Effect of saccharin addition on formation, wear and corrosion resistance of electrodeposited Ni-Cr coatings

In Ni-Cr coatings deposited onto AISI 1040 steel substrates, the initiation potentials for coating formation were determined by means of cyclic voltammetry (CV), as illustrated in Figure 1. Notably, the open circuit potential of the solution displayed a positive value in Figure 1, suggesting that the current gradually decreased from 0.28 amps upon the application of a voltage. This decline was not indicative of deposition but rather the formation of hydrogen gas [28]. Upon applying a negative potential of 0.12 V, the current became null and remained constant until reaching −0.6 V. The graph reveals that, after attaining a cathodic potential of −0.6 V, a rapid reduction in cathodic current was observed, signalling the commencement of deposition. In section “B,” it’s evident that after the drop in current, the curve deviated from the path to −1.4 V and returned from this value. This phenomenon stems from the formation of the Ni-Cr alloy coating during the initial deposition, with continuous modification of the substrate during subsequent cycles. Dissolution occurred on the substrate, and it’s noteworthy that accumulation appeared on the platinum wire once the CV cycle was completed.

Fig. 1.

The dissolution peaks, as depicted in section “A,” were noted during the return potentials. The figure underscores that the dissolution peak was most pronounced in the bath without saccharin, with the intensity of the dissolution peak decreasing as saccharin concentration increased. In other words, the migration of the coating to the substrate intensified with higher saccharin content, indicating improved efficiency. In summary, the deposition process was observed to occur within the potential range of 0.12 V to −0.6 V on the test samples.

Figure 2 presents SEM images of the Ni-Cr coatings, along with EDS results for the entire region shown in the figure. The morphology of the Ni-Cr coatings exhibited a characteristic spherical nodular structure, as commonly observed in dominant Ni coatings [1]. Figure 2a notably reveals the presence of micro-cracks, a typical characteristic of Ni-Cr alloy coatings [29]. These cracks may be attributed to volumetric changes resulting from hydrogen absorption or residual stresses stemming from the formation of multiple phases [30]. It’s worth noting that increasing the amount of Cr in the coating can elevate internal stresses and lead to the formation of cracks [31]. The use of additives is an effective strategy to mitigate this undesirable aspect of Ni-Cr coatings [15].

Fig. 2.

Figures 2b to 2d present SEM images of coatings from baths containing 0.5 g/l saccharin, 1 g/l saccharin, and 2 g/l saccharin, respectively. Notably, the coating in the bath with 0.5 g/l saccharin exhibited a reduction in grain size and the complete closure of cracks (Figure 2b). In the case of the bath with 1 g/l saccharin, the grains displayed a more compact surface, albeit slightly larger than those in the bath with 0.5 g/l saccharin (Figure 2c). This suggests an enhancement in corrosion resistance and surface roughness. However, as the saccharin content increased, pinholes began to appear, indicating a potential adverse effect on corrosion resistance.

Particularly noteworthy is the increase in surface smoothness with higher saccharin content in the electrolyte. This might be attributed to the presence of sulphur in the saccharin molecule, which can release sulphur compounds during electroplating. In the Ni-Cr alloy coating, these dissociated sulphur compounds can lead to volume expansion [32]. Such expansion can induce compressive stresses in the coating, offsetting internal stresses generated during electrodeposition. In particular, the addition of saccharin in the right proportions can reduce internal stresses and minimize crack formation in the coating [20].

Table 4 provides noteworthy insights into the composition of saccharin, particularly the presence of elements. One of the most striking observations from the table is the substantial increase in the concentration of chromium (Cr) with the addition of saccharin. It becomes evident that the quantity of Cr incorporated into the coating nearly triples with each incremental increase in saccharin concentration. This finding aligns with existing literature, which supports the notion that the inclusion of saccharin in the bath is conducive to enhanced accumulation of auxiliary metals [10].

In this subsection, Figure 3 illustrates the XRD analysis of the coatings, with the corresponding peak angles and DB card numbers presented in Table 5. Notably, the most striking feature is the presence of specific diffraction peaks associated with nickel. At approximately 44.5 (1 1 1), 51.8 (2 0 0), and 76.3 degrees (2 2 0), which correspond to the 2θ values where Ni peaks typically form, an alloy of (Ni₂Cr₂₃)0.16 was detected. This observation is consistent with findings in the literature [33]. It’s worth noting that coatings formed in the presence of 2 g/l saccharin displayed different peak patterns compared to other samples. Specifically, the Cr peak at 51.8 degrees was absent in this coating, while the Cr₇Ni₃ alloy phase emerged at 42.6 degrees. Furthermore, the width of the peak at 76.3 degrees increased with higher saccharin concentrations.

