Effect of the Addition of Molybdenum on the Structure and ... - MDPI

coatings Article

Effect of the Addition of Molybdenum on the Structure and Corrosion Resistance of Zinc–Iron Plating Daichi Kosugi 1 , Takeshi Hagio 1,2 1

2 3

*

ID

, Yuki Kamimoto 2 and Ryoichi Ichino 1,2,3, *

Department of Chemical Systems Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan; [email protected] (D.K.); [email protected] (T.H.) Green Mobility Research Institute, Institutes of Innovation for Future Society, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan; [email protected] Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Correspondence: [email protected]; Tel.: +81-52-789-3352

Academic Editors: Kwang Ho Kim and Qimin Wang Received: 24 November 2017; Accepted: 14 December 2017; Published: 16 December 2017

Abstract: Zn–Ni plating is indispensable in various industries because of its high corrosion resistance. However, Ni has been reported to trigger allergies; thus, an alternative Ni-free plating is desired. Zn–Fe plating is considered to be a promising candidate, albeit its corrosion resistance still needs to be improved. The corrosion resistance of Zn–Fe plating is expected to increase by the addition of Mo as the third alloying element as it is more noble than Zn and Fe. In this study, Zn–Fe–Mo plating with a corrosion resistance nearly equivalent to that of the Zn–Ni plating was fabricated. Zn–Fe–Mo plating was electrically deposited from continuously-agitated plating baths prepared by mixing ZnSO4 , FeSO4 , Na2 MoO4 , Na3 C6 H5 O7 , and Na2 SO4 using Fe or Ni plates as the substrate. The surface morphology, composition, crystal phase, and electronic state of Mo of the platings were investigated by scanning electron microscopy equipped with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The anti-corrosion performance was evaluated by Tafel extrapolation method. Formation of plating comprising a Mo containing alloy phase was found to be crucial for improving corrosion resistance. The Zn–Fe–Mo plating demonstrates promise for replacing anti-corrosion Zn–Ni platings. Keywords: Zn–Ni plating; Zn–Fe plating; anti-corrosion performance; Mo addition; alloy formation

1. Introduction Metals, such as steel and cast iron, are typically used as the major components of architecture and machines in daily life [1]. In several cases, these metals are exposed to harsh environments, and protection against corrosion is a critical issue, particularly where high reliability is required. The application of corrosion-resistant alloys, including stainless steels, is an effective solution; however, the high cost hinders its applications for daily use [2]. Another promising method to improve corrosion resistance is protection by Zn plating or so-called galvanization [3]. It protects the underlying steel or cast iron by the sacrificial corrosion protection effect, and its considerably lower cost has led to its widespread use in various fields. Currently, protective coatings with a high corrosion resistance are desired to extend the life of industrial products for realizing a sustainable society and for withstanding a more severe environment to meet industrial demands. Alloying with iron-group elements, such as Fe and Ni, is well known in

Coatings 2017, 7, 235; doi:10.3390/coatings7120235

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improving the corrosion resistance of Zn platings [4–8]. By the formation of alloys of Zn with iron-group elements, the standard electrode potential of the alloy becomes closer to that of the substrate metal. The smaller potential difference between the plating and substrate decreases the driving force for the corrosion of the plating, which, in turn, improves its corrosion resistance. In particular, Zn–Ni plating has demonstrated to exhibit five to six times greater corrosion resistance compared with that of Zn plating according to salt spray tests [8]. Thus, Zn–Ni plating has become indispensable for various applications, including aircraft and automotive parts, where conventional Zn plating cannot be applied [9]. Meanwhile, in recent years, Ni allergies have become a social concern. In fact, allergic contact dermatitis in children is caused by nickel [10]. Nickel ions released from various alloys are potent allergens or haptens that can trigger skin inflammation [11]. Therefore, an alternative nontoxic Ni-free plating with a corrosion resistance equivalent to or higher than that of Zn–Ni plating is desired. Zn–Fe plating is considered to be promising because of its cost-effectiveness and nontoxicity. However, it exhibits insufficient corrosion resistance, corresponding to only one-third of that of the Zn–Ni plating [12]. Meanwhile, the addition of noble elements such as Mo and W into plating has been reported to improve the corrosion resistance via the stabilization of the passive film [13,14]. Mo and W cannot be electrically deposited from solutions alone, but these elements have been reported to undergo co-deposition with iron-group elements [15,16]. This result indicated that these elements are also possibly co-deposited into Zn–Fe platings; however, studies of Zn–Fe–Mo plating have been rarely reported thus far to the best of our knowledge [17–21]. In this study, Zn–Fe–Mo plating was electrically deposited by the addition of Mo into the plating bath, and its effect on corrosion resistance was investigated. Furthermore, the possibility of using Zn–Fe–Mo platings to replace conventional Zn–Ni platings is discussed. 2. Materials and Methods 2.1. Bath Preparation Table 1 shows the composition of the bath, which was prepared by mixing zinc sulfate heptahydrate (ZnSO4 ·7H2 O, Nacalai Tesque, Inc., Kyoto, Japan), iron (II) sulfate heptahydrate (FeSO4 ·7H2 O, Nacalai Tesque, Inc.), sodium molybdate dihydrate (Na2 MoO4 ·2H2 O, Kishida Chemical Co., Ltd., Osaka, Japan), trisodium citrate dihydrate (C6 H5 Na3 O7 ·2H2 O, Nacalai Tesque, Inc.), and sodium sulfate (Na2 SO4 , Nacalai Tesque, Inc.). Typically, 60 mL of distilled water (DI) was first added into a 100 mL Pyrex glass beaker, and C6 H5 Na3 O7 ·2H2 O, Na2 SO4 , Na2 MoO4 ·2H2 O, ZnSO4 ·7H2 O, and FeSO4 ·7H2 O were added in the order mentioned. Second, the pH was adjusted using sodium hydroxide (NaOH, Nacalai Tesque, Inc.) and sulfuric acid (H2 SO4 , Nacalai Tesque, Inc.) solutions, respectively, and the total bath volume was adjusted to 100 mL by the addition of distilled water. All procedures were carried out under continuous agitation. Finally, oxygen was removed from the bath by bubbling with Ar for 30 min before use. Table 1. Bath compositions used in this study.

