Corrosion Science 53 (2011) 2861–2871

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Corrosion Science

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Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

M. Bucko a, J. Rogan a, S.I. Stevanovic b, A. Peric-Grujic a, J.B. Bajat a,⇑a Faculty of Technology and Metallurgy, University of Belgrade, P.O. Box 3503, YU-11120 Belgrade, Serbiab ICTM – IEC, P.O. Box 473, 11001 Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 11 November 2010Accepted 6 May 2011Available online 14 May 2011

Keywords:A. ZincA. AlloyB. AFMB. EISB. XRDC. Electrodeposited films

0010-938X/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.corsci.2011.05.039

⇑ Corresponding author. Tel.: +381 11 3303 694; faE-mail address: jela@tmf.bg.ac.rs (J.B. Bajat).

a b s t r a c t

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings,deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotom-etry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemicalimpedance spectroscopy (EIS) and measurement of corrosion potential (Ecorr). XRD analysis disclosed thatzinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm�3

NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloycoating deposited at c.d. of 30 mA cm�2 as compared to that deposited at 80 mA cm�2.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Electrodeposited coatings of zinc and its alloys, particularly Zn–Fe, Zn–Co and Zn–Ni, are widely used for sacrificial protection offerrous materials in industry nowadays. Several authors have re-ported that, in aggressive environments where sodium chlorideand sulphur dioxide are present, Zn–Mn alloy coatings can showeven better corrosion resistance [1–3]. Zn–Mn coatings, thoughbeing less thermodynamically noble than Zn, show a passivatingbehaviour in such environments due to the formation of insolublecompounds on the corroding surface. Depending on aggressivemedia, various compounds were found in passive layer, such asoxides MnO, Mn0.98O2, Mn5O8 [4] and c-Mn2O3 [3], or basic saltslike Zn4(OH)6SO4�xH2O [5] and Zn5(OH)8Cl2�H2O [4].

Zn–Mn layers can be electrodeposited from various electrolyticsolutions, such as simple sulphate solution [2], a sulphate solutioncontaining citrates [1], a chloride solution [6], a fluorborate solu-tion [7], an acidic solution containing EDTA [8]. However, deposi-tion of these alloys often has important drawbacks, for examplein the terms of low current efficiency [8] and solution stability[1]. Literature data report attempts by earlier authors to obtainZn–Mn electrodeposits from pyrophosphate solution [9]. However,the first successful attempt to deposit Zn–Mn alloy from this kindof solution was made only a few years ago [10]. The main differ-ence between pyrophosphate and other plating solutions is thatpyrophosphate solution is alkaline, with pH higher than 9. It is wellknown that alkaline Zn plating solutions have several advantages

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x: +381 11 3370 387.

over the acidic ones, among which the most important is highercorrosion resistance of Zn deposits [11].

Sylla et al. have established the influence of deposition potentialon some properties of Zn–Mn coatings electrodeposited from pyro-phosphate solution [10], such as chemical and phase compositionand morphology. They found that the change of deposition poten-tial in cathodic direction significantly influences morphology of thedeposits, since it increases the rate of hydrogen evolution reactioni.e. the formation of H2 bubbles on the cathodic surface. However,corrosion resistance of Zn–Mn coatings deposited from pyrophos-phate solution remains undisclosed. The objective of the presentpaper was to study the influence of deposition c.d. on corrosionbehaviour of Zn–Mn alloys obtained from pyrophosphate solution,i.e. to reveal the relation of phase composition and morphology ofdeposits, with their corrosion resistance. The protective property ofthe coatings was assessed by recording the free corrosion potentialas a function of time in corrosive media, and by electrochemicalimpedance spectroscopy. The metallic coatings were characterizedusing various techniques, specifically atomic force microscopy,atomic absorption spectrophotometry and X-ray diffraction.

2. Experimental procedures

2.1. Material

Electrodeposition of Zn–Mn alloys was performed galvanostat-ically, at current densities of 10–140 mA cm�2, on steel panels.Deposition times were managed to obtain layers with 10 lmthickness.

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The composition of the plating solution was as follows:1 mol dm�3 K4P2O7 + 0.017 mol dm�3 ascorbic acid + 0.05 moldm�3 ZnCl2 + 0.05 mol dm�3 MnCl2�4H2O. Pyrophosphate ions, usedas complexing agent, reduce the difference between zinc(II) andmanganese(II) deposition potentials and hinder precipitation of Znand Mn hydroxides, although pH of the solution is 9.3 [10]. All solu-tions were made from analytical grade chemicals (Sigma–Aldrich)and double distilled water.

The electrodeposition process was carried out in a classicalthree-electrode glass cell, at the temperature of 20 ± 2 �C in aeratedconditions without stirring. Working electrodes were steel panels.Prior to each experiment, the steel surfaces were abraded succes-sively with emery papers of the following grades: 600, 1000,1200 and then degreased in a saturated solution of NaOH in etha-nol, pickled with a HCl 50% by volume for 30 s and finally rinsedwith distilled water. Cylindrical Zn foil (high purity zinc) placedclose to the cell walls was used as a counter electrode, providinggood current distribution in the cell, since working electrode wasplaced in the middle of the cell. The reference electrode was a sat-urated calomel electrode (SCE) in all measurements.

