Effects of Zinc Surface Acid-Based Properties on ... - ACS Publications

Article pubs.acs.org/JPCC

Effects of Zinc Surface Acid-Based Properties on Formation Mechanisms and Interfacial Bonding Properties of Zirconium-Based Conversion Layers P. Taheri,†,‡ K. Lill,§ J.H.W. de Wit,†,‡ J.M.C. Mol,‡ and H. Terryn*,†,∥ †

Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands § Henkel AG & Co. KGaA, Henkelstr. 67, 40191 Dusseldorf, Germany ∥ Vrije Universiteit Brussel, Department of Electrochemical and Surface Engineering, Pleinlaan 2, B-1050 Brussels, Belgium ‡

ABSTRACT: This study investigates the surface characteristics and deposition procedure of zirconium-based conversion layers on pure zinc substrates. The topography and composition of a set of pure zinc samples treated in alkaline, neutral, and acid solutions before and after deposition of the conversion layers were evaluated using X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). Additionally, the depth profiles of the elements across the thickness of conversion layers as well as the deposition kinetics were characterized by means of Auger Election Spectroscopy (AES) depth profiling and recording the Open Circuit Potential (OCP), respectively. The acid-based properties of the obtained conversion layers were probed by adsorption of succinic acid molecules. The results revealed that the surface composition and deposition procedure of conversion layers strongly depend on the initial surface composition and roughness. Additionally, the adsorption of the succinic molecules was found to correlate to the surface hydroxyl fraction on the conversion layers, which in turn varied with the initial hydroxyl fraction on the based zinc substrates.

1. INTRODUCTION Before an organic coating is applied, metal surfaces are often subjected to a conversion treatment to improve the corrosion resistance and adhesive properties of the overlying organic coating. Chromate and phosphate-containing conversion layers are commonly used for this purpose. However, they are being increasingly replaced with various alternatives because of several health, environmental, energy, and process disadvantages.1−4 Substantial recent research has been focused on the development of alternative chromate-free conversion layers. Application of zirconium oxide on metal surfaces is a promising treatment technique that came into view as a potential replacement for the traditional treatments.5,6 The zirconium (Zr)-based pretreatment showed a good corrosion protection performance and an adhesion promotion on a variety of metallic substrates.7−10 The zirconium-based conversion layers have been applied successfully for galvanized steels as well.11,12 The metal substrate is also subjected to surface pretreatments prior to the application of the organic coatings to clean the surface and/or obtain the desirable surface composition for an improved coating adhesion.13−15 However, the studies showed that the effectiveness of conversion layers may rely heavily on the initial metal surface properties.16−19 Walmsley et al.20 showed that the preferential nucleation of the zirconium−titanium oxide film and its growth occurred on and around intermetallic particles, resulting in reduced cathodic activity of the particles. Verdier et al.21 revealed that the film nucleation more likely occurs on the © 2012 American Chemical Society

cathodic parts of the surface. Additionally, the surface roughness has been found to affect the deposition procedure of Zr-based conversion layers.12,22 The electron donation between the metal surface and the top polymer coating plays a major role in interfacial reactions and consequently in (de)adhesion properties of the polymer coating systems.23−25 Additionally, due to the diffusion of water, oxygen, and aggressive species through the coating, corrosion occurs at the interface, leading to the failure of interfacial bonding initiated by cathodic delamination, which is a localized electrochemical process.26,27 To determine the characteristics of the oxide surface, an understanding of the acid−base properties of the reactive sites is required. Since self-assembling molecules (SAMs) of similar functional groups have similar pKa values, they can be used to predict trends in acidity or basicity.28 Due to the presence of −COOH functional groups in succinic acid ((CH2)2(COOH)2), they can be used as probe molecules to characterize the acid−base nature of the active sites on metal surfaces.29−31 In this work, the depth profiling and deposition kinetics of the formed Zr-based conversion layers on differently pretreated zinc samples are evaluated by means of Auger Electron Spectroscopy (AES) and Open Circuit Potential (OCP) Received: September 29, 2011 Revised: February 25, 2012 Published: March 19, 2012 8426

dx.doi.org/10.1021/jp209422d | J. Phys. Chem. C 2012, 116, 8426−8436

The Journal of Physical Chemistry C

Article

scans were recorded with 29.35 eV pass energy and 0.1 eV step size. The measurements were done at 45° takeoff angles with respect to the sample surface. The spectra obtained on the untreated pure oxide layers were shifted to set the C−C/C−H components of the C 1s peak at a binding energy of 284.8 eV to correct the sample charging.32 Auger Electron Spectroscopy (AES) measurements were performed using a PHI 650 SAM with a LaB6 cathode. A cylindrical mirror analyzer (CMA) and a 5 keV ion-gun (PHI 04303) were used. The base pressure in the analysis chamber was 3 × 10−10 Torr. A primary electron beam with energy of 5 keV and a current of 1 μA was incident on the sample surface at 30° to the normal. Ion sputtering was performed to obtain the depth profile. The surface was rastered using a 2 keV Ar ion beam over an area of 4 × 4 mm2. The emission current was 20 mA, and Ar pressure was 10 mPa. The ion beam was used at intervals of 1 min and impinged on the sample surface at 50° to the normal. The recorded spectra were analyzed with Multipak 6.1A software. To evaluate the bonding characteristics of the adsorbed components on the metallic samples, a Thermo-nicolet Nexus FTIR apparatus was used equipped with a mercury-3 cadmium− telluride liquid-nitrogen cooled detector and a nitrogen-purged measurement chamber. The FTIR measurements of the adsorbents were taken by reflection of the incident beam at an angle of incidence of 80° using p-polarized radiation. The FTIR measurement of the sample with the adsorbed molecules was conducted versus the backgrounds collected from the bare zinc sample.

