Electrochemical corrosion evaluation of SnSb electrodeposited coatings |
Ana Aline C. AlcanforI; Natalia G. SousaI; Othon S. CamposII I. Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceará, Campus do Pici, 60440-900 Fortaleza - CE, Brasil Received: 06/13/2024 *e-mail: adriana@ufc.br SnxSb(1−x) coatings were electrodeposited from 1ChCl:2EG on copper. The corrosion resistance of the electrodeposits was evaluated in 0.1 mol dm-3 of NaCl. The characterization was performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Voltammetric profiles showed that the electrodeposition potential shifted towards less positive values with increasing Sb3+ concentration. SEM images revealed that the coatings exhibited grains and clusters. EDS analyses showed that the Sb content increased with the Sb3+ concentration. XRD results indicated the formation of the SbSn phases such as Cu2Sb, Cu6Sn5, and Cu6(Sn,Sb)5. The potentiodynamic polarization (PP) curves showed that Sn and Sn77Sb23 presented a passive potential region, while Sn37Sb63 had an active dissolution in the electrolyte. Immersion tests were performed for 24 h and XRD results revealed Cu, Sn, SnO, SbSn and Sb2O4 phases. Considering 48 h, Cu, CuSn, SnO2 and SnSb phases were identified. The polarization resistance values of 4.60, 14.69 and 1.81 kΩ cm2 were achieved for Sn, Sn77Sb23, and Sn37Sb63, respectively. The EIS results suggested that adding Sb3+ improves corrosion resistance for SnSb samples with higher Sn content. For Sn77Sb23, the charge transfer resistance was 10.5 kΩ cm2, for Sn and Sn37Sb63 1.15 and 1.46 kΩ cm2, respectively. INTRODUCTION Currently, tin (Sn), antimony (Sb) and SnxSb(1−x) alloys are promising materials for a variety of applications. These include their use as alloys for welding,1,2 semiconductors,3 thermoelectrics,4 anode materials for metal-ion batteries,3 anticorrosive coatings5 and as electrocatalysts for electrochemical reduction of CO2.6 The electrodeposition technique, one of the several methods for preparing these coatings, is particularly attractive for industrial practice. It offers a low cost, easy handling, the ability to control the thickness of the electrodeposited coating, and the capacity to obtain electrodeposited layers on metallic substrates of different geometries and shapes. Most importantly, the electrodeposits have excellent adherence to the substrate.6-11 However, the electrodepositions of Sn and Sb present several problems when carried out in an aqueous-based plating solution since there is a known hydrolysis in a neutral solution that Sn2+ undergoes (Equation 1).12-14 This reaction leads to a formation of SnO, which is unstable in an aqueous solution and brings other hydrolysis reactions to form the SnO2 (Equation 2). Finally, the Sb3+ cations can present problems with solvating in water once their solubility can be improved only by using strong acidic solutions (Equation 3). ![]() In this context, deep eutectic solvents (DES) offer a promising alternative for preparing electroplating solutions containing Sn2+ and Sb3+ ions since, unlike the traditional aqueous electroplating solutions, many chemical reactions that typically occur in water do not take place in DES. These organic solvents often contain only a small percentage of water, and consequently, the hydrolysis reactions involving Sn2+ and Sb3+ ions, as mentioned earlier, are less likely to occur. Also, the solubility of these metal salts is very high in these solvents as the DES polarity can be tuned by choosing the appropriate components to achieve polar or apolar behavior. In this context, SnCl2 and SbCl3 are highly soluble in DES-based formulation, and, therefore, electroplating solutions using these solvents can be prepared without adding any complexing addictive to maintain the Sn2+ and Sb3+ ions stable in the solution. Nowadays, there is an increasing interest in developing ambiently more sustainable processes, which means searching for new processes and technologies that mitigate the environmental impacts of traditional industrial processes. Since the beginning of the first decade of the current century, DES have been investigated as an environmentally friendly alternative to water plating solutions for the electrodeposition of transition metals and their metallic alloys because their ions are stable in these solvents. In addition, DES are widely available, relatively inexpensive, biodegradable, and non-toxic15 and, therefore, electrodeposited metallic layers can be achieved from these solvents without the addition of complexing agents, which gives to the electrodeposition of individual metal or metallic alloys in DES a potential to become, in the