2.2. Recording of corrosion potential

The corrosion behaviour of the Zn–Mn coatings was first stud-ied by following the Ecorr during the exposure to aerated0.5 mol dm�3 NaCl solution, until the Ecorr of steel substrate wasreached. Steel panels (20 � 20 mm) were used as substrate for al-loy coating deposition. The deposit thickness was 10 lm, deter-mined by Coating Thickness Measuring Instrument DUALSCOPEMPOR. The 10 lm thick pure zinc coatings, deposited at variousc.ds, were tested also, for comparison. The Zn coatings were ob-tained from an electrolytic bath with the following composition:3 mol dm�3 KCl + 0.42 mol dm�3 H3BO3 + 0.45 mol dm�3 ZnCl2.The Zn coating deposited at 30 mA cm�2 was homogeneous,smooth and bright and showed the best corrosion resistanceamong Zn coatings tested, so results for this Zn deposit are pre-sented for comparison.

2.3. Electrochemical impedance spectroscopy measurements

All EIS tests were carried out in aerated 0.5 mol dm�3 NaCl solu-tion at room temperature (20 ± 2 �C). The 0.5 mol dm�3 NaCl solu-tion is appropriate as a model medium for corrosion investigations,due to the presence of corrosion activators (chloride ions). Theelectrolyte was prepared with analytical grade chemicals and dou-ble distilled water. The three electrode system was employed withthe testing sample (1 cm2 area) as working electrode, Pt panel asauxiliary electrode and SCE as reference electrode. EIS of the sam-ples was measured after 2, 24, 48 and 72 h of immersion in themodel solution. EIS tests were performed using a potentiostatZRA Reference 600, Gamry Instruments. The impedance spectrawere recorded in the frequency range between 100 and 0.01 Hz,at 10 frequency points per decade, with 10 mV amplitude perturb-ing signal. EIS 300 software was used for the fitting.

2.4. Characterization of surfaces

Chemical composition of electrodeposited Zn–Mn alloys wasdetermined by atomic absorption spectrophotometry, using atom-ic spectrophotometer PYE Unicam SP9, Philips.

The morphology of the deposits was examined by optical micro-scope Olympus CX 41, with microscope camera Olympus UC 30. Inadition, atomic force microscopy (AFM) was used, with NanoScope3D (Veeco, USA), microscope operated in tapping mode underambient conditions. In AFM, etched silicon probes with spring con-stant 20–80 Nm�1 were used.

Structural analysis of the electrodeposited Zn–Mn films and ofthe corrosion products that formed on the coatings after 48 h in0.5 mol dm�3 NaCl solution, was made by X-ray powder diffraction(XRD) on Ital Structure APD2000 diffractometer in a Bragg–Brentano geometry using CuKa radiation (k = 0.15418 nm) andstep-scan mode (range: 30�–90� 2h, step-time: 0.50 s, step-width:0.02�). The program PowderCell [12] was used for an approximatephase analysis. Unit cell parameters were obtained by the least-squares method using the program LSUCRIPC [13].

3. Results

3.1. Structural properties of the Zn–Mn coatings

Fig. 1 shows micrographs, obtained by optical microscope, ofthe coatings deposited at various c.ds from pyrophosphate solu-tion. Adherent Zn–Mn deposits of good quality were generally ob-tained at c.ds from 30 to 120 mA cm�2. Deposits obtained at c.dslower than 30 mA cm�2, do not equally cover entire steel surface,even for long deposition times, so protective behaviour of Zn–Mncoating cannot be accomplished. Light areas on the sample depos-ited at 10 mA cm�2 (Fig. 1a) represent areas with poorer substratecoverage. On the other hand, deposits obtained at c.ds higher than120 mA cm�2 (Fig. 1d) have non-homogenous appearance and fur-thermore, after achieving thickness of a few micrometres, they donot adhere well to the substrate. Optical microscopy revealed thatcoatings with the best appearance were deposited at c.ds in therange of 30–80 mA cm�2 (Figs. 1b and c). So, for further investiga-tion, two coatings were chosen, namely those deposited at 30 and80 mA cm�2, in order to elucidate the dependence of structure,morphology and corrosion resistance of the coatings, on depositionc.d.

The phase diagram of Zn–Mn alloys is very complex, showing18 different phases [14]. However, the structure of electrodepos-ited Zn–Mn coatings is much simpler, commonly showing up tofour different phases, depending on the plating solution composi-tion and deposition parameters [15]. Usually, the observed phasesessentially depend on the manganese content in the alloy [8].

XRD patterns of the Zn–Mn coatings deposited from pyrophos-phate solution at 30 and 80 mA cm�2 are shown in Fig. 2. Manga-nese contents in these alloys, determined by atomic absorptionspectrophotometry, are 4.7 and 11.6 at.%, respectively. The XRDpatterns of both deposits show the same mixture of the two crys-talline phases: e-ZnMn phase and g-ZnMn phase, both having hex-agonal close packed (HCP) structure, space group P63/mmc. Theobtained unit cell parameters for sample deposited at 30 mA cm�2

are a = 0.2669 and c = 0.4928 nm (ICSD 643930). The unit cellparameters for sample deposited at 80 mA cm�2 are a = 0.2753and c = 0.4430 nm (ICSD 105018). For both examined samples,axial ratios and solubility limits correspond to the mixture ofHCP g- and e-phase in the Zn–Mn system [12]. The differencesin the unit cell parameters are the result of different amounts ofMn in each of the phases.