measurements. The surface composition of the differently pretreated zinc samples with and without conversion layers is assessed by X-ray Photoelectron Spectroscopy (XPS). Moreover, the local acid−base properties are characterized by deposition of succinic acid molecules and evaluation of the bonding characteristics by means of Fourier Transform Infrared Spectroscopy (FTIR) and XPS. Additionally, Atomic Force Microscopy (AFM) is utilized in this study to establish a correlation between the surface topographical aspects and the deposition mode of the Zr-based conversion layer.

2. EXPERIMENTAL SECTION The substrate used in this work was a commercially pure zinc sheet (99.95%) supplied by Goodfellow. The samples were mechanically grinded with SiC paper in subsequent steps and then polished to different grade diamond paste (9, 6, 3, 1, and 0.025 μm). Consequently, the samples were cleaned ultrasonically in ethanol and water for two minutes. Afterward, they were dried under a stream of compressed nitrogen gas. The zinc substrates were pretreated in different conditions according to the experimental parameters summarized in Table 1. A set of Table 1. Experimental Parameters Used for the Pretreatments of the Zinc Substrate sample code sample sample sample sample sample

1 2 3 4 5

solution

pH

temperature (°C)

potential (V)

duration (Min)

0.05 M HCl deionized water 1 M Na2CO3 deionized water 0.2 M H3BO3 + 0.1 M NaOH

5.4 7.0 11.5 7.0 12.3

25 65 25 25 25

0.8

30 30 30 30 30

3. RESULTS AND DISCUSSION 3.1. Surface Composition and Topography of the Untreated Pure and Differently Pretreated Zinc Samples. To assess the surface composition, XPS measurements were performed on differently pretreated samples. Figure 1

differently pretreated samples was subjected to the treatment in an aqueous hexafluorozirconic acid conversion bath (pH = 4.0) for 90 s at room temperature. Then, they were rinsed with deionized water and dried with compressed nitrogen gas. Subsequently, the adsorption of succinic acid molecules was conducted on the conversion layers formed on differently pretreated samples. Tetrahydrofuran (THF) including 0.1 wt % of succinic acid was used as the supportive solution for the adsorption procedure. The molecules were adsorbed for 30 min, and then the samples were rinsed by THF for 5 s to remove the nonadsorbed molecules. The OCP measurements were conducted in a conventional three-electrode cell using an EG&G 273 potentiostat. The reference was a saturated calomel electrode (SCE), connected to the main compartment through a salt bridge. All the potentials given in the present work refer to this electrode. A flat platinum plate was utilized as counter electrode (CE), and the zinc sample was used as the working electrode. Tapping mode AFM measurements were performed with a NanoWizardII atomic force microscope (JPK Instruments, Germany) using a silicon cantilever CSC37A (Mikromasch, Estonia) with a nominal force/spring constant of 0.65 N/m. The AFM images were obtained on the samples at room temperature and open air. The rms roughness of the studied samples was analyzed by JPK image processing software v.3. Version 3.3.25. To characterize the surface composition of the samples, XPS analysis was conducted with a PHI 1600/3057 instrument using incident X-ray radiation (Mg Kα1,2 = 1253.6 eV). The vacuum pressure was approximately 5 × 10−9 Torr. Narrow multiplex

Figure 1. XPS, O 1s peak fitting of the untreated pure zinc substrate.

shows the XPS, O 1s peak fitting of the untreated pure zinc sample. It can be seen that the O 1s peak is resolved into three subpeaks located around 530, 531.5, and 533 eV. They can be due to the contribution of O2−, OH−, and COx components, respectively.33 The surface hydroxyl fraction can be obtained from the O 1s peak fitting after a subtraction of the surface contamination contribution. The correction of the hydroxyl fraction was done according the procedure described by Wielant et al.34 Figure 2 shows the hydroxyl fraction on the untreated pure and differently pretreated zinc samples as determined after the 8427



Article

acquired for the conversion layer applied on the untreated pure zinc sample is shown in Figure 4a. It can be seen that the initially large carbon signal decays to the near background after a few nanometers. The large superficial carbon content comes from the surface contamination formed on the surface. Furthermore, it can be seen that O and Zr are the major constituents of the outer regions of the coating with an underlying Zn material. The oxygen profile increases initially, reaching a maximum at 10 nm and decaying to background values after 30 nm. The oxygen at the outer regions mainly is expected to be associated with the oxides and water of hydration. The increase of Zr concentration together with the O in the film materials shows that Zr mainly is present in the form of oxide/hydroxide. It is evident that the Zn signal starts from a low concentration level and increases monotonically to a depth of 90 nm. After this depth, the Zn signal approaches a steady-state value representative of pure Zn. It is significant that the increase in Zn signal occurs within the Zr-rich area. In this case, it is probable that the local alkaline environment etches the zinc substrate matrix, which leaves the zinc substrate protruding into the conversion layer.20 It therefore can be inferred that in a deeper area the layer is built up by a mixture of Zn−Zr oxides. Figure 4a also shows that F is also detected within and underneath the Zr-coated layer. The fluoride concentration in the film is low compared to the oxygen, suggesting that zirconium is mostly present as an oxide rather than fluoride. However, it is known that during film formation the presence of free fluoride ions dissolves the native zinc oxide,12 leading to metal dissolution and formation of Zn2+, which in turn leads to formation of the complex presented by the following reaction

