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The effect of pH of plating bath on electrodeposition and properties of protective ternary Zn–Fe–Mo alloy coatings
Surface & Coatings Technology 299 (2016) 81–89
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The effect of pH of plating bath on electrodeposition and properties of protective ternary Zn–Fe–Mo alloy coatings J. Winiarski a,⁎, A. Leśniewicz b, P. Pohl b, B. Szczygieł a a b
Department of Advanced Material Technologies, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, PL–50370 Wrocław, Poland Analytical Chemistry and Chemical Metallurgy Division, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, PL–50370 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 17 February 2016 Revised 19 April 2016 Accepted in revised form 29 April 2016 Available online 30 April 2016 Keywords: Zn–Fe–Mo alloy coating Electrodeposition X-ray diffraction Anodic dissolution Cyclic voltammetry
a b s t r a c t Ternary Zn–Fe–Mo alloy coatings electrodeposition in a sulphate bath with tri-sodium citrate in the pH range 4.8–6.0 was studied. The increase of the plating bath pH from 4.8 to 6.0 leads to the iron content increase from 1.5 up to 7.9 wt% and it is accompanied by a significant decrease in the current efficiency from 76 to 51%. It was demonstrated that molybdenum co-deposition starts at pH N 4.8, because binary Zn–Fe alloy coating without a significant amount of molybdenum was obtained from the plating solution at pH 4.8. The Mo content increases from 0.8 wt% at pH 5.1 to 2.7 wt% at pH 5.7 and 6.0. X-ray diffraction analysis as well as anodic linear sweep voltammetry measurements showed that within the considered plating solution pH range molybdenum modifies zinc microstructure to a very large extent. The deposited at pH 5.4–6.0 Zn–Fe–Mo coatings contain a solid solution of iron and molybdenum in zinc and probably FeZnx phase, where 6.67 b x b 13. Potentiodynamic polarization revealed that the deposited at pH 5.7 and 6.0 Zn–Fe–Mo coatings are characterized by a significantly lower corrosion rate in 0.5 mol dm−3 NaCl solution. This was confirmed by an electrochemical impedance spectroscopy test, where Zn–Fe(7.9 wt%)–Mo(2.7 wt%) alloy coating reached the highest resistance of charge transfer reaction during 24 hours of exposure to NaCl solution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The protective properties of zinc coatings can be easily improved by co-deposition of zinc with such metals as: cobalt, nickel, iron or manganese [1–3]. Among these elements, iron improves the corrosion resistance of Zn–Fe coatings and it may provide good paintability and weldability [2]. The modification of Zn–Fe alloy coatings performance can be achieved in various ways i.a. by additional conversion coatings [4], by selecting an appropriate bath composition or by changing the deposition parameters, which allows to obtain coatings characterized by different phase composition and morphology [5]. Electrodeposition of zinc composite coatings [6] or phosphorous doping of zinc alloy coatings [7] can be an alternative, however, the effect of dispersed particles on the improvement in corrosion resistance is often debatable. Introduction of a third alloying element, with higher corrosion resistance and better physico-mechanical properties than that of zinc matrix, is another possibility and may allow the elimination of additional harmful chromating. Molybdenum could be one of these elements. It is known from the literature that molybdenum has the positive impact on the passivity of stainless steel. On the other hand, co-deposition of molybdenum with nickel or iron leads to the improvement in microhardness, wear resistance and corrosion resistance of electroplated binary alloy ⁎ Corresponding author. E-mail address: [email protected] (J. Winiarski).
http://dx.doi.org/10.1016/j.surfcoat.2016.04.073 0257-8972/© 2015 Elsevier B.V. All rights reserved.
coatings [1,8,9]. Therefore introduction of molybdenum to zinc coating would be an advantageous option. Although it is possible to obtain binary Zn–Mo alloy coatings [10] with relatively high molybdenum content (from 0.5 to 8 wt%), there are problems with obtaining homogenous deposits characterized by good protective properties. The solution proposed in the current work is a deposition of ternary Zn–Fe–Mo alloy coatings with improved corrosion resistance and better mechanical properties than those of binary Zn–Fe and pure Zn coatings. There are literature reports on deposition of ternary Zn–Ni–Mo and Zn– Co–Mo alloy coatings [1,11–13] from weak acid citrate-sulphate baths, which is a good starting point to take up this research task. It has been shown that Zn–Co–Mo coatings, even with 4 wt% Mo and 6 wt% Co content, are characterized by relatively better physico-mechanical properties than zinc coating [1]. Moreover, the results of our further research [14,15] lead to the conclusion that molybdenum has a beneficial effect on the corrosion resistance of Zn–Co–Mo alloy coatings due to its ability to form oxides on the coating surface. Therefore it was clear that the change of a metal that induces molybdenum co-deposition (iron, instead of cobalt) should also allow to obtain coatings with satisfactory and new useful properties. Unfortunately, apart from the above mentioned works little information about ternary Zn–Fe–Mo alloy coatings can be found in the literature. Our preliminary study on ternary Zn– Fe–Mo alloy coatings [16] allowed us to determine a basic plating bath composition and an appropriate range of plating parameters. It was further observed that deposition of bright and homogeneous coatings was
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possible only in a relatively narrow plating solution pH range, cathodic current density, temperature, and at stable hydrodynamic conditions. All these variables determine the content of alloying elements in Zn– Fe–Mo coating and allow to obtain alloys with predefined chemical and phase composition. There is no doubt that the observed effect of the bath pH on the content of Mo and Fe in the alloy results from a strong influence of this parameter on the type of metal ion complexes present in the plating solution and their equilibria. Due to the fact that acidity of plating solution is a crucial parameter, the aim of the present work was to find important dependences between plating pH and chemical composition, phase structure and protective properties of these ternary alloy coatings. In order to solve the described problem, cyclic voltammetry (CV) and chronopotentiometry were employed to investigate electrodeposition process of ternary Zn–Fe–Mo alloy coatings at different pH values of the plating bath. The changes in morphology and chemical composition of the coatings were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The effect of the plating solution pH on the phase structure of the deposits was investigated by means of anodic linear sweep voltammetry (ALSV) and by X-ray diffraction (XRD). The corrosion resistance of Zn–Fe–Mo alloy coatings in chloride environment was determined by means of potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). 2. Experimental details 2.1. Materials The coatings for SEM, EDXS and XRD analyses were deposited on pure copper discs of a diameter of 14.8 mm, pressed into a Teflon® holder (geometric area of working electrode 1.72 cm2). This was necessary, in order to differentiate the signals that came from the substrate. For CV and ALSV experiments a copper tip was also used (geometric area of working electrode 0.204 cm2). The samples for corrosion measurements were deposited on AISI 1015 steel (nominal composition (wt%): C–0.17; Mn–1.40; P–0.040; N–0.012; Cu–0.55, Fe–balance) in accordance with the expected mechanism of their protective activity. 2.2. Pretreatment of substrates The surface of copper electrodes was polished with successive grades of abrasive paper up to 1200 grit. Then the electrodes were rinsed in an ultrasonic bath, degreased in acetone and activated in a 10 wt% H2SO4 solution for 10 s at room temperature. Each step was preceded by thorough rinse with distilled water. Before CV and ALSV measurements the surface of Cu tip was mechanically polished (1 μm grade diamond powder), degreased in acetone in an ultrasonic bath and activated in a 10 wt% H2SO4 solution. 2.3. Electrodeposition Zn–Fe–Mo alloy coatings were deposited from a citrate-sulphate bath with the composition (mol dm−3): ZnSO4–0.2; FeSO4–0.2; C6H5Na3O7 (tri-sodium citrate) – 0.2; Na2SO4–0.1 and Na2MoO4–0.01. The reference Zn and Fe coatings were deposited from the baths with a similar composition, but without Na2MoO4. Analytical grade chemicals and double distilled water were used for preparation of the solutions. The pH of the baths was adjusted to 5.7 for Zn and Fe plating and to 4.8–6.0 for Zn–Fe–Mo plating using a solution of diluted H2SO4 or NaOH. All the coatings were deposited on a rotating disc electrode (RDE) to ensure constant hydrodynamic conditions. The electrodeposition was performed in a 200 cm3 vessel at the temperature of 25 ± 1 °C under galvanostatic conditions at a cathodic current density jc = − 20 mA cm− 2. The rotational speed (ω) of RDE was set at 800 rpm. The parameters of electroplating were established during preliminary experiments [16].
