Introduction

Two major factors that reduce service life of engineering materials are corrosion and erosion in harsh environments. An effective solution for such problem is application of suitable coatings on the surface of materials to protect them against such harmful factors (Ref 1). Electrodeposition technique is known as a versatile method for applying protective coatings on various conductive substrates (Ref 2).

It is a well-known fact that Cr coating exhibits reasonable corrosion and wear resistance due to its active–passive behavior and low coefficient of friction, respectively (Ref 3). The major drawback of hard chromium electroplating process is carcinogenic nature of Cr+6 ion which is the main constituent of Cr electroplating bath. This fact forces different environmental agencies to ban industrial use of Cr+6 electroplating baths (Ref 4,5,6); such restriction inspires scientists to innovate different environmentally friendly alternatives for chromium coatings. For instance, application of alternative processes like application of Cr+3 baths, vacuum deposition, high velocity oxy-fuel (HVOF) and plasma spaying techniques or using alternative materials like nanostructure Ni, Ni–P and different metallic matrix composite coatings has been proposed, but none of them provides coatings with the same properties as those of hard chromium coatings that were electrodeposited from Cr+6 baths (Ref 7,8,9,10). Moreover, different practical procedures have been proposed to deal safely with the application and disposal of Cr+6 baths (Ref 4,5,6, 11). In this regard, it would not be fair to leave the subject of chromium electroplating unless a practical environmentally friendly alternative can be found to replace it.

As a general fact, conventional hard chromium coating has a smooth surface and includes microcracks which can act as lubricant storage regions; when the coating is used in an oil-based medium (Ref 7), the wear and corrosion behaviors of such coatings can be improved simultaneously if such cracks are eliminated and replaced by a solid compounds which have lubricity. Molybdenum disulfide (MoS2) is recognized as a good solid lubricant because of its unique lamellar crystal structure (Ref 12). In this regard, incorporation of MoS2 particles (as chemically stable compound that has lubricity) into Cr metallic matrix would be promising since the resulting composite coating would show better characteristics than those of conventional Cr coating.

In the last decades, different types of composite Cr coatings have been produced by electrodeposition technique by adding various types of ceramic particles in electroplating baths. For example, it was reported that incorporation of solid particles like ZrO2 (Ref 13), Al2O3 (Ref 14), WC (Ref 15) and SiC (Ref 1) into Cr matrix would enhance the corrosion and wear properties of the resulting electrodeposits. Although it has been reported earlier that MoS2 particles can be used to produce different metallic matrix composite coatings like nickel, zinc and Ni-Co-based ones by electrodeposition technique (Ref 12, 16, 17), there is not enough published papers dealing with the electrodeposition of Cr-MoS2 composite coatings. So, the purpose of the present work is to produce Cr-MoS2 composite coatings by electrodeposition technique and to evaluate the effects of the processing parameters (bath solid content and type of the surfactant) on the morphology, structure, chemical composition, microhardness and corrosion behavior of the electrodeposited coatings.

Experimental Procedure

The details of the experimental conditions that were used to electrodeposit composite Cr-MoS2 coatings are presented in Table 1. A conventional two-electrode cell was utilized for all the electrodeposition experiments in which a copper plate with dimensions of 2 × 3 cm2 was used as the cathode and a lead plate with the same dimensions was used as the anode. The effects of the type of surfactant and MoS2 concentration in bath on the properties of the electrodeposited coatings were investigated. To do so, different amounts of MoS2 particles (Sigma-Aldrich) with particle size less than 0.2 µm (Fig. 1a and b) were added to the conventional Cr+6 bath. Cetyltrimethylammonium bromide (CTAB), alkyldimethylbenzylammonium chloride (BAC) and sodium dodecyl sulfate (SDS) compounds were added to the bath as different surfactants. All the surfactants and the remaining chemicals mentioned in Table 1 were purchased from Merck and used without any further purification. Before each electrodeposition process, the prepared bath containing MoS2 particles was stirred for 30 min with a magnetic stirrer followed by ultra-sonication for 1 h. The well-dispersed suspension was used immediately for the electrodeposition process, while it was stirred constantly during the process using a magnetic stirrer with rotation speed of 100 rpm.