The g-phase is a solid solution of less than 1 mass% of Mn in Zn[16]. According to the phase diagram for Zn–Mn alloys, e-phase isstable at temperatures higher than 200 �C, between approximately11 and 58 at.% Mn. This means that the electrodeposition processproduces a metastable, nonequilibrium structure. However,e-phase was observed to be the main component of the electrode-posited Zn–Mn layers for wide range of Mn contents (15–45 at.%),obtained independently in various electrolytic solutions [8,15,17].Taking into account that the stoichiometry of e-phase is MnZn3

(25 at.% Mn) [16], it is understandable that Zn–Mn coatings inves-tigated in this work and having 4.7 and 11.6 at.% Mn, presentthe mixtures of e-phase and some other phase with lower Mn

Fig. 1. Micrographs of Zn–Mn alloys deposited at: (a) 10; (b) 30; (c) 80 and (d) 140 mA cm�2.

Fig. 2. XRD patterns of Zn–Mn coatings electrodeposited at 30 and 80 mA cm�2

from pyrophosphate solution.

M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871 2863

percentage, i.e. in this case g-phase. It can be concluded that thecontent of e-phase increases with the increase in Mn percentage,so in the Zn–Mn alloy deposited at 30 mA cm�2 the main phaseis g-phase, whereas the alloy deposited at 80 mA cm�2 is madeup mostly of the e-phase.

The grain size of the coatings was determined from X-ray dif-fraction patterns. The grain sizes of both investigated depositswere calculated using the angular width of the (0 0 2) peak at itsFull-width at Half Maximum (FWHM) and Scherrer formula [18]given by Eq. (1):

d ¼ 0:9kbcosh

ð1Þ

where k, b and h are the wavelength of Cu-Ka (0.15418 nm), theintegral width and the diffraction angle, respectively. The pseudo-Voigt function was used for curve fitting analysis required forFWHM determination by the program PowderCell [12]. Instrumen-tal line broadening was measured using a milled single crystal of c-Bi2O3. The results obtained indicate that the coatings are made up ofnanocrystalline grains, with the size of 29 nm for Zn–Mn alloydeposited at 30 mA cm�2 and 19 nm for alloy deposited at80 mA cm�2.

3.2. Morphology of the deposits

The surface morphology studies were based on AFM measure-ments. Typical 3D images, representing the influence of depositionc.d. on the surface topography of Zn–Mn coatings, are presented inFigs. 3 and 4.

The surface topography investigations on the micrometre level(5 lm � 5 lm scanned surface area) have shown that both coat-ings consisted of sub-micron sized granular agglomerations(Fig. 3). It is clearly visible that Zn–Mn coating deposited at30 mA cm�2 is more compact and has a more homogeneous struc-ture (more uniform agglomeration size). Besides, coating depositedat 80 mA cm�2 has cavities about 400 nm wide, with the shape ofchutes, where the coating is significantly thinner. The presence ofthese chutes could be related to the fact that when depositionc.d. is so high, hydrogen bubbles are evolved from the cathode sur-face to a great extent and they disturb the deposition of uniformlayer. Sylla et al. [10] observed similar influence of hydrogen bub-bles on the morphology of Zn–Mn coatings obtained from pyro-phosphate solution at higher deposition c.ds.

The ultrafine-grained morphology was determined from the500 nm � 500 nm scanned surface area. It can be observed fromthe corresponding AFM images (Fig. 4) that sub-micron sized crys-tallite agglomerations of both plated Zn–Mn coatings, presented inFig. 3, include a number of smaller particles. XRD measurementsshowed that the average crystalline grain size of both depositswas not greater than 29 nm. AFM images in Fig. 4 show particlesbigger than 29 nm, so it can be concluded that the shapes observed

Fig. 3. AFM images of Zn–Mn coatings deposited from pyrophosphate solution at: (a) 30 and (b) 80 mA cm�2. The scanned area is 25 lm2.

Fig. 4. AFM images of Zn–Mn coatings deposited from pyrophosphate solution at: (a) 30 and (b) 80 mA cm�2. The scanned area is 0.25 lm2.

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in AFM images present agglomerations of a few nanocrystallinegrains. The size of these agglomerations was determined fromAFM image profile line analysis for both samples. The measure-ment was based on the assumption that the surface irregularitiesindicated by the profile line, represented the agglomerate dimen-sion. An example of the profile line on both Zn–Mn samples, depos-ited at 30 and 80 mA cm�2, is presented in Fig. 5. Fig. 6 shows thedistribution of agglomeration size for both coatings examined.Agglomeration sizes were determined from AFM image by ran-domly chosen 100 agglomerations on a surface of 5 � 5 lm. Itcould be seen from Fig. 6 that the majority of the agglomerationsof the coating deposited at 80 mA cm�2 is approximately twice asbig as the majority of the agglomerations of the coating depositedat 30 mA cm�2. Based on the results presented in Figs. 3–6 it couldbe concluded that smaller crystalline grains, deposited at80 mA cm�2, tend to form bigger agglomerates, which furthermerge in clusters, and result in a more coarse morphology.

Observations about the surface topography of the deposits atnanometre level can be explained on the basis of the electrocrys-tallization theory. It is postulated that with the increase in deposi-tion c.d., because of the rise in the overpotential, free energy for the

formation of new nuclei in Zn–Mn alloy deposits increases, andthat leads to higher nucleation rate and hence formation of coat-ings with smaller grain size [19]. However, after reaching somecritical value of c.d., further increase in c.d. results in formationof discrete large agglomerates. The formation of big and discreteagglomerates is attributed to the fast growth of grains and agglom-eration on the nuclei as compared to the nucleation rate [19,20].This is the probable reason for the appearance of discrete largeclusters at the surface of the Zn–Mn deposit obtained at80 mA cm�2 (Fig. 4b), which further caused the coarsening of thedeposit.