future, an industrial electroplating process associated to the sustainable development. Therefore, the interest in the electrodeposition of metals and alloys from DES is rising, as demonstrated in the literature review papers related to this subject, which have already been published.16-18 Furthermore, Abbott18 has used pilot projects for electroplating Ni, Fe and Zn to show that plating solutions based on DES have real potential for application in industrial practice. Considering the deposition of Sn coatings in ethaline, Brandão etal. 19 used oxidized multi-walled carbon nanotubes (ox-MWCNT), pristine multi-walled carbon nanotubes (P-MWCNT) and reduced graphene oxide (rGO) over a metallic matrix using a glassy carbon electrode (GC) for comparison. The cyclic voltammograms showed a reduction peak around 0.6 V vs. Ag. The chronoamperometric analysis showed a 3D instantaneous process with diffusion-controlled growth. The Sn coating presented a topography by atomic force microscopy (AFM) showing round particles with different roughness in the function of the carbon electrode: the GC along ox-MWCNT presented higher values of roughness in comparison to P-MWCNT and rGO electrodes. Concerning the electrodeposition of Sn alloys, Gao et al. 20 studied the electrodeposition of Sn-Bi alloy coatings using ethaline with boric acid at 90 ºC over a Cu electrode. The morphology of the alloy varied with the applied potential from a compact layer with small grain sizes to an unevenly distributed large grain, which shows the effect of the potential on the Sn-Bi alloy morphologies. Anicai et al. 21 presented SnIn electrodeposition using reline on copper, whose coating had 10-65 wt. % of In. The authors present that the In content in the electrodeposited layer is inversely proportional to the applied current density. The morphology showed irregular particles covering the substrate under direct electrodeposition conditions at 3 mA cm-2. Moreover, corrosion electrochemical tests in 0.5 mol L-1 NaCl demonstrated that the electrodeposited SnIn coatings acted as an anticorrosion barrier. Lastly, the authors performed a thermogravimetric analysis, which showed that the electrodeposited coatings had a melting point of 400.7 K (Sn:In 50:50%) and 391.8 K (Sn:In 48:52%), which justifies the SnIn use as a lead-free welding solder. Presently, there are few reports in the literature related to the electrodeposition of SnSb alloys from DES. For instance, Su et al. 22 successfully electrochemically prepared submicrometric SnSb alloy powders on a titanium substrate in DES medium formed by choline chloride-ethylene glycol (ChCl-EG, 1:2 molar ratio) containing 0.2 mol dm-3 SbCl3 and 0.2 mol dm-3 SnCl2 at 343 K. These authors showed that the applied electrodeposition potential had a significant effect on the alloy composition, but little influence on the morphology. Ma and Prieto23 reported that the electrodeposition of pure phase SnSb as alloy-type anode material exhibits high stability for sodium-ion batteries. These authors electrodeposited SnSb thins films from ethylene (1:2 by weight ChCl:EG), and the electrochemical tests demonstrated that independent of SnSb composition, the electrode retained 95% of its initial capacity after 300 cycles at 0.5 A g-1. This concern arose from the inadequate disposal of Pb waste arising from the manufacturing process and manufactured products.24,25 The interest in the welding process of SnSb alloys is due to their good thermal fatigue resistance and high fracture strength properties, as pointed out by Wang et al.1 Furthermore, these authors investigated the welding reliability of SnSb with different Sb contents on the Cu substrate. Therefore, due to the outstanding properties of SnSb alloys and the concern with the creation of a sustainable electroplating process for these alloys, this work aimed to electrodeposited SnxSb(1−x) alloys from a plating solution based on 1ChCl:2EG, without the addition of complexing agents, and to evaluate the effect of Sb content on the corrosion behavior of the SnxSb(1−x) electrodeposited coatings.
EXPERIMENTAL Chemicals and electrolyte preparation All chemicals in this study were used as received without any further purification. The eutectic mixture was prepared by mixing choline chloride (ChCl, Sigma-Aldrich®, ≥ 99%) and ethylene glycol (EG, Sigma-Aldrich®, ≥ 99.8%) in a molar ratio 1:2 (1ChCl:2EG) at 353 K until the formation of a colorless liquid. Next, anhydrous salts of tin(II) chloride and antimony(III) chloride (SnCl2 and SbCl3, Sigma-Aldrich®, ≥ 99%) were added to 1ChCl:2EG to form the plating solutions. The compositions of all plating solutions are listed in Table 1.