Fig. 3.

Table 6 provides XRD models, revealing that the grains in the samples are crystalline in nature. The table also facilitates a comparison of the average crystallite grain size. The Scherrer equation was employed to determine the average grain size [34], utilizing the primary peak (111) for all samples. Scherrer equation: 1 D=k·λβcosθ \[D=\frac{\text{k}\cdot \lambda }{\beta \cos \theta }\] where D is the average particle diameter (in Å), λ is the X-ray radiation wavelength (in Å), K is a constant, θ is the peak angle, and β is the full width at half maximum (FHWM) of the respective XRD peak [34]. Grain size measurements were converted to nanometers (nm). According to the literature, the grain size of the coating obtained from a saccharin-free bath measures 32.42 nm, and it is well-established that the addition of saccharin leads to a reduction in the average crystallite grain size [8, 35]. The smallest grain size was observed in the bath with 0.5 g/l saccharin. As the saccharin content increased, grain sizes progressively increased. Notably, the sample containing 2 g/l saccharin resulted in a grain size identical to that of the sample without saccharin. During electrodeposition, saccharin molecules adhere to active growth sites on the crystal surface, enhancing cathodic polarization in the plating bath. This reinforcement has a positive impact by reducing the rate of nucleation and increasing the formation of nuclei. As a direct consequence, there is a notable reduction in the crystal grain size of the NiCr alloy coating [20]. However, with the continued addition of saccharin to the electrolyte, it caused the grains to grow instead of thinning. This situation is encountered in the literature [36].

To assess the impact of saccharin addition on the resulting hardness and elasticity modulus, nanoindentation hardness measurement experiments were conducted. Figure 4 illustrates the nanoindentation loading-displacement curves, while Table 7 presents the data obtained from the nanoindentation tests.

Fig. 4.

The nanoindentation results revealed that the Ni-Cr alloy coating exhibited a hardness of 1.76 GPa. However, with increasing concentrations of saccharin, the hardness increased to 2.93 GPa. Notably, the load-displacement curve for the sample with 2 g/l saccharin appeared notably narrower compared to the others, indicative of greater hardness. This increase in hardness can be attributed to the smoother surfaces obtained with higher saccharin concentrations in the bath. Furthermore, the growing amount of Cr in the coating was believed to have a positive effect on hardness [31]. EDS analysis corroborated this, showing an increase in the accumulation of Cr with higher saccharin content. The observed trend of increasing hardness with the addition of saccharin aligns with findings in the literature [37].

Another significant finding in this study is that adding saccharin, up to 2 g/l, led to a drop in the modulus of elasticity. While saccharin contributed to improved surface properties, it resulted in reduced adhesion of the coating to the substrate. The more compact surface structure correlated with the observed increase in hardness.

Furthermore, this study examined residual stresses to understand why the hardness did not change as expected with variations in grain size. A positive sign signifies tensile stress, while a negative sign indicates compressive stress. Without saccharin in the bath, the sample displayed tensile stress. However, with saccharin added, the residual stress shifted toward compression. It’s well-known that hardness decreases with tensile stress [38], offering an explanation for the hardness increase with the addition of saccharin. Equation (2) was used to calculate the residual stresses [39] considering the distribution of hardness (H) and Young’s modulus (E), and the maximum indentation depth (h) was determined using the Oliver and Pharr method [40].

If the abbreviations in the table are explained: hc (contact depth), Pmax (max load), s (slope of the unloading curve (dP/dh), A (contact area), hmax (maximum depth), Er (reduced modulus of elasticity). In addition to crystal structure and morphology, surface roughness plays a crucial role in coatings [41]. Figure 5 presents a graph depicting the average surface roughness, highlighting a noticeable reduction in surface roughness with increasing saccharin concentrations. Specifically, the Ra value decreased from 0.32 for the sample without saccharin to 0.1 for the sample with 2 g/l saccharin. This represents a reduction in roughness by more than threefold due to the addition of saccharin. A similar trend was observed for Rz, with a threefold reduction compared to the coating achieved without the inclusion of saccharin in the bath. The primary reason for this phenomenon is the ability of saccharin to flatten the coating surface. With high free adsorption energy, deposition occurs more rapidly on protruding portions of the coating. This results in an increased presence of additives in areas with greater accumulation. This, in turn, reduces electron transfer, leading to enhanced accumulation in regions with lower levels. Consequently, a more uniform coating is achieved. Additionally, coating thicknesses were measured as 10±1 μm.

