Experimental investigations of electrodeposited Zn–Ni, Zn–Co, and Ni–Cr–Co–based novel coatings on AA7075 substrate to ameliorate the mechanical, abrasion, morphological, and corrosion properties for automotive applications

3.1 Analysis of micro-hardness and Taber abrasion resistance on coated and uncoated materials

The microhardness of the binary and ternary coatings applied on the substrate material was measured and tabulated in Table 2.

Table 2 shows the microhardness of both binary and ternary coated and uncoated normal and annealed state of specimens. It is clearly evident from the table that the coated specimens have higher hardness than the uncoated specimens in both normal and heat-treated (annealed) state. Particularly, the ternary-coated specimen has higher hardness than the binary-coated specimen in both conditions [12].

For “non-heat-treated material,” 50% of hardness enhancement is observed for the Zn–Co coating and 61% of improvement is observed for Zn–Ni coating compared to the uncoated condition. In the case of Zn–Ni binary coating, the increase in hardness could be due to the formation of fine grains with a meager porosity and the absence of microcracks [31]. The improved grain refinement could also be a reason for the hardness enhancement. The presence of smaller grains impedes the dislocation motion and increases microhardness. In the case of Zn–Co binary coating, the improvement in microhardness is the transformation of the whole Zn phase into a CoZn intermetallic one [10].

The ternary-coated specimen is observed to be higher (82%) among the coated specimens than the uncoated material. In the case of “heat-treated specimens,” the minimum enhancement of hardness is observed as 65% for the Zn–Co alloy coating and a 69% improvement of hardness is observed for the Zn–Ni binary coating [32]. Similar to the non-heat-treated material, a higher hardness (92%) improvement is observed for ternary (Ni–Cr–Co) coating compared to the uncoated substrate material [33].

The presence of Cr as Cr₂O₃ in Ni–Cr–Co coating with a binding energy of 576 eV at the outer layer in the ternary Ni–Cr–Co coating could be the reason for higher hardness than other binary depositions. Stress-relieving, recrystallization, and precipitation of deposited metals are the reasons for the increased hardness [22].

It is clearly evident from the analysis that the higher hardness enhancement is observed for the ternary coating compared to the uncoated and binary-coated specimens in both the status.

Comparing the non-annealed and annealed materials, the uncoated specimen has more than double-fold enhancement in the hardness of the heat-treated material. A similar kind of hardness improvement is also observed for both the binary- and ternary-coated materials. It may be concluded that the heat-treated coated and uncoated specimens were found to have higher hardness than the non-annealed specimens due to relieving stress and affirmative alloy coating on the substrate material.

Microhardness results indicated that Ni–Cr–Co coatings showed higher hardness than the other two binary electrodeposited coatings. The following order of performance of coatings was observed from the microhardness studies: Ni–Cr–Co > Zn–Ni > Zn–Co.

The abrasion wear resistance of coated and uncoated materials of both the status such as heat treated and non-heat-treated specimens are provided in Table 3.

The Taber wear index value is an indicator of the wear resistance of the material. The lower the value, the higher will be the wear resistance. Based on the observation, the uncoated substrate material has lower abrasion resistance as it has a higher index value. The heat-treated uncoated specimen is also having a higher index value, which signifies the poor resistance to wear. In general, the coated specimens are exhibiting better wear resistance in both the heat-treated and non-heat-treated conditions. It has been observed from the table data that the ternary coating has higher wear resistance than the binary-coated material. It has 64% higher resistance in the non-heat-treated status and 67% higher resistance in the heat-treated condition compared to the uncoated specimens. Among the binary-coated materials, Zn–Ni-coated material has higher wear resistance. It has 61% higher abrasion resistance compared to the uncoated specimen. On the other hand, it exhibits 53% higher wear resistance compared to the uncoated specimen in the case of heat-treated status. The annealed coated specimens still have better wear resistive properties compared to the non-annealed specimens. The wear resistance has been increased to almost two-fold in the heat-treated specimens. The formation of NiZn₃ and Ni₂Zn₁₁ phase structure in the Zn–Ni deposits contributes to wear resistance. The submerging of Co particles in the galaxy of Zn atoms has led to improved wear resistance for the Zn–Co-coated material. The presence of Cr₂O₃ at the surface of Ni–Cr–Co coatings has improved the abrasion resistance in comparison with other electrodeposited coatings. The following order of performance of coatings was observed from the abrasion studies: Ni–Cr–Co > Zn–Ni > Zn–Co.