A platinum plate with a geometric area of 6 cm2 was used as the anode. Deposition time varied between 21 and 37 min taking into account the current efficiency of electroplating. This approach allowed us to obtain coatings with a comparable thickness (~10 μm). The thickness of the coatings was measured by means of gravimetric method and magnetic induction method using a Fischer FMP40 thickness gauge. The current efficiency (η) of the electrodeposition was defined as the amount of coulombs required for the reaction (Qi) divided by the total amount of coulombs passed (Qtotal) according to Eq. (1): η = Qi / Qtotal. Knowing the weight of the deposited alloy, and the content (wt%) of alloying elements, needed to reduce the metals electric charges (Qi) were calculated. The Qtotal was calculated from the quotient of current (I) and time of electrolysis (t) according to Eq. (2): Qtotal = I × t. The general assumption was that each of the metals is reduced on the electrode to its metallic form. 2.4. Testing methods Morphology of the deposits was examined using a VEGA II SBH (TESCAN) scanning electron microscope. Concentration of alloying elements in the deposits were measured using energy dispersive X-ray spectroscopy. Phase composition and structure of Zn–Fe–Mo alloys were analyzed by X-ray diffraction using a Philips X'PERT system with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. The 2 theta angle (2θ) ranged from 3° to 100° at a scan rate of 0.05° per second. Identification of the phases was carried out by comparison of the experimental patterns with the reference patterns stored in the Powder Diffraction Files database (International Centre for Diffraction Data PDF-2 base) and with the experimental patterns of copper substrate and pure zinc coating. Electrochemical tests were carried out using a three-electrode electrochemical cell. A platinum wire was used as a counter electrode. A saturated calomel electrode (SCE) or silver/silver chloride electrode (Ag|AgCl) only for corrosion measurements, both with a single junction Luggin capillary, was applied as the reference electrode. In order to avoid any confusions, all potentials were referred to SCE (EAg|AgCl = −0.042 V vs SCE) throughout the article. Cyclic voltammetry experiments were performed in a suitable amount of plating bath. The CV scan was initiated at open circuit potential (OCP) in the negative direction and reversed at the potential slightly more negative than that for galvanostatic (−20 mA cm−2) deposition in the positive direction up to +0.2 V vs SCE. The scan rate was set to 5 mV s−1. ALSV measurements were performed in a 0.5 mol dm−3 NH4Cl solution at 25 °C. Ammonium chloride was chosen due to its ability to easily form complexes of zinc. Deposits were dissolved by sweeping the potential from OCP and ending at +0.2 V vs SCE with a sweep rate of 1 mV s−1 and at rotational speed of 2000 rpm. Before each CV and ALSV experiment the plating or stripping solution was purged with high purity nitrogen for 15 min. At this time, the gas flow was maintained over the liquid. Corrosion measurements were performed in a non-deaerated 0.5 mol dm−3 NaCl solution with a neutral pH. The geometric area of working electrode and counter electrode exposed to the NaCl solution was 1 cm2 and 1.4 cm2, respectively. The potential measured after 30 min of immersion of the sample in the NaCl solution was adopted as an open circuit potential at which dc measurement was performed. After this time the variation in the potential was smaller than 1 mV min− 1. Potentiodynamic polarization curves were recorded starting at −0.1 V and ending at +0.15 V relative to OCP at a potential change rate of 0.5 mV s−1. Electrochemical impedance spectroscopy (EIS) was performed at a corrosion potential after 24 h of the sample exposure to a non-deaerated 0.5 mol dm− 3 NaCl solution. Impedance spectra were recorded in the frequency range from 10 kHz to 10 mHz, ten points per decade. The amplitude of sinusoidal signal was set to 5 mV. During the EIS measurements the electrochemical cell was kept in a Faraday cage. All polarization measurements were performed using a computer-controlled Reference600 (Gamry) Potentiostat/
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Galvanostat/ZRA. For CV, ALSV and chronopotentiometry measurements correction of IR drop was considered, but it proved to be irrelevant due to the relative high conductivity of the plating solutions. The acquired data were curve fitted and analyzed using the Echem Analyst (Gamry) software. 3. Results and discussion 3.1. The effect of pH on electrodeposition and current efficiency Fig. 1 shows the potential-time dependence for the galvanostatic (− 20 mA cm−2) deposition of Zn–Fe–Mo alloys on the platinum tip at different pH (4.8–6.0). In the first seconds of polarization the rise of potential was observed due to the nucleation process. For longer deposition time, the potential became almost stable which may indicate a uniform growth of the coating. After the nucleation process, the structure of electrode became homogeneous and constant in time. Reactions occurring on the surface of the coating reach a certain state of equilibrium. A pH increase in the plating bath leads to a shift of the deposition potential towards more negative values (Fig. 1). This behavior may be due to the blocking of the cathode surface by metal oxides or hydroxides that arises as a result of strong alkalization of the electrolyte near the cathode surface. Acidity of plating bath is an important parameter in Zn–Fe–Mo alloy deposition, because it also determines the ionic forms of molybdenum [11]. Therefore electrodeposition of ternary Zn–Fe–Mo alloy coatings on the copper electrode at different pH values of plating bath was studied by cyclic voltammetry. In the pH range 4.8–6.0 electrodeposition of coatings takes place at the potentials between − 1.13 V vs SCE and − 1.32 V vs SCE (Fig. 1). Therefore, the reverse potential for each pH was set at slightly more negative value (usual by a tens mV) than that registered during galvanostatic deposition in Fig. 1. From the CV curves presented in Fig. 2 it is evident that with increasing pH of the plating bath up to 6.0 the deposition of the alloy starts at more negative potentials and the current density at the switching potential decreases. This ions in the may be due to the increasing concentration of free MoO2− 4 plating solution. According to [11] in the Zn–Ni–Mo plating electrolytes with pH close to 4.5 molybdenum citrate complexes predominate and no considerable amount of Mo in the alloy was found. On the other hand, at pH close to 5.7 the uncomplexed MoO24 − ions are the main ionic form of molybdenum. Therefore the percentage of Mo in the alloy increases. Based on the literature reports on the mechanism of
Fig. 1. Potential–time dependence for the galvanostatic deposition of ternary Zn–Fe–Mo alloy coatings on the copper electrode at different pH values of plating bath (ω = 800 rpm, T = 25 °C, jc = −20 mA cm−2).
Fig. 2. Cyclic voltammograms for copper electrode in Zn–Fe–Mo plating baths at different pH values. Scan rate 5 mV s−1, ω = 800 rpm, T = 25 °C.
deposition of binary alloy coatings containing an iron group metal and molybdenum, which takes into account formation of the passive layer [17], it can be assumed that if the reaction rate of oxide formation on the cathode may exceed the rate of its further reduction, the surface of the cathode may be blocked. This would explain the changes observed in the course of the cathodic parts of the CV curves in Fig. 2. From the anodic parts of the registered in the plating baths at pH 4.8; 5.1 and 5.4 voltammograms it is evident that a rise in pH leads to the appearance of additional dissolution peaks (Fig. 2). Such changes are probably related to the increase of iron and molybdenum content in the deposits and to the ability of these metals to form intermetallic compounds with zinc or between themselves. At pH 5.4 one can see three, relatively well separated, peaks. Generally, the peak placed at more negative potential (−0.97 V vs SCE) can be associated with the dissolution of zinc from a Zn matrix. The peak recorded at more positive potential (− 0.86 V vs SCE) can be related to dezincification of the zinc-iron phase. The remains of the intermetallic phase should be dissolved as the last components of the alloy coating. This oxidation reaction can be associated with the anodic peak at −0.64 V vs SCE (Fig. 2). According to the proposed above interpretation, the obtained at pH 5.4 coating should be characterized by the most diverse phase composition. It should also be noted, that at the potentials more positive than −0.30 V vs SCE a weak peak appears (graph inset in Fig. 2). This peak is absent at pH 4.8. It appears at pH 5.1, reaches maximum current at pH 5.4 and then decreases at pH 6.0. This anodic peak may be related to oxidation of some residues of the alloy constituents or to other reaction that occurs at the metal/electrolyte interface. To verify these assumptions, the CV measurement was repeated at pH 5.4 (in a fresh portion of plating bath) in the range of potentials from OCP to −1.255 V vs SCE and terminated at −0.5 V vs SCE. Copper tip was subjected to SEM/EDXS observation (Supplementary material 1). Further abrupt increase of the current at the potentials more positive than 0.1 V vs SCE (Fig. 2) was related to the dissolution of copper substrate in the plating baths. An increase of the bath pH above 5.4 causes a significant change in the shape of the recorded CV characteristics. The obtained at pH 5.7 and 6.0 deposits start to dissolve at more positive potentials (approx. − 0.9 V vs SCE) than those observed at lower pH values. The CV curve recorded at pH 5.7 is characterized by only one distinct anodic peak at −0.76 V vs SCE (Fig. 2). This peak is shifted towards more noble values at pH 6.0. It is also clear that for pH 5.7 and 6.0 a very weak dissolution peak appears just behind the main dissolution peak (between − 0.65 and − 0.60 V vs SCE). Its presence can be associated with oxidation of iron from the residue of its phase. This is possible
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due to the relatively high iron content in the alloy and because this peak coincides with that recorded at pH 5.4. Fig. 3 shows selected cathodic parts of the CV voltammograms revealing that an increase in the plating bath pH causes significant changes in electrodeposition process. The shape of the cathodic curves for pH 4.8–5.1 is similar. The abrupt increase in the current density value below −1.08 V vs SCE is caused by hydrogen evolution reaction that overlaps the reduction of metals. The current density increases abruptly at even more negative potentials when the pH value rises. At pH 5.7 and 6.0 an appreciable change in the course of the CV curves appears (Fig. 3). There is a new cathodic peak c1 at approx. −0.6 V vs SCE. The presence of this peak may be related to reduction of MoO2− 4 ions to Mo2O5 or to MoO2 in containing Na2MoO4 citrate electrolytes [18]. Formation of the electrically passive layer of molybdenum oxides is possibly taking into account a further local drop of current density (c1 peak in “pH 6.0” curve in Fig. 3). The changes in pH of the plating bath influenced the current efficiency (η) of deposition process (Fig. 4). Thus η slightly decreases between pH 4.8 and 5.4 from 76 to 72%, while at pH N 5.4 the efficiency decreases rapidly to approx. 50% at pH 6.0. This observation is in agreement with that of previously investigated Zn–Ni–Mo coatings [11]. As it could be expected, the decrease in current efficiency also reduces the deposition rate. In the pH range 4.8–6.0 the deposition rate decreases from 26 to 16.5 μm h−1 (Fig. 4).
Fig. 4. The effect of pH of the plating bath on the current efficiency (η) and deposition rate of Zn–Fe–Mo alloy coatings (T = 25 °C, jc = − 20 mA cm−2, ω = 800 rpm, copper substrate).