Figure 2. Hydroxyl (OH) fraction obtained from O 1s and C 1s peak deconvolutions of the untreated pure and differently pretreated zinc samples.

correction of hydroxyl fraction. It can be seen that the hydroxyl fraction increases gradually from sample 1 to 5. The low portion of hydroxyl fraction formed on sample 1 can be explained by the low pH (5.4) of 0.05 M HCl solution used for the pretreatment. On the other hand, the relatively high hydroxyl fraction formed on samples 3 and 5 can be ascribed to the alkaline solutions used for the pretreatments. It can be seen that the hydroxyl fraction detected on the sample pretreated in water at 25 °C is considerably more than that of the sample pretreated in water at 65 °C. It is noticeable that temperature plays a substantial role in the oxide/hydroxyl formation on zinc surfaces. Kotnik35 and Gilbert36 reported that zinc hydroxyl is usually stable in cold water (0−30 °C), whereas in hot water (30−90 °C) the surface product is mainly zinc oxide. Figure 3 shows the AFM topographies (50 × 50 μm) and surface roughnesses of the untreated pure and differently pretreated zinc samples. A comparison of the images shows that the samples are subjected to a nanoscaled topographical rearrangement after the pretreatments. It can be seen that the samples 1 and 2 that were treated in acid solution and hot water, respectively, present a porous surface compared to that of untreated pure sample. This is expected to originate from the presence of hexagonal layers of a ZnO wurzite-type structure on these samples.37 The rougher surface obtained for sample 2 compared to that of sample 1, regardless of a lower oxide portion on sample 2, can be correlated to the higher pretreatment temperature for sample 2. In this case, the high pretreatment temperature might lead to a higher growth rate of the wurzitestructured oxides and consequently a higher surface roughness. On the other hand, the alkaline pretreated samples show less porosity on the surface. The decrease in the surface roughness from the samples with a low hydroxyl fraction toward those with a high hydroxyl fraction can be due to the dissolution of ZnO as zinc oxides transform into zinc hydroxyls. Valtiner et al.37 proved the possibility to create smooth single-crystalline surface topographies as a result of a kinetic stabilization of certain surface structures within the alkaline medium. 3.2. Surface Composition and Film Formation Mechanism of Zr-Based Conversion Layers on the Untreated Pure and Differently Pretreated Zinc Samples. Auger depth profiling of the formed conversion layers was performed on differently pretreated samples. The spectrum

3Zn 2 + + ZrF62 − → 3ZnF2 + Zr4 +

(1)

17

Lunder et al. showed that the formation of the complex presumably is accompanied by oxygen reduction and hydrogen evolution O2 + 2H2O + 4e− → 4OH−

(2)

2H+ + 2e− → H2

(3)

Figure 4b shows the fluoride depth profiling across the differently pretreated zinc samples. It is evident that the fluoride intensity decreases mildly across the depth. Although fluoride is supposed to remove the zinc compounds,16 a part of the formed components may remain in the formed layer during the Zr deposition procedure, which leaves fluorine on the samples. It can be seen that the concentration of fluoride is high on samples 4 and 5, which possess high hydroxyl fractions. This means that the fluoride is rather reactive with zinc hydroxyls compared to the oxides. Moreover, the high fluoride concentration on sample 2 presenting a high surface roughness shows that the surface roughness plays an important role in formation of fluoride compounds within the conversion layer. This supports the study conducted by Karlsson et al.22 who proved the effects of surface roughness on fluoride concentration in the Zr conversion layer. Figure 4c presents the zirconium depth profiles obtained for differently pretreated zinc samples. It can be seen that the Zr-based conversion layer has reacted to different extents on the differently pretreated samples. A comparison of Figures 4b and c reveals a decrease in zirconium concentration in the film accompanied by an increase of fluorine concentration. This suggests that there is competition between zirconium deposition and surface fluorination in which the fluoride 8428



Article

Figure 3. AFM topography of the (a) untreated pure zinc sample, (b) sample 1, (c) sample 2, (d) sample 3, (e) sample 4, (f) sample 5, and (g) surface roughnesses.

hinders the zinc surface from deposition of the zirconium compounds. To explain the competition, four hypotheses are

reported. First, since the fluorides remove base metal compounds, the zirconium deposition may be prohibited by 8429



Article

was estimated. Figure 5 shows the thickness of zirconium oxide films formed on the samples. It can be seen that the thickness of

Figure 5. Thickness of the Zr-based conversion layers formed on the untreated pure and differently pretreated samples (the hydroxyl fraction increases from sample 1 to 5).