Table 1 Experimental conditions that were used to electrodeposit composite Cr-MoS2 coatings
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Fig. 1
figure 1

(a) SEM micrograph and (b) particle size distribution of MoS2 powders

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Chronopotentiometry tests were carried out in a conventional three-electrode cell containing baths with different chemical compositions; an Ag/AgCl reference electrode, a platinum counter electrode and the copper substrate as the working electrode were used to obtain the chronopotentiograms with an Ivium potentiostat/galvanostat device. The zeta potential (surface charge) values and the particle size distribution of the MoS2 particles, suspended in different electroplating baths, were determined using a SZ-100 Horiba Zeta Potential analyzer. Surface morphologies and the cross sections of the electrodeposited composite coatings were investigated using scanning electron microscopy (Cambridge 360). The observed coatings thickness values were used to calculate the cathodic current efficiencies (CCE) of the baths with different chemical compositions (%CCE = t/te × 100 where t is the experimentally measured coating thickness value and te is the theoretical coating thickness value calculated according to the Faraday’s equation (t0 = 417.3 µm based on the relevant parameters as shown in Table 1)). An energy-dispersive x-ray system (Tescan Vega 3) was used to perform coatings surface chemical analysis. The average particle content in each composite coating (that was electrodeposited in its specific experimental condition) was estimated by analyzing its respective SEM micrographs taken from three different areas at the surface (or cross section) of the coating with magnification of 2500 × using an image analyzer software (ImageJ). The microhardness and roughness measurements were taken using koopa Haresh MH1 and MitutoyoSuftest 201 devices, respectively. Microhardness of the samples was evaluated according to Vickers microhardness test procedure using 200 g load with dwell time of 15 s. All the electrochemical corrosion tests were done in a conventional three-electrode cell, containing 3.5 wt.% NaCl electrolyte at room temperature using an Ivium potentiostat/galvanostat device; an Ag/AgCl electrode, a platinum rode and the electroplated substrate were used as the reference, counter and working electrodes, respectively. The potentiodynamic polarization tests were performed from − 0.3 to + 0.3 V with respect to open-circuit potential (OCP) at the scan rate of 1 mV/s. The electrochemical impedance spectroscopy (EIS) tests were conducted using an applied alternating current (AC) voltage with amplitude of 10 mV (with respect to OCP) in the frequency range of 0.01 Hz to 100 kHz. The corrosion inhibition efficiency values (CIE%) of the coatings were determined based on Eq 1 and 2 for Tafel and EIS results, respectively,

$${\text{CIE}}\% = [(i_{{{\text{Corr}}\,{\text{Pure}}\,{\text{Cr}}}} {-}i_{\text{Corr}} )/(i_{{{\text{Corr}}\,{\text{Pure}}\,{\text{Cr}}}} )] \times 100$$
(1)
$${\text{CIE}}\% \, = [((1/R_{{{\text{ct}}\,{\text{pure}}\,{\text{Cr}}}} ) - (1 /R_{\text{ct}} )) /(1 /R_{{{\text{ct}}\,{\text{pure}}\,{\text{Cr}}}} )] \, \times 100$$
(2)

where iCorr Pure Cr and Rct pure Cr are the corrosion current density and charge transfer resistance of pure chromium coating, while iCorr and Rct are those of composite coating, respectively. The results of the potentiodynamic polarization and EIS tests were analyzed by IviumSoft software. The x-ray diffraction (XRD) patterns were obtained in a Bruker D8 diffractometer using Cu Kα radiation at the scan rate of 5° min−1.

Results and Discussion

Suspension Stability of Cr-MoS2 Electroplating Bath

The zeta potential values attributed to the MoS2 particles suspended in the baths containing different types of surfactant are presented in Fig. 2. As seen, the particles acquire negative surface charges in the absence of any surfactant in the bath. Moreover, at fixed concentration of MoS2 particles in the bath (2 g/l), application of anionic surfactant (SDS) results in having more negative zeta potential values, while application of cationic surfactants (BAC and CTAB) leads to have less negative zeta potential values. In addition, at the same amount of the surfactant concentration in the bath (0.1 g/l), as the concentration of MoS2 particles in the bath increases, the zeta potential values are less deviated from that of the particles which were dispersed in the bath with no surfactant.