Type of Bath Base Zn Fe Zn–Fe Zn–Fe–Mo

Composition of Bath (mol dm−3 ) ZnSO4

FeSO4

Na2 MoO4

C6 H5 Na3 O7

Na2 SO4

– 0.2 – 0.2 0.2

– – 0.2 0.2 0.2

– – – – 0.01–0.1

0.2 0.2 0.2 0.2 0.2

0.1 0.1 0.1 0.1 0.1

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2.2. Preparation of Zn–Fe–Mo Platings by Electrodeposition All electrochemical experiments were carried out using a three-electrode system. The electrodeposition of the Zn–Fe–Mo alloy was carried out at a fixed amount of electric charge of 50 C using the bath prepared in Section 2.1. The plating bath was stirred at a constant speed of 300 rpm using a magnetic stirrer at room temperature (25 ◦ C). A potential/galvanostat (HZ-7000, Hokuto Denko Co., Tokyo, Japan) was utilized to control the electrolysis potential and analyze data. A Ni plate (NI-313374, Nilaco, Tokyo, Japan) or an Fe plate (B-60-P01, Yamamoto-Ms, Tokyo, Japan) and a Pt coil were used as the cathode and anode, respectively. Ag/AgCl in saturated KCl was used as the reference electrode. The cathode was degreased with ethanol, pickled with 10% H2 SO4 , and masked with an insulation tape (PES-01, AS ONE Corporation, Osaka, Japan), leaving a space of 400 mm2 (20 mm × 20 mm) for electrodeposition prior to the experiment. 2.3. Characterization and Evaluation of the Platings Scanning electron microscopy (SEM; JSM-6330F, JEOL, Tokyo, Japan) equipped with energy-dispersive spectroscopy (EDS; JED2140-GS, JEOL) was employed to examine the surface morphology and composition of the plating. The composition was determined by analyzing two to three areas near the center of the plating and their average value was adopted. X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) was employed to examine the crystalline phase. X-ray photoelectron spectroscopy (XPS, ESCA-3300, Shimadzu Corporation, Kyoto, Japan) was employed to examine the electronic state of Mo. Ar+ etching was performed using an ion source operated at 2 kV and 20 mA for 3 min to confirm the electronic state of Mo inside the plating. Current efficiency (Ceff ) was calculated from the mass change and plating composition, as shown in Equation (1): Ceff =

ZZn nZn F + ZFe nFe F + ZMo nMo F Q

(1)

Here, ZX (X = Zn, Fe, or Mo) is the ionic valence of each element, nX is the molar amount obtained from EDS analysis, F is the Faraday constant, and Q is the amount of electric charge passed. The corrosion resistance was evaluated by anode polarization tests. A 3 wt.% sodium chloride (NaCl, Nacalai Tesque, Inc., Kyoto, Japan) solution degassed with Ar was used as the test solution, with a sweep rate of 1 mV s−1 for the polarization measurement. The potential at 0.1 mA cm−2 in the anode polarization curve was defined as the corrosion potential (Ecorr ). Zn, Zn–Fe, and Zn–Ni plating were prepared and evaluated as a reference. 3. Results and Discussions 3.1. Effect of Bath pH on Zn–Fe–Mo Platings Zn–Fe–Mo plating was electrically deposited at various pH values. The current density was fixed at 10 mA cm−2 . A black precipitate with low adhesion was formed on the substrate during electrodeposition at a pH greater than 5.7. Thus, the pH range was selected from 4.0 to 5.7. Figure 1 shows the SEM images of the platings deposited in the baths at different pH values. The surface morphology clearly changed. The plating obtained from the bath at pH 4 comprised granules of ca. 2 µm (Figure 1a), whereas that obtained from pH 4.5 to 5.5 changed to a fibrous structure (Figure 1b–d). Finally, that from pH 5.7 exhibited a smooth structure comprising of fine grains (Figure 1e). Figure 2 shows the composition of the platings deposited at various pH values. Ni substrates were used in this experiment to avoid the detection of Fe from the substrate. A majority of the platings exhibited a high Zn content, with only trace Fe and Mo, indicating that the current conditions are categorized as “anomalous co-deposition” [22]. Although Mo was barely observed for the plating prepared by electrical deposition at pH between 4.0 and 5.2, the amount of Mo increased with further increasing pH. A plating with approx. 3 at.% Mo could be obtained at a pH of 5.7, probably related to the difference in the present form of Mo. In solution, Mo exists as oxyanions, which is known to

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form various polyoxyanion at low pH and mono-oxyanions at pH greater than around 6. Therefore, the deposition of Mo is thought to occur with the decrease in the size of oxyanions, which allows for the easier deposition of Mo [23,24]. Coatings 2017, 7, 235 4 of 13 Coatings 2017, 7, 235

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(a) (a)

(b) (b)

(d) (d)

(e) (e)

(c) (c)

Figure 1. SEM images of the Zn–Fe–Mo platings deposited various values. (b) Figure 1. 1. SEM SEM images imagesof ofthe theZn–Fe–Mo Zn–Fe–Moplatings platings deposited at various pH values. (a) pH 4.0; (b) pH pH deposited at at various pHpH values. (a) (a) pHpH 4.0;4.0; (b) pH 4.5; 4.5; (c) pH 5.0; (d) pH 5.5; and (e) pH 5.7. 4.5; (c) pH 5.0; (d) pH 5.5; and (e) pH 5.7. (c) pH 5.0; (d) pH 5.5; and (e) pH 5.7.

Figure Figure 2. 2. The The content content of of Fe Fe and and Mo Mo in in Zn–Fe–Mo Zn–Fe–Mo plating plating deposited deposited at at various various pH pH values. values. Figure 2. The content of Fe and Mo in Zn–Fe–Mo plating deposited at various pH values.

