Abstract
The nanocrystalline, bright, ternary Zn–Ni–Fe alloy electrodeposit has been developed using additives such as tetradecyltrimethylammonium bromide and thiosemicarbazide on mild steel. The microstructure, corrosion resistance, hardness and tribological behaviour of bright Zn–Ni–Fe alloy deposit were analysed. The study revealed that additives modify the morphology, orientation and composition of the Zn–Ni–Fe alloy deposit. The morphological, orientation and chemical composition changes play a significant role by improving corrosion resistance and reducing wear and friction of deposit. Simultaneously, the effect of iron and TSC in plating bath on the corrosion resistance and microhardness of electrodeposited Zn–Ni–Fe coating has been investigated.
Keywords
Introduction
Zinc and its alloy electrodeposition have been extensively used to provide sacrificial protection to steel against corrosion. The alloying of zinc with iron group metals (Fe or Co or Ni) remarkably improves the anticorrosion and mechanical properties of zinc coating. The electrodeposition of binary or ternary zinc alloys with iron group metal is an anomalous codeposition, where less noble metal zinc is deposited preferentially.
In recent years, binary Zn–Ni alloy coatings have gained more significance due to its excellent corrosion resistance in aqueous sodium chloride solution and improved mechanical characteristics than pure zinc coating [1-3]. Binary Zn–Fe alloy deposits are also being used in automobile and aerospace industries mainly due to their low cost, good corrosion resistance, weldability, formability and paintability [4,5]. In addition, Zn–Ni and Zn–Fe alloy coatings are good alternatives for toxic cadmium coating. It is more interesting to bring together the properties of Zn–Ni and Zn–Fe coating by electrodepositing ternary Zn–Ni–Fe alloy deposit in order to enhance the consistency of zinc coating under aggressive environment. The Zn–Ni–Fe alloy coating contain both nickel and iron in zinc matrix and coating exhibits good corrosion resistance and other functional properties than compared to pure Zn, Zn–Ni and Zn–Fe alloy coatings [6-8].
The additives used in zinc and its alloy plating bath are usually brighteners and surfactants. The brighteners give a bright appearance to the deposit. The surfactants increase the solubility and dispersion of the brightener in the bath. The use of specific plating bath additives influences the grain size, orientation and chemical composition of the deposit [9]. These changes affect on corrosion resistance and other properties of alloy deposit [1-4]. The examination of results revealed that the bright Zn–Ni–Fe electrodeposit in the presence of additives has been found to be more beneficial compared to dull Zn–Ni–Fe as well as bright Zn or Zn–Ni or Zn–Fe alloy coating [10-12]. Chitharanjan et al. have electrodeposited bright, ternary Zn–Ni–Fe alloy deposit with good corrosion resistance in the presence of additives such as gelatin, sulfanilic acid and ascorbic acid [10]. Karahan et al. described that the inclusion of Fe or Fe–Ni into the zinc matrix increases the corrosion resistance of Zn–Ni–Fe alloy coating in the presence of gelatin [11]. The literature pertaining to the use of additives (other than gelatin) in Zn–Ni–Fe alloy electrodeposition and their effect on grain size, orientation, corrosion behaviour and tribological properties of coating are very limited.
Karahan et al. also reported that the increase of gelatin content as well as the decrease of the iron content of plating bath increases the corrosion resistance of Zn–Ni–Fe alloy deposit [11]. The study revealed that the brightening additive and iron composition of plating bath greatly influence on protection ability and other properties of Zn–Ni–Fe deposit.
In the present work, Zn–Ni–Fe alloy coating has been electrodeposited on mild steel from an acid sulphate bath using additives such as tetradecyltrimethylammonium bromide (TDTAB) and thiosemicarbazide (TSC). The influence of additives on chemical composition, microstructure and properties of Zn–Ni–Fe alloy deposit has been evaluated. The effect of iron and TSC in plating bath on corrosion resistance and microhardness of electrodeposited Zn–Ni–Fe coating was also examined.