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Voltammetric measurements The voltammetric experiments were conducted on a potentiostat/galvanostat model PGSTAT30 (Autolab, Metrohm-Eco Chemie) connected to a computer, using NOVA® software version 2.1.4. To investigate the electrochemical behavior of the Sn2+ and Sb3+ species in solution and to electrodeposit the SnxSb(1−x) coatings, a glass electrochemical cell with a Teflon® lid was used, which was coupled to a thermostatic bath for temperature control under atmospheric conditions. Before the obtention of the cyclic voltammograms, the working electrode was sanded on SiC silicon carbide 600 grit sandpaper, washed with Milli-Q water (18.2 MΩ cm), and dried with airflow. Initially, to characterize the surface of the working electrode in the 1ChCl:2EG mixture, cyclic voltammetry was obtained between -0.4 and -1.5 V. Next, to evaluate the electrochemical reduction of Sn2+ and Sb3+ species (baths I-III), linear sweep voltammetries were performed in the electrochemical potential range between -0.4 and -1.0 V. All voltammograms were recorded at 10 mV s-1 and 353 K. Cu disc (geometric surface area of 0.023 cm2), a Pt plate (with 1.5 cm2) and an Ag(s)/AgCl(s) wire immersed in 1ChCl:2EG mixture, were used as the working, auxiliary and reference electrodes, respectively. Electrodeposition and characterization All SnxSb(1−x) coatings were electrodeposited on Cu discs (geometric area 0.18 cm2) to characterize the surface morphologies, chemical compositions, and crystalline structures. The electrodeposits were obtained under potentiostatic control at −0.55 V and 353 K, with electric charge control to obtain coatings with a nominal thickness of 2 μm, estimated from Faraday's equation. The morphologies and chemical compositions of the obtained coatings were analyzed by a field emission gun-scanning electron microscopy (FEG-SEM, FEI-Quanta 450) coupled to an energy-dispersive X-ray spectrometer (EDS, Oxford Instruments INCA X-MAX). The particle size of electrodeposited coatings was determined using the ImageJ-Fiji software.26 Finally, X-ray diffraction (XRD) analyses were carried out using a Bruker diffractometer, model D8 Advance, equipped with a LynxEye detector. The measurements were made with a Cu tube, operating at 40 kV and 40 mA, a range of 20 to 100 degrees, with a step size of 0.02 degrees and a counting time of 0.2 s step-1. The experimental data was treated using PANalytical® X'Pert HighScore Plus® software,27 and the crystalline phases were indexed using the crystallography PDF-22004 card files from the Inorganic Crystal Structure Database (ICSD). Electrochemical corrosion tests An electrochemical cell composed of three electrodes was used for corrosion tests. Ag(s)/AgCl(s)/Cl- (saturated KCl) was used as the reference electrode, a platinum foil of 1 cm2 was the counter electrode, and the Cu recovered with the SnxSb(1−x) coatings was the working electrode. Before each experiment, the open circuit potential (Eocp) was recorded as a function of time for 1 h. The potentiodynamic polarization curves were obtained in 0.1 mol dm-3 NaCl at 298 K and 0.5 mV s-1 and between −0.25 and +1.00 V around the Eocp. All experiments were performed in duplicate. After sample preparation, the morphologies of the tested samples were evaluated after 24 and 48 h immersion in a 0.1 mol dm-3 NaCl solution. The effect of the potential applied in a 0.1 mol dm-3 NaCl medium, −0.1 and 0.2 V for 1 h was also evaluated. A copper disc with a geometric area of 0.28 cm2 was used for these experiments. The electrochemical impedance spectroscopy results were obtained in 0.1 mol dm-3 NaCl at 298 K, applying 0 V to the Eocp in the frequency range of 20 kHz to 6 mHz with a sinusoidal amplitude of 12 mV. Twelve points were collected by frequency decade. The validation of results considered Lissajous method.