3.2 Analysis of tensile strength of coated and uncoated materials

The tensile test was conducted as per the ASTM E8 standard to evaluate the tensile strength of the binary and ternary deposition on the substrate materials. The mechanical strength studies have been undertaken after immersing the tensile specimens in Harrison’s solution in order to simulate the industrial environment in real-time applications. The tensile specimen used for the present work is shown in Figure 3.

Figure 3

Tensile test sample as per ASTM E8 Standard.

As demonstrated by the results of the tensile test, Ni–Cr–Co deposits exhibit excellent adhesion to AA7075 PDC substrates as depicted in Figure 4(a–c). In a similar fashion to the microhardness trend, the material that has been coated but has not been immersed has a higher tensile strength. However, for binary coatings, the enhancement percentage is in the range of 1 and 2. In comparison with the uncoated or substrate materials, ternary-coated materials have a maximum enhancement of 5% in tensile strength. In the case of deposits, there is an increase in tensile strength over uncoated deposits as a result of the cohesive strength of the coating, its load-bearing capacity, and the interlocking between the coating and anchoring points of the substrate. As a result of immersion in Harrison’s solution for a week, the tensile properties of both coated and uncoated materials have deteriorated. For uncoated materials, the tensile strength has been reduced to a maximum of 7%, while for Zn–Ni binary-coated materials, the tensile strength has been reduced to a minimum of 3%. After immersion in the solution for a month, coated materials show a reduction in tensile strength of around 30%. After 1 week and 1 month of immersion, the uncoated material exhibits a higher reduction in tensile strength. The ternary-coated material, on the other hand, exhibits a lesser reduction in tensile strength as compared to the substrate and binary-coated materials. In light of the test results, it seems that the ternary coating is capable of protecting the substrate to some extent regardless of the duration of the immersion.

Figure 4

(a–c) Tensile strength of coated and uncoated deposited materials with and without immersion. (a) As-plated, (b) 1 week after immersion, and (c) 1 month after immersion.

Due to the presence of trimetallic atoms in Ni–Cr–Co, the surface coverage of this material has been enhanced to a greater degree. Furthermore, the chromium oxide layer has a better corrosion prevention characteristic, which may contribute to Harrison’s solution’s improved corrosion resistance as a result of the ternary electrodeposition.

The stress–strain curve for the uncoated AA7075 PDC substrate shows variations depending on the amount of stress applied and the resulting deformation as exhibited in Figure 5a. At low levels of stress, the material exhibits linear elastic deformation, which means that the deformation is reversible when the stress is removed. This is represented by the linear region on the stress–strain curve, known as the elastic region which indicates how stiff the material is. For uncoated AA7075, the modulus of elasticity is relatively high, indicating a stiff material. As the stress increases, the material begins to undergo plastic deformation, which is irreversible when the stress is removed. This is represented by the non-linear region on the stress–strain curve, known as the plastic region. The slope of this region decreases as the material undergoes more deformation, indicating that it becomes easier to deform as it approaches its ultimate tensile strength (UTS). At this point, the material undergoes necking, which is a localized reduction in cross-sectional area. The stress required to cause necking is referred to as the ultimate strength or tensile strength. The stress–strain curve for uncoated AA7075 also shows a region beyond the UTS, known as the post-necking region. In this region, the material continues to deform until it eventually breaks. The slope of this region decreases as the material undergoes more deformation until it reaches its breaking point.

Figure 5

(a–d) Stress–strain curves of as-plated series. (a) Uncoated, (b) Zn–Ni deposited on AA7075 PDC, (c) Zn–Co deposited on AA7075 PDC, and (d) Ni–Cr–Co deposited on AA7075 PDC.

The mechanism of the stress–strain curve for uncoated AA7075 is related to the material’s microstructure, which consists of a mixture of Al and Zn. As stress is applied, dislocations in the material’s crystal structure move, allowing the material to deform. At low levels of stress, dislocations move only a small amount before returning to their original position, resulting in elastic deformation. As the stress increases, dislocations move further, resulting in plastic deformation. In summary, the stress–strain curve for the uncoated AA7075 PDC substrate shows variations in its response to external stress, which can be attributed to the material’s microstructure. The curve exhibits a linear elastic region, a non-linear plastic region, a necking region, and a post-necking region, culminating in eventual failure as exhibited in Figure 5a.