Experimental
Optimised bath composition and operating conditions for electrodeposition of Zn, Zn–Ni, Zn–Fe and Zn–Ni–Fe alloys on mild steel.
The cathodic current efficiency (CCE) of the dull and bright plating bath (Table 1) was calculated based on Faraday's law [10]. The chemical composition of ZNFdull and ZNFbright deposit as well as coating obtained in the presence of different concentrations of iron (4–24 gL−1 FeSO4·7H2O) and TSC (80–240 mgL−1) in ZNFbright bath were analysed using atomic absorption spectrometer (GBC, Australia). The alloy was electrodeposited on a copper plate for 10 min. A known weight of deposit was stripped into a known volume of 10% HCl solution and then analysis of iron and nickel content was carried out.
The surface morphology of the coating was characterised by recording the scanning electron microscope (SEM) using FESEM (JSM-840A, JEOL). The average grain size and orientation of the deposit were examined by X-ray diffraction (XRD) studies using PANalytical X'pert PRO powder diffractometer with graphite monochromatized Cu Kα radiation (0.1540 nm) as a source. Surface profiles of ZNFdull and ZNFbright coating were examined using a Stylus profilometer (Talysurf Plus, Rank Taylor Hobson Ltd., U.K.). Five tracings at different locations of coating were made and the average value of surface roughness (Ra) of the deposit has been calculated.
The dull and bright coatings were electrodeposited for 10 min at 4 Adm−2 and subjected to corrosion studies by electrochemical methods using CHI660C electrochemical workstation in a conventional three electrode electrochemical cell. The coated mild steel surface with an exposed surface area of 1 × 1 cm acts as a working electrode. The saturated calomel electrode (SCE) and Pt wire (1 mm dia.) serve as reference and counter electrodes, respectively. Before each measurement, a coated specimen was immersed in 3.5% NaCl solution for about 10 min to establish steady-state open circuit potential (OCP). About 30 samples of deposits were used for each measurement in corrosion studies.
The ZNFbright coatings were electrodeposited for 20 min at 4 Adm−2 in the presence and absence of additive as well as at different concentrations of iron and TSC in ZNFbright bath. Microhardness of the coatings were determined by using Vickers indenter (microhardness tester FM-800 FUTURE-TECH) with a test load of 50 g for 5 s. Five measurements at different locations of each coating surface were performed and an average value has been reported as microhardness of the deposit.
The dull and bright Zn–Ni–Fe alloy deposits were electrodeposited on steel cathode for 20 min to analyse the tribological behaviour of coating. Tribological experiments were carried out using the ball on disc tribometer (Reciprocating friction monitor-282-M111, Ducom, Bengaluru) at 500 mN load. The coated sample was mounted on to the sample holder. The coated samples were sliding against a stationary steel ball (DIN 100Cr6) of 2 mm diameter (rms roughness ≈ 1−2 nm) and no lubricant was used (dry run). The ball was cleaned with acetone in an ultrasonic bath prior to the experiment. The tests were carried out at 27 ± 2°C with a sliding speed of 0.05 cms−1. During testing, the wear and friction coefficient of deposit was continuously recorded with the help of a personal computer.
Results and discussion
Surface morphology and orientation of deposit
The SEM images of the Zn–Ni–Fe deposits obtained in the presence and absence of additives are given in Figure 1. The SEM image of ZNFdull coating (absence of additives) showed randomly distributed coarse grains (Figure 1(a)), whereas ZNFbright coating obtained in the presence of additives contain nanocrystalline grain with size less than 50 nm (Figure 1(b)). This indicates that the additives modify the conventional coarse grained, ZNFdull deposit to smooth, compact, nanocrystalline bright ZNFbright deposit (Figure 1(b), inset).
SEM images of (a) ZNFdull and (b) ZNFbright deposits.