3.3. Recording of Ecorr

Sacrificial protection ability of the Zn–Mn coatings on steel sub-strate was evaluated by following of immersion potentials of thesamples. Fig. 7 shows plots of Ecorr vs. time, for Zn–Mn deposits ob-tained at 30 and 80 mA cm�2, as well as for pure Zn coating, forcomparison. The three samples of each coating were monitoredsimultaneously, and Ecorr values recorded were in the range of±30 mV. The average Ecorr values are presented in Fig. 7. The Ecorr

Fig. 5. Profile lines obtained from AFM images of Zn–Mn coatings deposited at 30 and 80 mA cm�2. The scanned area is 0.25 lm2. Markers on profile line correspond to themarkers on agglomerate borders.

Fig. 6. Agglomeration size distribution of Zn–Mn coatings electrodeposited at: (a)30 and (b) 80 mA cm�2.

M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871 2865

of bare steel substrate in 0.5 mol dm�3 NaCl solution is�640 mVSCE. The Ecorr values of both examined Zn–Mn alloys aremore negative than the Ecorr of steel substrate, so Zn–Mn depositsoffer sacrificial cathodic protection. As can be seen, time taken toreach the Ecorr of steel substrate was much longer for both exam-ined Zn–Mn alloy coatings, as compared to the pure Zn coating.This result clearly proves that pyrophosphate alkaline solutioncan be successfully used for the electrodeposition of Zn–Mn alloysin order to increase the corrosion stability of Zn coatings, so it jus-tifies further efforts in research of deposition of Zn–Mn coatingsfrom this solution.

However, Fig. 7 reveals that there is a significant difference incorrosion behaviour between Zn–Mn coatings deposited at differ-ent c.ds. Initial Ecorr values for Zn–Mn samples deposited at 30and 80 mA cm�2, are �1072 and �1089 mVSCE, respectively. Inthe initial period of 24 h of immersion, Ecorr values for both sam-ples reach the value of approximately �1060 mVSCE. After thispoint, plateau is achieved, i.e. the value of Ecorr is almost constant,which discloses that some protective layer is probably formed oncorroding sample [4]. As Fig. 7 shows, protective layer on Zn–Mnsample deposited at 30 mA cm�2 withstood appreciably longertime (140 h), than on the sample deposited at 80 mA cm�2 (80 h).XRD analysis, as well as EIS, was used in order to investigate theobserved influence of current density on the corrosion stability ofthese two coatings. The abrupt change of the immersion potentialin positive direction, for prolonged immersion times, indicates thatcontact of the electrolyte with Fe-substrate is made through possi-ble pinholes of the Zn–Mn electrodeposits, which means that pro-tective layer on Zn–Mn coating is disturbed and degradation ofcoating and substrate occurs.

3.4. Characterization of corrosion products

The XRD patterns for Zn–Mn alloys deposited at 30 and80 mA cm�2, after exposure to 0.5 mol dm�3 NaCl solution during

Fig. 7. The dependence of Ecorr in 0.5 mol dm�3 NaCl solution on time, for Zn–Mnalloys deposited on steel at 30 and 80 mA cm�2, and for pure Zn (10 lm thickness).

2866 M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871

48 h at corrosion potential, are presented in Figs. 8a and b, respec-tively. The samples are composed of the substrate a-Fe and g- ande-Zn–Mn phases, indicating that the alloy coatings have not dis-solved completely during the testing period. In addition, the exis-tence of the zinc hydroxide chloride, Zn5(OH)8Cl2�H2O(ZHC), asthe only corrosion product, was confirmed in both cases. This com-pound has a hexagonal lattice and may be regarded as a doublesalt, 4Zn(OH)2�ZnCl2�H2O, with very low solubility product of10�14.2 mol dm�3 [21]. At higher chloride concentrations and near

Fig. 8. XRD patterns of Zn–Mn coatings deposited at: (a) 30; (b) 80 mA cm�2,obtained after 48 h of immersion in 0.5 mol dm�3 NaCl solution.

neutral acidities of corrosion media a stability regime for ZHC ex-ists, as shown by the thermodynamic diagram for the chloride–zinc–water system (Fig. 9) [22].

According to the many literature data, Zn corrosion in NaClsolution occurs with the formation of mixture of zinc oxide, zinchydroxide chloride and zinc hydroxide carbonate [4,23,24]. Thismixture of corrosion products possesses a porous structure, sothe real passivation of Zn cannot be accomplished [25,26]. On theother hand, in recent papers, ZHC has been reported as the maincorrosion product on Zn–Mn alloy coatings treated in chloride cor-rosion media, and furthermore, due to its low solubility and com-pactness, it was shown to present an efficient protective passivelayer [21]. Boshkov et al. [21] have explained the role of Mn inthe formation of ZHC on Zn–Mn alloy surface: since manganeseis electrically more negative element than zinc, it dissolves firstas Mn2+ ions, causing evolution of hydrogen and consequently aslight increase of the pH value:

Mn!Mn2þ þ 2e ð2Þ

2Hþ þ 2e! H2 ð3Þ

Zn! Zn2þ þ 2e ð4Þ

After the total dissolution of manganese from the surface layerof the coating, in which Mn atoms are distributed randomly andhomogenously, the process continues with the dissolution of thezinc (Eq. (4)). Because of the higher pH value in the near-surfaceregion, Zn2+ ions react immediately with the medium, accordingto the following formal equation, and ZHC forms.