Figure Figure 33 shows shows the the XRD XRD patterns patterns of of the the platings platings deposited deposited at at various various pH pH values. values. Intense Intense peaks peaks Figure 3 shows the XRD patterns of the platings deposited at variousstructure. pH values. Intense peaks were observed for all platings, indicative of the deposition of a crystalline Reflections were observed for all platings, indicative of the deposition of a crystalline structure. Reflections only only were observed for all platings, indicative of the deposition of a crystalline structure. Reflections only corresponding corresponding to to the the Zn Zn phase phase and and the the substrate substrate were were observed observed in in the the platings platings deposited deposited at at corresponding to the Zn phase and the substrate were observed in the platings deposited at pH 4.0–5.5; pH 4.0–5.5; however, with increasing pH, a slight shift toward low angles was observed, in addition pH 4.0–5.5; however, with increasing pH, a slight shift toward low angles was observed, in addition however, with increasingincorporation pH, a slight shift toward into low angles was observed, in additionlead to peak to to peak peak broadening. broadening. The The incorporation of of Fe Fe atoms atoms into the the Zn Zn lattice lattice was was anticipated anticipated to to lead to to aa broadening. The incorporation of Fe atoms into the Zn lattice was anticipated to lead to a distorted Zn distorted distorted Zn Zn structure. structure. Peaks Peaks other other than than Zn Zn were were observed observed for for the the plating plating obtained obtained at at pH pH 5.7. 5.7. These These structure. Peaks other than Zn were observed for the plating obtained at pH 5.7. These are expected to are are expected expected to to correspond correspond to to reflections reflections from from aa Fe Fe33Mo-based Mo-based alloy alloy phase phase since since similar similar peaks peaks have have correspond to reflections from a Fe Mo-based alloy phase since similar peaks have been observed for been been observed observed for for Fe Fe33Mo Mo alloys alloys in in 3previous previous studies studies [25,26]. [25,26]. Fe3 Mo alloys in previous studies [25,26]. Figure 4 shows the anodic polarization curves Figure 4 shows the anodic polarization curves of of platings platings deposited deposited at at various various pH pH values. values. The The plating deposited at pH 4.0 exhibited a sharp increase in current density at around −960 mV, plating deposited at pH 4.0 exhibited a sharp increase in current density at around −960 mV, whereas whereas that that deposited deposited at at pH pH values values of of 4.5 4.5 exhibited exhibited aa slight slight increase increase at at around around −1100 −1100 mV mV and and aa drastic drastic increase at around −960 mV. At pH 5.0 and 5.5, a gradual increase between −1100 mV to −920 mV increase at around −960 mV. At pH 5.0 and 5.5, a gradual increase between −1100 mV to −920 mV and and aa similar similar behavior behavior to to that that at at pH pH 4.0 4.0 was was observed observed with with aa sharp sharp change change at at around around −990 −990 mV, mV, respectively. respectively. This indicated a low corrosion potential for the fibrous structure. For the platings deposited at pH 5.7,

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Figure 4 shows the anodic polarization curves of platings deposited at various pH values. The plating deposited at pH 4.0 exhibited a sharp increase in current density at around −960 mV, whereas that deposited at pH values of 4.5 exhibited a slight increase at around −1100 mV and a drastic increase at around −960 mV. At pH 5.0 and 5.5, a gradual increase between −1100 mV to −920 mV and a similar behavior to that at pH 4.0 was observed with a sharp change at around −990 mV, respectively. Coatings 2017, 7, 235 5 of 13 Coatings 2017, 7,a235 of 135.7, This indicated low corrosion potential for the fibrous structure. For the platings deposited at 5pH a change at at −950 700mV mVwere wereobserved. observed.The Theplating plating with supposed a change −950mV mVand andaadrastic drasticchange change at at − −700 with thethe supposed a change at −950 mV and a drastic change at −700 mV were observed. The plating with the supposed Fe3Fe Mo-based alloy phase exhibited higher 3Mo-based alloy phase exhibited highercorrosion corrosionresistance resistancethan thanthose thoseofofthe theothers otherscomprising comprisingthe Fe3Mo-based alloy phase exhibited higher corrosion resistance than those of the others comprising Znthe phase. From From these these experiments, a pHaof selected for for thethe remaining experiments. Zn phase. experiments, pH5.7 ofis5.7 is selected remaining experiments. the Zn phase. From these experiments, a pH of 5.7 is selected for the remaining experiments.

Figure XRDpatterns patterns of theZn–Fe–Mo Zn–Fe–Mo plating deposited various Figure 3.3. depositedat variouspH pHvalues. values. Figure 3.XRD XRD patternsofofthe the Zn–Fe–Mo plating plating deposited atatvarious pH values.

Figure 4. Anodic polarization curves of the Zn–Fe–Mo plating deposited at various pH values. Figure Anodicpolarization polarizationcurves curvesof of the the Zn–Fe–Mo Zn–Fe–Mo plating values. Figure 4. 4. Anodic platingdeposited depositedatatvarious variouspH pH values.

3.2. Effect of Current Density on Zn–Fe–Mo Platings 3.2. Effect of Current Density on Zn–Fe–Mo Platings 3.2. Effect of Current Density on Zn–Fe–Mo Platings A cathodic polarization measurement was carried out to consider the range of the current A cathodic polarization measurement was carried out to consider the range of the current density. Figurepolarization 5 shows themeasurement measured cathodic polarization at athe pHrange of 5.7 of and a sweep of A cathodic was carried out tocurves consider the currentrate density. density. Figure 5 shows the measured cathodic polarization curves at a pH of 5.7 and a sweep rate of 5 mV/s. The deposition of Zn and Fe started at approximately −1080 and −980 mV from baths Figure 5 shows measured polarization at a pH of 5.7 and sweep of 5baths mV/s. 5 mV/s. The the deposition of cathodic Zn and Fe started at curves approximately −1080 and a−980 mVrate from ZnoforZn Feand alone (Zn and at Feapproximately baths), respectively. Meanwhile, when using the plating bathZn Thecontaining deposition Fe started − 1080 and − 980 mV from baths containing containing Zn or Fe alone (Zn and Fe baths), respectively. Meanwhile, when using the plating bath containing both Zn and Fe (Zn–Fe bath), a small peak at −980 mV and a second large peak were containing both Zn and Fe (Zn–Fe bath), a small peak at −980 mV and a second large peak were observed at −1080 mV, corresponding to the deposition of Fe and Zn, respectively. Finally, from the observed at −1080 mV, corresponding to the deposition of Fe and Zn, respectively. Finally, from the bath containing Zn, Fe, and Mo, two peaks were again observed at −800 mV, possibly corresponding bath containing Zn, Fe, and Mo, two peaks were again observed at −800 mV, possibly corresponding to the co-reduction of Fe and Mo, and at −1200 mV, related to Zn deposition. These shifts in potential to the co-reduction of Fe and Mo, and at −1200 mV, related to Zn deposition. These shifts in potential are likely to be explained by the presence of noble Mo, shifting the reduction of Fe to higher potentials are likely to be explained by the presence of noble Mo, shifting the reduction of Fe to higher potentials