RESULTS AND DISCUSSION Electrochemical analyses The insert in Figure 1 shows the cyclic voltammetry, at 353 K, obtained from 1ChCl:2EG on a copper surface in the potential range between -0.4 and -1.5 V. In this potential range, there is an increase in the cathodic current density at potential values from -1.2 V. This process is associated with the reduction of ethylene glycol hydroxyl groups, choline chloride cationic ions and traces of water.28 The electrochemical behaviors of Sn2+ and Sb3+ species were investigated by linear sweep voltammetry using a Cu electrode immersed in baths I-III, applying a potential range between -0.4 and -1.0 V at 10 mV s-1 and 353 K. The recorded voltammograms are shown in Figure 1. For bath I (black line), a single well-defined cathodic peak can be observed at -0.56 V, which is attributed to the electrochemical reduction of Sn2+ to metallic Sn. A similar voltammetric profile was reported29-31 in a previous investigation for the electrodeposition of Sn.
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Since the Sb content increases in baths II and III (blue and red lines), the voltammograms change regarding cathodic peak potential and total electrochemical charge. A similar system was used for Sb electrodeposition in the literature,32 but a Pt disc was used as the working electrode. In this context, the peak potential of Sb3+ reduction at 298 K was located before -0.4 V since this electrode presents more electrochemical stability than the Cu substrate. Once undesired electrochemical reactions occur before -0.4 V in DES, the voltammogram for bath III (red line) presents a so-called "semi-peak" between -0.4 and -0.5 V due to the diffusion process of Sb3+ reduction. The subsequent peak around -0.58 V is related to the Sn2+ reduction, which was slightly shifted towards more negative potentials than pure Sn2+ bath I (black line). In bath II (blue line), having less Sb3+ than bath III, the cathodic peak seen in bath III does not appear, having the Sn2+ reduction process at -0.58 V. Although the hypothesis of co-deposition of the Sn2+ and Sb3+ was reported in the literature.6 Morphology, composition, and structural results The SEM images shown in Figure 2 present an evolution of the grain structure from a cuboid-like in pure Sn to a pentagonal-like structure in Sn37Sb63. In Figure 2a, it is possible to see the body-centered tetragonal, once the cuboid structure is elongated in metallic islets. When Sb is added, there is a transition in the structure, as shown in Figure 2b, in which those islets do not appear due to the rhombohedral structure of Sb, which modulates the alloy grain size. Still, in this image, they are tiny alloy grains with pentagonal and hexagonal shapes, which could be related to the atomic position of the elements in the alloy that mixes both cuboid (from Sn) and rhombohedral (from Sb) structures. This observation is clearer in Figure 2c, which shows many pentagonal-like structures in the SnSb and other regular shapes, such as trigonal and cuboid. With the increase in the Sb content in the coatings, the rhombohedral structure is predominant, which causes those variations in the alloy grains. Compared with the pure Sn image (Figure 2a), Figures 2b and 2c exhibit a regular and compact layer, which is not seen in the pure Sn image. The Sn islets are deposited over an Sn layer; the electrodeposited Sn layer can be related to the voltammetric results shown in Figure 1, which shows a small peak at circa −0.47 V. Although the system is similar, having Sn2+ in ethaline using a copper electrode, the Sn(ads) → Sn(abs) step was not observed within the potential window of the copper electrode. Therefore, this Sn layer before the bulk Sn deposition was observed in both voltammetric and SEM experiments.
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Furthermore, Figure 2a shows the grain clusters in pure Sn deposits. When Sb3+ is added to the electrolytic solution (Figure 2b), a reduction in the average size to 1.05 µm was observed, in addition to a non-uniform overlayer.32 In Figure 2c, obtained from an equimolar solution of Sn2+ and Sb3+, grains with distinct shapes (circular, pentagonal, and hexagonal) were observed, covering the entire surface. This organization of the morphology was related to the reduction in the average particle size (0.49 µm), approximately 10 times smaller than the coating containing only Sn (Figure 2a). Azpeitia et al. 33 found a similar morphology for Sn electrodeposition in 1ChCl:2U on a copper substrate at 343 K, applying a potential of −0.75 V. The atom percentages of Sn and Sb in each electrodeposited coating are displayed in Table 2. From this table, it can be observed that increasing the concentration of Sb3+ species in the plating solution composition led to an increase in the Sb content in the coating and that the ratio between the Sb and Sn content in the coatings is higher than this ratio in the plating solution, indicating that under the used operational conditions, the electrodeposition of SnxSb(1−x) is classified as normal, based on the comparative analysis of the ratio Sn2+/Sb3+ in the bath and Sn/Sb in the electrodeposited layer.
