowest Qdl values and highest Rct values in comparison to the values exhibited by all other coatings. 3.2. Coating microstructure To understand the correlation between corrosion parameters and CNT amount in Ni-CNT coatings, microstructural analysis of the coatings was conducted using the EBSD technique. Four different coatings were analysed: pristine Ni coating, Ni-CNT3, Ni-CNT8 and Ni-CNT10 composite coatings. Fig. 7(a) shows the EBSD maps of the coatings. It is apparent from Fig. 7(a) that the incorporation of CNTs promoted columnar grain growth parallel to the coating growth direction. Grain size distribution histogram provided in Fig. 7(b) indicates towards grain size refinement and development of bimodal grain size distribution with increasing CNT incorporation into the coatings. Inverse pole figure (IPF) maps shown in Fig. 7(c) suggest strong [110] texture parallel to the growth direction for all the coatings. It is apparent that CNT addition enhanced [110] fibre texture and promoted columnar grain growth. This is very clear in the case of Ni-CNT10 coating. Kernel average misorientation (KAM) is

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Table 3 Ccoat , Rcoat , Qdl and Rct values obtained from EEC simulation of EIS data. Sample

Ni Ni-CNT1 Ni-CNT2 Ni-CNT3 Ni- CNT 4 Ni- CNT 5 Ni- CNT 6 Ni- CNT 7 Ni- CNT 8 Ni- CNT 9 Ni-CNT10

Parameters Rs (ohm cm2 )

Ccoat (μF/cm2 )

6.71 6.61 7.24 7.53 6.29 7.45 6.90 6.54 6.80 5.24 4.32

7.36 5.06 4.66 4.90 3.14 5.67 4.44 5.65 5.53 6.90 7.52

× × × × × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

Rcoat (ohm cm2 )

Qdl (μS-secn))

N

Rct (ohm cm2 )

𝜒2

9.156 10.11 7.762 9.11 13.45 10.16 9.70 9.90 10.96 13.47 10.6

248.00 60.27 31.73 21.5 108.3 273.9 180.5 36.68 21.57 35.21 160.6

0.69 0.77 0.82 0.84 0.79 0.68 0.79 0.85 0.84 0.84 0.67

7129 29,490 48,850 110,600 43,740 7914 13,240 21,640 141,600 66,260 3128

2.64 3.35 2.75 5.46 1.98 5.68 1.77 5.97 2.13 4.94 3.38

× × × × × × × × × × ×

10−3 10−3 10−3 10−4 10−3 10−3 10−3 10−4 10−3 10−3 10−3

a measure of the average misorientation between selected points within the same grain, larger the misorientation higher the dislocation density within the grain [38]. KAM data shown in Fig. 8(a) indicates that Ni-CNT3 coating has large portion of highly strained regions within individual grains and around grain boundaries due to high dislocation density. With increasing CNT concentration form Ni-CNT3 to Ni-CNT8 coating, LAGBs and dislocations come in contact, resulting in dislocation annihilation along {111} slip plane and ⟨110⟩ slip direction. Dislocation annihilation along {111} ⟨110⟩ slip system enhances the degree of misorientation resulting in the formation of coincidence site lattice (CSL) or High angle grain boundaries (HAGBs). Fig. 8(b) shows that Ni and Ni-CNT8 coatings have large fraction of KAM less than 0.5° misorientation followed by Ni-CNT10. A shift in the peak maxima for Ni-CNT3 coating towards higher KAM can also be noticed.

Fig. 6. (a) Nyquist plots of Ni and Ni-CNT composite coatings, (b) Bode plots Ni and Ni-CNT composite coatings, (c) Electrochemical equivalent circuit (EEC) used for simulation of EIS data.