Zn–Ni coatings deposited on the AA7075 substrate improve the material’s corrosion resistance, wear resistance, and adhesion. The stress–strain curve for Zn–Ni-coated AA7075 shows an increase in the UTS and a smoother transition from elastic to plastic deformation, compared to uncoated AA7075 as exhibited in Figure 5b. The coating prevents the formation of microcracks and distributes stress more evenly, leading to an increase in UTS and improved mechanical strength.

Zn–Co coatings deposited on AA7075 improve corrosion resistance, wear resistance, and adhesion. The stress–strain curve for Zn–Co-coated AA7075 shows an increase in UTS and a slightly steeper slope in the elastic region compared to uncoated AA7075 as exhibited in Figure 5c. The coating’s enhanced adhesion to the substrate improves the mechanical strength of the material, leading to an increase in UTS as exhibited in Figure 5c.

Ni–Cr–Co coatings deposited on AA7075 improve wear resistance, adhesion, and high-temperature stability. The stress–strain curve for Ni–Cr–Co-coated AA7075 shows a slight increase in UTS and a steeper slope in the elastic region compared to uncoated AA7075 as exhibited in Figure 5d. The coating’s ability to reduce friction and prevent wear leads to an improvement in the material’s mechanical strength.

Overall, each coating has a unique impact on the mechanical properties of AA7075, as seen from the stress–strain curves. Zn–Ni and Zn–Co coatings improve corrosion resistance, wear resistance, and adhesion, leading to an increase in UTS and improved mechanical strength. Ni–Cr–Co coatings improve wear resistance, adhesion, and high-temperature stability, resulting in a slight improvement in UTS and mechanical strength.

The tensile test was conducted on the uncoated and coated specimens after immersing them in Harrison’s solution for a period of 1 week and 1 month. The results of the tests are provided in Table 4.

It has been observed that the appearance of the fracture planes existed within the deposit, which signifies the cohesive type of fracture. In all the cases, the fracture is found nearer to the interface between the deposit and the substrate, but still inside the deposit. No external peeled-off layer is noticed. Ni–Cr–Co deposit has remarkable adhesion on AA7075 PDC substrates, showing good bonding between the deposit and substrate, evidenced by the result of the tensile test.

Similar to the microhardness trend, the tensile strength is also higher for the coated non-immersed materials. However, the percentage enhancement is in the order of 1 and 2 for the binary coatings. The ternary-coated material has a maximum of 5% enhancement in the tensile strength compared to the uncoated material or substrate material. The reasons for the increase in tensile strength of deposit over uncoated material are due to the cohesive strength of the surface deposit, its load-bearing capacity, and the interlocking of deposit with anchoring points of the substrate, which is made by LST.

The tensile property of both the coated and uncoated materials has deteriorated while they are immersed in Harrison’s solution for a week. The tensile strength has been reduced to a maximum extent of 7% for uncoated material and a minimum (3%) for Zn–Ni binary-coated material.

The percentage reduction in tensile strength is found to be around 30% in the case of coated materials after they are immersed in the solution for a month. The uncoated material is found to have a higher reduction in tensile strength for the 1-week and 1-month immersion periods. On the other hand, the ternary-coated material has a lesser reduction in tensile strength compared to the substrate and binary-coated materials. It is understood from the test results that the ternary coating protects the substrate to some extent irrespective of the immersion periods.

The presence of tri-metallic atoms in Ni–Cr–Co has enhanced the surface coverage to a larger extent. Also, the chromium oxide layer possesses a better corrosion prevention characteristic and these aspects could be the reasons for ternary electrodeposition enriching the performance against the corrosion in Harrison’s solution.

It is demonstrated that the tensile strength (MPa) of Ni–Cr–Co on AA7075 PDC is higher than the other deposits (582.24 of Ni–Cr–Co > 566.07 of Zn–Co > 560.05 of Zn–Ni > 553.64 of uncoated condition) due to the existence of hard Cr particles along with its strong bonding between the intermetallic atoms such as Ni and Co [32].