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X-ray diffraction analyses were performed to investigate the effect of electrolytic bath composition on the crystal structure of SnxSb(1−x) coatings. Figure 3 shows the XRD patterns for the electrodeposits of Sn, Sn77Sb23 and Sn37Sb63 obtained on the Cu surface from baths I, II and III, applying a potential of -0.55 V at 353 K. In all diffractograms in Figure 3, the peaks corresponding to the Cu substrate are indexed at 2θ = 43.2, 50.4, 74.0 and 89.9º with a crystal structure cubic (space group Fm-3m, 225, ICDD 00-003-1005). For the diffractograms in Figures 3a-3b, a pure Sn phase is indexed at 2θ = 30.6, 31.9, 43.8, 44.8, 55.3, 62.5, 63.7, 64.5, 73.1 and 79.4º, referring to planes (200), (101), (220), (211), (301), (112), (400), (321), (411) and (312), respectively, with a tetragonal crystal system (space group I41/and, 141, ICDD 03-065-0296). In addition, a diffraction peak of the intermetallic phase Cu6Sn5 with a monoclinic crystal structure (space group C2/c, 15, ICDD 00-045-1488) is indexed for the diffractograms of Figures 3a-3b. The pure Sn and intermetallic Cu6Sn5 phases were also reported by Zhao et al. 34
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Additionally, for diffractograms of Figures 3b-3c, it was observed that the presence of Sb3+ in the electrolytic baths II and III favored the formation of the SnSb alloy, evidenced by the pure phase SbSn, with a rhombohedral crystal system (space group R-3m, 166, ICDD 00-033-0118).35 Finally, for the diffractogram of Figure 3c, phases referring to the intermetallic Cu2Sb and Cu6(Sn, Sb)5 were indexed, with tetragonal crystal structure (space group P4/nmm, 129, ICDD 01-087-1176) and hexagonal crystal system (space group P63/mmc E, 194, ICDD 00-49-1055), for Cu2Sb and Cu6(Sn, Sb)5, respectively. The formation of these CuSn and CuSb intermetallic phases is attributed to the metallic bonding between the Cu atom on the surface of the Cu electrode and the first SnSb electrodeposited layer on the Cu surface. Considering the different phases and morphologies obtained in the coatings, a test was carried out to evaluate the applicability of the coatings for corrosion. Electrochemical corrosion tests Potentiodynamic polarization (PP) The potentiodynamic polarization curves obtained for the SnxSb(1−x) coatings in 0.1 mol dm-3 NaCl are shown in Figure 4. Initially, it can be noted that the corrosion potential (Ecorr) of the SnxSb(1−x) alloys slightly shift towards more positive values compared to the Sn coating and with the Sb content in the electrodeposit. This behavior is attributed to Sb being a nobler metal than Sn. However, the Ecorr values for Sn and Sn77Sb23, shown in Table 3, were statistically similar, considering the standard deviation at a 95% confidence level. For the cathodic branches, the achieved current densities are related to the oxygen reduction reaction (ORR), which is the cathodic reaction of the corrosion process in a neutral medium. For the anodic branches, it can be noted that the PP anodic curves obtained for the Sn and SnxSb(1-x) alloys are similar and characterized by two potential regions. The first region is related to the current increase with applied potential until it reaches a maximum current, attributed to the active dissolution of both electrodeposited coatings. In contrast, the second region is evidenced by the current plateaus, which are associated with the formation of tin oxides (SnO or SnO2)36-38 and antimony oxides (Sb2O3 or Sb2O4) on the electrodeposited SnxSb(1−x) coatings. These oxides can act as a physical barrier against the corrosion of both coatings. These results are in close agreement with those reported by Dias et al. ,36 who investigated the corrosion behavior of SnSb solder alloys, varying the Sb content between 2 and 10 wt. %.