the conversion layers yields almost a parabolic gradient with the hydroxyl fraction, meaning that the zirconium mainly reacted with the hydroxyl. Nordlien et al.20 and Andreatta et al.16 showed that the cathodic areas are the preferential Zr deposition sites. Consequently, it can be assumed that the hydroxyls possessing a passive behavior in aqueous solution are the favored sites for Zr deposition. However, the thickness of the Zr layer formed on sample 2, which has a relatively low hydroxyl fraction (OH = 16.78%), reaches 87 nm. This, together with the large variation in the conversion film thickness, i.e., between 50 and 87 nm, can be explained by the rough surface of sample 2 as shown in Figure 3c. Lunder et al.17 showed that the nucleation and growth of Zr-based conversion layers are stimulated on the rough surfaces. The mechanism of the Zr deposition on the hydroxyl fraction as the favored site may raise the assumption that during the film formation the free fluoride ions dissolve the native zinc hydroxide and cause Zn metal dissolution as represented by the following reaction Zn(OH)2 + x F− → ZnFx(2 − x) + 2OH−

(4)

This reaction increases the interfacial pH locally, and the alkalinization favors the precipitation of zirconium, which is feasible only in a high pH range.39,40,42 Consequently, the precipitation of the Zr-containing oxide conversion film may occur according to the following equation17

Figure 4. Auger depth profiling of the formed conversion layers: (a) Zr, Zn, F, O, and C profiles on the untreated pure zinc, (b) F, and (c) Zr profiles on differently pretreated samples.

ZrF62 − + 4H2O → ZrO2 ·H2O + 6HF + O2 −

the fluorination. Second, fluoride ions may be adsorbed at the sample surface, preventing zirconium from reaching the surface.38 Third, fluoride ions also can form stable complexes with zirconium ions, which decrease their reactivity with respect to the sample surface.39,40 Fourth, fluoride ions have a detrimental effect on film formation by inhibiting the deposition reaction on the hydroxyl ions liberated at the interface.41 In the later case, substitution of the hydroxyl groups at the metal surface by fluoride has been shown to have no detrimental effect on the Zr film formation. From the quantitative AES depth profiles, the thickness of the conversion films formed on differently pretreated samples

(5)

The film formation on the hydroxyl is expected to be limited for progressive growth since it is a self-extinguishing process and the deposition of the conversion layer is restricted to the near vicinity of the hydroxyl. Consequently, the Zr layer forming process implies substantial variations in the extent of deposition on differently pretreated samples as shown in Figure 5. This mechanistic process is basically different from other conversion methods such as chromating, which involves redox reactions between the chromate ions and the metal surface.43−45 To evaluate the deposition kinetic of the Zr-based conversion layers, the open circuit potential (OCP) was monitored during an extended deposition period (0−300 s). Figure 6 shows the 8430



Article

The existence of the plateau region in the OCP regime suggests that the surface is substantially covered by the conversion layer after about 150 s of immersion, resulting in a depletion of reactive hydroxyl sites and driving force for growth of the conversion layer. This proves the above assumption that the film growth is limited as the deposition of the Zr-based conversion layer is a self-extinguishing process. 3.3. Surface Topography, Composition, and Acid− Base Properties of the Formed Zr-Based Conversion Layers on the Untreated Pure and Differently Pretreated Zinc Samples. Figure 7 shows AFM topographies (50 × 50 μm) and surface roughnesses of the untreated pure and differently pretreated zinc samples after deposition of the Zr-based conversion layers. The results show again that a nanostructured surface was obtained on the surfaces after the conversion treatment. It can be seen that the surface roughness of all samples except samples 2 and 5 became lower than the original value after the Zr deposition. It can be due to the fact that the deposited layer virtually fills the cavities of the based substrate. Additionally, the clusters formed on these samples appear to be fine in size. This can be related to the fact that the hydroxyls as the deposition sites are homogenously distributed over the surface, leading to the formation of smoother surfaces. Samples 2 and 5, which, respectively, present a high initial surface roughness and hydroxyl fraction, exhibit a small increase in surface roughness after the deposition of the Zr conversion layer. The large clusters formed on sample 2 can be due to the initial rough surface obtained after the pretreatment resulted in an inhomogeneous nucleation and growth of the Zr-based conversion layer on the surface. This confirms the wide layer thickness range of sample 2 shown in Figure 5. In this case, the feature to fill up the cavities became less significant as the thickness of the Zr conversion layer (48 nm) is not considerably more than the initial surface roughness (41 nm). On the other hand, the increase in surface roughness of sample 5 (OH = 51.96%) compared to the initial level can be correlated to the densely packed hydroxyls acting as the nucleation sites which resulted in an increase in cluster size. Figure 8a shows the XPS survey spectrum of the Zr-based conversion layer deposited on an untreated pure zinc sample. The presence of Zr, Zn, and F is clearly shown in the spectrum. The corresponding Zr 3d and Zn 3p peaks can be used to calculate the fragments of different zirconium and zinc components formed in the conversion layer vicinity. As shown in Figure 8b, the Zr 3d peak can be deconvoluted into two individual components: Zr 3d5/2 and Zr 3d3/2 at 183.79 and 185.62 eV, respectively. On the other hand, reactions 1 and 5 show that Zr is expected to consist of oxides and fluorides. Ebert et al.46 showed that the Zr 3d5/2 subpeak relates to zirconium oxides. Zirconium oxides are mainly in the form of three compounds: ZrO2, ZrOx, and the Zr compound with the metallic core level, i.e., Zr−Zn alloy.47−49 On the other hand, AES results showed that fluoride compounds are detected in the Zr-reached zone, indicating formation of zirconium fluoride. This together with the F 1s peak at 684.6 eV (Figure 8a) suggests a significant contribution of ZrF4 in the Zr 3d3/2 peak. The values obtained for Zr 3d5/2 and Zr 3d3/2, i.e., 183.79 and 185.62 eV, respectively, are in good agreement with other investigations.46,47 Figure 8c shows that the Zn 3p peaks are simulated with four components. Zn 3p(1) located at 91.64 eV and Zn 3p(2) at 88.14 eV subpeaks. They originate, respectively, from Zn 3p1/2 and Zn 3p3/2 photoelectrons and can be attributed to the zinc