The XRD pattern of the deposits obtained from bath ZNFdull, and ZNFbright are shown in Figure 2(a) (* marks peak corresponding to the steel substrate). The preferred orientations of the deposits were determined by calculating the texture coefficient (Tc). The orientation with maximum texture coefficient value was the preferred orientation of the deposit [1]. The texture coefficient (Tc) values calculated for each peak of the XRD pattern are given in Figure 2(b). The ZNFdull deposit obtained in the absence of additives is a solid solution of iron and nickel in zinc and shows the presence Fe4Zn9 intermetallic phase. The average grain size of Zn–Ni–Fe dull deposit is 85 nm calculated using the Scherrer equation. The maximum Tc value of 39% is observed for (101) plane, hence pyramidal (101) plane is the preferred orientation of the ZNFdull deposit. (101) is the lower energy plane formed in the absence of additives when compared to (100) and (110) planes [13].
(a) XRD pattern and (b) Tc values as a function of crystallographic planes of the deposits.
The XRD pattern of ZNFbright deposit shows that additives reduce the average grain size to 40 nm from 85 nm of ZNFdull deposit. The presence of additives in the electrodeposit promotes the growth of (100) and (110) planes and diminishes the growth of (101), (102) and (103) planes. (100) and (110) planes require higher energy for its formation and deposition occurs at higher overpotential [13]. During the deposition, adsorption of additives (TDTAB + TSC) on mild steel surface acts as a barrier and hinders the deposition which leads to the growth of higher energy prismatic (100) and (110) planes in bright coating.
The maximum 56% of crystals grow along (100) plane, hence prismatic (100) plane was the preferred orientation of ZNFbright deposit. The additives change the preferred orientation of the ZNFbright deposit to prismatic (100) plane from pyramidal (101) plane (ZNFdull). The addition of Fe and Ni into zinc matrix promotes the development of (100) texture (Figure 2(b)) as reported by Karahan and Guder [12]. The study confirms the influence of additives on (100) texture of Zn–Ni–Fe deposit.
CCE and chemical composition
The CCE of ZNFdull and ZNFbright baths at 4 Adm−2 were found to be 74% and 80% respectively. The additives improve the CCE of bright Zn–Ni–Fe alloy plating bath. The chemical composition of the ZNFdull deposit obtained in the absence of additives was found to be 88.35% Zn, 10.35% Ni and 1.3% Fe whereas in the presence of additives, ZNFbright coating contain 95.34% Zn, 3.70% Ni and 0.96% Fe. This confirms that additives (TDTAB + TSC) lower the iron and nickel (alloy metal) content of the bright Zn–Ni–Fe deposit. The extent of reduction of nickel and iron content of the deposit depends on the interaction of both the additives (TDTAB and TSC) with Ni and Fe ions in the bath solution during the deposition process.
The effect of iron and TSC in plating bath on Fe and Ni content of Zn–Ni–Fe alloy deposit has been analysed. The nickel (1.9–4.7%) and iron (0.5–1.4%) content of the ZNFbright deposit increased with the iron content of the bath (Figure 3(a)). The presence of TDTAB alone in ZNFbright bath lowers the Ni (1%) and Fe (0.5%) content of the deposit. The addition of different concentration of TSC to plating bath (with TDTAB) increases the nickel (1.4–5.2%) and iron (0.81–1.28%) content of ZNFbright deposit (Figure 3(b)). Thiosemicarbazide initially gets associated with Ni and Fe ion in plating bath solution forming adducts which reduced at the cathode during deposition [9,14]. This leads to the increase of Ni and Fe content of the deposit with increasing TSC concentration. This confirms the influence of TSC in bath solution on the alloy composition. Hence TSC as well as iron content in plating bath influences on rising alloy metal content of the ZNFbright deposit.
The effect of (a) iron and (b) TSC in plating bath on the chemical composition of the ZNFbright coating.