Fig. 2
figure 2

Zeta potential values attributed to MoS2 particles suspended in electrodeposition baths with different solid contents and types of surfactant

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Critical factors that should be kept in mind during electroplating of composite coatings are the stability of suspension and surface charge of the particles that are suspended in the electrolyte. These factors can affect dramatically the amount of particles that will incorporate into the growing composite coatings. Such factors can be related to the zeta potential values (that are indicative of the amount and type of the surface charges on the suspended particles in an electrolyte) in such a way that particles in stable suspensions usually have high zeta potential values (Ref 18). In this regard, it can be concluded that at fixed amount of MoS2 concentration in the bath (2 g/l), although SDS is slightly more effective surfactant than CTAB to stabilize the suspension (comparing their respective absolute zeta potential values), it may retard electrophoretic migration of the particles toward cathode because of the creation of high negative surface charges. Moreover, at fixed concentration of the surfactant in the bath, as the concentration of MoS2 particles increases, the stability of the electroplating bath decreases.

Cr-MoS2 Electroplating Bath Efficiency

Figure 3 illustrates the cross sections of the coatings that were electrodeposited at various experimental conditions and the estimated coatings thickness values are presented in Table 2. As seen, the thickness of the coating decreases as the concentration of MoS2 particles in the bath increases; moreover, the type of surfactant affects the resulting coating thickness in such a way that thinner coatings are deposited from the baths containing cationic surfactants in comparison with the other ones. The influence of the presence of solid particles in different types of electroplating baths on their cathodic current efficiency (CCE) has been reported previously (Ref 19,20,21,22). Depending on the type of the particles and nature of electroplating bath, addition of solid particles to the bath may cause an increase or decrease in the cathodic current efficiency; for instance, it has been shown that chromium electroplating bath efficiency increases as the concentration of Al2O3 particles in the bath increases, and such behavior has been related to the increases in the hydrogen evolution over potential at such experimental conditions (Ref 19). Chromium electroplating process suffers from its low cathodic current efficiency (occurrence of high degree of hydrogen evolution during electrodeposition of Cr) (Ref 23); it has been reported that during Cr electrodeposition from Cr+6 baths, trichromate ions (Cr3O102−) will first discharge at the cathode surface as Cr(OH)2 in a successive steps including election transfer, loss of oxygen and reaction with H+ ions. At the next step, a complex ion will be produced as a result of hydrogen bonding between bisulfate ions and Cr(OH)2 according to Eq 3

$${\text{Cr(OH)}}_{2} + {\text{HSO}}_{4}^{ - } \to^ {{\delta +}}{\text{Cr}} - {\text{OH}} \cdots {\text{OSO}}_{3}^{ - } +{\text{H}}_{2}$$
(3)
Fig. 3
figure 3

Cross sections of the coatings that were electrodeposited at various experimental conditions, (a) pure chromium, (b) Cr-MoS2 electrodeposited from bath containing 2 g/l MoS2 and no surfactant, (c) Cr-MoS2 electrodeposited from bath containing 2 g/l MoS2 and CTAB surfactant, (d) Cr-MoS2 electrodeposited from bath containing 2 g/l MoS2 and BAC surfactant and (e) Cr-MoS2 electrodeposited from bath containing 2 g/l MoS2 and SDS surfactant

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Table 2 Estimated thickness values attributed to the coatings that were electrodeposited at various experimental conditions
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Finally, metallic chromium will be precipitated on the cathode according to Eq 4 (Ref 24)

$$^{\delta + } {\text{Cr}} - {\text{OH}} \cdots {\text{OSO}}_{3}^{ - } + 2{\text{H}}^{ + } + 2{\text{e}}^{ - } \to {\text{Cr}} + {\text{HSO}}_{4}^{ - } + {\text{H}}_{2} {\text{O}}$$
(4)

In this regard, the feasibility of Cr electrodeposition is mainly dependent of H+ activity at the cathode surface, since MoS2 compound is considered as a good catalyst for hydrogen evolution reaction (HER) (Ref 25, 26), so that would be logical to expect having its influence on the Cr electrodeposition efficiency and the resulting coatings thickness.