Fig. 5.

Upon reviewing the friction coefficient graph (Figure 6), it is seen that the addition of saccharin starts with a lower friction coefficient value. Here, the lowest friction coefficient value is in the sample containing 1 g/l saccharin. However, the sample containing 2 g/l saccharin is almost at the same points compared to the sample without saccharin addition. Due to the initial values not being distinctly visible on the graph, their approximately specific values (0 g/l saccharin initial value = 0.45, 2 g/l saccharin initial value = 0.44) are explicitly provided here. As the sliding distance increases, the surface hardness also rises, and correspondingly, the friction coefficient exhibits a continuous increase. Despite having lower roughness, the rise in the friction coefficient over the sliding distance is likely caused by the harder particles detaching from the surface. These particles become trapped between the abrasive ball and the substrate, functioning as abrasives. Conversely, the saccharin-free Ni-Cr sample initiates with a friction coefficient of 0.47 and maintains a stable trajectory.

Fig. 6.

When examining the SEM image of the microstructure of the worn surface (Figure 7a), it’s evident that micro cracks, oriented perpendicular to the wear track, are present. In regions where these micro cracks intensify due to repeated loading, plastic deformation occurs. The size of the plastic deformation measures approximately 40–50 μm in some areas, leading to the effective spilling of the coatings from the surface. Additionally, a complex wear mechanism is observed, characterized by micro scratches and micro smears in the form of wide grooves on the sample surface, parallel to the wear track [42].

Fig. 7.

In the SEM image of the worn surface of the Ni-Cr coating achieved with 0.5 g/l saccharin (Figure 7b), an oxidation-type wear mechanism is evident in the central and inner regions. Around the edges of the wear marks, surface tearing due to fatigue can be observed, resulting from tensile stresses behind and compression stresses ahead of the abrasive ball. It’s also possible that these tears are caused by wear particles, as some areas of the surface exhibit wear debris. In the outermost region, a flaking-type wear mechanism occurs through delamination, a consequence of fatigue resulting from the repeated application of loads [26].

Upon examining the SEM wear images depicted in Figures 7c and 7d, it’s noticeable that both exhibit similar wear behaviour. Unlike the other samples, micro cracks oriented perpendicular to the direction of the abrasion scar intensify. Moreover, a darker contrast colour is apparent on the surface, suggesting the formation of an oxidation-type wear mechanism. The increased number of micro cracks is attributed to the higher surface hardness of these samples. Increased hardness reduces toughness and increases fragility. Furthermore, the mismatch between the coating layer and the substrate’s hardness or the difference in hardness and modulus of elasticity between the substrate and the Ni-Cr coating layer may account for these observed differences [43].

Upon reviewing Table 8, it’s evident that the addition of saccharin results in a significant reduction in wear volume losses, with decreases of 1.87 and 3 times observed. This reduction can be attributed to the increased hardness achieved through the addition of saccharin. As the surface hardness of the material increases, the abrasive ball penetrates the surface to a lesser extent, and the removal of abrasive particles during wear is diminished. This, in turn, leads to an enhancement in wear resistance [44, 45]. The coefficient of friction (COF) is the ratio of the friction force to the force pressing on them and is dimensionless.

The corrosion behaviour of both the substrates and coatings was investigated in a 3.5% NaCl solution. The Tafel extrapolation method was used to determine corrosion currents, corrosion potentials, and corrosion rates, which are presented in Figure 8 and Table 9. Notably, all electrodeposited Ni-Cr coatings displayed superior corrosion resistance compared to the AISI 1040 steel substrate. In comparison to AISI 1040, the Ni-Cr alloy coating exhibited a corrosion potential that was 51 mV more positive. Additionally, the addition of saccharin to the Ni-Cr alloy coating resulted in an improvement in corrosion resistance across all baths. Tafel curves and corrosion rates indicated better performance in all baths with added saccharin compared to the bare AISI 1040 steel substrate. However, the enhancement in corrosion resistance was not directly proportional to the saccharin concentration in the bath. The optimum corrosion resistance was achieved with a 1 g/l saccharin concentration in the bath. This particular sample exhibited approximately three times greater corrosion resistance than AISI 1040 steel. Saccharin played a role in diminishing both anodic and cathodic electrochemical reactions responsible for corrosion by altering the coating kinetics. Furthermore, the primary reason for the change in corrosion resistance can be attributed to the elimination of defects on the coating surface facilitated by the addition of saccharin. The additives in the coating served as a physical barrier by filling cracks and micron-sized gaps, partially obstructing reactive areas and reducing cathodic reactions [13]. Additionally, the presence of saccharin within the grain boundaries made the coating less permeable because of its enhanced compactness, offering another explanation for the improvement in corrosion resistance with the addition of saccharin [17]. The existence of imperfections or flaws in the coating may allow the corrosive medium to reach the substrate surface, resulting in an inefficient coating layer that negatively impacts the corrosion protection of the coating [46].