3.2.1. Grain boundary character distribution (GBCD) GBCD plots for the coatings are shown in Fig. 9(a). Fraction of LAGBs is highest in Ni-CNT3 coating whereas HAGBs are dominant in pure Ni and Ni-CNT10 coatings. With initial CNT addition into the coatings, CSLs percent has come down 21.3% in Ni-CNT3 coating possibly due to dislocation rearrangement. At the higher concentrations of CNT, CSLs percent increased to 41.7% for Ni-CNT8 and later dropped to 32.0% in Ni-CNT10 coating. The misorientation angle plot is shown in Fig. 9(b). The frequency distribution suggests that larger portion of misorientation is the range of (0–5°) and at 60°. In all the coatings, low angle grain boundaries with less than 5° misorientation contribute at least a minimum of 35%, whereas in the case of Ni-CNT3 coating it is highest at 68.5%. For the case of 60° misorientation angle, Ni-CNT8 exhibits the highest percent when compared to the other coatings. Fig. 10(a) shows that in all the coatings, boundaries with 60° misorientation promote growth around ⟨111⟩ rotation axes followed by ⟨110⟩. Boundaries with 60° ⟨111⟩ misorientation are generally considered as Σ3 CSLs (coherent boundaries) and are pure symmetrical twist boundaries. Misorientation plot (Fig. 9(b)) also shows a raise in the frequency around 38.9° and grain boundaries with ⟨110⟩ axes of rotation (Fig. 10(a)) suggesting the formation of Σ9 CSLs. The corresponding multiples of a random distribution (MRD) values suggests that Ni-CNT8 has dominant share of Σ3 CSLs as coherent twin boundaries. The CSLs distribution with respect to sigma (Σ) values is shown in the Fig. 10(b and c). CSLs observed in coatings are Σ3 and Σ3n (Σ9 and Σ27a). In the coatings, the distribution of Σ3 was relatively higher when compared to the other CSLs. The relative percentage of CSL boundaries are provided in Table 4 (which can be read as “In pristine Ni coating, among the CSLs 89% are Σ3 boundaries”). A fluctuation in the Σ3 CSLs boundary percentage as a function of CNT concentration in the matrix was observed. The fluctuation of Σ3 CSLs boundary percentage was due to “multiple twinning” or “Σ3 regeneration”. In Ni-CNT8 coating higher order Σ3n combined to generate Σ3 (Σ3n + Σ3n+1 → Σ3), therefore resulting in high percent Σ3 and a corresponding drop in Σ3n . Whereas in the case of Ni-CNT10 coating a reversed phenomenon was

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 7. (a) EBSD mapping of Ni and Ni-CNT coatings, (b) Histogram of grain size distribution, (c) Inverse pole figure maps of Ni and Ni-CNT coatings.

noticed i.e. multiple twinning (Σ3n + Σ3n+1 → Σ3n+2 ) which resulted in relatively more Σ9 and Σ27a boundaries. 3.2.2. Grain boundary plane distribution (GBPD) GBPD of Σ3, Σ9 and Σ27a CSLs for Ni and Ni-CNT coatings are shown in Fig. 11. The Σ3 CSLs with misorientation around [111] correspond to symmetric twist boundaries [39,40]. Highest peak intensity for the coatings is at (111) plane in the stereographic projection with the MRD values maximum for Ni-CNT8 (715.8) and minimum for Ni-CNT10 (300.7). (111) plane misorientation around [111] direction suggests {111}/{111} coherent twins. Stereographic projection shows that with the CNT addition the contours around (111) have changed from concave to convex which indicates reduction in the number of asymmetrical tilt grain boundaries [30]. Ni-CNT10 coating shows high

intensity peaks of (112) as well as (100). Σ3 CSL boundaries interfering about (112) planes are incoherent twins suggesting that the twins in Ni-CNT10 are highly incoherent. Stereographic projection of Σ9 plane distribution shows high intensity peaks along ⟨110⟩ zone suggesting presence of tilt boundaries. In general, Σ9 has many asymmetric tilt boundaries along with {114} and {221} symmetric tilt boundaries. The MRD values are ranging between 7.4 and 60.8 for Ni-CNT8 and Ni-CNT10 coatings respectively. Stereographic projection provided the following information: (i) Ni coating have maximum intensity at (1−1 4) symmetrical tilt on the 110 tilt zone, (ii) presence of {110}/{114} asymmetrical tilt and (2 −2 1) symmetrical tilt in Ni-CNT3 coating, (iii) Ni-CNT8 coating has (1 −1 4) symmetrical tilt as well as {111}/{115} asymmetrical tilt and lower distribution about {221} and (iv) Ni-CNT10 coating has lowest