Figure 4 illustrates the results of the chronopotentiometry tests and the respective %CCE values attributed to the baths containing different amounts of MoS2 particles and type of surfactant. As seen, by increasing the particle content in the bath containing no surfactant, the value of the %CCE decreases (Fig. 4a(I)). It would be related to the enhanced hydrogen evolution at the cathode surface due to the presence of more MoS2 particles in the bath. Such hypothesis would be proven by comparing the chronopotentiometry test results obtained during the electrodeposition process of the samples from baths containing no surfactant and different amounts of MoS2 particles (Fig. 4a(II)). As can be seen in Fig. 4a(II), by increasing the concentration of MoS2 particles in the bath, the deposition potential shifts to more positive values close to that of hydrogen reduction which is indicative of enhanced hydrogen evolution (HER) at such experimental conditions. The same behavior (i.e., enhanced HER at the cathode surface (decrease in %CCE) as a result of increase in MoS2 concentration in the bath) was observed in the case of the presence of different surfactants in the bath (Fig. 4b, c and d), but it is worthy to note the following points. First, application of the anionic surfactant (SDS) in the bath results in having more %CCE values with respect to those of the baths containing cationic surfactants (BAC and CTAB). This would be related to the accumulation of less amount of MoS2 particles at the cathode surface in the former case due to the creation of higher negative charge on the surfaces of MoS2 particles than in the latter case (Fig. 2). Second, those baths that contain CTAB surfactant exhibit less %CCE than those that contain BAC surfactant. It has been reported that those surfactant compounds which have higher hydrophilic–lipophilic balance (HLB) and lower critical micelle concentration (CMC) values provide more stable colloidal suspensions than the other ones. So, CTAB compound would provide more stable suspension with respect to BAC since it satisfies the above conditions (HLBCTAB = 21.4, CMCCTAB = 0.90–0.98 mM, HLBBAC = 20.74, CMCBAC = 8.79 mM) (Ref 27,28,29,30,31,32). Moreover, it creates more positive charge on the surface of MoS2 particles than BAC surfactant (Fig. 2). In this regard, at the presence of CTAB in bath, more MoS2 particle accumulation will occur at the cathode surface in comparison with the presence of BAC; this leads to have more HER and less values of %CCE for the former case. Finally, it should be noted that, in the absence of surfactant in the bath, chromium cannot be electrodeposited from the baths containing more than 7 g/l MoS2, while the feasibility of Cr electrodeposition process is lost when the concentration of MoS2 particles in the bath is more than 4.5 and 2 g/l for anionic and cationic surfactants, respectively. In other words, application of the surfactant results in having less amount of allowable solid content in the bath in comparison with those containing no surfactant. It can be related to the occurrence of more severe hydrogen evolution at the cathode surface during electrolyzing of more stable suspensions, especially in the case of the presence of cationic surfactants.

Fig. 4
figure 4figure 4

Cathodic current efficiency (I) and chronopotentiometry (II) test results of the electrodeposition experiments that were carried out in baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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Morphological Characteristics of Cr-MoS2 Composite Coatings

The SEM micrographs from the surfaces and cross sections of different samples that were electrodeposited from baths containing different amounts of MoS2 particles and no surfactant are illustrated in Fig. 5. As seen, MoS2 particles are well distributed through the surfaces and cross sections of the coatings (Fig. 5b and c). In addition, by increasing the MoS2 content from 0 to 2 g/l in such baths, coatings with finer morphology will be electrodeposited. Moreover, those coatings that were electrodeposited from baths containing more than 2 g/l MoS2 have coarser morphology than the other ones (Fig. 5a). Such observations are in agreement with the surface roughness values of the samples (Fig. 6) in which the coating surface roughness decreases as the concentration of MoS2 particles in the bath increases up to 2 g/l, while higher surface roughness values are obtained for the coatings that were electrodeposited from baths containing more than 2 g/l MoS2 particles. The effect of the presence of the solid particles in the bath on the resulting coatings morphology has been also reported for different types of composite coatings (Ref 33). For instance, it has been reported that the presence of SiC particles in Ni electroplating bath alters the coating morphology by increasing the number of nucleation sites and reducing the crystals growth (Ref 34). Moreover, in the case of the electrodeposition of Cr-MoO2 composite coatings, it has been shown that the presence of MoO2 particles in the bath alters the coating surface morphology by affecting the hydrogen evolution process occurring at the cathode surface (Ref 21). So, the decrease in the surface roughness would be related to the blocking effect of MoS2 particles, attached to the cathode surface, on the growth of the Cr deposits nuclei, and the increase in surface roughness would be attributed to the enhanced hydrogen evolution at the cathode surface in baths containing high amounts MoS2.