3.2. SEM and EDXS analyses Fig. 5 shows SEM microphotographs of the surface of Zn–Fe–Mo alloy coatings deposited at pH 4.8–6.0. The coatings deposited at pH 4.8 and 5.1 (Fig. 5a and 5b) are not homogenous and are characterized by non–uniform thickness and a granular structure. The size of the granules is tens μm. The granules are not dense and look as if they are porous, especially at pH 4.8 (Fig. 5a). Between pH 5.1 (Fig. 5b) and 5.4 (Fig. 5c) a significant change in the coatings morphology takes place. The size of the crystal agglomerations abruptly decreases to a few micrometers, the surface becomes more homogenous and smooth (Fig. 5c). The coating deposited at pH 5.4 represents a flake-like structure, where the size of the flakes amounted to a single micrometers. A homogenous and smooth coating was deposited at 5.7 (Fig. 5d). This coating was free from noticeable defects. Increasing pH to 6.0 causes that the granular structure is more pronounced and the coatings become cracked, probably due to the presence of high tensile stresses inside the coating (Fig. 5e). In addition, numerous large pores were also observed. Their formation can be associated with a significant decrease in the current efficiency and therefore large amounts of evolved hydrogen. The rough structure of the surface of both deposits (Fig. 5d and 5e) is due to the mechanical pretreatment of the copper substrate and this may indicate a weak ability for microsmoothing of the used plating baths. Fig. 6 presents the results of EDXS analysis of ternary Zn–Fe–Mo alloy coatings as a function of the plating bath pH. The analyses were performed pointwise across the sample, therefore, this work presents an average content of iron and molybdenum in the deposits. It was demonstrated that, except zinc and 0.9 wt% Fe, the deposit obtained at pH 4.8 does not contain a considerable amount of molybdenum. Starting from pH 5.1 the molybdenum content increases from 0.8 to 2.7 wt% at pH 5.7 and remains constant up to pH 6.0. The iron content is much higher, i.e. 1.5 wt% at pH 5.1, and it rises to 7.9 wt% at pH 6.0.
3.3. The effect of plating pH on the phase structure of the coatings
Fig. 3. Selected parts of the cathodic CV curves at different pH values. Scan rate 5 mV s−1, ω = 800 rpm, T = 25 °C, copper substrate.
X-ray diffraction of deposited at various pH Zn–Fe–Mo alloy coatings was performed. The pure zinc coating experimental diffractogram corresponds to the metallic zinc standard no 04-0831. The diffraction peaks at 2θ: 39.65°, 43.87°, 54.34°, 71.20°, 82.61° and 87.05° indicate metallic zinc presence (Fig. 7a). There are also peaks that correspond to the copper substrate, moreover, some of them overlap the zinc peaks. Analysis of the experimental diffraction patterns for pure Cu substrate (Fig. 7b) allows to state that the copper XRD pattern is similar to that of the metallic copper standard number 04–0836. Relative intensities of the experimental peaks are comparable to those given in the standard, however the peak positions are slightly shifted, which is a
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Fig. 5. SEM micrographs of ternary Zn–Fe–Mo alloy coatings deposited from citrate-sulphate baths at pH: 4.8 (a), 5.1 (b), 5.4 (c), 5.7 (d) and 6.0 (e). Deposition parameters: jc = −20 mA cm−2, ω = 800 rpm, T = 25 °C, copper substrate.
common phenomenon occurring for differently processed alloys and metals. Analysis of the diffraction patterns of deposited from solutions at different pH Zn–Fe–Mo alloy coatings (Fig. 7c–7g) leads to the conclusion that the relationship between the coating phase composition and the plating bath acidity is obvious. The main XRD peaks corresponding to metallic Zn become less intense and slightly wider with the pH increase. The most intense reflections are present in the XRD pattern for the deposited at pH 4.8 alloy coating. For the other examined coatings a
decrease of the measured signal intensity with an acidity increase is observed. At the diffraction patterns of the deposited at pH ranged from 4.8 to 5.4 Zn–Fe–Mo coating the most intensive peak position is similar to that of the diffractogram of Zn coating (Fig. 7a), however all the characteristic for metallic Zn reflections are shifted towards higher values of the diffraction angle, which can be related to the phase change, i.e. possible formation of new phases or to molybdenum and iron that may incorporate in the crystal lattice of zinc matrix. The presence of a few additional peaks in the diffraction patterns of the deposited from
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Fig. 6. The effect of pH of the plating solution on the composition of ternary Zn–Fe–Mo alloy coatings deposited on copper substrate from citrate-sulphate bath (jc = −20 mA cm−2, ω = 800 rpm, T = 25 °C).
solutions of pH between 5.4 and 6.0 coatings (for example at 2θ: 37.65°, 55.38°, 70.80°, 86.85° and several reflections around 44°), nevertheless not very intensive, indicates an existence of other constituents in the alloy coating. The diffractograms of the coatings deposited from solutions with pH 5.7 and 6.0 (Fig. 7f and 7g) show the absence of several peaks corresponding to zinc coating and copper substrate, which may be caused by differences in Zn–Fe–Mo alloy coating compositions or texture. The peaks corresponding to pure iron or molybdenum were not found in the diffraction patterns, which could be ascribed to the fact that the crystal lattice of Zn matrix was partly replaced by iron or molybdenum and formed solid solution or other phases with zinc. It is known from the literature that the possible phases in electrodeposited Zn–Fe coatings with low iron content (b10 wt%), apart from the solid solution of iron in zinc matrix (namely the η phase), are the ζ (FeZn13) and δ phases (FeZn13–FeZn6.67) [19,20]. Based on the comparison of the available reference patterns of iron–zinc phases (with stoichiometry close to FeZn13) with those recorded for Zn–Fe–Mo coatings it is difficult to definitely confirm the presence of ζ or δ phase. Fig. 8 presents the ALSV voltammograms obtained during dissolution of the deposited at pH 4.8–6.0 ternary Zn–Fe–Mo alloy coatings. In the pH range 4.8 to 5.4 the deposits start to dissolve at similar potentials, which are close to −1.08 V vs SCE. It is also seen that the shape and position of the dissolution peaks is similar for coatings obtained at pH 4.8 and 5.1 while between pH 5.1 and 6.0 they change significantly (Fig. 8). When the pH is close to 4.8 and 5.1 only one dissolution peak is observed. The dissolution peak registered for “pH 5.4” is noticeably wider (Fig. 8). Moreover, the current after reaching the maximum value does not decrease to zero. At approx. −0.62 V vs SCE a small oxidation peak is visible, which could be assigned to zinc from iron–zinc phase oxidation. This assumption can be confirmed in the next voltammogram recorded for the coating deposited at pH 5.7. From Fig. 8 it is clear that dissolution of the coatings deposited at pH 5.7 and 6.0 starts at more noble potentials (−1.02 and −0.92 V vs SCE, respectively). At pH 5.7 there are three, relatively weak distinguishable peaks at −0.91, −0.62 and −0.56 V vs SCE. In view of the fact that this coating contains approx. 4.9 wt% Fe, it seems likely that a considerable amount of zinc may be incorporated in an intermetallic phase FeZnx (where 6.67 b x b 13), as mentioned previously. Therefore, the peaks at −0.91 and − 0.62 V vs SCE may be related to zinc oxidation. So the last one at −0.56 V vs SCE may be due to oxidation of iron. The interpretation of the recorded for the coating deposited at pH 6.0 ALSV voltammogram is difficult, despite the relative range of the potentials at which coatings oxidation occurs is very similar for other pH values. It can only be assumed that the phase composition of deposited at pH 6.0 Zn–Fe–Mo
Fig. 7. X-ray diffraction patterns for reference samples: Zn coating deposited at pH 5.7 (a), Cu substrate (b) and for ternary Zn–Fe–Mo alloy coatings deposited at pH: 4.8 (c), 5.1 (d), 5.4 (e), 5.7 (f) and 6.0 (g).
alloy coating is more complicated. For comparison purposes ALSV voltammograms of the deposited at pH 5.7 Zn and Fe coatings (Fig. 8) were also recorded. Fig. 8 clearly shows that pure Zn coating starts to dissolve at −1.08 V vs SCE and has the main dissolution peak at about −0.82 V vs SCE. During anodic dissolution of Fe coating several dissolution peaks in the potential range from −0.8 to −0.2 V vs SCE were recorded. The presence of multiple peaks indicates that either iron forms several phases, or (more likely) it does not pass directly in an ionic form to a stripping solution (0.5 M NH4Cl). The latter may indicate that oxidation reaction of iron is a multi-step process and a presence of intermediates on the cathode surface is possible. Because of the above doubts, the exact interpretation of the Fe voltammogram is not a subject of this work. An important conclusion is, that under the applied test conditions the oxidation of the Fe in 0.5 M NH4Cl solution takes place, and the registered current response derived from oxidation reaction of iron is
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Fig. 9. Potentiodynamic polarization curves recorded for Zn–Fe–Mo alloy coatings deposited at pH 4.8–6.0 after 30 min of immersion in 0.5 mol dm−3 NaCl solution. Scan rate 0.5 mV s−1. AISI 1015 steel substrate.
Fig. 8. ALSV voltammograms for alloy coatings deposited from citrate-sulphate bath at pH 4.8–6.0 and for Zn and Fe coatings (jc = − 20 mA cm−2, T = 25 °C). Stripping parameters: sweep rate 1 mV s−1, ω = 2000 rpm, T = 25 °C, 0.5 M NH4Cl solution, copper substrate.