5Zn2þ þ 8OH� þ 2Cl� ! Zn5ðOHÞ8Cl2ðsÞ ð5Þ

When intensities of diffraction maxima related to corrosion-treated Zn–Mn alloys are compared (Fig. 8), it can be observed thatafter 48 h of exposure the intensities of the lines corresponding tothe mixture of g-Zn–Mn + e-Zn–Mn phase were reduced moreconsiderably for Zn–Mn alloy deposited at 80 mA cm�2. This sug-gests that this alloy corrodes more rapidly and that protective ac-tion of ZHC is lower as compared to the alloy deposited at30 mA cm�2. Fig. 8 shows similar intensities of the lines corre-sponding to the ZHC at both Zn–Mn samples. Therefore, it was sug-gested that lower protective action of ZHC on sample deposited at80 mA cm�2 is due to its higher porosity as compared to the sam-ple deposited at 30 mA cm�2.

Fig. 9. Stability domains for ZHC in aerated aqueous solutions with varying chloridecontent and pH values, at 25 �C and with concentration of zinc ionic species of0.1 mol dm�3 [22].

M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871 2867

3.5. Electrochemical impedance spectroscopy

The Nyquist impedance plots and Bode plots for Zn–Mn coatingdeposited at 30 mA cm�2, as a function of immersion time in0.5 mol dm�3 NaCl solution, are shown in Fig. 10. It seems thatthe Nyquist curves are composed of one depressed semicircle atmiddle frequencies (Fig. 10a). However, Bode plots show clearlythat there are two time constants present in the impedance spec-tra, which are independent of the immersion time. The first timeconstant is observed at middle frequencies, and the second at thehighest frequency values, as shown in Fig. 10b. The two time con-

Fig. 10. (a) Nyquist plot, (b) Bode plot (phase angle vs. log f) and (c) Bode plot (logmodulus Z vs. log f) for Zn–Mn alloy deposited at 30 mA cm�2, after 2, 24, 48 and72 h of immersion in 0.5 mol dm�3 NaCl solution.

stants at high and medium frequency could be associated withpore resistance of corrosion product film and charge transfer resis-tance, respectively. Several authors had found similar results for Znin the chloride containing media [27,28].

The shape of impedance plots at high and medium frequenciesis similar for all immersion times. Contrary, at low frequency val-ues, the shape of Nyquist plots changes rapidly during immersionperiod, i.e. the extension of middle-frequency semicircle loop isdifferent for various immersion times. For immersion time of 2 h,an inductive loop can be clearly seen in Nyquist plot. For longertimes of immersion in chloride media, the inductive loop disap-pears, but it can also be seen that the middle-frequency loop in Ny-quist plots does not have the shape of regular semicircle, related tosimple Randles type time constant. Contrary, the shape of this loopat low frequencies reveals the coupling of semicircle loop with aline related to diffusion processes.

The Nyquist and Bode plots for Zn–Mn alloy deposited at80 mA cm�2 are given in Fig. 11. The impedance of this coatingshows similar features to that deposited at 30 mA cm�2, i.e. twotime constants at high and medium frequencies, where the middlefrequency semicircle is coupled with inductive loop for shorterimmersion time, and with the shape related to diffusion for longerimmersion times in corrosive media. It should be noted that forcoating deposited at 80 mA cm�2, the inductive loop is still presenton plots even after 24 h of immersion.

3.5.1. Inductive behaviour at short immersion timeImmediately after the electrodeposition of the coatings and dry-

ing in the air, the samples were immersed in 0.5 mol dm�3 NaClsolution. Consequently, at the beginning of EIS measurements, bareZn–Mn alloy surface was exposed to corrosive media, covered lo-cally with thin layer of ZnO/Zn(OH)2 or Mn-oxides, as a result ofatmospheric corrosion of Zn–Mn alloys in a short period [29,30].The appearance of inductive loop in EIS plots for short immersiontime shows that the surface was not stable during the recording ofimpedance response, i.e. the dissolution of Zn–Mn alloy has oc-curred. As it was described above, manganese dissolves first, asMn2+ ions, and after its dissolution from the surface layer of thecoating, the process continues with the dissolution of the zinc.According to the fact that Zn content in the Zn–Mn alloys is muchhigher than Mn content, the inductive loop is probably related tothe anodic reaction of zinc. A model describing the process of dis-solution of pure zinc and various Zn materials in NaCl was pro-posed in the literature [31,32] and it is shown in Eqs. (6)–(11).According to this model, three intermediate species, Znþad, Zn2þ

ad

and ZnOHad, are adsorbed at the interface, and they are character-ized by three inductive loops in EIS diagram. Hamlaoui et al.showed that the first inductive loop (the loop at the highest fre-quency among three inductive loops) is related to the potentialrelaxation of the Znþad adsorbate, the second loop to Zn2þ

ad and thevery low frequency inductive loop to ZnOHad [32]. In this work,one inductive loop was observed in Nyquist plots for short immer-sion times.

Path 1

Zn ¡ eþ Znþad ð6Þ

Znþad ! eþ Zn2þad ð7Þ

Zn2þad ! Zn2þ

sol ð8Þ

Path 2

Zn ¢ eþ Znþad ð6Þ

Znþ Znþad ! 2eþ Znþad þ Zn2þsol ð9Þ

Fig. 11. (a) Nyquist plot, (b) Bode plot (phase angle vs. log f) and (c) Bode plot (logmodulus Z vs. log f) for Zn–Mn alloy deposited at 80 mA cm�2, after 2, 24, 48 and72 h of immersion in 0.5 mol dm�3 NaCl solution.

