Electrodeposition and Properties of ZnFeNi Alloys

CHINESE JOURNAL OF PHYSICS

VOL. 46 , NO. 1

FEBRUARY 2008

Electrodeposition and Properties of ZnFeNi Alloys Ismail Hakki Karahan∗ Department of Physics, Faculty of Science and Arts, Kilis 7 Aralik Universty, 79100-Kilis, Turkey (Received June 6, 2007) Four Zn-Fe-Ni films were electrodeposited under potentiostatic deposition conditions on aluminum and steel plates. Their composition, corrosion property, and structure were evaluated as a function of the FeSO4 concentration in the electrolyte. Cyclic voltammetry of the individual metals was performed in conjunction with the SEM technique to study the electroplating process of the films. The results show that Zn-Fe-Ni alloys can be produced with good adhesive and anticorrosive properties, and all films have the same crystallographic structure as zinc, but with different crystallographic orientations. PACS numbers: 81.15.Pq, 68.55.Jk, 81.65.Kn

I. INTRODUCTION

Electrodeposited Zn-Fe-Ni alloys are valuable for their leveling action [l], and electrodeposited Ni-Zn has been widely studied [2–8], as has also the electrodeposited Fe-Zn co-deposit [9–14]. The co-deposition of iron group metals with zinc has several unique features, as reported in Refs. [15, 16]. Alloy electrodeposition is widely used in the production of new materials that require specific mechanical, chemical, and physical properties [17–19]. This technique has been demonstrated to be very convenient because of its simplicity and low cost in comparison with other methods, such as mechanical alloying, sputtering, and vapour deposition [20]. The electrodeposition method is suitable for producing reproducible electrodeposited films at the same conditions. Zn coatings have been applied as the first protection layer on steel, mainly in the automobile industry [21]. In particular, Zn-Ni and Zn-Fe crystalline electrodeposits are promising alternatives to pure Zn and galvannealed steel, due to their improved mechanical properties and corrosion resistance [22, 23]. Therefore, it was felt that it would be interesting to collect the properties of Zn-Ni and Zn-Fe alloys in one alloy via the electrodeposition of a Zn-Fe-Ni ternary alloy. The first objective of this study is to electrodeposit Zn-Fe-Ni alloy coatings onto steel substrates in a useful form from a citrate bath. The second objective is to study the corrosion behavior of Zn-Fe-Ni alloy deposits on low carbon steel substrates, evaluated in comparison to the corresponding pure Zn coating. The composition and structure of these alloys were investigated.

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ELECTRODEPOSITION AND PROPERTIES . . .

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TABLE I: Composition of solutions for alloy electrodeposition. Composition of solutions Zn ZnCl2 . (gl−1 ) 40 −1 NiCl2 .6H2 O (gl ) FeSO4 .7H2 O (gl−1 ) C6 H5 Na3 O7 ·2H2 O (gl−1 ) (sodium citrate) 25 H3 BO3 (gl−1 ) 16 NH4 Cl (gl−1 ) 25 −1 Gelatine (gl ) 1 Solution pH 4 Temperature 45 Voltage (V) -2.5

ZnFeNi 40 10 5 25 16 25 1 4 45 -2.5

ZnFeNi 40 10 10 25 16 25 1 4 45 -2.5

ZnFeNi 40 10 20 25 16 25 1 4 45 -2.5

ZnFeNi 40 10 30 25 16 25 1 4 45 -2.5

II. EXPERIMENT

Zn-Fe-Ni alloys were obtained by electrodeposition from the solutions shown in Table I. The plating time was 10 min, after which the cathode was withdrawn, washed with distilled water, and dried. All these electrolytes were prepared from Merck pro-analysis grade chemicals and 18 MΩ cm Milli-Q distilled water. A commercial grade gelatin (a protein with C, N, H, and O) was added to the plating bath to improve the corrosion resistance. The working electrodes (WE) were aluminum foils used for X-Ray Diffraction (XRD) and Atomic Absorption Spectrometer (AAS) measurements and AISI 4140 steel disks used for corrosion. The chemical composition of AISI 4140 steel is tabulated in Table II. To show the reproducibility of the samples, all samples were produced twice under the same conditions and analyzed. The WE were polished with silicon carbide papers from 3 through 1 to 0.5 µm and velvet, rinsed twice with distilled water, washed in acetone, rinsed twice with distilled water again, and then dried in air. The counter electrode, for polarization and corrosion measurements was a Pt gauze electrode. The reference electrode used in all experiments was a saturated calomel electrode (SCE). All the potentials are in reference to the SCE. TABLE II: Chemical composition of AISI 4140 low alloy steel. Chemical composition of AISI 4140 low alloy steel (%) Element C Mn Si Cr Ni Mo V Wt.% 0.36 0.80 0.005 0.914 0.30 0.85 0.075