Mo in the coatings as well as their morphology). In the case of the deposited at pH 4.8, 5.1 and 5.4 coatings the cathodic reaction takes place without explicit limitations, while limitations are apparent in the anodic oxidation of the alloy (Fig. 9). One would expect that with increasing pH (and thus increasing content of Fe and Mo in the deposits) the corrosion potential (Ecorr) should take more noble values. Between pH 5.1 and 5.4 the content of Fe and Mo in the Zn–Fe–Mo coatings increases, but the corrosion potentials measured for these coatings are almost the same and close to −1.06 V vs SCE. This behavior may be due to differences in coating morphology, different degree of surface development or changes in the phase composition. Increasing the pH from 5.4 to 6.0 is accompanied by a considerable increase in iron and molybdenum content in the alloy. This results in a clear shift of Ecorr towards more positive values by approx. 300 mV. The shift in the corrosion potential also causes changes in the shape of the recorded polarization curves. Despite this the corrosion potentials of Zn–Fe–Mo alloys are still more negative than that of steel (Ecorr = − 0.66 V vs SCE), which proved that these coatings offer cathodic protection to the steel substrate. From Fig. 9 it is evident that the anodic oxidation of the deposited at pH 5.7 and 6.0 Zn–Fe–Mo alloy coatings proceeds without any major constraints. The slopes of the anodic branches are lower than those of the cathodic ones. Due to the fact that in an open air 0.5 mol dm−3 neutral solution of NaCl oxygen reduction (O2 + 2H2O + 4e− ↔ 4OH−) is the main cathodic reaction on zinc surface [21], therefore oxygen reduction is the rate determining step. Because of the diffusion constraints and the differences in the shape of polarization curves, an extrapolation was performed only on the cathodic branches. Corrosion current densities (jcorr), corrosion potentials and slopes of cathodic curves (βc) are shown in Table 1. Corrosion current densities rise initially from 6.0 to
helpful in the interpretation of the other voltammograms for the deposited at pH 5.4 and 5.7 Zn–Fe–Mo coatings.
Table 1 Results of corrosion measurements for samples with Zn–Fe–Mo alloy coatings deposited at different pH values. pH
Ecorr (V vs SCE)
jcorr (μA cm−2)
βc (mV dec−1)
4.8 5.1 5.4 5.7 6.0
−1.04 −1.07 −1.06 −0.80 −0.75
6.0 15.7 18.8 2.9 3.5
−42 −36 −34 −56 −47
3.4. Potentiodynamic polarization measurements Fig. 9 shows the polarization curves recorded for the samples with deposited on AISI 1015 steel at various pH values Zn–Fe–Mo alloy coatings. On the basis of Fig. 9 it is clear, that the shape of the polarization curves is affected by pH of the plating solution (the amount of Fe and
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18.8 μA cm−2 with increasing pH of the plating solution from 4.8 to 5.4. This is probably due to the surface inhomogeneity of the deposits. Above pH 5.4 the coatings become smooth and the corrosion current density decreases markedly. In the plating solution 5.7–6.0 pH range, the deposits contain considerable amounts of molybdenum and iron and are characterized by homogenous structure which results in a clear reduction in the corrosion rate in NaCl solution. The lowest values of jcorr were calculated for the Zn–Fe–Mo coating deposited at pH 5.7 (2.9 μA cm−2) and 6.0 (3.5 μA cm−2). 3.5. Electrochemical impedance spectroscopy experiment The results of dc polarization measurements revealed differences in the corrosion resistance of the investigated zinc alloy coatings depending on the plating bath pH. Therefore EIS was employed to indicate the differences in the coating performance after longer exposure to a corrosive solution. The impedance plots in the Bode and Nyquist representation (Fig. 10a and 10b) are presented here only for selected exposure time (24 h) and samples, for better clarity of the figures. The representation of the phase angle clearly exhibits two time constants for all of the tested coatings (Fig. 10a). The Bode plot shows that as a result of the increased Fe and Mo content, the shape of the impedance plots changes. The impedance modulus at low frequency (| Z |0.01 Hz) shows higher values for Zn–Fe–Mo coating deposited at pH 6.0 (about 2.4 kΩ cm2) than that for deposited at pH 4.8 (about 1.8 kΩ cm2) and at pH 5.4 (about 1.3 kΩ cm2). This is also visible in the Nyquist plot (Fig. 10b). For the all investigated coatings it can be assumed that the highfrequency time constant is due to the formation of a porous layer of corrosion products while the low-frequency time constant could be related to the corrosion process. This interpretation is consistent with the previous analysis of the electrochemical behavior of Zn–Co–Mo and Zn– Co alloy coatings on steel [14]. The experimental impedance spectra were fitted using the presented in Fig. 11 electric equivalent circuit. In this model, Rs corresponds to the resistance of NaCl solution. Rpore and constant phase element (CPE1) may be assigned to the resistance of the electrolyte in the pores of the corrosion products layer and the pseudo-capacitance of the porous corrosion products layer. Rct is the charge transfer resistance and CPE2 is the double layer pseudocapacitance. The impedance of the CPE is well known and it was previously described [14]. An analysis of the Nyquist plot shape (Fig. 10b) and the calculated values of CPE parameters presented in Table 2 (n – an exponent of a pseudo-capacitance) revealed that CPE does not reflect the pure capacitance and thus the surface is not homogenous. That is why it was necessary to use CPE instead of a capacitor. This electric
Fig. 11. Electric equivalent circuit used for fitting the experimental impedance spectra of Zn–Fe–Mo alloy coatings after 24 h exposure of the samples to a non-deaerated 0.5 mol dm−3 NaCl solution (AISI 1015 steel substrate).
equivalent circuit ensured a good fit to the experimental data (χ2 in the order of 10−3 - 10−4 and the fitting lines in Fig. 10a and 10b). The residual errors of each of the parameters were as follows: 0.4–0.5% for Rs, 1.8–7.9% for CPE1-Y0, 1.8–9.4% for CPE1-n, 1.0–16% for Rpore, 0.4– 4.6% for CPE2-Y0, 1.2–2.4% for CPE2-n and 1.5–3.6% for Rct. According to Table 2 the corrosion potentials after 24 h exposure are more noble for the coatings deposited at higher pH, but still more negative than that of steel substrate. Rpore for the deposited at pH 4.8 and 6.0 coatings are almost the same, but more than six times lower than that of the deposited at pH 5.4 coating. This may be due to the coatings nonuniformity and different structure of the passive film. After the values of parameters Y0 and n of CPE and the corresponding resistance (R) are calculated from the equivalent circuit, the capacitance could be simply calculated using Eq. (3): C = [(Y0 R)1/n] / R [22]. According to the data presented in Table 2 and Eq. (3) the capacitance of porous corrosion products layer (Cpore) and the capacitance of the electric double layer (Cdl) were calculated. The Cpore values for the deposited at pH 4.8; 5.4 and 6.0 coatings are as follows: 111; 248 and 147 μF cm2. Because capacitance depends on dielectric constant, permittivity of free space and thickness of the passive layer, thus the observed increase in the coating capacitance may be an evidence of a decrease in the film thickness or an increase in the dielectric constant (due to the increase in the porosity of the corrosion products layer or due to defects appearing in the alloy coating). Cdl values do not exhibit such a variation, as they are between 1.79 and 1.99 mF cm2. It is noteworthy that charge transfer resistance for the deposited at pH 6.0 coating is higher (3.4 kΩ cm2) than those of the other coatings (Table 2) which proved better corrosion resistance of the coatings with higher Fe and Mo content. 4. Conclusions In summary, the following conclusions can be drawn from the presented results:
Fig. 10. Impedance spectra in the Bode (a) and Nyquist (b) representation of zinc alloy coatings deposited at pH 4.8; 5.4 and 6.0 recorded after 24 h exposure of the samples to a nondeaerated 0.5 mol dm−3 NaCl solution (AISI 1015 steel substrate). Solid lines represent the calculated spectra.
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Table 2 Equivalent circuit parameters determined by fitting the impedance spectra of samples with Zn–Fe–Mo alloy coatings deposited at pH 4.8; 5.4 and 6.0 after 24 h of immersion in 0.5 mol dm−3 NaCl solution. pH
Ecorr (V vs SCE)
Rs (Ω cm2)
CPE1-Y0 (Ω−1 cm−2 s−n)
CPE1-n
Rcoat (Ω cm2)
CPE2-Y0 (Ω−1 cm−2 s−n)
CPE2-n
Rct (Ω cm2)
4.8 5.4 6.0
−0.979 −0.965 −0.760
25 22 22
3.50 × 10−4 4.61 × 10−4 6.46 × 10−4
0.76 0.73 0.68
75 404 67
1.24 × 10−3 1.64 × 10−3 1.31 × 10−3
0.65 0.82 0.78
1939 903 3364
1. From a bath containing: ZnSO4, FeSO4, C6H5Na3O7, Na2SO4 and Na2MoO4 ternary Zn–Fe–Mo alloy coatings containing considerable amounts of molybdenum in the pH range 5.4–6.0 can be deposited. However, deposition of bright and homogenous coatings is possible only at pH close to 5.7. 2. Chemical composition of the coatings changes significantly between pH 4.8 and 6.0. In the above mentioned pH range the Zn–Fe–Mo coatings contain from 0.8 to 2.7 wt% of molybdenum and from 1.5 to 7.9 wt% of iron. Co-deposition of Mo causes an increase of the Fe amount in the coating. An increase in the Mo and Fe content leads to the shift of corrosion potential towards more noble potentials up to −0.75 V vs SCE. 3. With an increase in the plating solution pH the microstructure of Zn– Fe–Mo alloy coatings differs from the structure of the zinc coating even more. Molybdenum and iron were not detected in their pure form in the investigated coatings. Because of a high content of iron in the deposited at pH 5.4–6.0 alloys an intermetallic phase with a stoichiometry close to FeZnx (where 6.67 b x b 13) may appear. 4. Deposited at pH 5.7–6.0 Zn–Fe–Mo coatings are characterized by higher corrosion resistance in NaCl solution than those of the coatings obtained at lower pH values. The presence of considerable amount of Mo in the alloy causes that the calculated corrosion rates are close to 3 μA cm−2 and the charge transfer resistance after 24 h of exposure reaches the highest value for the coatings deposited at pH 6.0. It was also established, that the Zn–Fe–Mo alloy coating corrosion rate in NaCl solution is limited by cathodic constraints. 5. The corrosion potential values determined for the tested Zn–Fe–Mo alloy coatings regardless the plating bath pH (and thus the content of Fe and Mo in the alloy) are more negative than the corrosion potential of steel, which ensures the cathodic protection of the steel substrate.
Acknowledgements The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education (grant number S50150/ Z0317) for the Department of Advanced Material Technologies within the Faculty of Chemistry of Wrocław University of Science and Technology, Poland.