Figure 6. OCP responses of differently pretreated zinc samples in the Zr conversion bath.

OCP response due to the immersion of the untreated pure sample and samples 1, 2, and 4 in the Zr-based pretreatment solution. These samples were selected to evaluate the effects of surface hydroxyl and roughness on the deposition process. Two typical regions are evident in every OCP graph. Within region I, i.e., the first 25 s of immersion, the OCP decreases. This presumably is attributed to the removal of the natural hydrated oxide layer from the surface due to the interaction of the fluorides present in the solution and the gradual surface activation according to reaction 4. Afterward, the OCP increases for an immersion time of 25−300 s in region II. Subsequent increases in values of OCP may correspond to precipitation and growth of the Zr-based coating (reaction 5). The results show that the OCP level of the pure zinc sample (OH = 24.60%) decreases from an initial value of about −1.03 mVSCE to a minimum value of −1.04 mVSCE at 45 s. The OCP of sample 4 (OH = 42.32%) starts from a higher value as a result of a higher passivation provided by hydroxyl than the oxide contributing to obtain a noble OCP level. Then OCP of this sample decreases sharper to the minimum than the untreated pure sample, resulting in a higher amount of OCP decrease in region I. In the case of sample 4, the OCP reaches the minimum in a shorter period, i.e., 23 s, than the other samples indicating higher fluoride reactivity. This presumably is related to the higher hydroxyl fraction existing on this sample interacting with fluoride. On the other hand, the initial OCP level of sample 2 is substantially higher than that of the other samples, starting from −0.93 mVSCE, decreasing to −1.00 mVSCE. It is evident that the OCP level of this sample is always the highest among the samples investigated. Additionally, the OCP decrease of the sample is substantially higher than the other samples, although the hydroxyl fraction existing on the surface is relatively low (OH = 16.78%). This is probably correlated to the high surface roughness keeping the surface reactive during the dissolution and conversion processes. It can be seen in Figure 5 that the OCP level of sample 4 in region II increases and reaches a higher potential than that of sample 1 and the untreated pure one as a consequence of a higher extent of deposition. The high OCP level of sample 2 in region II implies a high deposition rate on this sample. However, the deposition rate on sample 2 decreases after about 150 s as indicated by a change in slope of the OCP curves. 8431



Article

Figure 7. AFM topography of Zr-based conversion layers formed on (a) untreated pure zinc sample, (b) sample 1, (c) sample 2, (d) sample 3, (e) sample 4, (f) sample 5, and (g) surface roughnesses after 90 s.

atoms linked to oxygen in zinc oxide.50 Zn 3p(3) located at 92.75 eV and Zn 3p(4) at 89.84 eV subpeaks, respectively, are

attributed to Zn 3p1/2 and Zn 3p3/2 photoelectrons assigned to Zn−O−C and/or Zn−O−H bonds.51,52 8432



Article

Figure 8. XPS (a) survey, (b) Zr 3d, and (c) Zn 3p spectra of the Zr-based conversion layer deposited on untreated pure zinc substrate.

surfaces among sample 1, untreated pure sample, and samples 3 and 4 (Figure 2) results in a decrease in zirconium oxide. This proves AES results (Figure 4c) regarding the role of a high hydroxyl fraction in lowering the amount of zirconium compounds. However, the thickness of the converted layer increases with hydroxyl fraction as indicated by Figure 5. This contradictory behavior together with the observed increase in zinc oxide level in the formed layer with hydroxyl fraction (Figure 9) suggest the hypothesis that the amount Zn impurities within the Zr-based conversion layer increases by the initial hydroxyl fraction. Figure 10 schematically shows the role of the hydroxyl fraction in the formation of Zr-based conversion layers. It can be

The integrated peak areas of Zr 3d5/2 and Zn 3p(1) + Zn 3p(2) representing, respectively, the amount of zirconium and zinc oxides obtained for the untreated pure and differently pretreated samples are plotted in Figure 9. It can be seen that

Figure 10. Schematic drawings of conversion layers formed on a sample with (a) low and (b) high hydroxyl fractions.

seen that the conversion layer formed on the sample with a low hydroxyl fraction (Figure 10a) is thin, while the layer formed on the sample with a high hydroxyl fraction is thick (Figure 10b). However, the amount of Zr compounds within the formed layer is lower for the latter case as a consequence of a higher level of Zn impurities. In this case, an intensive interaction between fluoride and hydroxyl is proposed. Consequently, the surface