Corrosion studies
The developed coating was largely employed for sacrificial protection of the iron and steel product against corrosion. Hence the corrosion resistance of the coating was analysed by adopting Tafel extrapolation and impedance technique in 3.5% NaCl. During corrosion, pseudo-passive layer of zinc hydroxide [Zn(OH)2], zinc oxide (ZnO) and zinc hydroxyl chloride [Zn5(OH)8Cl2·H2O] were formed on coating surface which prevent further dissolution [15].
Tafel extrapolation
Figure 4(a) shows the Tafel curves recorded for ZNFdull, Zbright, ZFbright, ZNbright and ZNFbright deposits in the potential range −0.2 to +0.2 V from OCP. The corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate (CR) and anodic/cathodic Tafel slopes (βa and βc) derived from each Tafel plots are listed in Table 2. The ZNFdull coating obtained in the absence of additives showed more negative Ecorr, higher Icorr and corrosion rate compared to bright deposit. Hence ZNFdull coating easily undergoes corrosion. SEM image confirms the free dissolution of coarse grains of diameter greater than 1 μm (Figure 4(b)).
(a) Tafel curves and SEM images of (b) ZNFdull and (c) ZNFbright deposits after Tafel studies. Corrosion parameters from Tafel plots.
In the presence of additives, more positive corrosion potential, lesser corrosion current and lower anodic Tafel slope were observed for bright alloy coating in comparison with the dull coating. The study confirms the slow dissolution of zinc from bright alloy deposit surface during corrosion.
The alloying of zinc with iron and nickel respectively to form binary ZFbright and ZNbright alloy coating shows a positive shift in Ecorr value and lower Icorr. Thus ZFbright and ZNbright coatings exhibit lesser corrosion compared to Zbright deposit. Further, in ZNFbright alloy deposit both nickel and iron are incorporated in zinc matrix which shows more positive Ecorr, much lower Icorr than ZFbright and ZNbright coatings and thus exhibit higher corrosion resistance. Due to smaller iron and nickel content of deposit the shift in corrosion potential values are very closer in different alloy deposits compared to pure zinc coating.
The corrosion rate of ZNFbright deposit (8.10 μgh−1) is approximately four times lesser than that of ZNFdull deposit (33.81 μgh−1). Thus ZNFbright coating undergoes dissolution less easily compared to ZNFdull deposit as shown by the SEM image (Figure 4(c)). This confirms that the presence of additives improves the corrosion resistance of the bright Zn–Ni–Fe alloy deposit.
Electrochemical impedance
The corrosion behaviour of the coatings has been evaluated by electrochemical impedance technique at OCP in the frequency range 10 mHz to 100 kHz with a 5 mV sine wave as an excitation signal. EIS data are fitted to a suitable equivalent circuit by using ZSimp-Win 3.21 software. EIS data are presented as Nyquist (−Z″ versus Z′) and Bode (frequency versus phase angle/|Z|) plots as shown in Figure 5(a,b) respectively. The proposed electrically equivalent circuit (EEC) is given in Figure 5(c) and impedance parameters are listed in Table 3. In order to obtain the exact fitting capacitance element Cdl is related to constant phase element (CPE) Qdl in EEC. The CPE represent all the frequency dependent electrochemical phenomena, namely double layer capacitance and diffusion process. The coefficient (n) of CPE, n = 1 gave a real capacitive sense to the electrical element and n < 1 shows a non-homogeneous distribution of current due to the surface roughness and defects [16].
(a) Nyquist and (b) Bode plots of ZNFdull and ZNFbright deposits and corresponding (c) EEC. Impedance data.