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.04.073.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The effect of pH of plating bath on electrodeposition and properties of protective ternary Zn–Fe–Mo alloy coatings J. Winiarski a,⁎, A. Leśniewicz b, P. Pohl b, B. Szczygieł a a b
Department of Advanced Material Technologies, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, PL–50370 Wrocław, Poland Analytical Chemistry and Chemical Metallurgy Division, Faculty of Chemistry, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, PL–50370 Wrocław, Poland
a r t i c l e
i n f o
Article history: Received 17 February 2016 Revised 19 April 2016 Accepted in revised form 29 April 2016 Available online 30 April 2016 Keywords: Zn–Fe–Mo alloy coating Electrodeposition X-ray diffraction Anodic dissolution Cyclic voltammetry
a b s t r a c t Ternary Zn–Fe–Mo alloy coatings electrodeposition in a sulphate bath with tri-sodium citrate in the pH range 4.8–6.0 was studied. The increase of the plating bath pH from 4.8 to 6.0 leads to the iron content increase from 1.5 up to 7.9 wt% and it is accompanied by a significant decrease in the current efficiency from 76 to 51%. It was demonstrated that molybdenum co-deposition starts at pH N 4.8, because binary Zn–Fe alloy coating without a significant amount of molybdenum was obtained from the plating solution at pH 4.8. The Mo content increases from 0.8 wt% at pH 5.1 to 2.7 wt% at pH 5.7 and 6.0. X-ray diffraction analysis as well as anodic linear sweep voltammetry measurements showed that within the considered plating solution pH range molybdenum modifies zinc microstructure to a very large extent. The deposited at pH 5.4–6.0 Zn–Fe–Mo coatings contain a solid solution of iron and molybdenum in zinc and probably FeZnx phase, where 6.67 b x b 13. Potentiodynamic polarization revealed that the deposited at pH 5.7 and 6.0 Zn–Fe–Mo coatings are characterized by a significantly lower corrosion rate in 0.5 mol dm−3 NaCl solution. This was confirmed by an electrochemical impedance spectroscopy test, where Zn–Fe(7.9 wt%)–Mo(2.7 wt%) alloy coating reached the highest resistance of charge transfer reaction during 24 hours of exposure to NaCl solution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The protective properties of zinc coatings can be easily improved by co-deposition of zinc with such metals as: cobalt, nickel, iron or manganese [1–3]. Among these elements, iron improves the corrosion resistance of Zn–Fe coatings and it may provide good paintability and weldability [2]. The modification of Zn–Fe alloy coatings performance can be achieved in various ways i.a. by additional conversion coatings [4], by selecting an appropriate bath composition or by changing the deposition parameters, which allows to obtain coatings characterized by different phase composition and morphology [5]. Electrodeposition of zinc composite coatings [6] or phosphorous doping of zinc alloy coatings [7] can be an alternative, however, the effect of dispersed particles on the improvement in corrosion resistance is often debatable. Introduction of a third alloying element, with higher corrosion resistance and better physico-mechanical properties than that of zinc matrix, is another possibility and may allow the elimination of additional harmful chromating. Molybdenum could be one of these elements. It is known from the literature that molybdenum has the positive impact on the passivity of stainless steel. On the other hand, co-deposition of molybdenum with nickel or iron leads to the improvement in microhardness, wear resistance and corrosion resistance of electroplated binary alloy ⁎ Corresponding author. E-mail address: [email protected] (J. Winiarski).
http://dx.doi.org/10.1016/j.surfcoat.2016.04.073 0257-8972/© 2015 Elsevier B.V. All rights reserved.
coatings [1,8,9]. Therefore introduction of molybdenum to zinc coating would be an advantageous option. Although it is possible to obtain binary Zn–Mo alloy coatings [10] with relatively high molybdenum content (from 0.5 to 8 wt%), there are problems with obtaining homogenous deposits characterized by good protective properties. The solution proposed in the current work is a deposition of ternary Zn–Fe–Mo alloy coatings with improved corrosion resistance and better mechanical properties than those of binary Zn–Fe and pure Zn coatings. There are literature reports on deposition of ternary Zn–Ni–Mo and Zn– Co–Mo alloy coatings [1,11–13] from weak acid citrate-sulphate baths, which is a good starting point to take up this research task. It has been shown that Zn–Co–Mo coatings, even with 4 wt% Mo and 6 wt% Co content, are characterized by relatively better physico-mechanical properties than zinc coating [1]. Moreover, the results of our further research [14,15] lead to the conclusion that molybdenum has a beneficial effect on the corrosion resistance of Zn–Co–Mo alloy coatings due to its ability to form oxides on the coating surface. Therefore it was clear that the change of a metal that induces molybdenum co-deposition (iron, instead of cobalt) should also allow to obtain coatings with satisfactory and new useful properties. Unfortunately, apart from the above mentioned works little information about ternary Zn–Fe–Mo alloy coatings can be found in the literature. Our preliminary study on ternary Zn– Fe–Mo alloy coatings [16] allowed us to determine a basic plating bath composition and an appropriate range of plating parameters. It was further observed that deposition of bright and homogeneous coatings was
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possible only in a relatively narrow plating solution pH range, cathodic current density, temperature, and at stable hydrodynamic conditions. All these variables determine the content of alloying elements in Zn– Fe–Mo coating and allow to obtain alloys with predefined chemical and phase composition. There is no doubt that the observed effect of the bath pH on the content of Mo and Fe in the alloy results from a strong influence of this parameter on the type of metal ion complexes present in the plating solution and their equilibria. Due to the fact that acidity of plating solution is a crucial parameter, the aim of the present work was to find important dependences between plating pH and chemical composition, phase structure and protective properties of these ternary alloy coatings. In order to solve the described problem, cyclic voltammetry (CV) and chronopotentiometry were employed to investigate electrodeposition process of ternary Zn–Fe–Mo alloy coatings at different pH values of the plating bath. The changes in morphology and chemical composition of the coatings were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The effect of the plating solution pH on the phase structure of the deposits was investigated by means of anodic linear sweep voltammetry (ALSV) and by X-ray diffraction (XRD). The corrosion resistance of Zn–Fe–Mo alloy coatings in chloride environment was determined by means of potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). 2. Experimental details 2.1. Materials The coatings for SEM, EDXS and XRD analyses were deposited on pure copper discs of a diameter of 14.8 mm, pressed into a Teflon® holder (geometric area of working electrode 1.72 cm2). This was necessary, in order to differentiate the signals that came from the substrate. For CV and ALSV experiments a copper tip was also used (geometric area of working electrode 0.204 cm2). The samples for corrosion measurements were deposited on AISI 1015 steel (nominal composition (wt%): C–0.17; Mn–1.40; P–0.040; N–0.012; Cu–0.55, Fe–balance) in accordance with the expected mechanism of their protective activity. 2.2. Pretreatment of substrates The surface of copper electrodes was polished with successive grades of abrasive paper up to 1200 grit. Then the electrodes were rinsed in an ultrasonic bath, degreased in acetone and activated in a 10 wt% H2SO4 solution for 10 s at room temperature. Each step was preceded by thorough rinse with distilled water. Before CV and ALSV measurements the surface of Cu tip was mechanically polished (1 μm grade diamond powder), degreased in acetone in an ultrasonic bath and activated in a 10 wt% H2SO4 solution. 2.3. Electrodeposition Zn–Fe–Mo alloy coatings were deposited from a citrate-sulphate bath with the composition (mol dm−3): ZnSO4–0.2; FeSO4–0.2; C6H5Na3O7 (tri-sodium citrate) – 0.2; Na2SO4–0.1 and Na2MoO4–0.01. The reference Zn and Fe coatings were deposited from the baths with a similar composition, but without Na2MoO4. Analytical grade chemicals and double distilled water were used for preparation of the solutions. The pH of the baths was adjusted to 5.7 for Zn and Fe plating and to 4.8–6.0 for Zn–Fe–Mo plating using a solution of diluted H2SO4 or NaOH. All the coatings were deposited on a rotating disc electrode (RDE) to ensure constant hydrodynamic conditions. The electrodeposition was performed in a 200 cm3 vessel at the temperature of 25 ± 1 °C under galvanostatic conditions at a cathodic current density jc = − 20 mA cm− 2. The rotational speed (ω) of RDE was set at 800 rpm. The parameters of electroplating were established during preliminary experiments [16].