S 0.07

Cu 0.143

P 0.034

The electrochemical behavior of the electrodeposited specimens was analyzed in a 3 wt.% NaCl aqueous solution at room temperature in a Pyrex glass cell. The corrosion

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FIG. 1: Cyclic voltammetry of an AISI steel electrode in the presence of ZnCl2 .5H2 O 40 g/l, FeSO4 .6H2 O 10 g/l, NiCl2 .5H2 O 10 g/l, C6 H5 Na3 O7 ·2H2 O 25 g/l, H3 BO3 16 g/l, NH4 Cl 25 g/l, Gelatine 1 g/l solution 45 ◦ C. Scan rate: .1 V/s.

behavior of the samples was investigated by a potentiodynamic polarization technique. Polarization and cyclic voltammetry measurements were performed with an electrochemical analyzer/workstation (Model 1100, CH Instruments, USA) with a three-electrode configuration. The exposed area of the specimens was about 1 cm2 . The specimens were covered with a cold setting resin and immersed into the solution until a steady open circuit potential (ocp) was reached. After equilibration, polarization started at a rate of 1 mV/sn. A Rigaku difractometer was used to analyze the structure of the alloys. The X-ray difractometer was operated at 30 kV and 30 mA with CuKα radiation. The film composition was determined via an AAS (Perkin-Elmer), after dissolving the deposits in concentrated hydrochloric acid and diluting the solution with distilled water to 100 ml. The results obtained via the AAS are shown in Table III. The error bars for the elemental compositions presented in Table III are ±0.1%. The morphology of the deposits was analyzed by scanning electron microscopy (SEM). TABLE III: Chemical composition of deposits. Electrolyte Zn 1.6 1.6 1.6 1.6

(g/l) Fe 0.2 0.4 0.8 1.2

Ni 0.4 0.4 0.4 0.4

% at Electrolyte Zn Fe 72.7 9.1 66.7 16.7 57.1 28.6 50 37.5

Ni 18.2 16.7 14.3 12.5

% at Film Zn Fe 93 3 92 5 88 10 80 17

Ni 4 3 2 3

III. RESULTS AND DISCUSSION

Before alloy deposition, a study of zinc deposition was made. The results for this individual metal were obtained through the structural, electrochemical, and morphological

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FIG. 2: XRD patterns of Zn-Fe-Ni deposits with various alloy content.

FIG. 3: Cell parameters for the ZnFeNi deposits with different iron percentages.

techniques applied to the study of electrodeposition of Zn-Fe-Ni alloys. Fig. 1 shows the cyclic voltammograms (CV) of Zn-Fe-Ni alloy on steel substrate. During the forward scan, a cathodic current increase could be observed as the potential was extended to −1.124 V and above for the alloy. These current increases are due to the formation of metals on the surface of the substrate. As the potential was scanned forward to a more negative value, a drastic increase in the current profile confirms the hydrogen evolution process that usually occurs at higher negative potentials. During the reverse scan, at −0.12 V a stripping peak indicates the dissolution of the compounds into the solutions. From the data obtained through CV, we can conclude that the deposition of Zn-Fe-Ni alloy occurs as the deposition potentials are forwarded to values more than −1.12 V. One well defined anodic peak appeared at E = −0.12 V corresponding to the re-oxidation of deposits. The XRD patterns of the electrodeposited Zn-Fe-Ni films are given in Fig. 2. The phases of the electrodeposited Zn-Fe-Ni alloy are very complicated and depend on the chemical compositions. These XRD patterns indicate that the films are of a hexagonal close packed (HCP) structure. The lines are distorted as a consequence of iron and nickel present in the zinc lattice. The main peaks arising from the (002), (100), (101), and (102) planes were observed at 2θ ∼ = 36, 40, 43, and 54◦ , respectively. However, in the Zn93 Fe3 Ni4 film the (002) peak is absent. The (102) peak is only observed in the Zn80 Fe17 Ni3 film. In