Figure 9. Integrated peak areas of Zr 3d5/2 and Zn 3p(1) + Zn 3p(2) representing, respectively, the amount of zirconium and zinc oxides obtained for the untreated pure and differently pretreated samples. (The hydroxyl fraction increases from sample 1 to 5.)

an increase in amount of zirconium oxides is accompanied by a decrease in the level of zinc oxides on all samples as expected. Additionally, an increase in hydroxyl fraction of the initial 8433



Article

samples according to the procedure described in the experimental part. Figure 12 shows the infrared spectra of the

is activated intensively for the Zr deposition resulting in an increase in the thickness of the formed layer. The intensive interactive removal rate of the zinc components by fluoride followed by nucleation and growth of the conversion layer resulted in trapping and deposition of a high portion of zinc compounds within the conversion layer. Although, in this study, the formation of the fluoride compounds in the interface between zinc and zirconium oxide is not confirmed, the Zn and Zr competition (XPS observation) together with the F and Zr competition (AES observation) to deposit within the conversion layer accomplishes the so-called “blocking” effects of fluoride for Zr deposition. Indeed, the fluoride acts as a mediator to increase the Zn and consequently decrease the Zr fractions. As shown, the decrease in Zr content due to an increase in the initial hydroxyl fraction is not valid for samples 2 and 5. On the other hand, Figure 5 showed that these samples possess the highest thickness of conversion layer among the samples. Additionally, Zn is present mainly in the deep area where the base substrate exists rather than the superficial top layer (Figure 4a). On the other hand, the XPS, Zn 3p spectra are expected to partially originate from the deep Zn-rich area located underneath the formed conversion layer. Consequently, it is probable that the extraordinary high thickness of the conversion layers on samples 2 and 5 masks the Zn-rich area located far beyond the detectable XPS depth. Consequently, a higher Zr fraction appears in XPS spectra. Figure 11 shows the hydroxyl (OH−) fraction on conversion layers deposited on the untreated pure and differently

Figure 12. Infrared spectra of succinic acid molecules adsorbed on conversion layer deposited on the untreated pure zinc samples.

adsorbed succinic acid molecules on conversion layer of the untreated pure zinc sample. The presence of the bands around 1445 and 1595 cm−1 indicates that the adsorption process resulted in the formation of carboxylate salts coordinatively bonded to the surfaces.54,55 The peaks at 1445 and 1595 cm−1 can be assigned to symmetric νs(COO−) and asymmetric νas(COO−) carboxylate stretching vibration bands, respectively. The frequency shift of the carboxylate stretchings, Δν(COO) = (υas(COO−) − υs(COO)), is 150 cm−1 for this sample. This value indicates a bridging bidentate coordination state of the compounds formed on the studied oxide layer.56 Additionally the small band at 1779 cm−1 is assigned to the carbonyl compounds (CO).33 The presence of carbonyl indicates that the succinic acid molecules are partially adsorbed with one end. However, our previous studies showed that succinic acid molecules are fully deprotonated and adsorb with both ends.57 This can be correlated to a variation in the semiconductor properties of the zinc surface upon the deposition of the Zrbased conversion layers. Figure 13 shows the amount of adsorbed succinic acid molecules versus the hydroxyl fraction of the formed conversion layers on the untreated pure and differently pretreated zinc

Figure 11. Hydroxyl (OH−) fraction values calculated from O 1s and C 1s peak deconvolutions of the untreated pure and differently pretreated zinc samples. (The hydroxyl fraction of the initial surfaces increases from sample 1 to 5.)

pretreated zinc samples. The surface hydroxyl fraction was calculated through the method utilized for the differently pretreated zinc samples. As mentioned in reaction 5, ZrO2·H2O is expected to form due to the conversion treatment. However, occurrence of a hydration procedure is probable due to the formation of water.53 A comparison of Figures 9 and 11 shows an obvious coherent relation between the hydroxyl fraction and the amount of zinc content in the conversion layer. This might be related to a higher affinity of zinc oxide to take up the adsorbed water and form hydroxyl. Thus, it can be inferred that the zinc content is a determining factor in the final hydroxyl fraction formed on the Zr-based conversion layer. To reveal the acid−base properties of the formed conversion layers, succinic acid molecules were adsorbed on different

Figure 13. O−CO−/(Zr 3d + Zn 3p) values representing the amount of adsorbed succinic acid molecules on the untreated pure and differently pretreated zinc samples versus the hydroxyl fraction of the formed conversion layers (as discussed, the amount of zinc content increases by the hydroxyl fraction). 8434



■

samples calculated from the C 1s peak deconvolution according to the procedure described in our previous work.57 To subtract the equipment variations affecting the signal intensities obtained for different samples, the O−CO− subpeak area was divided by the integrated Zr 3d + Zn 3p peak area. It is clear that the amount of the adsorbed succinic acid molecules roughly conforms to the zinc content and consequently the hydroxyl fraction. This certifies our previous findings regarding the involvement of surface hydroxyl fraction, i.e., the basicity of the metal substrate, in the adsorption of succinic acid molecules.33 The deviation in the relative amount of hydroxyl and the adsorbents on sample 5 may originate from the various surface roughnesses and Zr/Zn contents. A comparison of the results with our previous study57,58 shows that the Zr conversion layer adsorbs a higher amount of succinic acid than the zinc substrate because of having the higher hydroxyl fraction as the favored sites for the molecular adsorption. Figure 14 schematically shows the adsorption modes of succinic acid molecules on the formed conversion layer. It can

Article

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was carried out under the project number MC6.06254 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl).