A platinum plate with a geometric area of 6 cm2 was used as the anode. Deposition time varied between 21 and 37 min taking into account the current efficiency of electroplating. This approach allowed us to obtain coatings with a comparable thickness (~10 μm). The thickness of the coatings was measured by means of gravimetric method and magnetic induction method using a Fischer FMP40 thickness gauge. The current efficiency (η) of the electrodeposition was defined as the amount of coulombs required for the reaction (Qi) divided by the total amount of coulombs passed (Qtotal) according to Eq. (1): η = Qi / Qtotal. Knowing the weight of the deposited alloy, and the content (wt%) of alloying elements, needed to reduce the metals electric charges (Qi) were calculated. The Qtotal was calculated from the quotient of current (I) and time of electrolysis (t) according to Eq. (2): Qtotal = I × t. The general assumption was that each of the metals is reduced on the electrode to its metallic form. 2.4. Testing methods Morphology of the deposits was examined using a VEGA II SBH (TESCAN) scanning electron microscope. Concentration of alloying elements in the deposits were measured using energy dispersive X-ray spectroscopy. Phase composition and structure of Zn–Fe–Mo alloys were analyzed by X-ray diffraction using a Philips X'PERT system with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. The 2 theta angle (2θ) ranged from 3° to 100° at a scan rate of 0.05° per second. Identification of the phases was carried out by comparison of the experimental patterns with the reference patterns stored in the Powder Diffraction Files database (International Centre for Diffraction Data PDF-2 base) and with the experimental patterns of copper substrate and pure zinc coating. Electrochemical tests were carried out using a three-electrode electrochemical cell. A platinum wire was used as a counter electrode. A saturated calomel electrode (SCE) or silver/silver chloride electrode (Ag|AgCl) only for corrosion measurements, both with a single junction Luggin capillary, was applied as the reference electrode. In order to avoid any confusions, all potentials were referred to SCE (EAg|AgCl = −0.042 V vs SCE) throughout the article. Cyclic voltammetry experiments were performed in a suitable amount of plating bath. The CV scan was initiated at open circuit potential (OCP) in the negative direction and reversed at the potential slightly more negative than that for galvanostatic (−20 mA cm−2) deposition in the positive direction up to +0.2 V vs SCE. The scan rate was set to 5 mV s−1. ALSV measurements were performed in a 0.5 mol dm−3 NH4Cl solution at 25 °C. Ammonium chloride was chosen due to its ability to easily form complexes of zinc. Deposits were dissolved by sweeping the potential from OCP and ending at +0.2 V vs SCE with a sweep rate of 1 mV s−1 and at rotational speed of 2000 rpm. Before each CV and ALSV experiment the plating or stripping solution was purged with high purity nitrogen for 15 min. At this time, the gas flow was maintained over the liquid. Corrosion measurements were performed in a non-deaerated 0.5 mol dm−3 NaCl solution with a neutral pH. The geometric area of working electrode and counter electrode exposed to the NaCl solution was 1 cm2 and 1.4 cm2, respectively. The potential measured after 30 min of immersion of the sample in the NaCl solution was adopted as an open circuit potential at which dc measurement was performed. After this time the variation in the potential was smaller than 1 mV min− 1. Potentiodynamic polarization curves were recorded starting at −0.1 V and ending at +0.15 V relative to OCP at a potential change rate of 0.5 mV s−1. Electrochemical impedance spectroscopy (EIS) was performed at a corrosion potential after 24 h of the sample exposure to a non-deaerated 0.5 mol dm− 3 NaCl solution. Impedance spectra were recorded in the frequency range from 10 kHz to 10 mHz, ten points per decade. The amplitude of sinusoidal signal was set to 5 mV. During the EIS measurements the electrochemical cell was kept in a Faraday cage. All polarization measurements were performed using a computer-controlled Reference600 (Gamry) Potentiostat/
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Galvanostat/ZRA. For CV, ALSV and chronopotentiometry measurements correction of IR drop was considered, but it proved to be irrelevant due to the relative high conductivity of the plating solutions. The acquired data were curve fitted and analyzed using the Echem Analyst (Gamry) software. 3. Results and discussion 3.1. The effect of pH on electrodeposition and current efficiency Fig. 1 shows the potential-time dependence for the galvanostatic (− 20 mA cm−2) deposition of Zn–Fe–Mo alloys on the platinum tip at different pH (4.8–6.0). In the first seconds of polarization the rise of potential was observed due to the nucleation process. For longer deposition time, the potential became almost stable which may indicate a uniform growth of the coating. After the nucleation process, the structure of electrode became homogeneous and constant in time. Reactions occurring on the surface of the coating reach a certain state of equilibrium. A pH increase in the plating bath leads to a shift of the deposition potential towards more negative values (Fig. 1). This behavior may be due to the blocking of the cathode surface by metal oxides or hydroxides that arises as a result of strong alkalization of the electrolyte near the cathode surface. Acidity of plating bath is an important parameter in Zn–Fe–Mo alloy deposition, because it also determines the ionic forms of molybdenum [11]. Therefore electrodeposition of ternary Zn–Fe–Mo alloy coatings on the copper electrode at different pH values of plating bath was studied by cyclic voltammetry. In the pH range 4.8–6.0 electrodeposition of coatings takes place at the potentials between − 1.13 V vs SCE and − 1.32 V vs SCE (Fig. 1). Therefore, the reverse potential for each pH was set at slightly more negative value (usual by a tens mV) than that registered during galvanostatic deposition in Fig. 1. From the CV curves presented in Fig. 2 it is evident that with increasing pH of the plating bath up to 6.0 the deposition of the alloy starts at more negative potentials and the current density at the switching potential decreases. This ions in the may be due to the increasing concentration of free MoO2− 4 plating solution. According to [11] in the Zn–Ni–Mo plating electrolytes with pH close to 4.5 molybdenum citrate complexes predominate and no considerable amount of Mo in the alloy was found. On the other hand, at pH close to 5.7 the uncomplexed MoO24 − ions are the main ionic form of molybdenum. Therefore the percentage of Mo in the alloy increases. Based on the literature reports on the mechanism of
Fig. 1. Potential–time dependence for the galvanostatic deposition of ternary Zn–Fe–Mo alloy coatings on the copper electrode at different pH values of plating bath (ω = 800 rpm, T = 25 °C, jc = −20 mA cm−2).
Fig. 2. Cyclic voltammograms for copper electrode in Zn–Fe–Mo plating baths at different pH values. Scan rate 5 mV s−1, ω = 800 rpm, T = 25 °C.
deposition of binary alloy coatings containing an iron group metal and molybdenum, which takes into account formation of the passive layer [17], it can be assumed that if the reaction rate of oxide formation on the cathode may exceed the rate of its further reduction, the surface of the cathode may be blocked. This would explain the changes observed in the course of the cathodic parts of the CV curves in Fig. 2. From the anodic parts of the registered in the plating baths at pH 4.8; 5.1 and 5.4 voltammograms it is evident that a rise in pH leads to the appearance of additional dissolution peaks (Fig. 2). Such changes are probably related to the increase of iron and molybdenum content in the deposits and to the ability of these metals to form intermetallic compounds with zinc or between themselves. At pH 5.4 one can see three, relatively well separated, peaks. Generally, the peak placed at more negative potential (−0.97 V vs SCE) can be associated with the dissolution of zinc from a Zn matrix. The peak recorded at more positive potential (− 0.86 V vs SCE) can be related to dezincification of the zinc-iron phase. The remains of the intermetallic phase should be dissolved as the last components of the alloy coating. This oxidation reaction can be associated with the anodic peak at −0.64 V vs SCE (Fig. 2). According to the proposed above interpretation, the obtained at pH 5.4 coating should be characterized by the most diverse phase composition. It should also be noted, that at the potentials more positive than −0.30 V vs SCE a weak peak appears (graph inset in Fig. 2). This peak is absent at pH 4.8. It appears at pH 5.1, reaches maximum current at pH 5.4 and then decreases at pH 6.0. This anodic peak may be related to oxidation of some residues of the alloy constituents or to other reaction that occurs at the metal/electrolyte interface. To verify these assumptions, the CV measurement was repeated at pH 5.4 (in a fresh portion of plating bath) in the range of potentials from OCP to −1.255 V vs SCE and terminated at −0.5 V vs SCE. Copper tip was subjected to SEM/EDXS observation (Supplementary material 1). Further abrupt increase of the current at the potentials more positive than 0.1 V vs SCE (Fig. 2) was related to the dissolution of copper substrate in the plating baths. An increase of the bath pH above 5.4 causes a significant change in the shape of the recorded CV characteristics. The obtained at pH 5.7 and 6.0 deposits start to dissolve at more positive potentials (approx. − 0.9 V vs SCE) than those observed at lower pH values. The CV curve recorded at pH 5.7 is characterized by only one distinct anodic peak at −0.76 V vs SCE (Fig. 2). This peak is shifted towards more noble values at pH 6.0. It is also clear that for pH 5.7 and 6.0 a very weak dissolution peak appears just behind the main dissolution peak (between − 0.65 and − 0.60 V vs SCE). Its presence can be associated with oxidation of iron from the residue of its phase. This is possible
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due to the relatively high iron content in the alloy and because this peak coincides with that recorded at pH 5.4. Fig. 3 shows selected cathodic parts of the CV voltammograms revealing that an increase in the plating bath pH causes significant changes in electrodeposition process. The shape of the cathodic curves for pH 4.8–5.1 is similar. The abrupt increase in the current density value below −1.08 V vs SCE is caused by hydrogen evolution reaction that overlaps the reduction of metals. The current density increases abruptly at even more negative potentials when the pH value rises. At pH 5.7 and 6.0 an appreciable change in the course of the CV curves appears (Fig. 3). There is a new cathodic peak c1 at approx. −0.6 V vs SCE. The presence of this peak may be related to reduction of MoO2− 4 ions to Mo2O5 or to MoO2 in containing Na2MoO4 citrate electrolytes [18]. Formation of the electrically passive layer of molybdenum oxides is possibly taking into account a further local drop of current density (c1 peak in “pH 6.0” curve in Fig. 3). The changes in pH of the plating bath influenced the current efficiency (η) of deposition process (Fig. 4). Thus η slightly decreases between pH 4.8 and 5.4 from 76 to 72%, while at pH N 5.4 the efficiency decreases rapidly to approx. 50% at pH 6.0. This observation is in agreement with that of previously investigated Zn–Ni–Mo coatings [11]. As it could be expected, the decrease in current efficiency also reduces the deposition rate. In the pH range 4.8–6.0 the deposition rate decreases from 26 to 16.5 μm h−1 (Fig. 4).
Fig. 4. The effect of pH of the plating bath on the current efficiency (η) and deposition rate of Zn–Fe–Mo alloy coatings (T = 25 °C, jc = − 20 mA cm−2, ω = 800 rpm, copper substrate).