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or Fe alone (Zn and Fe baths), respectively. Meanwhile, when using the plating bath containing both Zn and Fe (Zn–Fe bath), a small peak at −980 mV and a second large peak were observed at −1080 mV, corresponding to the deposition of Fe and Zn, respectively. Finally, from the bath containing Zn, Fe, and Mo, two peaks were again observed at −800 mV, possibly corresponding to the co-reduction of Fe and Mo, and at −1200 mV, related to Zn deposition. These shifts in potential are likely to be explained Coatings 2017, 7, 235 6 of 13 by the presence of noble Mo, shifting the reduction of Fe to higher potentials through co-reduction Coatings 2017, 7, 235 6 of 13 of through Mo while shifting theofZn deposition to lower potential by avoiding deposition of Zn via co-reduction Mo while shifting the Zn deposition to lowerthe potential by avoiding thethe adsorption oxyanions on the while substrate surface. deposition of Zn via theof adsorption of oxyanions on the substrate through of co-reduction Mo shifting the Zn deposition to surface. lower potential by avoiding the deposition of Zn via the adsorption of oxyanions on the substrate surface.

Figure5.5.Cathodic Cathodicpolarization polarization curves curves obtained Figure obtainedfrom fromdifferent differentbaths. baths. Figure 5. Cathodic polarization curves obtained from different baths.

From the results obtained from the cathodic polarization curves, current densities of 1, 2, 5, and From the results obtained from cathodic polarization curves,current current densities of 1,5,2,and 5, and From results obtained from6the the cathodic polarization densities of 1, 10 mA cm−2the were selected. Figure shows the SEM images curves, of the platings deposited at 2,different −2 were selected. Figure 6 shows the SEM images of the platings deposited at different current 10 mA cm −2 10 mA densities. cm were At selected. Figuredensities, 6 shows the images of the deposited at different current low current the SEM plating surface wasplatings comprised of bulky granules densities. At low current densities, the plating surface was comprised of bulky granules (Figure current densities. At low current densities, the plating surface was comprised of bulky granules (Figure 6a). The size decreased with increasing current density, probably because a high current6a). Thedensity size decreased with increasing current density, probably because a high current density promoted (Figure 6a). The size decreased with increasing current density , probably because a high current promoted the nucleation on the substrate surface. the density nucleation on thethe substrate surface. promoted nucleation on the substrate surface.

(a) (a)

(b) (b)

(c) (c)

(d) (d)

−2 Figure deposited at at various various current currentdensities. densities.(a) (a)11mA mAcm cm −2; ; Figure6.6.SEM SEMimages imagesof of the the Zn–Fe–Mo Zn–Fe–Mo plating plating deposited −2 ; Figure 6. SEM images of the Zn–Fe–Mo plating deposited at various current densities. (a) 1 mA cm −2 −2 −2 (b) −2; ;(c) (b)22mA mAcm cm (c)55mA mA cm cm−−22;; and and (d) (d) 10 10 mA mA cm cm−2..−2 − 2 (b) 2 mA cm ; (c) 5 mA cm ; and (d) 10 mA cm .

Figure the plating plating composition compositionand andcurrent currentdensity. density.One Oneplot plot Figure77shows showsthe the relationship relationship between between the corresponds to one sample. The content of Mo in the plating decreased with increasing current corresponds to one sample. The content of Mo in the plating decreased with increasing current density, rate of of the the plating platingand andan aninsufficient insufficientfeed feedrate rate density,possibly possiblycorresponding corresponding to to aa high high deposition deposition rate ofofthe Mo ions. The extremely high deposition rate must have led to the depleted Mo layer near the the Mo ions. The extremely high deposition rate must have led to the depleted Mo layer near the substrate surface, thereby leading to the slow reduction of Mo [27]. In addition, the composition substrate surface, thereby leading to the slow reduction of Mo [27]. In addition, the composition ofof

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Figure 7 shows the relationship between the plating composition and current density. One plot corresponds to one sample. The content of Mo in the plating decreased with increasing current density, possibly corresponding to a high deposition rate of the plating and an insufficient feed rate of the Mo ions. The extremely high deposition rate must have led to the depleted Mo layer near the substrate surface, thereby leading to the slow reduction of Mo [27]. In addition, the composition of the platings, especially of2017, Fe,7,prepared at current densities of 2 and 5 mA cm−2 were found to be quite7 of scattered. Coatings 235 13 This seemed not to be caused by the difference in the analyzed area since the values before averaging scattered. not to plot. be caused the difference in the analyzed area before was within ±1This molseemed % for each Theby variation in composition may besince duethe to values their precipitated averaging was within ±1 mol % for each plot. The variation in composition may be due to their form. From the SEM image (Figure 6), the platings obtained by electrodeposition at current densities of precipitated form. From the SEM image (Figure 6), the platings obtained by electrodeposition at 2 and 5 mA cm−2 seemed to be comprised of a mixture of two types of granules while those of 1 and current densities of 2 and 5 mA cm−2 seemed to be comprised of a mixture of two types of granules − 2 10 mAwhile cm those seemed to be of one. This impliesofthat densities of 2 and 5 mA cm−2 of 1 and 10 comprised mA cm−2 seemed to be comprised one.current This implies that current densities are in of the2 and transition region. deposition ratioThe of these two ratio typesofof grains varied −2 are inThe 5 mA cm the transition region. deposition these two may typeshave of grains mayin this unstable transition region. have varied in this unstable transition region.