Fig. 12. A schematic model showing dependence of oxygen reduction rate on thethickness of corrosion product layer, during immersion of Zn–Mn coated specimensin 0.5 mol dm�3 NaCl.

2868 M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871

(Znþad acts as a catalyst of the zinc dissolution in reaction (9) [31].)Path 3

ZnþH2O! ZnOHad þ 1=2H2 ð10Þ

ZnOHad ! eþ ZnOHþsol ð11Þ

With the increase of immersion time, the inductive loop on EISplots was suppressed by development of protective surface layeron Zn–Mn alloys. However, the disappearance of the inductive loopwas much faster for coating deposited at lower c.d., which meansthat the layer with ability to inhibit dissolution of Zn is formedon this alloy more easily.

It can be concluded from the impedance responses that duringthe initial stage of corrosion, dissolution of alloy and fast oxygen

reduction occur on both samples. A schematic model referring tothis case is shown in the upper part of Fig. 12. The model proposedin Fig. 12 is based on the model that refers to the corrosion of Zn–coating, given by A. Yadav [33].

3.5.2. Diffusion control of process in prolonged immersionThe XRD analysis of corrosion product films for both deposited

Zn–Mn alloys, after 48 h in the testing solution, indicated that themain corrosion product on the coatings was zinc hydroxide chlo-ride (ZHC) layer. According to EIS plots, it was assumed that ZHClayer was porous, and a schematic model of the cross-section ofZn/porous film/solution system is shown in the lower part ofFig. 12. For the metal covered with a porous film, the anodic disso-lution is mainly limited in the pores. The anodic reaction of Zn dis-solution was shown in Eq. (4) and cathodic reduction of oxygen isgiven by:

O2 þ 2H2Oþ 4e! 4OH� ð12Þ

ZHC is known as insoluble compound that provides protectionto the Zn–Mn alloy surface [34], so the oxygen reduction on surfaceof ZHC layer can be greatly inhibited. As a result, cathodic oxygenreaction mainly occurs on bare metal, by oxygen diffusion throughthe tiny pores of ZHC, and this process may control the overall cor-rosion process.

Therefore, the diffusion line observed as extension of the middlefrequency semicircle loop at Nyquist plots for prolonged period incorrosive solution, is associated with the diffusion of oxygen tothe bare surface, as well as with diffusion of Zn and Mn ions fromcorroding alloy surface, through the pores of ZHC layer. Instead ofa straight line superimposed at 45� to both axes, which is typicalimpedance response of semi-infinite diffusion, Nyquist plots in thiswork showed that at the lowest frequencies there was a relaxationof the impedance towards the real axis. This occurs when the thick-ness of the diffusion layer is restricted to a finite length, i.e. in caseswhere one boundary imposes a fixed concentration for the diffusingspecies. Element related to the finite length diffusion is an excellentmodel for representation of oxygen diffusion through the thin pro-tective layer [35]. The dispersion relation for this element containsa cotangent-hyperbolic function in the admittance representation:

Y ¼ Y0ðjxÞ1=2 coth ½B jxð Þ1=2� ð13Þ

The parameters Y0 and B contain the thickness of the thin layer andthe diffusion coefficient [35].

3.5.3. Fitting of the impedance dataBased on the above discussion and according to the schematic

models of corrosion processes shown in Fig. 12, an impedance

M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871 2869

equivalent circuits are proposed for the EIS of Zn–Mn alloys in thetesting solution, as shown in Figs. 13a for short immersion timeand 13b for longer immersion time. Instead of a pure capacitance,the constant phase element (CPE), defined as Q, is used for theequivalent circuit in this study. CPE is given by

Q ¼ Y0ðjxÞn ð14Þ

Elements used in the equivalent circuit are as follows: Rs repre-sents the electrolyte resistance; Rf represents the electrolyte resis-tance in the pores of corrosion product, because it is necessary totake into account that the concentration of the species involvedin the reaction can differ in the pores from the bulk concentration;Qf represents the capacitance of porous corrosion product layer; Rct

represents the charge transfer resistance which is associated withboth the anodic reaction (Eq. (4)) and cathodic reaction (Eq. (12)),and Qdl represents double layer capacitance; Zw is the porousbounded Warburg impedance for finite-length diffusion.

Although Nyquist impedance plots show only one inductiveloop, the best fitting of data related to the inductive behaviourwas achieved with parallel combination of two branches Ra + La

and Rb + Lb in the equivalent circuit, as it is depicted in Fig. 13a. Pe-ter et al. explained in detail that this equivalent circuit is related tothe dissolution mechanism that involves two adsorbed intermedi-ates present at the same time on the electrode surface [36]. SinceZn dissolution can occur through three parallel paths, where eventhree different intermediates can be adsorbed at the same time, themodel proposed in Fig. 13a has appropriate physical meaning.

According to the equivalent circuit model, all measured imped-ance values were fitted. The calculated parameters for Zn–Mn coat-ings deposited at 30 and 80 mA cm�2 are listed in Tables 1 and 2,respectively. Goodness of fitting was at high level. The standarddeviations were in the order of 10�4, and the relative error for eachparameter was less than 5%. In Figs. 10 and 11, measured impedancediagrams are denoted with scattered symbols and fitted diagramswith lines.