3.2. SEM and EDXS analyses Fig. 5 shows SEM microphotographs of the surface of Zn–Fe–Mo alloy coatings deposited at pH 4.8–6.0. The coatings deposited at pH 4.8 and 5.1 (Fig. 5a and 5b) are not homogenous and are characterized by non–uniform thickness and a granular structure. The size of the granules is tens μm. The granules are not dense and look as if they are porous, especially at pH 4.8 (Fig. 5a). Between pH 5.1 (Fig. 5b) and 5.4 (Fig. 5c) a significant change in the coatings morphology takes place. The size of the crystal agglomerations abruptly decreases to a few micrometers, the surface becomes more homogenous and smooth (Fig. 5c). The coating deposited at pH 5.4 represents a flake-like structure, where the size of the flakes amounted to a single micrometers. A homogenous and smooth coating was deposited at 5.7 (Fig. 5d). This coating was free from noticeable defects. Increasing pH to 6.0 causes that the granular structure is more pronounced and the coatings become cracked, probably due to the presence of high tensile stresses inside the coating (Fig. 5e). In addition, numerous large pores were also observed. Their formation can be associated with a significant decrease in the current efficiency and therefore large amounts of evolved hydrogen. The rough structure of the surface of both deposits (Fig. 5d and 5e) is due to the mechanical pretreatment of the copper substrate and this may indicate a weak ability for microsmoothing of the used plating baths. Fig. 6 presents the results of EDXS analysis of ternary Zn–Fe–Mo alloy coatings as a function of the plating bath pH. The analyses were performed pointwise across the sample, therefore, this work presents an average content of iron and molybdenum in the deposits. It was demonstrated that, except zinc and 0.9 wt% Fe, the deposit obtained at pH 4.8 does not contain a considerable amount of molybdenum. Starting from pH 5.1 the molybdenum content increases from 0.8 to 2.7 wt% at pH 5.7 and remains constant up to pH 6.0. The iron content is much higher, i.e. 1.5 wt% at pH 5.1, and it rises to 7.9 wt% at pH 6.0.
3.3. The effect of plating pH on the phase structure of the coatings
Fig. 3. Selected parts of the cathodic CV curves at different pH values. Scan rate 5 mV s−1, ω = 800 rpm, T = 25 °C, copper substrate.
X-ray diffraction of deposited at various pH Zn–Fe–Mo alloy coatings was performed. The pure zinc coating experimental diffractogram corresponds to the metallic zinc standard no 04-0831. The diffraction peaks at 2θ: 39.65°, 43.87°, 54.34°, 71.20°, 82.61° and 87.05° indicate metallic zinc presence (Fig. 7a). There are also peaks that correspond to the copper substrate, moreover, some of them overlap the zinc peaks. Analysis of the experimental diffraction patterns for pure Cu substrate (Fig. 7b) allows to state that the copper XRD pattern is similar to that of the metallic copper standard number 04–0836. Relative intensities of the experimental peaks are comparable to those given in the standard, however the peak positions are slightly shifted, which is a
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Fig. 5. SEM micrographs of ternary Zn–Fe–Mo alloy coatings deposited from citrate-sulphate baths at pH: 4.8 (a), 5.1 (b), 5.4 (c), 5.7 (d) and 6.0 (e). Deposition parameters: jc = −20 mA cm−2, ω = 800 rpm, T = 25 °C, copper substrate.
common phenomenon occurring for differently processed alloys and metals. Analysis of the diffraction patterns of deposited from solutions at different pH Zn–Fe–Mo alloy coatings (Fig. 7c–7g) leads to the conclusion that the relationship between the coating phase composition and the plating bath acidity is obvious. The main XRD peaks corresponding to metallic Zn become less intense and slightly wider with the pH increase. The most intense reflections are present in the XRD pattern for the deposited at pH 4.8 alloy coating. For the other examined coatings a
decrease of the measured signal intensity with an acidity increase is observed. At the diffraction patterns of the deposited at pH ranged from 4.8 to 5.4 Zn–Fe–Mo coating the most intensive peak position is similar to that of the diffractogram of Zn coating (Fig. 7a), however all the characteristic for metallic Zn reflections are shifted towards higher values of the diffraction angle, which can be related to the phase change, i.e. possible formation of new phases or to molybdenum and iron that may incorporate in the crystal lattice of zinc matrix. The presence of a few additional peaks in the diffraction patterns of the deposited from
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Fig. 6. The effect of pH of the plating solution on the composition of ternary Zn–Fe–Mo alloy coatings deposited on copper substrate from citrate-sulphate bath (jc = −20 mA cm−2, ω = 800 rpm, T = 25 °C).
solutions of pH between 5.4 and 6.0 coatings (for example at 2θ: 37.65°, 55.38°, 70.80°, 86.85° and several reflections around 44°), nevertheless not very intensive, indicates an existence of other constituents in the alloy coating. The diffractograms of the coatings deposited from solutions with pH 5.7 and 6.0 (Fig. 7f and 7g) show the absence of several peaks corresponding to zinc coating and copper substrate, which may be caused by differences in Zn–Fe–Mo alloy coating compositions or texture. The peaks corresponding to pure iron or molybdenum were not found in the diffraction patterns, which could be ascribed to the fact that the crystal lattice of Zn matrix was partly replaced by iron or molybdenum and formed solid solution or other phases with zinc. It is known from the literature that the possible phases in electrodeposited Zn–Fe coatings with low iron content (b10 wt%), apart from the solid solution of iron in zinc matrix (namely the η phase), are the ζ (FeZn13) and δ phases (FeZn13–FeZn6.67) [19,20]. Based on the comparison of the available reference patterns of iron–zinc phases (with stoichiometry close to FeZn13) with those recorded for Zn–Fe–Mo coatings it is difficult to definitely confirm the presence of ζ or δ phase. Fig. 8 presents the ALSV voltammograms obtained during dissolution of the deposited at pH 4.8–6.0 ternary Zn–Fe–Mo alloy coatings. In the pH range 4.8 to 5.4 the deposits start to dissolve at similar potentials, which are close to −1.08 V vs SCE. It is also seen that the shape and position of the dissolution peaks is similar for coatings obtained at pH 4.8 and 5.1 while between pH 5.1 and 6.0 they change significantly (Fig. 8). When the pH is close to 4.8 and 5.1 only one dissolution peak is observed. The dissolution peak registered for “pH 5.4” is noticeably wider (Fig. 8). Moreover, the current after reaching the maximum value does not decrease to zero. At approx. −0.62 V vs SCE a small oxidation peak is visible, which could be assigned to zinc from iron–zinc phase oxidation. This assumption can be confirmed in the next voltammogram recorded for the coating deposited at pH 5.7. From Fig. 8 it is clear that dissolution of the coatings deposited at pH 5.7 and 6.0 starts at more noble potentials (−1.02 and −0.92 V vs SCE, respectively). At pH 5.7 there are three, relatively weak distinguishable peaks at −0.91, −0.62 and −0.56 V vs SCE. In view of the fact that this coating contains approx. 4.9 wt% Fe, it seems likely that a considerable amount of zinc may be incorporated in an intermetallic phase FeZnx (where 6.67 b x b 13), as mentioned previously. Therefore, the peaks at −0.91 and − 0.62 V vs SCE may be related to zinc oxidation. So the last one at −0.56 V vs SCE may be due to oxidation of iron. The interpretation of the recorded for the coating deposited at pH 6.0 ALSV voltammogram is difficult, despite the relative range of the potentials at which coatings oxidation occurs is very similar for other pH values. It can only be assumed that the phase composition of deposited at pH 6.0 Zn–Fe–Mo
Fig. 7. X-ray diffraction patterns for reference samples: Zn coating deposited at pH 5.7 (a), Cu substrate (b) and for ternary Zn–Fe–Mo alloy coatings deposited at pH: 4.8 (c), 5.1 (d), 5.4 (e), 5.7 (f) and 6.0 (g).
alloy coating is more complicated. For comparison purposes ALSV voltammograms of the deposited at pH 5.7 Zn and Fe coatings (Fig. 8) were also recorded. Fig. 8 clearly shows that pure Zn coating starts to dissolve at −1.08 V vs SCE and has the main dissolution peak at about −0.82 V vs SCE. During anodic dissolution of Fe coating several dissolution peaks in the potential range from −0.8 to −0.2 V vs SCE were recorded. The presence of multiple peaks indicates that either iron forms several phases, or (more likely) it does not pass directly in an ionic form to a stripping solution (0.5 M NH4Cl). The latter may indicate that oxidation reaction of iron is a multi-step process and a presence of intermediates on the cathode surface is possible. Because of the above doubts, the exact interpretation of the Fe voltammogram is not a subject of this work. An important conclusion is, that under the applied test conditions the oxidation of the Fe in 0.5 M NH4Cl solution takes place, and the registered current response derived from oxidation reaction of iron is
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Fig. 9. Potentiodynamic polarization curves recorded for Zn–Fe–Mo alloy coatings deposited at pH 4.8–6.0 after 30 min of immersion in 0.5 mol dm−3 NaCl solution. Scan rate 0.5 mV s−1. AISI 1015 steel substrate.
Fig. 8. ALSV voltammograms for alloy coatings deposited from citrate-sulphate bath at pH 4.8–6.0 and for Zn and Fe coatings (jc = − 20 mA cm−2, T = 25 °C). Stripping parameters: sweep rate 1 mV s−1, ω = 2000 rpm, T = 25 °C, 0.5 M NH4Cl solution, copper substrate.
