1 Introduction

Multilayered coatings have attracted considerable attention of several investigators during the last decade, mainly because of their improved mechanical, physical and/or electrochemical properties. Furthermore, the possibility of tailoring the structure of those materials in conformity with the demands of certain mechanical, optical and electrochemical applications, has led to increased flexibility in materials design. However, due to their complex structure, multilayered coatings usually demand a great number of parameters to be determined for a detailed characterization of their structure including the structural and chemical periodicity, the crystallography of the sub-layers, the surface and interface roughness and the coherency between the sub-layers [1,2,3].

Zn alloy single-layered electrodeposited coatings (such as Zn–Ni, Zn–Co and Zn–Fe coatings) have been extensively studied [4,5,6,7,8,9]. It has been shown that these coatings present very good anticorrosive properties, providing enhanced corrosion protection of ferrous substrates [9,10,11,12,13,14] and improved tribological behavior in comparison to the pure Zn electrodeposited coatings [15,16,17]. Therefore, Zn alloy coatings have been used in several industrial sectors including the automotive industry. Recent investigations though, have shown that multilayered coatings consisting of Zn alloy sub-layers may perform better as corrosion resistant coatings than equivalent single-layered coatings. Up to date, multilayered coatings consisting of pure Zn and Ni layers and Zn–Ni multilayered alloy coatings were produced with the aid of dual-bath and single-bath electrodeposition techniques [18,19,20,21]. The surface morphology and the corrosion behavior of those coatings have been studied and improved corrosion resistance of multilayered coatings in comparison to equivalent single-layered coatings has been reported. Investigations dealing with the morphological characteristics and the corrosion behavior of electrodeposited Zn–Co multilayered coatings have also been reported [22,23,24]. It has been shown that Zn–Co multilayered alloy coatings present better corrosion resistance than equivalent single-layered coatings. For the case of Zn–Fe multilayered coatings, the literature is limited and more concentrated in Zn–Fe multilayered alloy coatings produced by the single bath electrodeposition technique [25, 26]. In these studies [25, 26], special attention has been given in the understanding of the anomalous codeposition mechanism in Zn–Fe alloy electroplating and it has been suggested that the Zn based alloy multilayered coatings offer a combination of sacrificial and barrier corrosion protection to mild steel substrates.

During the past few years, may investigations have been published dealing with the multilayer coatings and their improved tribological properties, their corrosion resistance and many other surface properties [27,28,29,30,31,32,33,34,35,36,37,38]. In the present study, new Zn–Fe multilayered alloy coatings were deposited on mild steel substrates using the dual bath electrodeposition technique. The surface and cross-sectional morphology, the crystal structure, the surface roughness and the microhardness of Zn–Fe multilayered alloy coatings were investigated.

2 Materials and methods

Mild steel specimens with dimensions 40 × 20 × 0.5 mm3 were cut from a mild steel sheet, Table 1, machined to an average roughness (Ra) of 0.3 μm and annealed at 550 °C for 2 h in an Ar-inert atmosphere.

Table 1 Chemical composition (wt%) of mild steel substrates

After the stress-relief annealing, the specimens were left to cool inside the furnace until the temperature in the furnace araised 50 °C. Annealing was followed by cleaning with acetone in an ultrasonic bath for 25 min., rinsing by deionized water, activation in a proprietary solution consisted of HCl and various corrosion inhibitors and rinsing by deionized water. The mild steel specimens were then used as substrates for the deposition of Zn–Fe multilayered alloy coatings with the aid of the dual bath electrodeposition technique (DBT). The two solutions used in this study, Table 2, consisted of the same salts in different concentrations. Solution 1 was leading to the electrodeposition of Zn–1wt% Fe alloy layers, while solution 2 was leading to the electrodeposition of Zn–10wt% Fe alloy layers. Alternating Zn–1wt% Fe and Zn–10wt% Fe alloy layers were deposited on the steel substrates, in order to produce a variety of multilayered alloy coatings, Table 3.