Figure 7. The content of Fe and Mo in the Zn–Fe–Mo plating deposited at various current densities.

Figure 7. The content of Fe and Mo in the Zn–Fe–Mo plating deposited at various current densities. Figure 8 shows the result obtained from the XPS analysis of platings deposited at different

Figure shows the result obtained from theeVXPS analysis of platings at 229–234 differenteVcurrent current8 densities. The peaks around 227–228 corresponds to metal Modeposited (Mo (0)) and corresponds to Mo in the oxide or hydroxide state. Mo (0) was not observed on the plating densities. The peaks around 227–228 eV corresponds to metal Mo (Mo (0)) and 229–234 eVsurface. corresponds −2, changes were not observed even after Ar-ion etching; the oxide platings at 1state. and 2Mo mA(0) cmwas to Mo For in the ordeposited hydroxide not observed on the plating surface. For the platings −2 after Ar-ion etching, indicating that a however, Mo (0) was cm observed for plating at not 5 and 10 mA cm −2 , changes deposited at 1 and 2 mA were observed even after Ar-ion etching; however, Mo (0) relatively high current density is required to−reduce Mo, which is necessary for co-deposition to occur. 2 was observed for plating at 5 and 10 mA cm after Ar-ion etching, indicating that a relatively high The low generation rate of atomic hydrogen at the substrate surface may be responsible for this current density is required to reduce Mo, which is necessary for co-deposition to occur. The low phenomenon since Mo is proposed to be reduced by the generation of atomic hydrogen in previous generation rate of atomic atbeen the substrate surface may beinresponsible for this studies [27,28]. Similarhydrogen results have reported for electroplating a Ni–Mo system [27].phenomenon since Mo is proposed to be reduced by the generation of atomic hydrogen in previous studies [27,28]. Similar results have been reported for electroplating in a Ni–Mo system [27]. Figure 9 shows the relationship between the current density and current efficiency. The current efficiency increased with current density. As the amount of electric charge was constant, a low current density is equivalent to a long electrodeposition time. The possibility of oxygen dissolution from the atmosphere into the plating bath was suspected, leading to necessity of extra electric charge to reduce the dissolved oxygen in the bath. To confirm this hypothesis, electrodeposition using a bath with continuous Ar bubbling was conducted; however, the current efficiency was found to be of similar value. Thus, the dissolution of oxygen seemed not to be the reason. The decrease in the current efficiency at low current density may be related to the oxidation-reduction reaction between oxides or hydroxides of Mo and Mo oxyanion on the substrate, but further investigation is necessary to understand this phenomenon. Figure 10 shows the XRD patterns of the platings deposited at various current densities. With increasing current density, peaks corresponding to the alloy phase were observed. The peak of the alloy phase became more intense as the current density increased. Even though a large amount of Mo was detected in platings deposited at low current densities, Mo could not be reduced to form alloys and could only exist as oxides or hydroxides. Figure 8. XPS analysis results of the Zn–Fe–Mo platings deposited at various current densities.

corresponds to Mo in the oxide or hydroxide state. Mo (0) was not observed on the plating surface. For the platings deposited at 1 and 2 mA cm−2, changes were not observed even after Ar-ion etching; however, Mo (0) was observed for plating at 5 and 10 mA cm−2 after Ar-ion etching, indicating that a relatively high current density is required to reduce Mo, which is necessary for co-deposition to occur. The low generation rate of atomic hydrogen at the substrate surface may be responsible for this Coatingsphenomenon 2017, 7, 235 since Mo is proposed to be reduced by the generation of atomic hydrogen in previous 8 of 14 studies [27,28]. Similar results have been reported for electroplating in a Ni–Mo system [27].

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Figure 9 shows the relationship between the current density and current efficiency. The current efficiency increased with current density. As the amount of electric charge was constant, a low current density is equivalent to a long electrodeposition time. The possibility of oxygen dissolution from the atmosphere into the plating bath was suspected, leading to necessity of extra electric charge to reduce Coatings 2017, 7, 235 of 13 the dissolved oxygen in the bath. To confirm this hypothesis, electrodeposition using a bath 8with continuous Ar bubbling was conducted; however, the current efficiency was found to be of similar Figure 9 shows the relationship between the current density and current efficiency. The current value. Thus, the dissolution of oxygen seemed not to be the reason. The decrease in the current efficiency increased with current density. As the amount of electric charge was constant, a low current efficiency at low current density may be related to the oxidation-reduction reaction between oxides density is equivalent to a long electrodeposition time. The possibility of oxygen dissolution from the or hydroxides of Mo and Mo oxyanion on the substrate, but further investigation is necessary to atmosphere into the plating bath was suspected, leading to necessity of extra electric charge to reduce understand this phenomenon. the dissolved oxygen in the bath. To confirm this hypothesis, electrodeposition using a bath with continuous Ar bubbling was conducted; however, the current efficiency was found to be of similar value. Thus, the dissolution of oxygen seemed not to be the reason. The decrease in the current efficiency at low current density may be related to the oxidation-reduction reaction between oxides or hydroxides of Mo and Mo oxyanion on the substrate, but further investigation is necessary to Figure 8. XPS analysis results of the Zn–Fe–Mo platings deposited at various current densities. Figure 8. XPS results of the Zn–Fe–Mo platings deposited at various current densities. understand this analysis phenomenon.

Figure 9. The relationship between current density and current efficiency.

Figure 10 shows the XRD patterns of the platings deposited at various current densities. With increasing current density, peaks corresponding to the alloy phase were observed. The peak of the alloy phase became more intense as the current density increased. Even though a large amount of Mo was detected in platings deposited at low current densities, Mo could not be reduced to form alloys Figure 9. The relationship between current density and current efficiency. and could only exist oxides or hydroxides. Figure 9.asThe relationship between current density and current efficiency. Figure 10 shows the XRD patterns of the platings deposited at various current densities. With increasing current density, peaks corresponding to the alloy phase were observed. The peak of the alloy phase became more intense as the current density increased. Even though a large amount of Mo was detected in platings deposited at low current densities, Mo could not be reduced to form alloys and could only exist as oxides or hydroxides.