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 8. (a) Kernel average misorientation (KAM) distribution mapping (b) Graphical representation of KAM fractional distribution.

distribution about {111} symmetrical tilt compared to {221}, and a strong {112}/{552} symmetrical tilt boundary. As closely packed planes have the lowest energy therefore, {111} symmetrical tilt boundaries possess lower energy when compared to {221} and in case of the asymmetrical tilt boundaries, the order is {111}/{115} < {110}/{114} < {112}/{552} [30]. A high diffuse distribution in the case of Ni and Ni-CNT3 coatings validates that the boundaries are not ideal tilt boundaries in the same region. A low distribution of {221} suggest Σ3 regeneration due to decomposition of high energy Σ9 to low energy facets. Σ27a are relatively high energy planes in comparison to Σ3 and Σ9 CSL boundaries. Stereographic projection of Σ27a shows that in all the coatings, planes are orientated about [110] zones i.e. tilt boundaries. Similar to Σ9 even Σ27a also have a high diffuse distribution for Ni and Ni-CNT3 coatings. Of the two symmetrical tilt boundaries i.e. {115} and {552}, planes in pure Ni coating are symmetrical with (1 −1 5). CNT addition has changed Σ27a symmetrical tilt boundaries to (−5 5 2).

4. Discussion

Fig. 9. (a) Grain boundary character distribution (GBCD) of Ni and Ni-CNT coatings, (b) Misorientation distribution of Ni and Ni-CNT coatings.

A strong [110] texture in all coatings (Fig. 7(c)) along the growth direction suggests that for all the coatings, surface exposed to the corrosive environment contains (110) planes. Corrosive medium can reach the coating/substrate interface in the presence of large sized columnar grains via random HAGBs. Therefore, the difference in the observed corrosion behaviour of the coatings was primarily governed by the nature of the grain boundaries. Grain boundary energy depends on the degree of misorientation and therefore metals with high percentage of LAGBs show resistance towards degradation or corrosion. CSLs are special case of HAGBs which have lower energies. Therefore, to obtain better corrosion properties in terms of GBE a coating should predominantly possess low energy grain boundaries and conditions are: (i) Low angle grain boundaries [41] (ii) Σ3 CSLs as coherent twins [43] and (iii) Σ9 and Σ27 CSLs with low energy symmetrical and asymmetrical tilt boundaries [30,43,44]. CSL deviation plot shown in Fig. 12 and MRD values from GBPD from the Fig. 11 gives the details about CSLs as effective special boundaries. CSL deviation plot was calculated on the basis of Brandon’s criterion [45], the plot indicates the scatter of boundaries form the ideal position. Based on the above data, the following text describes the

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 10. (a) Angle/axis misorientation distribution about 38.9° and 60°, (b) Histogram representing the CSLs fraction in Ni and Ni-CNT coatings, (c) CSLs distribution maps.

correlation between the observed corrosion behaviour and the nature of boundaries in Ni, Ni-CNT3, Ni-CNT8 and Ni-CNT10 coatings.

Table 4 Relative percentage of CSL boundaries. Sigma%

Σ3 Σ9 Σ27a Σ27b

Coating Ni

Ni-CNT3

Ni-CNT8

Ni-CNT10

89.0 8.5 2.0 0.5

87.8 9.4 1.9 0.9

90.3 7.4 1.6 0.6

83.7 12.0 4.3 0.0

4.1. Ni coating Ni coating contained lowest percentage of LAGBs (41.2%) and highest percentage HAGBs (22.4%) in comparison of all other coatings. In Ni coating, Σ3 CSL fraction was about 32.4% of total grain boundaries, but the GBPD and MRD values suggested that the Σ3 CSLs were not

K.S. Jyotheender, A. Gupta and C. Srivastava

Materialia 9 (2020) 100617

Fig. 11. Stereographic projection Σ3, Σ9 and Σ27a representing grain boundary plane distribution (GBPD) of Ni and Ni-CNT coatings.