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FIG. 4: (a) SEM image of Zn film. (b) SEM image of Zn80 Fe17 Ni3 alloy. (c) SEM image of Zn92 Fe5 Ni3 alloy.

FIG. 5: Linear sweep voltammogram of electrodeposited Zinc and zinc alloys on an AISI 4140 substrate. (1: Zn80 Fe17 Ni3 , 2: Zn88 Fe10 Ni2 , 3: Zn92 Fe5 Ni3 ,, 4: Zn93 Fe3 Ni4 alloys)

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FIG. 6: Evolution of the corrosion potentials versus the Fe.

addition, a new peak appeared at 39.80◦ for the three films except for Zn93 Fe3 Ni4 . This phase was not identified. Similar to our previous study [24], no oxidation peak was observed in the XRD pattern. For all the Zn-Fe-Ni alloys, the (101) peak is the strongest one, while for that at pure zinc, the (100) peak is dominant. From the diffraction patterns the a and c cell parameters of the hexagonal structure were found. (Fig. 3) The c parameter first decreased with the increase in the iron content; but a sudden change was observed when the iron percentage was increased. On the other hand, the a parameter increased with the increase in iron content. However, for deposits with iron content greater than 10%, the lattice constant decreased at 0.26 nm. When an alloying metal replaces the atoms of another metal in the latter’s space lattice, the cell parameter of the latter changes. In many instances it varies approximately linearly with the proportion of the atoms of the substituting metal. This relation is known as Vegard’s law [25]. ZnFeNi alloys obeyed Vegard’s rule between 3 to 10 % iron content. This indicates that the solubility of iron in a zinc lattice decreases for an iron content larger than 10%. All the deposits obtained from the investigated solutions adhere well to the steel substrate. The metallic luster and brightness of the deposit decreased with the presence of iron. It was observed that the deposits are generally composed of fine grains. The surface morphology of the films was examined by SEM. Some of the SEM micrographs are shown in Fig. 4. Fig. 5 shows the corrosion property of the Zn coating and that of the deposited ZnFe-Ni alloys from different baths, in a 3 wt. % NaCl aqueous solution. It can be seen that the free corrosion potential of the Zn-Fe-Ni alloys are more positive than that of pure zinc, which indicates that the Zn-Fe-Ni coating is an ideal anodic protective coating for iron or steel products, and can provide longer protection for iron or steel products than pure zinc coatings, due to the smaller free corrosion potential difference between the Zn-Fe-Ni coating and that of the iron-base substrate. Compared with other Zn-Fe-Ni alloys, Zn88 Fe10 Ni2 was the noblest alloy among the others. For up to 10% iron, the corrosion potential shifts to positive with the iron content. The electrochemical characteristics deduced from the polarization curves are presented in Fig. 6. The figure presents the evolution of the corrosion potential versus the iron content. The evolution of the iron is consistent with the increase of the zinc hcp lattice parameter. Figure 3 and 6 may explain the mechanism of the

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good anticorrosion properties of Zn88 Fe10 Ni2 . Increasing the a parameter of the hcp zinc structure increased the corrosion potential of the film. A further increase in the iron content decreased both the lattice parameter and the corrosion potential of the ZnNiFe alloys.

IV. CONCLUSIONS

The protective Zn-Fe-Ni coatings were obtained from different chloride-sulfate baths, and the corresponding electroplating behavior and corrosion was investigated using cyclic voltammetry and sweep voltammetry methods. Structural analysis revealed that all the samples have the same crystallographic structure, but with different crystallographic orientation, which is a consequence of the small amount of iron and nickel. The Zn88 Fe10 Ni2 alloy has the biggest corrosion resistance.

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