■

REFERENCES

(1) Toxicological Profile for Chromium, ATDSRyTP-88y10, Agency for Toxic Substance, U.S. Public Health Service, Washington, DC, 1989. (2) Nordlien, J. H.; Walmsley, J. C.; Østerberg, H.; Nisancioglu, K. Surf. Coat. Technol. 2002, 153, 72−78. (3) Narayanan, T. Rev. Adv. Mater. Sci. 2005, 9, 130−177. (4) Van Wazer, J. R. Phosphorous and its Compounds; Interscience Publishers Inc.: New York, 1967; Vol. II. (5) Adhikari, S.; Unocic, K. A.; Zhai, Y.; Frankel, G. S.; Zimmerman, J.; Fristad, W. Electrochim. Acta 2011, 56, 1912−1924. (6) Schram, T; Goeminne, G; Terryn, H Trans. Inst. Met. Finish. 1995, 73, 91−95. (7) Li, H.; Liang, K.; Mei, L.; Gu, S.; Wang, S. J. Mater. Sci. Lett. 2001, 20, 1081−1083. (8) Fedrizzi, L.; Rodriguez, F. J.; Rossi, S.; Deflorian, F.; Maggio, R. Electrochim. Acta 2001, 46, 3715−3724. (9) Gusmano, G.; Montesperelli, G.; Rapone, M.; Padeletti, G.; Cusma, A.; Kaciulis, S.; Mezzi, A.; Maggio, R. Surf. Coat. Technol. 2007, 201, 5822−5828. (10) Maggio, R.; Fedrizzi, L.; Rossi, S. J. Adhes. Sci. Technol. 2001, 15, 793−808. (11) Puomi, P.; Fagerholm, H. M.; Rosenholm, J. B.; Sipilä, R. Surf. Coat. Technol. 1999, 115, 79−86. (12) Puomi, P.; Fagerholm, H. M.; Rosenholm, J. B.; Jyrkäs, K. Surf. Coat. Technol. 1999, 115, 70−78. (13) Berger, R.; Bexell, U.; Stavlid, N.; Grehk, T. M. Surf. Interface Anal. 2006, 38, 1130−1138. (14) Draou, K.; Bellakhal, N.; Cheron, B .G.; Brisset, J. L. Mater. Chem. Phys. 1997, 51, 142−146. (15) Hong, R.; Huang, J.; He, H.; Fan, Z.; Shao, J. Appl. Surf. Sci. 2005, 242, 346−352. (16) Andreatta, F.; Turco, A.; de Graeve, I.; Terryn, H.; de Wit, J. H. W.; Fedrizzi, L. Surf. Coat. Technol. 2007, 201, 7668−7685. (17) Lunder, O.; Simensen, C.; Yu, Y.; Nisancioglu, K. Surf. Coat. Technol. 2004, 184, 278−290. (18) McGovern, W. R.; Schmutz, P.; Buchheit, R. G.; McCreery, R.L. J. Electrochem. Soc. 2000, 147, 4494−4501. (19) Yu, Z.; Ni, H.; Zhang, G.; Wang, Y. Appl. Surf. Sci. 1992, 62, 217−221. (20) Nordlien, J. H.; Walmsley, J. C.; Østerberg, H.; Nisancioglu, K. Surf. Coat. Technol. 2002, 153, 72−78. (21) Verdier, S.; van der Laak, N.; Dalard, F.; Metson, J.; Delalande, S. Surf. Coat. Technol. 2006, 200, 2955−2964. (22) Karlsson, J. Licentiate Thesis, Centre for Industrial Engineering, and Development (CITU), Dalarna University, Sweden, 1996. (23) Fowkes, F. M. Adv. Chem. Ser. 1964, 43, 99−111. (24) Barthes-Labrousse, M. G. Vacuum 2002, 67, 385−392. (25) Davydov, A. A. In Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; Rochester, C. H., Ed.; Wiley: Chichester, England, 1990. (26) Stratmann, M.; Feser, R.; Leng, A. Electrochim. Acta 1994, 39, 1207−1214. (27) Stratmann, M.; Leng, A.; Furbeth, W.; Streckel, H.; Gehmecker, H.; Groβe-Brinkhaus, K. Prog. Org. Coat. 1996, 27, 261−267. (28) Matoga, M.; Laborde-Kummer, E.; Langlois, M. H.; Dallet, P.; Bosc, J. J.; Jarry, C.; Dubost, J. P. J. Chromatogr. A 2003, 984, 253− 260.

Figure 14. Schematic adsorption modes of succinic acid molecules to zinc and zirconium atoms formed on the conversion layer.

be seen that zinc atoms forming most of the surface hydroxyls are involved in the adsorption procedure rather than zirconium atoms. In this case, one-end and two-end adsorption modes are proposed. In the former case, one −COOH is subjected to a deprotonation procedure, and the other end remains unbonded; in the latter case, a full deprotonation occurs.