Fig. 8.

The open-circuit potentials of the steel substrate and Ni-Cr alloy coatings, including those with saccharin, in a 3.5% NaCl solution were recorded for a duration of 3600 seconds (Figure 9). It is known that the one on the positive side of the curves obtained exhibits superior corrosion resistance compared to the others [47]. Figure 9 illustrates that all the coatings displayed improved corrosion resistance compared to the bare AISI 1040 substrate. The more positive Ecorr observed in the coatings may result from various factors, such as the alloy composition of the coating, surface chemistry, reduced surface roughness, and oxidation product within a passive layer produced by the coating. Furthermore, the corrosion resistance of the coatings improved as saccharin was added to the bath, with the coating containing 1 g/l saccharin displaying the best corrosion resistance. Notably, both the coatings and the substrate experienced a rapid decrease in corrosion potential upon exposure to the NaCl solution, which stabilized after a while. The sample without the coating demonstrated the most significant initial drop in potential when exposed to a corrosive environment. Although the Ni-Cr coating initially exhibited a favourable potential, over time, this potential significantly declined. In contrast, the saccharin-added baths initially caused a decrease in potential, but they quickly formed an anti-corrosion layer, effectively protecting the material against corrosion. Consequently, it can be concluded that saccharin, forming an inert physical barrier, is a primary reason for the increased resistance to corrosion. The presence of sulphur accumulating at the grain boundaries was found to provide positive results against corrosion when maintained at a certain level. However, an excess of sulphur led to negative corrosion results. Additionally, the coatings’ active area decreased as a result of the addition of saccharin [13].

Fig. 9.

The impedance modulus graph, shown in Figure 10, is a known indicator of corrosion resistance, where an increasing impedance modulus signifies better corrosion resistance [48]. In line with this, it’s evident that the sample with the highest impedance modulus is the one with 1 g/l saccharin added to the bath, while the lowest impedance was recorded for the AISI 1040 substrate. This observation supports the presence of a protective barrier between the substrate and the coatings. In all the samples, impedance exhibited a sharp decrease with increasing frequency. However, as the frequency increased further, the impedance began to stabilize. In this region, samples with added saccharin were very close to each other.

Fig. 10.

Solution resistances (Rs), charge transfer resistances (Rp) of electrode reactions, and doublelayer non-ideal capacitances (Cp) were calculated from the EIS data and are provided in Table 10. It’s worth noting that the surface shape plays a significant role in altering Cp values. A high density of microcracks and an uneven structure result in a larger contact area. Based on the data, the sample containing 1 g/l saccharin exhibited the most favourable Cp value. However, the addition of more than 1 g/l saccharin resulted in a slight increase in the Cp value. Nevertheless, this increase is relatively modest and is approximately three times lower than the coatings without saccharin addition. This decrease in Cp can be attributed to the improvement in surface properties achieved with the addition of saccharin.

In summary, the steel substrate exhibited inferior corrosion resistance compared to the Ni-Cr alloy coatings in all three corrosion measurement methods. The corrosion resistance of the resulting coatings was enhanced by the addition of saccharin to the electrolytic Ni-Cr alloy bath. However, there was a slight reduction in corrosion resistance when saccharin was added to the bath at concentrations exceeding the optimal level (>1 g/l). In essence, these coatings effectively shielded the substrate from the corrosive environment, thus safeguarding it against corrosion. Saccharin played a key role in forming a physical barrier by filling cracks and gaps on the coating’s surface. Additionally, it minimized imperfections, resulting in a denser and less permeable Ni-Cr coating. The uniform structure achieved with saccharin not only improved the coating’s corrosion resistance by inhibiting matrix dissolution but also increased its strength. It’s worth noting that the decline in corrosion resistance with 2 g/l saccharin can be attributed to the presence of pinhole gaps in the coating. This observation aligns with the SEM images depicted in Figure 2, illustrating a more compact surface and the presence of pinhole cavities due to the addition of 2 g/l saccharin.