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The corrosion resistance of the investigated samples was evaluated by the polarization resistance (Rp) derived from the PP curves since the corrosion rate of a sample decreases with the increase of the Rp values. The Rp values were calculated from the fitted slope of the PP curves in the region of the Ecorr (± 10 mV), as shown in Figure 1S (Supplementary Material). The obtained Rp values are also displayed in Table 3. For SnxSb(1−x) samples, a three-fold increase and a two-and-a-half-fold decrease in the obtained Rp values were observed for Sn77Sb23 and Sn37Sb63, respectively, compared to the Sn coating. Therefore, the Rp values indicate that the Sn77Sb23 sample is the most corrosion-resistant among the tested samples. Figure 5 shows the SEM images and diffractograms for the Sn, Sn23Sb77, and Sn37Sb63 coatings after 24 and 48 h of immersion.
Analyzing these SEM images obtained for Sn after 24 and 48 h in 0.1 mol dm-3 NaCl solution (Figures 5a and 5d, respectively), it is observed that there is a reduction in the number of grains observed at 48 h, compared to 24 h, where it was possible to observe vacancies in the coating for the longest exposure time. For the SEM images of Sn77Sb23 coatings (Figures 5b and 5e), no significant modification was observed on the surface of this sample. The Sn37Sb63 coatings (Figures 5c and 5f) presented a dendritic morphology resembling pine trees32,37,38 with size of approximately 0.022 µm for 24 h and 0.19 µm for 48 h, demonstrated in Figure 2S (Supplementary Material). The analysis of XRD data achieved for the SnxSb(1-x) samples immersed in 0.1 mol dm-3 NaCl solution for 24 h, allowed the identification of five distinct crystalline phases: Cu, Sn, SnO, SbSn and Sb2O4. Characteristic peaks of the SnSb phase are also observed in the corresponding X-ray diffractograms of both Sn23Sb77 and Sn37Sb63 samples after 24 h of immersion. Considering 48 h of immersion, the analysis of XRD data allowed for the identification of four distinct crystalline phases: Cu, CuSn, SnO2 and SnSb. Characteristic peaks of the SnSb phase were observed in both Sn23Sb77 and Sn37Sb63 materials for 48 h of immersion. Identifiable peaks related to the SnO phase were observed in both samples mentioned, and identifiable peaks related to the Sb2O4 phases were found only in the Sn37Sb63 material. Analyzing the SnSb polarization curves and the XRD analysis of the samples after immersion (Figures 4 and 5) showed that the formation of Sn and Sb oxides occurred in all coatings. For Sn and Sn77Sb23, the first level appeared due to the high Sn content. For the Sn27Sb63 coating, the first level referring to Sn did not appear because all the Sn present in the coating was used to combine with Sb. In this last league, Sb is in excess (63%). For the coatings, Figure 4 shows at potential with values close to −0.1 V, a maximum current density, and stability in the current density values at 0.2 V. Polarization tests were conducted by applying the mentioned potential for 1 h to evaluate the influence of the potential applied to the Sn and SnSb coatings. Figure 6 shows the SEM images and diffractograms for the Sn, Sn23Sb77, and Sn37Sb63 coatings, obtained by polarizing the electrode for 1 h, at −0.1 and 0.2 V. At −0.1 V, Figures 6a-6c, the analysis of XRD data allowed for identifying five distinct crystalline phases: Cu, CuSn, SnO, SnO2 and SnSb. Characteristic peaks of the SnSb phase were observed in the Sn23Sb77 and Sn37Sb63 coatings at −0.1 V, confirming the formation of the SnSb alloy. Identifiable peaks referring to the SnO phases were observed in the two samples mentioned. Figures 6d-6f shows the SEM images and diffractograms for the Sn, Sn23Sb77, and Sn37Sb63 coatings obtained by polarizing the coating at 0.2 V for 1 h. Therefore, the analysis of XRD data allowed for identifying eight distinct crystalline phases: Cu, Cu2O, Cu3Sn, SnO, SbSn, SnSb, Sb2O3 and SbO2. Characteristic peaks of the SnSb phase were observed in the Sn23Sb77 and Sn37Sb63 coatings. Identifiable peaks related to the SbO2 and Sb2O3 phases were also detected. For the coatings containing only Sn (Figures 6a and 6d), it was observed that the negative potential ended up removing the grains observed in Figure 5. In contrast, at 0.2 V, a morphology like that obtained in Figure 5 was observed.