Table 2 Chemical composition of electrodeposition baths and electrodeposition parameters
Table 3 Characteristics of the produced Zn–Fe multilayered alloy coatings

The surface and the cross sectional morphology of the produced Zn–Fe multilayered alloy coatings were examined with the aid of a Jevanert metallurgical microscope (MM), a Jeol 6100 scanning electron microscope (SEM) equipped with a Noran TS 5500 energy dispersive spectrometer (EDS) and an Image Pro image analysis system. The crystal structure of the same coatings was investigated using a Siemens D 5000 X-ray diffractometer (XRD) with Cu Ka radiation and a graphite monochromator. Zn–1wt% Fe and Zn–10wt% Fe single-layered coatings with 5 μm and 10 μm thickness were also electrodeposited on mild steel substrates from baths 1 and 2 respectively, and examined with the X-ray diffractometer in order to supplement the crystallographic investigation of the produced Zn–Fe multilayered alloy coatings. The average grain size was estimated using the Debye–Schrerrer relation [39].

The average surface roughness (Ra) of the produced Zn–Fe multilayered alloy coatings was studied with the aid of a Time TR 230 VDH profilometer. Eleven (11) surface roughness measurements in random directions were performed on the surface of each multilayered coating. The microhardness of the Zn–Fe multilayered alloy coatings was investigated with the aid of a Shimadzu HMV 2000 microhardness tester equipped with a Vickers indenter. Microhardness measurements were carried out on the surface and in the cross section of the produced coatings. The applied load was 10 g for 15 s. For the selected applied load, any substrate effects on the hardness of the multilayered coatings were avoided, since the maximum indentation depth was smaller than the one-tenth of the coatings thickness, which is the threshold value for the substrate interference in coating’s thickness [40].

3 Results and discussion

All the Zn–Fe multilayered coatings investigated in the present study had as top-layer (near the surface sub-layer) a Zn–10wt% Fe alloy sub-layer. Surface micrographs of the produced Zn–Fe multilayered alloy coatings are presented in Fig. 1a–d. As can be seen in those figures, the surface morphology of the produced multilayered coatings showed a typical nodular structure. The size of the formed nodules was observed to be smaller in Fig. 1a (periodicity length: 2 μm, ML1) than that in Fig. 1b–d (periodicity lengths: 4–8 μm, ML2, ML3, ML4). The above observation could be attributed to the lower top-layer thickness (1 μm) of the multilayered coating shown in Fig. 1a (periodicity length: 2 μm, ML1), in comparison to the top-layer thickness (2 μm) of multilayered coatings shown in Fig. 1b–d (periodicity lengths: 4–8 μm, ML2, ML3, ML4). Based on the above observations, it could be suggested that the surface morphology of the produced Zn–Fe multilayered coatings was directly influenced by the top-layer thickness and indirectly connected to their periodicity length (total thickness of two adjacent alternate sub-layers).

Fig. 1
figure 1

SEM surface micrographs of Zn–Fe multilayered alloy coatings with periodicity lengths: a 2 μm (ML1), b 4 μm (ML2), c 6 μm (ML3) and d 8 μm (ML4)

Figure 2a–d show the surface morphology of the Zn–Fe multilayered alloy coatings after metallographic preparation (polishing and chemical etching). In these figures, the presence of two different areas can be observed. The light colour areas correspond to η-phase (Zn–Fe solid solution) and the dark colour areas correspond to δ1 phase (intermetallic FeZn7). Quantitative investigations performed on coatings surfaces showed that the light colour areas (η-phase solid solution) covered the 60–65% of the surface, while the dark colour areas (δ1-phase, intermetallic FeZn7) covered the 40–35% of the coatings surface.

Fig. 2
figure 2

Surface morphology of Zn–Fe multilayered alloy coatings with periodicity lengths: a 2 μm (ML1), b 4 μm (ML2), c 6 μm (ML3) and d 8 μm (ML4), after metallographic preparation (polishing and chemical etching). MM micrographs

Cross sectional micrographs of the Zn–Fe multilayered alloy coatings are presented in Fig. 3a–d. As it can be observed, the Zn–1 wt% Fe and Zn–10 wt% Fe alloy sub-layers were continuous, without detectable cracks or other flaws. The chemical composition of each sub-layer was measured with the aid of an EDS apparatus connected to the SEM. Observing the interfaces between the Zn–1 wt% Fe and Zn–10 wt% Fe sub-layers the absence of noticeable pores, cracks or other discontinuities can also be noticed, indicating good adhesion between the alloy sub-layers. Interface roughness was observed between the Zn–1 wt% Fe and Zn–10 wt% Fe alloy sub-layers. This observation could be attributed to the surface roughness of each Zn–1 wt% Fe and Zn–10 wt% Fe alloy sub-layer, which was formed during the electrodeposition of those layers. Interface roughness is believed to contribute to the interface adhesion between the alloy sub-layers, although in some cases (if it is too high), could lead to unfavourable high surface roughness.