The proposed EEC is Rs(Cf(Rf(Qdl(Rct(CF RF))))). The contribution of each element in the circuit is as follows. Rs is the resistance of electrolyte present between the coating surface and the reference electrode. The high-frequency contribution (Cf – Rf) is attributed to the dielectric character of thin corrosion product layer (Cf) and electrical leakage due to ionic conduction through its pores (Rf). The medium-frequency couple (Qdl – Rct) is ascribed to double layer capacitance (Qdl) at the electrolyte/coating interface below the porous corrosion product layer and Rct is charge transfer resistance (corrosion rate). The lower frequency couple (CF – RF) is associated with the redox processes occurring at the substrate/coating interface. The total polarisation resistance (RP) is the sum of Rf, Rct and RF i.e. RP = Rf + Rct + RF.
The ZNFbright coating obtained in the presence of additives showed larger Rct and RP values and smaller Qdl compared to ZNFdull coating. This confirms the superior anticorrosion property of the developed ternary ZNFbright alloy deposit. The three time constants obtained in Nyquist plots are also observed in frequency versus phase angle Bode plot (Figure 5(b)) which confirms the existence of three relaxation process during corrosion. The first time constant is in the high-frequency range 105 Hz (small rising part), the second time constant appears between 103 and 102 Hz, and the third time constant at the low-frequency range 101–10−1 Hz. In Bode plots at high-frequency response describes the behaviour of the coating when it is in contact with corrosive media. ZNFdull and ZNFbright coatings minimum phase angle with less resistance was observed which corresponds to the first time constant (105 Hz). There is no considerable corrosion product layer on the coated surface at initial (10 min) stage of immersion in 3.5% NaCl solution, hence the phase angle is minimum with a smaller resistance value. This small rising part might also be due to the equipment. The second time constant (103 and 102 Hz) corresponds to the charge transfer resistance peak. The third time constant (101–10−1 Hz) is associated with processes occurring at the substrate/coating interface. Further, the frequency against impedance modulus |Z| Bode plot revealed that ZNFbright coating showed higher |Z| value, and the observed trend is similar to the Tafel and Nyquist plot analysis study.
The corrosion behaviour of metallic coating depends on the morphology, orientation and chemical composition of the deposit. The smooth, compact and nanocrytalline, bright coating contain a larger number of nearest neighbouring atoms for binding, hence higher energy is required for breaking of metallic bond and dissolution. The nanocrystalline coating contains a large number of active atoms on the surface which accelerate the formation of an oxide protective layer which prevents further oxidation of zinc from the deposit surface as reported by Zhang [17]. Hence, nanocrystalline ZNFbright coating containing lower Ni and Fe content showed the highest corrosion resistance than coarse-grained ZNFdull coating with higher content of Ni and Fe. The atoms in different crystallographic planes have different binding energy due to different coordination of atoms in different crystallographic planes. Hence dissolution occurs at a relatively different rate from different crystallographic planes. The low index crystallographic planes require higher energy for dissolution of atoms due to the higher binding energy of atoms [18]. The ZNFbright coating are having a preferred orientation along (100) low index plane. This accounts for the higher corrosion resistance of bright deposit than compared to dull deposit.
The nanocrystalline ZNFbright alloy coatings obtained in the presence of additives exhibit higher corrosion resistance than bright, nanocrystalline pure zinc coating (Zbright) (Table 2). In the presence of additives alloying of zinc with nickel and iron (noble metals) shifts the corrosion potential to more noble direction from −1.005 V (Zbright) to −0.973 V (ZNFbright). Hence, dissolution of zinc becomes more difficult in the presence of iron and nickel in ZNFbright coating. Thus, chemical composition (iron and nickel content) of the coating influences on corrosion behaviour of nanocrystalline ZNFbright alloy deposit.
Nyquist plots recorded for ZNFbright coatings obtained at different concentrations of iron (4, 8, 12 and 16 gL−1 FeSO4·7H2O) and TSC (80, 160, 240 mgL−1) in the plating bath is shown in Figure 6. The EEC given in Figure 5(c) is used to fit the impedance data in Figure 6. The total RP value was calculated from each Nyquist plot and plotted against iron and TSC concentration in the plating bath. Figure 7 shows that the RP values of ZNFbright coating increase with iron and TSC.