Fig. 5
figure 5

SEM micrographs from the surfaces and cross sections of the coatings that were electrodeposited from baths containing no surfactant; a) and b) surface morphologies at low and high magnifications, respectively, c) cross sections at low and high magnifications

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Fig. 6
figure 6

Surface roughness values attributed to the coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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Figure 7 illustrates surface morphologies and cross sections of the coatings that were electrodeposited from baths containing different types of surfactant. As seen, MoS2 particles are well distributed through the surfaces and cross sections of the coatings. Moreover, increasing the MoS2 concentration in the bath results in the formation of deposits with coarser morphology. By comparing the surface roughness values attributed to the coatings that were electrodeposited at various experimental conditions, one can say that at the same amount of particle concentration in the bath, finer morphologies (lower values of surface roughness) are attributed to those samples that were electrodeposited from baths containing no surfactant. In this regard, it can be concluded that the presence of surfactant in the bath would result in having more MoS2 particles at the cathode surface (especially in the case of cationic surfactants) which in turn leads to promote hydrogen evolution and formation of coatings with rough surfaces.

Fig. 7
figure 7

SEM micrographs from the (a) surfaces and (b) cross sections of the Cr composite coatings that were electrodeposited from baths containing different surfactants and MoS2 particle contents

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Chemical Composition of Cr-MoS2 Composite Coatings

The EDX spectra obtained from the surfaces and cross sections of the coatings that were electrodeposited at various experimental conditions are illustrated in Fig. 8. As seen, the dominant element that is present in the deposits is Cr and the presence of the peaks attributed to the Mo and S elements is indicative of the incorporation of MoS2 particles into the metallic chromium matrix. The amounts of MoS2 particles at the surface of composite coatings that were electrodeposited at various experimental conditions are presented in Fig. 9. The results show that by increasing the MoS2 content in the bath, more particles will incorporate into the resulting electrodeposits. Such behavior has been also reported in the case of electrodeposition of other types of composite coatings (Ref 35). Moreover, the highest and lowest values of particle content at the surface of the coatings are attributed to those that were electrodeposited from baths containing CTAB and SDS surfactants, respectively (1.7 and 0.04 vol.%). This would be related to the cationic and anionic nature of the former and latter surfactants that enhances and retards the electrophoretic migration of the particles toward the cathode, respectively. It is in agreement with the results of zeta potential measurements (Fig. 2) that show more positive charge on the surface of MoS2 particles suspended in the baths containing cationic surfactant (CTAB) in comparison with the case of particles that were suspended in the baths containing anionic surfactant (SDS). The effects of surfactant type on the degree of particle incorporation into the other types of composite coatings have been also reported previously; for instance, it has been shown that application of cationic surfactant increases drastically the volume fraction of particles (such as SiC, PTFE and diamond) in various electrodeposited composite coatings (Ref 36,37,38).

Fig. 8
figure 8

Results of EDX analysis attributed to the coatings that were electrodeposited from baths containing (a) no surfactant and 4.5 g/l MoS2 particles, (b) 0.1 g/l CTAB surfactant and 2 g/l MoS2 and (c) 0.1 g/l BAC surfactant and 2 g/l MoS2

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Fig. 9
figure 9

Calculated vol.% values of MoS2 particles on the surfaces of the composite coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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The amount of MoS2 particles which are distributed through the thickness of different composite coatings is presented in Fig. 10. By comparing these results with the respective ones as shown in Fig. 9, one can see that those experimental conditions that led to increase in the particle content at the coating surface (i.e., increasing the particle concentration in bath and using cationic surfactant) result in increasing the particle content through the coating thickness too. So, one may conclude that MoS2 particles are well distributed through the metallic Cr matrix at such experimental conditions.