Fig. 3
figure 3

Cross sectional morphology of Zn–Fe multilayered alloy coatings with periodicity lengths: a 2 μm (ML1), b 4 μm (ML2), c 6 μm (ML3) and d 8 μm (ML4). SEM micrographs

Figure 4a and b show the X-ray diffraction spectra taken from the Zn–1 wt% Fe and Zn–10 wt% Fe single-layered coatings respectively. In Fig. 4a (Zn–1 wt% Fe coating), the detected XRD peaks corresponding to η-phase (Zn–Fe solid solution) can be mainly observed. In the same figure (Fig. 4a), some small XRD peaks corresponding to δ1-phase (intermetallic FeZn7) can also be noticed. In Fig. 4b (Zn–10 wt% Fe coating), XRD peaks corresponding to both η-phase (Zn–Fe solid solution) and δ1-phase (intermetallic FeZn7) can be observed. The intensity of δ1-phase XRD peaks in Fig. 4b (Zn–10 wt% Fe coating), was noted to be higher in comparison to their intensity in Fig. 4a (Zn–1 wt% Fe coating). Furthermore, the intensity of η-phase XRD peaks shown in Fig. 4b (Zn–10 wt% Fe coating) was observed to be lower compared to their intensity in Fig. 4a (Zn–10 wt% Fe coating). The above observation could be considered as a clear evidence for the higher concentration of δ1-intermetallic phase in Zn–10 wt% Fe layers in comparison to the concentration of the same phase (δ1-phase) in Zn–1 wt% Fe layers. It should be noted that η-phase corresponds to the Zn–Fe solid solution with maximum solubility of 0.2 at.% Fe and hexagonal close packed (hcp) crystal structure, while δ1-phase is an intermetallic phase (FeZn7), which also has hexagonal close packed (hcp) crystal structure.

Fig. 4
figure 4

X-ray diffraction patterns taken from a Zn–1wt% Fe and b Zn–10 wt% Fe single-layered alloy coatings. Where η: η-phase (Zn–Fe solid solution) and δ1: δ1-phase (intermetallic Fe Zn7)

Figure 5a–d show XRD spectra taken from the produced Zn–Fe multilayered alloy coatings. In these figures, XRD peaks corresponding to η-phase (solid solution) and δ1-phase (intermetallic FeZn7) can be observed. Comparing the spectra from Figs. 4 and 5, it is strongly believed that the crystallographic information shown in the Zn–Fe multilayered alloy coatings XRD spectra (Fig. 5) originates from several Zn–10 wt% Fe and Zn–1 wt% Fe sub-layers and not only from the top Zn–10 wt% Fe layer. In XRD patterns (Figs. 4b, 5c) there are many very minor peaks that are not assigned. This noise may be attributed to the fact that the X-ray beam penetrates deeper than the thickness of the upper layer. This leads to the fact the X-ray diffractometers are enriched with some very small peaks that may be attributed to the interface of the two layers.

Fig. 5
figure 5

X-ray diffraction patterns taken from Zn–Fe multilayered alloy coatings with periodicity lengths: a 2 μm (ML1), b 4 μm (ML2), c 6 μm (ML3) and d 8 μm (ML4). Where η: η-phase (solid solution) and δ1: δ1-phase (intermetallic Fe Zn7)

According to the equilibrium phase diagram, Fig. 6, the Zn–1 wt% Fe sub-layer would be expected to consist of η-phase (Zn–Fe solid solution) and ζ-phase (intermetallic Fe–Zn13), while Zn–10 wt% Fe sub-layer would be expected to consist only of δ1-phase(intermetallic Fe–Zn7). However, based on the above observations from the XRD analysis (Figs. 4, 5), it could be suggested that the electrodeposited Zn–Fe multilayered alloy coatings do not follow the Fe–Zn equilibrium phase diagram, confirming previous results which support the predominance of non-equilibrium conditions during the electrodeposition of Zn–Fe alloy coatings [39,40,41,42].