Nyquist plots for ZNFbright coating at different concentration of (a) TSC and (b) iron in plating bath. Total polarisation resistance (RP) value of ZNFbright deposit obtained at different concentrations of (a) iron and (b) TSC in plating bath.
The iron and TSC present in the plating bath have a considerable effect on increasing iron and nickel content of the nanocrystalline, bright deposit. With an increase in nickel and iron content of the deposit resistance for dissolution of zinc from coating surface increases and the deposit become more noble. Thus, increase in iron and nickel content of the deposit improves the corrosion resistance of developed ZNFbright coating at different concentrations of iron and TSC in the plating bath.
Microhardness
Vickers microhardness (HV) of the ZNFdull and ZNFbright coating were found to be 145 ± 5.8 and 138 ± 6.53 HV respectively. The nanocrystalline bright Zn–Ni–Fe coating with (100) texture shows almost similar microhardness as coarse-grained ZNFdull coating. The observed results are in contrast with literature reports where deposit with smaller grain size and (100) texture exhibited higher hardness [19-21]. Thus microhardness value of ZNFbright deposit was not only influenced by grain size and texture but also depends on the chemical composition of the coating. The additives reduce the nickel and iron hard metal content of the deposit when compared to dull coating. Hence, bright ZNFbright coating showed lower microhardness.
ZNFbright coatings were electrodeposited for 20 min at different concentration of iron and TSC in ZNFbright alloy plating bath and microhardness of the deposit was measured. The increase of microhardness of the ZNFbright deposit with an iron concentration of bath is shown in Figure 8(a). This trend is attributed to increase of hardmetals (Ni and Fe) content of the ZNFbright deposit at higher concentration of iron in the plating bath (Figure 3(a)). Figure 8(b) shows that in the absence of TSC, the ZNFbright bath (TDTAB alone) gave the deposit with lower microhardness value of 115 HV. The addition of TSC to bath increases the microhardness of the ZNFbright coating which is attributed to the rise in Ni and Fe content of the ZNFbright deposit (Figure 3(b)).
Microhardness of ZNFbright coating obtained at different concentrations of (a) iron and (b) TSC in plating bath.
Friction and wear
The tribological contact of two solid bodies (coating and steel ball) results in wear and friction phenomenon. The wear and friction behaviours of ZNFdull and ZNFbright deposits are shown in Figure 9(a,b). The additives refine the grain size and smoothen the ZNFbright deposit by reducing the surface roughness to 34 nm from 238 nm of ZNFdull coating. The sliding on smooth, fine-grained deposit with smaller roughness results in lesser wear and friction. Thus, smooth, fine grained, bright coating obtained in the presence of additives showed better wear and friction performance when compared to coarse-grained ZNFdull deposit as reported by Li et al. [21].
(a) Sliding wear behaviour and (b) friction coefficients of ZNFdull and ZNFbright deposits.
Conclusions
The new brightener system TDTAB and TSC gave nanocrystalline bright Zn–Ni–Fe alloy electrodeposit with a preferred orientation of (100) plane. The additives improve the corrosion resistance, lowers wear and friction in bright Zn–Ni–Fe coating when compared to dull coating. The bright Zn–Ni–Fe coating showed lower microhardness over dull deposit due to lesser Fe and Ni content (chemical composition) of the deposit. The addition of iron and TSC to bright Zn–Ni–Fe bath rises the Fe and Ni content of the deposit thereby enhances the microhardness and corrosion resistance of bright Zn–Ni–Fe coating.
Footnotes
Acknowledgements
The authors thank the Department of Chemistry, Bangalore University for providing the research facilities. We acknowledge Mr Shamsundar, Indian Institute of Science for the technical support.
Disclosure statement
No potential conflict of interest was reported by the authors.