Fig. 12. Σ3, Σ9 and Σ27a CSLs deviation on the basics of Brandon’s criterion.

completely special boundaries (coherent twins) [30,42]. Ni coating had (1 −1 4) symmetrical tilt boundary for Σ9 and (1 −1 5) for Σ27a CSLs. Therefore, the coating possesses only one low energy symmetrical tilt boundary. Above grain boundary characteristics suggest that grain boundaries of pure Ni coatings did not meet the criteria required for providing high corrosion resistance performance. This was also observed in the corrosion measurement results. 4.2. Ni-CNT3 coating Ni-CNT3 coating contained ~70% of LAGBs and 7.9% of HAGBs. The fraction Σ3 CSLs in the Ni-CNT3 coating was less in comparison to the other coatings, it covered nearly 18.7% of total grain boundaries. GBPD of Σ9 CSLs suggests that Ni-CNT3 coating has a low energy asymmetrical tilt boundaries and high energy symmetrical tilt boundaries. Boundaries with symmetrical tilt are highly susceptible for corrosion.

Above characteristics suggest that Ni-CNT3 coatings should exhibit reasonably high corrosion resistance performance. This was also observed in the corrosion measurement results where Ni-CNT3 coatings exhibited very low corrosion rate. 4.3. Ni-CNT8 coatings Compared to pristine Ni coating, LABGs (43.4%) in this coating were in slightly higher fraction. Whereas the percentage of HAGBs (14.8%) was relatively lower. Ni-CNT8 coating contained very high fraction of Σ3 CSLs (Table 4) increase in Σ3 CSLs from Ni-CNT3 to Ni-CNT8 was due to Σ3 regeneration. Σ3 CSL deviation plot shows that larger fraction of boundaries has very small deviation (i.e. < 1°) and GBPD also suggests that it has the maximum MRD around 60°/⟨111⟩ misorientation. Therefore, Σ3 CSL in Ni-CNT8 are special boundaries i.e. coherent twins. Ni-CNT8 has both low energy symmetrical and asymmetrical tilt boundaries. Σ9 are also special boundaries in Ni-CNT8.

K.S. Jyotheender, A. Gupta and C. Srivastava

Thereby, Ni-CNT8 coating which has high percentage of LAGBs and CSLs which satisfies conditions to perform as special boundaries should exhibit high corrosion resistance performance. The corrosion performance results indeed showed that the Ni-CNT8 coating exhibits the best anti-corrosive properties. 4.4. Ni-CNT10 coatings Ni-CNT10 coating contains 47.9% LAGBs and 21% of HAGBs. Σ3 CSL deviation plot showed that this coating contains lowest fraction under small deviation and also lowest MRD value in GBPD suggesting the presence of incoherent twins. Due to multiple twinning, Ni-CNT10 coating contains high fractions of Σ9 and Σ27a. MRD maxima in GBPD for both Σ9 and Σ27a was seen in Ni-CNT10, but the planes are orientated about high energy symmetrical and asymmetrical tilt boundaries. Therefore, Σ9 and Σ27a in Ni-CNT10 are not special boundaries. Above grain boundary characteristics suggest that Ni-CNT10 coating did not meet any of the criterion for high corrosion resistance performance. This is consistence with the high corrosion rate exhibited by the Ni-CNT10 coating. 5. Conclusion The present study investigated the correlation between corrosion behaviour and microstructure of Ni-CNT composite coatings. The surface morphology and macro-texture did not indicate towards any significant difference between the composite coatings. The corrosion, behaviour however, exhibited interesting sensitivity to the amount of CNT contained in the composite coatings. EBSD analysis was conducted to investigate the microstructure of the coatings. IPF showed tendency of ⟨110⟩ orientated grains along the growth direction signifying formation of fibre texture. The electrochemical analysis revealed the possibility of two corrosion rate minima. The initial increases in the corrosion resistance was due to large fraction LAGBs. The later reduction was due to drop in LAGBs and re-arrangements to form low energy CSLs with increasing CNT concentration. Enhancement in the corrosion rate at very high CNT concentrations was due to increase in the relative amount of random HAGBs. Declaration of Competing Interest None. Acknowledgement The authors acknowledge the funding received from the Nano Mission Government of India and microscopy facilities in AFMM, IISc.

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