Fig. 10
figure 10

Calculated vol.% values of MoS2 particles in the cross sections of the composite coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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Microstructural Characteristics of Cr-MoS2 Composite Coatings

Figure 11 illustrates the XRD patterns attributed to the samples that were electrodeposited at various experimental conditions. The diffraction peaks at 2θ values of 44° and 81° are attributed to the (110) and (211) planes of crystalline chromium, respectively; the diffraction peaks related to the MoS2 particles are not present in the XRD patterns of the composite coatings which would be attributed to their very low incorporation into the electrodeposited coatings (less than 3 vol.% as can be seen in Fig. 9 and 10). Moreover, all the patterns exhibit characteristics of phases with very low crystallinity in which x-rays are scattered in different directions leading to have large bump distributed intensities in a wide range of 2θ instead of having high intensity narrow peaks as in the case of crystalline phases (Ref 38). Two important factors that cause broadening of diffraction peaks are the grain size and lattice microstrain according to Eq 5

$$W_{\text{f}} = \, 0.9\lambda /D\cos \theta + 4\varepsilon \tan \theta$$
(5)

where θ is the Bragg angle, Wf is the full width at half maximum (FWHM) value of the diffraction peak excluding the instrumental broadening effect, λ is the wavelength of x-ray radiation, D is the average crystallite (grain) size, and ε is the lattice microstrain (Ref 39). Based on Eq 5, smaller grain size and higher lattice microstrain values result in having diffraction peaks with more broadening (higher FWHM values). The calculated FWHM values obtained by fitting the pseudo-Voigt function to the experimental diffraction peaks at 2θ = 81° (Fig. 11) are shown in Fig. 12. As seen, the FWHM values attributed to the diffraction peaks of the samples that include more MoS2 particles (comparing the respective particle content in the coatings as shown in Fig. 9 and 10) are slightly higher than the other ones; in other words, incorporation of more MoS2 particles in the composite coatings results in the formation of metallic Cr matrix with finer grain size and higher lattice microstrain. The high values of lattice microstrain would also be related to the enhanced hydrogen evolution at the cathode surface at such experimental conditions since atomic hydrogen incorporation into the crystal structure of metallic coatings produces high residual stresses (Ref 23). Such behavior has been also reported in the case of electrodeposition of other types of composite and metallic coatings (Ref 40,41,42,43).

Fig. 11
figure 11

X-ray diffraction patterns of the coating that were electrodeposited from baths containing (a) 2 g/l MoS2 particles and no surfactant, (b) 1 g/l MoS2 particles and 0.1 g/l CTAB surfactant, (c) 1 g/l MoS2 particles and 0.1 g/l BAC surfactant and (d) 2 g/l MoS2 particles and 0.1 g/l SDS surfactant

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Fig. 12
figure 12

Calculated FWHM values attributed to the diffraction peaks at 2θ = 81° of the coating that were electrodeposited from baths containing (a) 2 g/l MoS2 particles and no surfactant, (b) 1 g/l MoS2 particles and 0.1 g/l CTAB surfactant, (c) 1 g/l MoS2 particles and 0.1 g/l BAC surfactant and (d) 2 g/l MoS2 particles and 0.1 g/l SDS surfactant

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Microhardness of Cr-MoS2 Composite Coatings

The microhardness values attributed to the samples which were electrodeposited at various experimental conditions are presented in Fig. 13. As seen, all the composite coatings have higher microhardness values than the conventional chromium coating. In addition, by increasing the MoS2 concentration in bath to an optimum value (1 and 2 g/l for the baths containing cationic surfactant and no surfactant, respectively), the microhardness values of the resulting coatings will increase; by increasing the MoS2 concentration in the baths above these optimum values, the microhardness of the resulting coating starts to decrease. Moreover, in comparison with baths containing cationic surfactants, application of anionic surfactant leads to have higher optimum MoS2 content in the bath above which the coatings microhardness value starts to decrease (1 and 2 g/l for cationic and anionic surfactants, respectively). The changes in microhardness values of different types of composite coatings as a result of the incorporation of solid particles into them have been reported previously. Depending on the case under study, different mechanisms have been attributed to the reason of such phenomena including change in metallic matrix crystallinity, dispersion strengthening and introduction of hard load bearing phases (particles) into the soft metallic matrix (Ref 40, 41, 44,45,46,47).