Figure 10. XRD patterns of the Zn–Fe–Mo platings deposited at various current densities.

Figure 10. XRD patterns of the Zn–Fe–Mo platings deposited at various current densities.

Figure 10. XRD patterns of the Zn–Fe–Mo platings deposited at various current densities.

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Figure Figure 11 11 shows shows the the anodic anodic polarization polarization curves curves of of platings platings deposited deposited at at various various current current densities. densities. Figure 11 shows the anodic polarization curves of platings deposited at various current densities. With increasing current density, the potential shifted to a more noble value. The presence of With increasing current density, the potential shifted to a more noble value. The presence of an an alloy alloy With increasing current density, the potential shifted to a more noble value. The presence of an alloy phase phase is is indicated indicated to to be be crucial, crucial,as asanticipated anticipatedin inSection Section3.1. 3.1. phase is indicated to be crucial, as anticipated in Section 3.1.

Figure 11. 11. Anodic atat various current densities. Figure Anodic polarization polarizationcurves curvesofofthe theZn–Fe–Mo Zn–Fe–Moplatings platingsdeposited deposited various current densities. Figure 11. Anodic polarization curves of the Zn–Fe–Mo platings deposited at various current densities.

3.3. Effect Effect of of Mo Mo Concentration Concentration of of Plating Plating Bath on on Zn-Fe-Mo Platings Platings 3.3. 3.3. Effect of Mo Concentration of Plating Bath Bath on Zn-Fe-Mo Zn-Fe-Mo Platings The SEM image of platings obtained when using plating baths with different Mo concentration The image of platings obtained when using using plating plating baths with different Mo concentration The SEM SEM image of platings obtained when baths with different concentration −3 −3, andMo −3. The is shown in Figure 12. The Mo concentration was 0.01 mol dm , 0.02 mol dm 0.1 0.1 mol dmdm − 3 − 3 −3 is shown in Figure 12. The Mo concentration was 0.01 mol dm , 0.02 mol dm , and mol −3 −3 −3. The. is shown in Figure 12. The Mo concentration was 0.01 mol dm , 0.02 mol dm , and 0.1 mol dm −3 plating obtained from the 0.01 mol dm −Mo containing bath showed smooth and homogeneous 3 The plating obtained from the 0.01 mol dm Mo containingbath bathshowed showedsmooth smoothand and homogeneous homogeneous −3 Mo plating 0.01 mol dm containing surface;obtained however,from smallthe cracks were observed byincreasing increasingthe theMo Moconcentration concentration to0.02 0.02mol moldm dm −−3 3 surface; however, small cracks were observed by to −3 surface; however, small cracks were observed by increasing the Mo concentration to 0.02 mol dm and a rough surface with even more cracks were observed by further increasing the Mo concentration and rough surface surface with with even even more more cracks cracks were were observed observed by by further further increasing increasing the the Mo Mo concentration concentration and aa rough to 0.1 0.1 mol dm dm−−33.. An increase in Mo concentration of the plating bath seems to have had had aa negative negative to mol An increase in Mo concentration of the plating bath seems to have −3 to 0.1 mol dmplating . An increase effect on the quality. in Mo concentration of the plating bath seems to have had a negative effect effect on on the the plating plating quality. quality.

(a) (a)

(b) (b)

(c) (c)

Figure 12. SEM images of Zn–Fe–Mo plating deposited at various Mo concentration. (a) 0.01 mol dm−3; Figure SEM of deposited at at various various Mo Mo concentration. concentration.(a) (a)0.01 0.01mol moldm dm−−33;; −3; and (c) −3. Figure 12. SEM images of Zn–Fe–Mo Zn–Fe–Mo plating deposited (b) 0.0212. mol dmimages 0.1 mol dmplating −3 −3 (b) 0.02 0.02 mol mol dm dm−3; ;and (b) and(c) (c)0.1 0.1mol moldm dm−. 3 .

The composition of Mo content in the platings are shown in Figure 13. An increase in the content The of Mo content in the platings are shown in Figure 13. An increase the content of FeThe andcomposition Mo in the platings was observed by increasing thein Mo concentration of thein bath. composition of Mo content in the platings are shown Figure 13. An increase inplating the content of Fe and Mo in the platings was observed by increasing the Mo concentration of the plating bath. 2− This because MoO were adsorbed onto the surface of the substrate along bath. with of Fe isand Mo inmore the platings was supplied observedand by increasing the Mo concentration of the plating This is because more MoO22−−were in supplied and adsorbed onto the surface of the substrate along with the increase in Mo concentration the plating bath. This is because more MoO were supplied and adsorbed onto the surface of the substrate along with the increase in Mo concentration in the plating bath. Figure in 14 shows the result obtained frombath. the XPS analysis of platings deposited from plating the increase Mo concentration in the plating Figure 14 shows the result obtained from the XPSwas analysis of platings deposited from plating bathsFigure with different Again, notofobserved on the plating After 14 showsMo the concentrations. result obtained from theMo XPS(0)analysis platings deposited fromsurface. plating baths baths with different Mo concentrations. Again, Mo (0) was not observed on the plating surface. After Ar-ion etching, Mo (0) was observed for plating deposited from baths containing 0.01 and 0.02 mol dm−3 with different Mo concentrations. Again, Mo (0) was not observed on the plating surface. After Ar-ion −3 Ar-ion etching, Mo (0) was observed for plating deposited from baths containing 0.01 and 0.02 mol dm −3 Mo, Mo, however, bathsfor containing 0.1 mol from dm−3 baths Mo. At low Mo 0.01 concentration, both and etching, Mo (0) not wasfrom observed plating deposited containing and 0.02 mol dmFe −3 Mo, however, notbyfrom baths of containing 0.1 coexist mol dm the Mo. At lowofMo both Fe thus and 2− shall hydroxide offrom Mo reduction MoO theconcentration, substrate Mo may however, not baths containing 0.1 mol dm−3 Mo. on At lowsurface Mo concentration, both and Fe and hydroxide 2− shall coexist on the surface of the substrate and Mo may thus hydroxide of Mo by reduction of MoO be reduced since the atomic hydrogen can be held at the unpaired 3d electrons on the Fe metals [28]. be the atomic hydrogen canthe be held at the 3d electrons the Fe with metalsexcess [28]. In reduced contrast,since at high Mo concentrations, surface of unpaired the substrate will be on covered In contrast, at high Mo concentrations, the surface of the substrate will be covered with excess