Further, it is observed that the presence of crystal orientation (XRD studies), phase content (XPS studies), and surface morphology of ternary Ni–Cr–Co have greatly contributed to the enhancement of tensile strength. The following order of performance of coatings is observed from the tensile test: Ni–Cr–Co > Zn–Co > Zn–Ni [33].

3.3 XRD and SEM image analysis of coated materials

The XRD pattern of both the binary- and ternary-coated materials is depicted in Figure 6(a–c).

Figure 6

(a–c) XRD patterns of (a) Zn–Ni (b) Zn–Co and (c) Ni–Cr–Co-coated materials.

The XRD analysis of the Zn–Ni binary coating is shown in Figure 6(a). The diffraction peaks of γ-NiZn₃ (831), γ-Ni₂Zn₁₁ (330), and 631 with 2θ values 38, 43, and 73° are confirming the presence of Zn–Ni binary deposit on AA7075 PDC substrate. A further peak of about 56, 68, and 83° is indicative of Zn deposits in the Zn–Ni coatings (102, 110, and 200). The coatings have enhanced hardness value due to the presence of intermetallic phases.

The XRD pattern of Zn–Co-coated material is depicted in Figure 6(b). The presence of three strong peaks such as Zn (110), Co (111), and CoZn (211) and two feeble peaks such as ε-CoZn₃ (220) and ε-CoZn₃ (301) is clearly visible. Besides, there is a very weak peak at 61.8° for α-Al (112) proving that Zn–Co coatings have been carried out on AA7075 PDC substrate.

The XRD pattern of Ni–Cr–Co ternary coating is displayed in Figure 6(c). It can be seen from the pattern that the Ni–Cr–Co ternary deposit is a solid solution with a body-centered cubic structure due to the formation peaks at lattice plane such as (110), (220), and (210) with a crystal lattice constant of 2.88 A°. Among them, the Co phase has clear, sharp diffraction peaks at θ of near 45.168, 65.792, and 77.392°, and their crystal plane index is (110), (200), and (210), respectively. The peaks in XRD are sharp, narrow, and regular and therefore, all the coatings Zn–Ni, Zn–Co, and Ni–Cr–Co are crystalline. The intensity of the peak is in accordance with the JCPDS card number 03-074-1264.

The SEM image has been taken on both the binary- and ternary-coated materials and is shown in Figure 7(a–c).

Figure 7

(a–c) SEM image of the (a) Zn–Ni (b) Zn–Co and (c) Ni–Cr–Co-coated materials.

The deposited surface displays many shallow pits due to hitting by progressive particles. The deficient and twisted particles are observed in the image. The uniformity in deposition attributes to a conservative structure, low porosity, and high deposition of coatings. The SEM image illustrated in Figure 7(a) shows the presence of smaller globular structure Zn–Ni atoms. Moreover, the atoms are overcrowded owing to the strong bonding between Zn and Ni and further effective adhesion of said coatings on LST AA7075 substrate. The uniformly dispersed Zn–Ni particles in the deposition are also observed. The surface morphology of binary Zn–Co coating on the PDC AA7075 substrate is shown in Figure 7(b). The evenly distributed dot-like structure and submerged Co particles in the galaxy of Zn atoms have been observed in the image. Further, it has been demonstrated that the incorporation of Co atoms in the Zn matrix is highly visible. To remove the oxide film on the AA7075 PDC, the zincating process was carried out. The deposition of Zn on the surface of Al created a suitable site for Zn nucleation. Since it is a chemical immersion coating, the aggressive nature of sodium hydroxide present in the zincating bath had thrown Zn atoms on AA7075 PDC, leading to a more uniform distribution of Zn atoms.

Good differentiation was achieved in the area between the laser track among the Ni–Cr–Co coatings and Al layers. The coverage of laser track and Ni–Cr–Co coatings is very obvious, confirming the better diffusion of Ni–Cr–Co coatings than Zn–Co-coated AA7075 PDC substrate. To understand the effectiveness of bonding by laser texturing, cross-section SEM has been carried out and shown in Figure 7(c). It is understood from the image that the effective adhesion of Ni–Cr–Co on AA7075 PDC had taken place and the same could also be the reason for the enhancement of microhardness, wear, and corrosion resistance of the said coating. The image shown in the SEM contained three parts: a) the substrate AA7075, b) the track of laser texturing, and c) the oxides of Ni–Cr–Co. It is established that the formation of unique and uniform oxide depositions on the top layer of Ni–Cr–Co has significantly contributed to higher retardation of metal attack on Ni–Cr–Co in the corrosive medium.