Fig. 13
figure 13

Microhardness values attributed to the coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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The increase in the microhardness value as a result of the increase in MoS2 concentration in the bath would be related to the incorporation of more MoS2 particles into the coatings (Fig. 9) or formation of coatings with finer grain size and more lattice microstrain (Fig. 12). It has been reported that cathodic charge which is consumed for the electrodeposition of metallic coatings has direct effect on the resulting coatings microhardness value, i.e., the more the cathodic charge, the more the microhardness value would be (Ref 48,49,50,51). In this regard, the reason for obtaining smaller microhardness values for the coatings that were electrodeposited from baths containing more MoS2 particles than the optimum values would be the consumption of lower amount of cathodic charge for Cr matrix deposition as a result of promoted hydrogen evolution at the cathode surface at such experimental conditions (using baths with high MoS2 concentration). Finally, having higher optimum MoS2 concentration in baths containing anionic surfactant than the baths containing cationic surfactants would be related to the less MoS2 particle accumulation at the cathode surface in the case of the former baths than the latter ones; this leads to the occurrence of promoted hydrogen evolution at the cathode surface (and consumption of less cathodic charge for Cr deposition) at higher MoS2 concentration in baths containing anionic surfactant with respect to the other ones.

Corrosion Behavior of Cr-MoS2 Composite Coatings

The potentiodynamic polarization test results attributed to the samples which were electrodeposited at various experimental conditions are presented in Fig. 14, and the corresponding corrosion current densities (calculated by Tafel extrapolation method) are shown in Fig. 15. As seen, in the case of the samples that were electrodeposited from baths containing no surfactant or cationic surfactants, increasing the MoS2 concentration in the bath up to their respective optimum value (2 and 1 g/l, respectively) leads to the formation of coatings with more positive corrosion potentials and less corrosion current densities (Fig. 15a, b and c). In addition, by increasing the MoS2 concentration in the baths containing anionic surfactant, the corrosion resistance of the resulting coatings will decrease (Fig. 15d); moreover, those coatings that were electrodeposited from baths containing BAC and SDS surfactants exhibit better corrosion resistance than the other ones.

Fig. 14
figure 14

Potentiodynamic polarization tests results of the coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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Fig. 15
figure 15

Corrosion current densities (ICorr), corrosion potentials (ECorr) and corrosion inhibition efficiencies (CIE%) attributed to the coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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The beneficial effects of the presence of solid particles on the corrosion behavior of various types of metallic matrix composite coatings have been reported previously. Such increase in the corrosion resistance has been related to different reasons depending on the case under study; for instance, it has been reported that incorporation of the solid particles increases the coatings corrosion resistance since particles act as inert physical barriers or make coatings surfaces smooth (reduction in active surface area) (Ref 52,53,54,55,56,57). In other studies, it has been shown that the presence of particles may improve the coatings corrosion resistance by making their microstructures as a fine grain ones (which can be easily passivated) or making them crack free (Ref 41, 56, 58, 59). In this regard, in the case of baths containing no surfactant, the increase in the coatings corrosion resistance as a result of the increase in the MoS2 concentration in the bath (up to 2 g/l) may be related to the increase in the particle incorporation into the coatings (increase in number of physical barriers), decrease in the coatings surface roughness (decrease in active surface area) and formation of fine grain metallic Cr matrix as the bath particle concentration increases. Since there is no significant increase in the particle incorporation into the coatings that were electrodeposited from the baths containing more than 2 g/l MoS2 particles and no surfactant (Fig. 9), so the low corrosion resistance of such coatings would be related to their higher surface roughness values (having more active surface) in comparison with that of the coating which was electrodeposited from bath containing 2 g/l MoS2 particles.