Mo in the coatings as well as their morphology). In the case of the deposited at pH 4.8, 5.1 and 5.4 coatings the cathodic reaction takes place without explicit limitations, while limitations are apparent in the anodic oxidation of the alloy (Fig. 9). One would expect that with increasing pH (and thus increasing content of Fe and Mo in the deposits) the corrosion potential (Ecorr) should take more noble values. Between pH 5.1 and 5.4 the content of Fe and Mo in the Zn–Fe–Mo coatings increases, but the corrosion potentials measured for these coatings are almost the same and close to −1.06 V vs SCE. This behavior may be due to differences in coating morphology, different degree of surface development or changes in the phase composition. Increasing the pH from 5.4 to 6.0 is accompanied by a considerable increase in iron and molybdenum content in the alloy. This results in a clear shift of Ecorr towards more positive values by approx. 300 mV. The shift in the corrosion potential also causes changes in the shape of the recorded polarization curves. Despite this the corrosion potentials of Zn–Fe–Mo alloys are still more negative than that of steel (Ecorr = − 0.66 V vs SCE), which proved that these coatings offer cathodic protection to the steel substrate. From Fig. 9 it is evident that the anodic oxidation of the deposited at pH 5.7 and 6.0 Zn–Fe–Mo alloy coatings proceeds without any major constraints. The slopes of the anodic branches are lower than those of the cathodic ones. Due to the fact that in an open air 0.5 mol dm−3 neutral solution of NaCl oxygen reduction (O2 + 2H2O + 4e− ↔ 4OH−) is the main cathodic reaction on zinc surface [21], therefore oxygen reduction is the rate determining step. Because of the diffusion constraints and the differences in the shape of polarization curves, an extrapolation was performed only on the cathodic branches. Corrosion current densities (jcorr), corrosion potentials and slopes of cathodic curves (βc) are shown in Table 1. Corrosion current densities rise initially from 6.0 to
helpful in the interpretation of the other voltammograms for the deposited at pH 5.4 and 5.7 Zn–Fe–Mo coatings.
Table 1 Results of corrosion measurements for samples with Zn–Fe–Mo alloy coatings deposited at different pH values. pH
Ecorr (V vs SCE)
jcorr (μA cm−2)
βc (mV dec−1)
4.8 5.1 5.4 5.7 6.0
−1.04 −1.07 −1.06 −0.80 −0.75
6.0 15.7 18.8 2.9 3.5
−42 −36 −34 −56 −47
3.4. Potentiodynamic polarization measurements Fig. 9 shows the polarization curves recorded for the samples with deposited on AISI 1015 steel at various pH values Zn–Fe–Mo alloy coatings. On the basis of Fig. 9 it is clear, that the shape of the polarization curves is affected by pH of the plating solution (the amount of Fe and
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18.8 μA cm−2 with increasing pH of the plating solution from 4.8 to 5.4. This is probably due to the surface inhomogeneity of the deposits. Above pH 5.4 the coatings become smooth and the corrosion current density decreases markedly. In the plating solution 5.7–6.0 pH range, the deposits contain considerable amounts of molybdenum and iron and are characterized by homogenous structure which results in a clear reduction in the corrosion rate in NaCl solution. The lowest values of jcorr were calculated for the Zn–Fe–Mo coating deposited at pH 5.7 (2.9 μA cm−2) and 6.0 (3.5 μA cm−2). 3.5. Electrochemical impedance spectroscopy experiment The results of dc polarization measurements revealed differences in the corrosion resistance of the investigated zinc alloy coatings depending on the plating bath pH. Therefore EIS was employed to indicate the differences in the coating performance after longer exposure to a corrosive solution. The impedance plots in the Bode and Nyquist representation (Fig. 10a and 10b) are presented here only for selected exposure time (24 h) and samples, for better clarity of the figures. The representation of the phase angle clearly exhibits two time constants for all of the tested coatings (Fig. 10a). The Bode plot shows that as a result of the increased Fe and Mo content, the shape of the impedance plots changes. The impedance modulus at low frequency (| Z |0.01 Hz) shows higher values for Zn–Fe–Mo coating deposited at pH 6.0 (about 2.4 kΩ cm2) than that for deposited at pH 4.8 (about 1.8 kΩ cm2) and at pH 5.4 (about 1.3 kΩ cm2). This is also visible in the Nyquist plot (Fig. 10b). For the all investigated coatings it can be assumed that the highfrequency time constant is due to the formation of a porous layer of corrosion products while the low-frequency time constant could be related to the corrosion process. This interpretation is consistent with the previous analysis of the electrochemical behavior of Zn–Co–Mo and Zn– Co alloy coatings on steel [14]. The experimental impedance spectra were fitted using the presented in Fig. 11 electric equivalent circuit. In this model, Rs corresponds to the resistance of NaCl solution. Rpore and constant phase element (CPE1) may be assigned to the resistance of the electrolyte in the pores of the corrosion products layer and the pseudo-capacitance of the porous corrosion products layer. Rct is the charge transfer resistance and CPE2 is the double layer pseudocapacitance. The impedance of the CPE is well known and it was previously described [14]. An analysis of the Nyquist plot shape (Fig. 10b) and the calculated values of CPE parameters presented in Table 2 (n – an exponent of a pseudo-capacitance) revealed that CPE does not reflect the pure capacitance and thus the surface is not homogenous. That is why it was necessary to use CPE instead of a capacitor. This electric
Fig. 11. Electric equivalent circuit used for fitting the experimental impedance spectra of Zn–Fe–Mo alloy coatings after 24 h exposure of the samples to a non-deaerated 0.5 mol dm−3 NaCl solution (AISI 1015 steel substrate).
equivalent circuit ensured a good fit to the experimental data (χ2 in the order of 10−3 - 10−4 and the fitting lines in Fig. 10a and 10b). The residual errors of each of the parameters were as follows: 0.4–0.5% for Rs, 1.8–7.9% for CPE1-Y0, 1.8–9.4% for CPE1-n, 1.0–16% for Rpore, 0.4– 4.6% for CPE2-Y0, 1.2–2.4% for CPE2-n and 1.5–3.6% for Rct. According to Table 2 the corrosion potentials after 24 h exposure are more noble for the coatings deposited at higher pH, but still more negative than that of steel substrate. Rpore for the deposited at pH 4.8 and 6.0 coatings are almost the same, but more than six times lower than that of the deposited at pH 5.4 coating. This may be due to the coatings nonuniformity and different structure of the passive film. After the values of parameters Y0 and n of CPE and the corresponding resistance (R) are calculated from the equivalent circuit, the capacitance could be simply calculated using Eq. (3): C = [(Y0 R)1/n] / R [22]. According to the data presented in Table 2 and Eq. (3) the capacitance of porous corrosion products layer (Cpore) and the capacitance of the electric double layer (Cdl) were calculated. The Cpore values for the deposited at pH 4.8; 5.4 and 6.0 coatings are as follows: 111; 248 and 147 μF cm2. Because capacitance depends on dielectric constant, permittivity of free space and thickness of the passive layer, thus the observed increase in the coating capacitance may be an evidence of a decrease in the film thickness or an increase in the dielectric constant (due to the increase in the porosity of the corrosion products layer or due to defects appearing in the alloy coating). Cdl values do not exhibit such a variation, as they are between 1.79 and 1.99 mF cm2. It is noteworthy that charge transfer resistance for the deposited at pH 6.0 coating is higher (3.4 kΩ cm2) than those of the other coatings (Table 2) which proved better corrosion resistance of the coatings with higher Fe and Mo content. 4. Conclusions In summary, the following conclusions can be drawn from the presented results:
Fig. 10. Impedance spectra in the Bode (a) and Nyquist (b) representation of zinc alloy coatings deposited at pH 4.8; 5.4 and 6.0 recorded after 24 h exposure of the samples to a nondeaerated 0.5 mol dm−3 NaCl solution (AISI 1015 steel substrate). Solid lines represent the calculated spectra.
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Table 2 Equivalent circuit parameters determined by fitting the impedance spectra of samples with Zn–Fe–Mo alloy coatings deposited at pH 4.8; 5.4 and 6.0 after 24 h of immersion in 0.5 mol dm−3 NaCl solution. pH
Ecorr (V vs SCE)
Rs (Ω cm2)
CPE1-Y0 (Ω−1 cm−2 s−n)
CPE1-n
Rcoat (Ω cm2)
CPE2-Y0 (Ω−1 cm−2 s−n)
CPE2-n
Rct (Ω cm2)
4.8 5.4 6.0
−0.979 −0.965 −0.760
25 22 22
3.50 × 10−4 4.61 × 10−4 6.46 × 10−4
0.76 0.73 0.68
75 404 67
1.24 × 10−3 1.64 × 10−3 1.31 × 10−3
0.65 0.82 0.78
1939 903 3364
1. From a bath containing: ZnSO4, FeSO4, C6H5Na3O7, Na2SO4 and Na2MoO4 ternary Zn–Fe–Mo alloy coatings containing considerable amounts of molybdenum in the pH range 5.4–6.0 can be deposited. However, deposition of bright and homogenous coatings is possible only at pH close to 5.7. 2. Chemical composition of the coatings changes significantly between pH 4.8 and 6.0. In the above mentioned pH range the Zn–Fe–Mo coatings contain from 0.8 to 2.7 wt% of molybdenum and from 1.5 to 7.9 wt% of iron. Co-deposition of Mo causes an increase of the Fe amount in the coating. An increase in the Mo and Fe content leads to the shift of corrosion potential towards more noble potentials up to −0.75 V vs SCE. 3. With an increase in the plating solution pH the microstructure of Zn– Fe–Mo alloy coatings differs from the structure of the zinc coating even more. Molybdenum and iron were not detected in their pure form in the investigated coatings. Because of a high content of iron in the deposited at pH 5.4–6.0 alloys an intermetallic phase with a stoichiometry close to FeZnx (where 6.67 b x b 13) may appear. 4. Deposited at pH 5.7–6.0 Zn–Fe–Mo coatings are characterized by higher corrosion resistance in NaCl solution than those of the coatings obtained at lower pH values. The presence of considerable amount of Mo in the alloy causes that the calculated corrosion rates are close to 3 μA cm−2 and the charge transfer resistance after 24 h of exposure reaches the highest value for the coatings deposited at pH 6.0. It was also established, that the Zn–Fe–Mo alloy coating corrosion rate in NaCl solution is limited by cathodic constraints. 5. The corrosion potential values determined for the tested Zn–Fe–Mo alloy coatings regardless the plating bath pH (and thus the content of Fe and Mo in the alloy) are more negative than the corrosion potential of steel, which ensures the cathodic protection of the steel substrate.
Acknowledgements The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education (grant number S50150/ Z0317) for the Department of Advanced Material Technologies within the Faculty of Chemistry of Wrocław University of Science and Technology, Poland.
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.04.073.
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