3.4 Analysis of XPS on binary- and ternary-coated materials

The analysis of XPS on binary- and ternary-coated materials has been carried out and the results are shown in Table 5 and Figure 8.

Figure 8

(a–c) XPS of coated materials (a) Zn–Ni, (b) Zn–Co, and (c) Ni–Cr–Co.

The results of XPS studies on the binary- and ternary-coated materials are provided in Table 5. It provides information about the species formed, binding energy, electron level, and the type of layer on XPS investigation of coated materials. Figure 8(a) shows the XPS analysis of the Zn–Ni deposit on AA7075 PDC. Two major Ni peaks in the high-resolution XPS arose from 2p signals located around 856 and 874 eV, which were the characteristics of 2p3 = 2 and 2p1 = 2, respectively. Moreover, there are two peaks referred to as satellite peaks around 2p3 = 2 (S) and 2p1 = 2 (3S) in the Ni 2p region. Remarkably, the presence of Ni₂Zn₁₁ demonstrated lower binding energy in comparison with -NiZn₃.

The results of XPS of Zn–Co on the AA7075 PDC substrate are shown in Figure 8(b). The existence of Co as Co₃O₄ in the inner layer of the coating is confirmed by O KLL with a binding energy of 1,008 eV, which has proximity to peaks of Co₂p₃ = 2 with the binding energy of 779 eV. It is justified that the Co could have existed as Co₃O4 in the coating. The binding energies resulting at 1,074, 853, and 839 eV are attributed to the presence of Zn metal in the coating. Further, the appearance of O KLL at 1,008 eV might be due to the formation of ZnO in the coating. It is peculiar to note that there is a sharing of oxygen atoms between Zn and Co metals which appeared as Zn and Co₃O₄ in the coating.

The XPS analysis on ternary-coated material is displayed in Figure 8(c). The XPS output shows the existence of Cr, Co, and Ni alloy composition on the surfaces of coatings. Adventitious C and N peaks have entered the laboratory atmosphere. The complex peaks of Co 2p, Ni 2p, and Cr 2p exhibit binding energies that correlated with the presence of Co₃Zn₃, NiO, and CrO₃, respectively. The sub-peaks in Zn 2p and Al 2p 1s peak revealed the zincating of the substrate material.

3.5 Analysis of salt spray on uncoated and coated materials

A salt spray test has been conducted on the substrate and the heat-treated-coated specimens. To analyze the corrosion resistance of materials, changes in appearance due to prolonged periods of exposure to a corrosive environment are considered. The results of the salt spray test on specimens are provided in Table 6.

The appearance and progress of corrosion of uncoated and Zn–Ni-coated samples under salt spray were recorded from time to time. After 60 min stay, it was noticed that 40% rust formed on uncoated AA7075. For the heat-treated sample, 1% rust area was formed after 610 h of stay in the salt spray chamber. Zn–Ni nano-crystalline coating could be obtained with higher current efficiency on a single ϒ phase. This ϒ phase shows the highest corrosion protection. The XRD analysis of Zn–Ni shows the ϒ phase of intermetallic NiZn₃, and Ni₂Zn₁₁ retards the corrosion formation. The homogeneity in phase composition leads to the up-gradation of corrosion protection properties. With various Ni content, the internal stress of deposit was found by the X-ray stress analyzer and concluded that a sudden increase in salt spray time was due to the penetration of corrosion products as a result of a shoot up in internal stress. The selective corrosion of the Zn–Ni deposit occurs in the following sequence deposit: a) dissolving of Zn, b) passivation, and c) crack propagation. Hence, it is understood that the enhancement of corrosion resistance for coated surfaces might be due to the formation of -NiZn₃ and -Ni₂Zn₁₁ [12].

It is observed from the table that the heat-treated Zn–Co electrodeposition on AA PDC 7075 has shown 1% of rust area formed after 550 h of stay in the salt spray chamber. The application of a reverse pulse current to suppress the zinc hydroxide formed the maximum percentage of Co in the coatings [38,39]. The controlled morphology and the composition of binary Zn–Co deposits minimize the internal stress and micro-cracks. The formation of refined grains of Co atoms is uniformly dispersed in the Zn matrix in the coatings. The reason for corrosion resistance improvement of binary Zn–Co deposits could be because the binary Zn–Co deposits consist of fine globular Co particles submerged in the galaxy of Zn atoms as ZnO shown in the SEM image.