The calculated parameters in Tables 1 and 2 indicate that thevalue of fractional exponent n of constant phase element Qf forboth samples is close to 1, proving that Qf possesses physicalmeaning of capacitance of corrosion product layer and it is directlyproportional to dielectric constant of layer and inversely propor-tional to its thickness. The Y0 value of Qf decreases significantlyafter 24 h of immersion tests for both samples, showing that thefilm of ZHC with some protective properties is formed. The slow in-crease in pore resistance, Rf, during the immersion test, and partic-ularly after 24 h, indicates that the number and the width of the

Fig. 13. Equivalent electrical circuits of corroding Zn–Mn alloys in 0.5 mol dm�3

NaCl testing solution: (a) for short immersion time, when inductive behaviour ispresent and (b) for longer immersion time, without inductive behaviour.

pores decreased, i.e. ZHC layer became less porous on both coat-ings. Higher pore resistance can block both the anodic and cathodicreactions on bare surface at the bottom of the pores. As can be seenfrom Tables 1 and 2, change in Qf and Rf values was followed by theincrease in charge transfer resistance, due to the blocking effect ofZHC film. When the two investigated alloy coatings are compared,it can be seen that for the coating deposited at 30 mA cm�2, valuesof Qf are lower and values of Rf are higher as compared to the coat-ing deposited at 80 mA cm�2. These values show that for the coat-ing deposited at lower c.d., the ZHC layer is formed in a shortertime and it is less porous than the layer on the coating depositedat higher c.d.

The use of Zw in fitting of EIS data for all immersion times, is jus-tified since the corrosion rate of zinc coatings is purely diffusioncontrolled at, or near, the corrosion potential [33]. In order to con-firm this assumption, Warburg element was added to the equiva-lent circuit used for fitting the impedance results at allimmersion times. In all cases, introduction of Warburg element re-sulted in better goodness of fitting. The Warburg admittance ofboth investigated Zn–Mn coatings decreased as the immersiontime increased, showing lower diffusion of reactant speciesthrough the pores of corrosion product. Therefore, the corrosionproduct layer developed a barrier against corrosion. This can beattributed to the precipitation of the corrosion product thoroughpores as immersion time increases. The values of parameter Y0 re-lated to porous bounded Warburg admittance, for longer immer-sion times (48 and 72 h), are lower for alloy coating deposited at30 mA cm�2 (Table 1) than for alloy coating deposited at80 mA cm�2 (Table 2). Higher Warburg impedance in longerimmersion times show that inhibiting effect on mass transport ofreactive species is more expressed for the coating deposited at30 mA cm�2, and this is another prove that protective layer on thiscoating is less porous.

As a conclusion, EIS measurements confirmed that protectivesurface layer was formed on Zn–Mn alloys deposited from pyro-phosphate solution during immersion in 0.5 mol dm�3 NaCl media.However, faster formation of protective layer, which is also lessporous, was on the alloy coating deposited at 30 mA cm�2, as com-pared to the alloy deposited at 80 mA cm�2. Consequently, coatingdeposited at lower c.d. corrodes with higher charge transfer resis-tance for corrosion reactions and with more expressed inhibitingaction of diffusion processes.

3.5.4. Impedance diagrams for pure Zn coatingThe EIS tests were also performed for pure Zn coating, in order

to confirm conclusions arisen from corrosion potential measure-ments, related to the comparison of corrosion resistances of pureZn coating and Zn–Mn alloy coatings. The Nyquist impedance plotsfor Zn coating, as a function of immersion time in 0.5 mol dm�3

NaCl solution, are shown in Fig. 14. One semicircle appears at highand middle frequencies, and it was followed by the second semicir-cle or ill-defined tail at low frequencies. Similarly to the case withZn–Mn alloy coatings, the first semicircle could be attributed to thecharge transfer reaction, associated with the coupled effect of theionic double layer capacitance and corrosion products film capaci-tance. On the other hand, the low frequency loop probably arisesfrom the charge transfer intermediate adsorption for immersiontime of 2 h and a finite thickness layer diffusion process for longerimmersion times [33,34]. It can be seen from Figs. 10a, 11a and 14that the magnitude of the impedance diagram for pure Zn coatingis significantly smaller as compared to the Zn–Mn alloy coatings.This confirms that the layer of corrosion products on Zn–Mn alloysprovides better protection of the substrate, as compared to thepure Zn coating.

Table 1Optimum fit parameters for Zn–Mn alloy deposited at 30 mA cm�2, depending on the immersion time.

Immersion time (h) Rs (X) Rf (X cm2) Qf Rct (X cm2) Qdl Zw

Y0 (10�6 S cm�2 sn) nf Y0 (10�6 S cm�2 sn) ndl Y0/10�3 (S cm�2 s1/2) B (s½)

2 12 12.3 54.6 0.85 461 616 0.83 818 0.724 11 14.6 51.0 0.91 756 1840 0.58 194 2.848 8 16.7 42.4 0.81 821 5225 0.64 27 2.272 14 24.9 29.4 0.83 1092 3540 0.69 22 3.5

Table 2Optimum fit parameters for Zn–Mn alloy deposited at 80 mA cm�2, depending on the immersion time.