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hydroxides hydroxides of of Mo, Mo, making making itit difficult difficult for for Fe Fe to to approach approach the the plating plating surface. surface. Thus, Thus, further further reduction reduction 2− shall coexist on the surface of the substrate and Mo may thus be reduced Mo by reduction of MoO of hydroxides of Mo must be prevented due to the lack of Fe; i.e., the lack of atomic of hydroxides of Mo must be prevented due to the lack of Fe; i.e., the lack of atomic hydrogen. hydrogen. Zn Zn since the atomic hydrogen can be held at the unpaired electrons on the Fe metals [28]. In contrast, must been as by hydroxide of Mo. must have have been prevented prevented as well well by the the hydroxide of 3d Mo. at high Mo concentrations, the surface between of the substrate be coveredin with of Mo, Figure 15 the concentration the plating bath Figure 15 shows shows the the relationship relationship between the Mo Mowill concentration in the excess platinghydroxides bath and and current current making it difficult for Fe to approach the plating surface. Thus, further reduction of hydroxides of efficiency. The current efficiency decreased by increasing the Mo concentration in the plating efficiency. The current efficiency decreased by increasing the Mo concentration in the plating bath. bath. Mo must be prevented due to the lack of Fe; i.e., the lack of atomic hydrogen. Zn must have been The The increasing increasing formation formation of of hydroxide hydroxide of of Mo Mo on on the surface surface prevents prevents Fe Fe deposition, deposition, resulting resulting in in aa prevented as well by the hydroxide of Mo. lower current efficiency [29]. The trend of the current efficiency seems to support this expectation. lower current efficiency [29]. The trend of the current efficiency seems to support this expectation.

Figure Figure 13. of Fe theZn–Fe–Mo Zn–Fe–Mo plating from aa plating bath with with different different Figure 13. 13. Content Content of Fe and and Mo Mo in in the the Zn–Fe–Mo plating deposited deposited from plating bath different Mo concentrations. Mo Mo concentrations. concentrations.

Figure Figure 14. 14. XPS XPS analysis analysis results results of of the the Zn–Fe–Mo Zn–Fe–Mo plating plating deposited deposited from from aaa plating plating bath bath with with different different Figure 14. XPS analysis results of the Zn–Fe–Mo plating deposited from plating bath with different Mo concentrations. Mo concentrations. Mo concentrations.

Figure 15 shows the relationship between the Mo concentration in the plating bath and current efficiency. The current efficiency decreased by increasing the Mo concentration in the plating bath. The increasing formation of hydroxide of Mo on the surface prevents Fe deposition, resulting in a lower current efficiency [29]. The trend of the current efficiency seems to support this expectation. Figure 16 shows the XRD patterns of the platings deposited from baths of various Mo concentrations. The intense peaks of alloy phase were detected as decreasing the Mo concentration. This also supports the idea that Mo in the plating existed in the Mo (0) form and lower Mo concentration led to enhanced formation of Mo (0). Figure 17 shows the anodic polarization curves of platings deposited from baths with various Mo concentrations. All three platings showed relatively noble potentials. A decrease in Mo concentration Figure Figure 15. 15. The The relationship relationship between between Mo Mo concentration concentration and and current current efficiency. efficiency.

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of theFigure plating corresponding intense peaksdeposited of the alloy shifted the potential 14. bath, XPS analysis results of to themore Zn–Fe–Mo plating fromphase, a plating bath with different to a moreMo noble value. Again, the results support the idea that the presence of an alloy phase is important. concentrations.

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Figure 16 shows the XRD patterns of the platings deposited from baths of various Mo Figure 16 shows the XRD patterns of the platings deposited from baths of various Mo concentrations. The intense peaks of alloy phase were detected as decreasing the Mo concentration. concentrations. The intense peaks of alloy phase were detected as decreasing the Mo concentration. This also supports the idea that Mo in the plating existed in the Mo (0) form and lower Mo This also supports the idea that Mo in the plating existed in the Mo (0) form and lower Mo concentration led to enhanced formation of Mo (0). concentration led to enhanced formation of Mo (0). Figure 17 shows the anodic polarization curves of platings deposited from baths with various Figure 17 shows the anodic polarization curves of platings deposited from baths with various Mo concentrations. All three platings showed relatively noble potentials. A decrease in Mo Mo concentrations. All three platings showed relatively noble potentials. A decrease in Mo concentration of the plating bath, corresponding to more intense peaks of the alloy phase, shifted the concentration of the plating bath, corresponding to more intense peaks of the alloy phase, shifted the potential to a more noble value. Again, the results support the idea that the presence of an alloy phase potential to a more noble value. Again, the resultsMo support the idea that the presence of an alloy phase is important. Figure Figure 15. 15. The The relationship relationship between between Mo concentration concentration and and current current efficiency. efficiency. is important.

Figure 16. XRD the plating deposited from a plating bathbath with different Mo Figure XRDpatterns patternsof theZn–Fe–Mo Zn–Fe–Mo plating deposited from a plating different Figure 16. 16. XRD patterns ofofthe Zn–Fe–Mo plating deposited from a plating bath withwith different Mo concentrations. Mo concentrations. concentrations.

Figure 17. Anodic polarization curves of the Zn–Fe–Mo platings deposited at various Mo Figure 17. 17.Anodic Anodic polarization curves of the Zn–Fe–Mo platings atdeposited various Mo Figure polarization curves of the Zn–Fe–Mo platings deposited various Moatconcentrations. concentrations. concentrations.