The analysis of salt sprays on ternary Ni–Cr–Co electrodeposition on AA7075 PDC is also provided in the table. The presence of trivalent Cr as an oxide in the phase of Cr as Cr₂O₃ at the outer layer has a binding energy of 576 eV in the ternary coating. Further, the maximum content of Zn (94.8%) with Co (5.2%) has led to 1% rust formation with a delayed time (660 h).

Compared with the binary deposit Zn–Ni and Zn–Co, the ternary Ni–Cr–Co deposit is found to have more Ni and Co concentration, as higher current density (500–550 A·ft⁻²) followed [40–42]. The reason for higher Ni and Co content in the ternary coating is the synergistic catalytic effect in the electrolyte bath [43–45]. The presence of a crystalline structure with the high alignment of Co and Ni atoms could significantly improve the corrosion resistance of Ni–Cr–Co coatings on AA 7075 PDC substrates [46–48]. In addition, the formation of the passive oxide layer on Cr has justified its corrosion resistance performance in the rankings [49–51]. All in all, the Al alloy AA7075 is broadly used in various industries owing to its high strength-to-weight ratio, which is comparable and replaceable to steel in many applications [52–54]. However, it has poor resistance to wear and corrosion compared to other Al alloys. To improve the mechanical, wear, and corrosion properties of AA7075, a novel coating method has been developed. Binary and ternary metals such as Zn–Ni, Zn–Co, and Ni–Cr–Co are electroplated on the substrate material (AA7075) after LST pre-treatment. According to the ternary coating results, the resistance of the specimens under non-heat-treated conditions is higher than the resistance of specimens under heat-treated conditions [55–57]. The Ni–Cr–Co ternary coating has higher tensile strength compared to other deposits [58–60]. The SEM images, XRD, and XPS findings on the coated materials have been corroborated with the analyses on mechanical and corrosion properties. The dendritic structure during solidification leads to deterioration in the mechanical properties of AA7075 PDC [61–63]. To improve the performance in automotive and aircraft applications, the present research work is focused to develop novel electrodeposition of binary and ternary coatings by LST on AA7075 PDC material for the enhancement of hardness, tensile strength, wear, and corrosion resistance of substrate material to lengthen the life span of AA7075 in actual applications [64–66]. The optimized parameters have been used for the surface coatings on AA7075 PDC in the present study [67–69]. Electrodeposited coatings of Zn–Ni, Zn–Co, and Ni–Cr–Co have become increasingly popular in recent years due to their unique properties and various applications [70–72]. Here are some of the applications of these novel coatings on AA7075.

1. Corrosion protection: Electrodeposited Zn–Ni, Zn–Co, and Ni–Cr–Co coatings have excellent corrosion resistance properties. These coatings can protect the underlying AA7075 from corrosion in various environments such as seawater, acidic and alkaline solutions, and humid and corrosive atmospheres [73–75].

2. Wear resistance: These coatings also exhibit high wear resistance, which is useful in applications that involve sliding, rolling, or abrasive wear. For instance, these coatings can be applied to components in the automotive, aerospace, and marine industries to enhance their wear resistance [76,77].

3. Thermal stability: Electrodeposited coatings of Zn–Ni, Zn–Co, and Ni–Cr–Co are also known for their high thermal stability. This property makes them suitable for use in high-temperature applications where other coatings may fail. For example, these coatings can be used on heat exchangers and other components that are exposed to high temperatures [78,79].

4. Decorative finishes: These coatings can also be used for decorative purposes due to their attractive appearance. The Zn–Ni, Zn–Co, and Ni–Cr–Co coatings can be easily plated onto AA7075 to provide a durable and aesthetically pleasing finish [80,81].

In summary, electrodeposited Zn–Ni, Zn–Co, and Ni–Cr–Co-based coatings have various applications on AA7075, including corrosion protection, wear resistance, thermal stability, decorative finishes, electrical conductivity, and biomedical applications. Hence, the order of corrosion resistance performances of the coating in 3.5% NaCl is as follows: Ni–Cr–Co > Zn–Ni > Zn–Co.