In the case of baths containing cationic surfactants, since increasing the MoS2 concentration from 0.5 to 1 g/l does not have significant effect on the resulting coatings roughness values (Fig. 6b and c), so the improved corrosion resistance of the coating that was electrodeposited from bath containing 1 g/l MoS2 may be attributed to its finer grain size and higher particle content than that of the coating which was electrodeposited from bath containing 0.5 g/l MoS2 particles. Moreover, those coatings that were electrodeposited from baths containing cationic surfactants and 2 g/l MoS2 particles exhibit less corrosion resistance than those which were electrodeposited from baths containing 1 g/l MoS2. In spite of the fact that the formers have more particle content than the latter ones, they also have higher surface roughness than the others; in this regard, it would be concluded that increasing surface roughness has significant influence on decreasing the coatings corrosion resistance.

Finally, in the case of baths containing anionic surfactants, since there is little particle incorporation into the coatings electrodeposited at such experimental condition (Fig. 9d), so the decrease in corrosion resistance as a result of increase in bath solid content would be related to the increase in coatings surface roughness values as the MoS2 concentration in the bath increases (Fig. 6d). Moreover, good corrosion resistance of the coatings that were electrodeposited from baths containing BAC or SDS surfactants would be related to their more intact structure (containing less microcracks) as one can see in Fig. 16.

Fig. 16
figure 16

SEM micrographs from the surface of the coatings that were electrodeposited from baths containing different types of surfactant and 1-2 g/l MoS2 particles. The microcracks are marked by arrows

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The AC impedance responses attributed to the samples which were electrodeposited at various experimental conditions are presented in Fig. 17. As seen, all the AC responses include single capacitive loops, with different diameters, indicating the occurrence of corrosion at the surface of a bare metal. The equivalent circuit that was used to fit such experimental data is shown in Fig. 18. In this circuit, Rs is the solution resistance, Rct is the charge transfer resistance, and CPE is the constant phase element attributed to the double layer formed at the electrode/electrolyte interface that its impedance value (ZCPE) is measured according to Eq 6

$$Z_{\text{CPE}} = 1 /Q(i2\pi f)^{n}$$
(6)

where Q is a constant, i = (− 1)1/2, f is the AC voltage frequency, and n is a content varying between 0 and 1 (Ref 60). It has been reported that n approaches unity as the electrode surface roughness decreases (Ref 61). The values of the equivalent circuit elements that are fitted to the experimental AC responses (Fig. 17) are presented in Table 3. As seen, the higher values of charge transfer resistance (Rct) are related to those samples that exhibited less corrosion current densities in their potentiodynamic polarization response (Fig. 15). Moreover, the samples with less surface roughness values (Fig. 6) have n values close to unity (Table 3). In this regard, one can find good consistency between the results of the EIS tests and those attributed to the potentiodynamic polarization and roughness measurement techniques.

Fig. 17
figure 17

AC responses of the coatings that were electrodeposited from baths containing different amounts of MoS2 particles and (a) no surfactant, (b) 0.1 g/l CTAB surfactant, (c) 0.1 g/l BAC surfactant and (d) 0.1 g/l SDS surfactant

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Fig. 18
figure 18

Equivalent circuit that was used to fit the experimental EIS data as shown in Fig. 17

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Table 3 Values of the equivalent circuit parameters fitted to the experimental data as shown in Fig. 17
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Conclusions

Chromium-MoS2 composite coatings were electrodeposited from baths containing different amounts of solid content and various types of surfactant. The results showed that

  1. 1.

    Incorporation of MoS2 particles into the metallic Cr matrix is very limited due to the enhanced hydrogen evolution occurring at the cathode surface in the presence of such particles.

  2. 2.

    Depending on the type of surfactant in the bath, chromium will not be electrodeposited when the concentration of the particles exceeds a particular value.

  3. 3.

    Application of cationic surfactant (especially with high HLB and low CMC values) leads to have more MoS2 particle incorporation into the Cr metallic matrix of the composite coatings.

  4. 4.

    The coatings that were electrodeposited at various experimental conditions include Cr matrix with very low crystallinity. Incorporation of more MoS2 particles into the coatings results in the formation of metallic Cr matrix with finer grain size and more lattice microstrain.

  5. 5.

    All the composite coatings exhibited smaller surface roughness and higher microhardness values in comparison with conventional Cr coatings.

  6. 6.

    The hardest and smoothest composite coating was electroplated from bath containing no surfactant and 2 g/l MoS2 particles.

  7. 7.

    The smooth coatings containing more MoS2 particles and less microcracks exhibit more corrosion resistance.