Fig. 6
figure 6

Fe–Zn equilibrium phase diagram [37]

The average grain size of the Zn–Fe multilayered alloy coatings was estimated using the well-known Debye–Schrerrer relation (1) from the XRD analysis:

$$\delta = \frac{\kappa \lambda }{B\cos \theta }$$
(1)

where δ is the average grain size, Β is the Full Width at Half Maximum (FWHM), θ is the Bragg’s angle, λ is the X-ray wavelength and κ is a constant [39]. Figure 7 presents the average grain size alloy as a function of the periodicity length of the studied Zn–Fe multilayered coatings. The average grain size was found in the range of 40–55 nm, being in agreement with previously reported results for Zn–Fe alloy electrodeposits [41, 42]. As can be observed in Fig. 7, the average grain size of Zn–Fe multilayered alloy coatings decreased slightly with the decrease of their periodicity length.

Fig. 7
figure 7

Average grain size of Zn–Fe multilayered alloy coatings as a function of their periodicity length

Figure 8 shows the surface roughness (Ra) of the studied Zn–Fe multilayered alloy coatings as a function of their periodicity length. The surface roughness of the multilayered coatings was found to increase with the increase of their periodicity length. The observed increase of the surface roughness might be due to the increase of Zn–1 wt% Fe and Zn–10 wt% Fe sub-layers thickness. It has been shown that increasing the thickness of Zn alloy coatings leads to increased surface roughness. The above observation is believed to be connected to the observed (Fig. 2) increase of the surface nodules size with increasing multilayered coatings periodicity length.

Fig. 8
figure 8

Average surface roughness (Ra) of Zn–Fe multilayered alloy coatings as a function of their periodicity length

Figure 9a presents the microhardness of the produced Zn–Fe multilayered alloy coatings as a function of their periodicity length. As it can be observed, the microhardness increased from 99 to 151 HVN as the periodicity length of the produced coatings decreased from 8 to 2 μm. These values were noticed to be higher than the microhardness of single Zn–1 wt% Fe alloy layers (60 HVN) and lower than the microhardness of Zn–10 wt% Fe alloy layers (165 HVN). It also must be said that the values of the microhardness were estimated on the surface of the upper layer. The diamond indenter penetrated in the surface layer but it might reach the inner layer with the different chemical composition. This leads to the assumption that the microhardness values are not responding only to the surface layer of the multilayer coating. The above mentioned Zn–Fe composite multilayered coatings microhardness results were found to be well described (R2 = 0.98) by a modified Hall–Petch relationship (2), as shown in Fig. 9b:

$${\text{H = H}}_{\text{o}} + {\text{K}}\Lambda ^{( - 1/2)} \quad [42]$$
(2)

where H is the hardness of the Zn–Fe multilayered alloy coatings, Λ (μm) is the half of their periodicity length, K and Ho are constants. The last relation was found to hold for the range of 0.5 ≤ Λ−1/2 ≤ 0.1. The observed increase of the multilayered coatings microhardness could be attributed to the presence of the interfaces between the Zn–1 wt% Fe and Zn–10 wt% Fe alloy sub-layers. The interfaces are considered to act in a similar way as grain boundaries in the Hall–Petch model, pilling up the dislocations and thus obstructing to their movement. The increase of interfaces number and the decrease of the distance between them, which were achieved by decreasing the multilayered coatings periodicity length, can lead to increased plastic deformation resistance and higher microhardness values. The values of M.

Fig. 9
figure 9

a Microhardness of the Zn–Fe multilayered alloy coatings as a function of their periodicity length, b Microhardness of Zn–Fe multilayered alloy coatings as a function of Λ(−1/2) (Λ: half of multilayer periodicity length, μm)

4 Conclusions

In the present study, the surface and the cross-sectional morphology, the crystal structure, and the microhardness of Zn–Fe multilayered alloy coatings electrodeposited on mild steel substrates were investigated. The main conclusions were the following:

  1. 1.

    Zn–Fe multilayered alloy coatings with periodicity length ranging between 2 and 8 μm were deposited. They consisted of Zn–1 wt% Fe and Zn–10 wt% Fe alternate alloy sub-layers.

  2. 2.

    The surfaces of the produced coatings were observed to have nodular structure. The cross section observations indicated that the multilayered coatings were free of noticeable pores, cracks or other discontinuities.

  3. 3.

    XRD investigations revealed the presence of η-phase (solid solution) and δ1-phase (intermetallic FeZn7) in the structure of Zn–Fe multilayered alloy coatings.

  4. 4.

    The microhardness of the produced multilayered coatings was found to increase with the decrease of their periodicity length.