Immersion time (h) Rs (X) Rf (X cm2) Qf Rct (X cm2) Qdl Zw

Y0 (10�6 S cm�2 sn) nf Y0 (10�6 S cm�2 sn) ndl Y0 (10�3 S cm�2 s½) B (s½)

2 14 12.6 52.6 0.98 315 340 0.74 196 1.224 13 13.6 51.6 0.98 543 3096 0.68 88 1.148 14 14.5 46.0 0.79 550 2470 0.69 62 3.872 13 15.9 39.2 0.89 753 2890 0.66 49 2.6

Fig. 14. Nyquist plot for Zn coating, after 2, 24, 48 and 72 h of immersion in0.5 mol dm�3 NaCl solution.

2870 M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871

4. Discussion

The Ecorr and EIS measurements, as well as the XRD analysis, re-vealed that Zn–Mn coating deposited at 30 mA cm�2 from pyro-phosphate solution, had significantly higher corrosion stability ascompared to the coating deposited from the same solution, at80 mA cm�2. The origin of its higher stability is easier formationof ZHC layer on the coating surface, which is of less porous struc-ture and plays the role of a pseudo-passive layer.

Better corrosion behaviour of Zn–Mn coatings, compared topure Zn coatings, in some aggressive environments, is ascribed tothe formation of passive layer on the coating, the process in whichmanganese plays significant role [3,21]. It was reported that Mncontent in the alloy, as well as alloy’s phase composition, deter-mined whether compact passive layer would be formed on theoverall coating surface area or not [22]. In the present study, bothinvestigated coatings are composed of the same two phases, i.e. ofg- and e-ZnMn phases, and the only difference among the coatingsis in the relative content of the phases. It is clear that for coatingdeposited at 30 mA cm�2, Mn content of 4.7 at.% is distributedmainly in g-phase, but even small amounts of e-ZnMn phase aresufficient for obtaining protective ZHC layer on the coating surface.The ZHC layer should form more easily on the coating composedmainly of e-ZnMn phase, which is richer in manganese. However,this was not the case for the coating deposited at 80 mA cm�2.Therefore, the change in the relative content of alloy phases, ob-

tained by two different deposition c.ds, was not a sufficient reasonfor the change in corrosion behaviour of the coatings.

The AFM analysis showed significant difference in surface mor-phology among the two investigated Zn–Mn coatings. As deposi-tion c.d. increased from 30 to 80 mA cm�2, the averagecrystalline size, determined by XRD technique, decreased from 29to 19 nm, but the size of crystalline agglomerates, determined byAFM image profile line analysis, increased. This could be a signifi-cant factor for higher corrosion resistance of the coating depositedat 30 mA cm�2, because higher grain boundary density in the coat-ing with smaller agglomeration size increases the rate of the for-mation of a stable and protective passive film [37]. Furthermore,discrete large clusters, as well as cavities where the coating wasthinner, probably related to the fast hydrogen evolution, appearedat the coating deposited at 80 mA cm�2. It seems that these surfaceirregularities have a negative effect on the stability of a ZHC pas-sive film at the surface of this coating. Investigations by otherauthors, on Zn coatings obtained with pulse electrodeposition,established the possibility that surface morphology is the main fac-tor determining corrosion behaviour of Zn deposits, especially inthe cases when corrosion stability depends on the stability of apassive film at the surface [19].

In order to improve the corrosion resistance of Zn–Mn coatingsobtained from pyrophosphate solution at higher c.ds (for example80 mA cm�2 and higher), some additives should be used, in orderto prevent hydrogen evolution reaction and enable deposition ofsmooth surfaces with homogeneous grain distribution.

5. Conclusions

The effect of deposition c.d. on corrosion behaviour of Zn–Mncoatings obtained from pyrophosphate solution has been studiedby electrochemical measurements, coupled with AFM investiga-tions and XRD analysis. The following conclusions could be made.

1. Results from Ecorr measurements of samples immersed in NaClsolution revealed that Zn–Mn coatings deposited from pyro-phosphate solution have improved corrosion resistance, ascompared with pure Zn coating.

2. The increased corrosion stability of Zn–Mn coatings was theconsequence of the passive layer formation, made up of ZHCcompound. However, deposition c.d. showed appreciable influ-ence on corrosion resistance of plated Zn–Mn coatings, in thecourse that coating obtained at higher c.d. showed lower corro-sion stability.

M. Bucko et al. / Corrosion Science 53 (2011) 2861–2871 2871

3. The increase in deposition c.d. resulted in the increase of Mncontent in Zn–Mn alloy coatings. The change in alloy phase’scontent was also observed. The main phase in the coatingdeposited at lower c.d. was g- phase, with a small amount ofe-ZnMn phase. The same two phases were present in the alloydeposited at higher c.d., but with the opposite relative contents.However, the change in the phase composition was not recog-nized as the main factor determining different corrosion behav-iour of Zn–Mn coatings investigated in this work.

4. The Zn–Mn alloy deposited at 30 mA cm�2 had 4.7 at.% Mn,which was sufficient to enable the formation of protectiveZHC layer on the corroded surface. Although the Zn–Mn coatingdeposited at 80 mA cm�2 contained even more manganese, theZHC layer on this coating was more porous. This was clearlyshown with EIS technique (lower Rf and Rct values). The appear-ance of discrete large crystalline agglomerations, as well as cav-ities related to the hydrogen evolution during theelectrodeposition process, were recognized as significant fac-tors that impeded the formation of passive ZHC layer at thecoating deposited at higher c.d. Zones on the surface whereZHC layer could not be formed, act as pores in protective layer,allowing penetration of electrolyte.

Acknowledgement

This research was financed by the Ministry of Education andScience Republic of Serbia, Contract No. III 45019.

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