3.4. Corrosion Potentials of the Zn–Fe–Mo Platings 3.4. Corrosion Potentials of the Zn–Fe–Mo Platings Figure 18 shows the relationship between the content of Mo in the plating and Ecorr, as well as Figure 18 shows the relationship between the content of Mo in the plating and Ecorr, as well as the Ecorr values for Zn, Zn–Fe, and Zn–Ni platings. A low Mo content led to high Ecorr; however, it was the Ecorr values for Zn, Zn–Fe, and Zn–Ni platings. A low Mo content led to high Ecorr; however, it was not proportional to the content of Mo in the plating. This indicated that the Mo content in the plating not proportional to the content of Mo in the plating. This indicated that the Mo content in the plating is not the primary factor improving the corrosion resistance of the plating. is not the primary factor improving the corrosion resistance of the plating. Figure 19 shows the relationship between the peak intensity ratio of the alloy and Zn phases Figure 19 shows the relationship between the peak intensity ratio of the alloy and Zn phases

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3.4. Corrosion Potentials of the Zn–Fe–Mo Platings Figure 18 shows the relationship between the content of Mo in the plating and Ecorr , as well as the Ecorr values for Zn, Zn–Fe, and Zn–Ni platings. A low Mo content led to high Ecorr ; however, it was not proportional to the content of Mo in the plating. This indicated that the Mo content in the plating Coatings 2017, 7, 235 factor improving the corrosion resistance of the plating. 12 of 13 is not the primary

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Figure 18. The between the the content content of of Mo Mo in in the the plating plating and and Ecorr corr.. Figure 18. The relationship relationship between

Figure 19 shows the relationship between the peak intensity ratio of the alloy and Zn phases from XRD and Ecorr to discuss the effect of alloy phase. Peaks corresponding to the alloy and Zn phases were observed at 2θ values of 40.7◦ and 42.8◦ , respectively, apparently indicating that the deposition of the alloy phase is crucial high corrosion resistance. Figure 18. Thefor relationship between the content of Mo in the plating and Ecorr.

Figure 19. The relationship between the peak intensity ratio of the alloy phase and Zn phase from the XRD and Ecorr.

4. Conclusions In this study, the addition of Mo into the Zn–Fe platings was considered to improve its corrosion resistance. following results werethe obtained. Figure 19. relationship between peak FigureThe 19.The The relationship between the peakintensity intensityratio ratioof ofthe thealloy alloyphase phaseand andZn Znphase phasefrom fromthe the  The Zn–Fe–Mo platings obtained by this experiment exhibited high corrosion resistance when XRD and E corr . XRD and Ecorr . the electrically-deposited layer formed a Fe3Mo-based alloy phase. 4. 4. Conclusions The best plating exhibited high corrosion resistance comparable with that of Zn–Ni platings, Conclusions revealing promise for Zn–Fe–Mo platings as potential alternatives for Zn–Ni platings.corrosion In Inthis thisstudy, study, the the addition addition of ofMo Mointo intothe theZn–Fe Zn–Feplatings platingswas wasconsidered consideredto toimprove improveits its corrosion  For the co-deposition of Mo, it isobtained. crucial to control the pH to approximately 5.7 to ensure that resistance. resistance. The Thefollowing followingresults results were were obtained. Mo does not form large polyoxides does not form hydroxides.  The Zn–Fe–Mo platings obtained byand thisZn experiment exhibited high corrosion resistance when • the Co-deposition not obtained effectively occur at low current densities, a highresistance Mo concentration Theelectrically-deposited Zn–Fe–Modoes platings by this experiment exhibited high and corrosion when the layer formed a Fe3Mo-based alloy phase. in the bath as Moexhibited (IV) layer cannot be efficiently reducedalloy tocomparable Mo (0). electrically-deposited formed a Fe3 Mo-based phase.  The best plating high corrosion resistance with that of Zn–Ni platings,

• revealing TheContributions: best plating high mainly corrosion comparable with that platings. of Zn–Ni platings, promiseexhibited for Zn–Fe–Mo platings asresistance potential Zn–Ni Author Daichi Kosugi performed the alternatives experiments for and analyses; Takeshi Hagio revealing promise for Zn–Fe–Mo platings as potential alternatives for Zn–Ni platings. considered the data and wrote the paper; Yuki Kamimoto gave important advice throughout the experiments;  For the co-deposition of Mo, it is crucial to control the pH to approximately 5.7 to ensure that and and designed the • Ryoichi For does theIchino co-deposition of Mo, it is crucial to control theform pH to approximately 5.7 to ensure that Mo Mo not conceived form large polyoxides andexperiments. Zn does not hydroxides. doesofnot form does large polyoxides Zn does not form hydroxides. Conflicts Co-deposition not effectively at of low current densities, and a high Mo concentration Interest: The authors declareand nooccur conflict interest. • in Co-deposition does not effectively occur at low current densities, the bath as Mo (IV) cannot be efficiently reduced to Mo (0). and a high Mo concentration in the bath as Mo (IV) cannot be efficiently reduced to Mo (0). References Author Contributions: Daichi Kosugi mainly performed the experiments and analyses; Takeshi Hagio considered data and wroteJ.;the paper; Yuki Kamimoto gave important advice throughout 1. Nesic,the S.; Postlethwaite, Olsen, S. An electrochemical model for prediction of corrosionthe of experiments; mild steel in and Ryoichi Ichino conceived and designed the experiments. aqueous carbon dioxide solutions. Corrosion 1996, 52, 280–294. 2. Lo, K.H.; Shek, C.H.; Lai, J.K.L. Recent in stainless steels. Mater. Sci. Eng. 2009, 65, 39–104. Conflicts of Interest: The authors declare nodevelopments conflict of interest. 3. Mackowiak, J.; Short, N.R. Metallurgy of galvanized coatings. Int. Met. Rev. 1979, 24, 1–19.

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Author Contributions: Daichi Kosugi mainly performed the experiments and analyses; Takeshi Hagio considered the data and wrote the paper; Yuki Kamimoto gave important advice throughout the experiments; and Ryoichi Ichino conceived and designed the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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