4. CONCLUSION This work investigated the deposition mechanism of Zr-based conversion layers on differently pretreated Zn surfaces and evaluated the obtained surface properties. It was found that the film formation proceeded by precipitation of the zirconium complexes, initiated by hydroxyl removal from the zinc surfaces. The Zr layer forming process showed a substantial variation on differently pretreated zinc samples. In this case, the surface hydroxyl as well as roughness was found to substantially affect the deposition procedure. During the deposition of the Zr-based conversion layer, fluoride was found to remove the hydroxyl fraction from the initial zinc surfaces, resulting in deposition of the zinc compounds within the conversion layer. In this case, a higher hydroxyl fraction resulted in more Zn impurities within the conversion layer. Additionally, the results showed that the initial hydroxyl on the base substrate is a determining factor in the final hydroxyl fraction formed on the Zr-based conversion layer. This in turn affects the acid−base properties of the conversion layers as shown by a variation in the amount of formed carboxylates due to the succinic acid adsorption. 8435



Article

(29) Chen, L.; Dudek, A.; Lee, Y. L.; Chang, C. H. Langmuir 2007, 23, 3123−3127. (30) Kanazawa, N.; Urano, K.; Kokado, N.; Urushigawa, Y. J. Colloid Interface Sci. 2001, 238, 196−202. (31) Free, M. L.; Miller, D. Langmuir 1997, 13, 4377−4382. (32) Barr, T. L. Thin Solid Films 1994, 253, 277−284. (33) Taheri, P.; Wielant, J.; Hauffman, T.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Mol, J. M. C.; Terryn, H. Electrochim. Acta 2011, 56, 1904−1911. (34) Wielant, J.; Hauffman, T.; Blajiev, O.; Hausbrand, R.; Terryn, H. J. Phys. Chem. C 2007, 111, 13177−13184. (35) Kotnik, L. J. An investigation of the properties of zinc corrosion products, PhD thesis, Case institute of technology, 1960. (36) Gilbert, P. T. J. Electrochem. Soc. 1952, 99, 16−21. (37) Valtiner, M.; Torrelles, X.; Pareek, A.; Borodin, S.; Gies, H.; Grundmeier, G. J. Phys. Chem. C 2010, 114, 15440−15447. (38) Tomlinson, C. E. Proc. Electrochem. Soc. 1995, 95, 159. ́ (39) Pourbaix, M.; Van Muylder, J. Atlas dEquilibres Electrochimiques; Gauthier-Villars et Cie: Paris, 1963; p 168 ́ (40) Pourbaix, M.; Van Muylder, J. Atlas dEquilibres Electrochimiques; Gauthier-Villars et Cie: Paris, 1963; p 223. (41) Verdier, S.; Delalande, S.; van der Laak, N.; Metson, J.; Dalard, F. Surf. Interface Anal. 2005, 37, 509−516. (42) Jolivet, J. P. De la solution aloxyde; Savoir Actuels (Ed.), InterEditions/CNRS Editions, 1994. (43) Brown, G. M.; Shimizu, K.; Kobayashi, K.; Thompson, G. E.; Wood, G. C. Corros. Sci. 1992, 33, 1371−1385. (44) Brown, G. M.; Shimizu, K.; Kobayashi, K.; Thompson, G. E.; Wood, G. C. Corros. Sci. 1993, 34, 1045−1054. (45) Brown, G. M.; Shimizu, K.; Kobayashi, K.; Thompson, G. E.; Wood, G. C. Corros. Sci. 1993, 35, 253−256. (46) Ebert, H.; Knecht, M.; Muhler, M.; Helmer, O.; Bensch, W. J. Phys. Chem. 1995, 99, 3326−3330. (47) Wang, Y. M.; Li, Y. S.; Wong, P. C.; Mitchell, K. A. R. Appl. Surf. Sci. 1993, 72, 237−244. (48) Davis, P. R. Surf. Sci. 1980, 91, 385−399. (49) Bardi, U.; Ross, P. N.; Somorjai, G. A. J. Vac. Sci. Technol. A 1984, 2, 40−49. (50) Parhi, P.; Manivannan, V. J. Alloys Compd. 2009, 469, 558−564. (51) Ben Amor, S.; Jacquet, M.; Fioux, P.; Nardin, M. Mater. Chem. Phys. 2010, 119, 158−168. (52) Ben Amor, S.; Jacquet, M.; Fioux, P.; Nardin, M. Appl. Surf. Sci. 2009, 255, 5052−5061. (53) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; CEBELCOR: Brussels, 1974. (54) Thissen, P.; Wielant, J.; Köyer, M.; Toews, S.; Grundmeier, G. Surf. Coat. Technol. 2010, 204, 3578−3584. (55) Kataby, G.; Cojocaru, M.; Prozorov, R.; Gedanken, A. Langmuir 1999, 15, 1703−1708. (56) Ohe, C.; Ando, N.; Sato, Y.; Urai, M.; Yamamoto, M.; Itoh, K. J. Phys. Chem. B 1999, 103, 435−444. (57) Taheri, P.; Hauffman, T.; Mol, J. M. C.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Terryn, H. J. Phys. Chem. C 2011, 115, 17054− 17067. (58) Taheri, P.; Hauffman, T.; Mol, J. M. C.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Terryn, H. Electrochim. Acta 2011, 56, 9317− 9323.

8436


Effects of Zinc Surface Acid-Based Properties on ... - ACS Publications

Recommend Documents