Electrochemical impedance spectroscopy (EIS) EIS experimental data were collected for Sn, Sn37Sb63, and Sn77Sb23 coatings at 298 K in open circuit potential after 1 h of immersion in 0.1 mol dm-3 NaCl. Figure 7 presents the Nyquist diagrams obtained. The EIS diagrams were obtained at open circuit potential after 1 h of immersion of the samples in 1 mol L-1 NaCl. The potentials of −0.37, −0.30, and −0.24 V, determined by OCP after 1 h, were applied to Sn, Sn77Sb23, and Sn37Sb63 during the EIS experiment. The evolution of open circuit potential values with immersion time is shown in Figure 3S (Supplementary Information).
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Since the highest corrosion resistance is associated with the largest capacitive arc, the EIS data indicate that, among the tested samples, the one with the highest Sb content (Sn37Sb63) is more susceptible to corroding in a neutral medium and that the binary coating with a higher Sn content (Sn77Sb23) presents the greatest corrosion resistance. These data corroborate the Rp values displayed in Table 3. The Sn77Sb23 coating was analyzed during 24 h of immersion in 0.1 mol dm-3 NaCl. The data corresponding to immersion times of 1, 2, 4, and 24 h are presented in Figure 8. Nyquist diagram profile showed no change from 4 to 24 h. A change in the impedance diagrams is observed for the first four hours, indicating an increase in susceptibility to corrosion of this coating in the first four hours. For immersion time between 4 and 24 h, no significant changes in the EIS diagrams are observed, suggesting that the corrosion resistance of the Sn77Sb23 reaches a constant value with the immersion time, which is attributed to the formation of SnO and Sb2O4 on the coating surface, acting as a barrier against the corrosion of the tested sample.
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EIS data (Figure 4S, Supplementary Material) reveal the presence of two-time constants regardless of the analyzed coating. Analysis of the Nyquist diagrams shows capacitive time constants with elongated characteristics, indicating the overlap of two-time constants. A first-time constant is observed in the high to medium-frequency region, and the second-time constant is characterized in the low-frequency region (below 0.1 Hz). A similar result was reported by Dias et al.36 The X-ray diffraction (XRD) obtained after experiments in 0.1 mol dm-3 NaCl reveal the formation of oxides as corrosion products. This result allows for the association of the two-time constants with the oxides/electrolyte interface and the coating/oxides interface. The resistance and capacitance of the corrosion product layer are linked to the time constants in a high frequency region. The capacitance and resistance of the electrical double layer between the interface of corrosion products and coating are linked to the time constants in a low frequency region. Taking into consideration the coating surface characteristics and the formation of oxides, the proposed model (Figure 7b) comprises the solution resistance, RS, in series with two parallel elements: charge-transfer resistance (RCT) and constant phase element (CPE). The first-time constant (RCT1-CPE1) couple corresponds to the oxides/electrolyte interface, while the second-time constant (RCT2-CPE2) couple, characterized in the low-frequency region, is associated with the coating/oxides interface. This configuration aligns with observations from other studies of Sn-based coatings in the literature.39-41 The CPE is essential as it accounts for the distribution of relaxation times arising from physical, chemical, or geometrical inhomogeneity.42,43 Equation 4 describes the CPE impedance (ZCPE), which is characterized by a non-ideal double-layer capacitance. ![]() where j is an imaginary number, ω is the angular frequency of sinusoidal perturbation, and Yo is the capacity parameter that considers the combination of properties related to the surface and electrochemical species and proportional to the electroactive area.40 The parameter α is the dimensionless non-linearity coefficient that ranges between 0 and 1. A value of 1 corresponds to an ideal capacitor, 0 corresponds to a resistor, and 0.5 can be associated with diffusion phenomena. Additionally, a value of −1, indicating inductive behavior.44,45 The quality of the fit was assessed using chi-squared values, which ranged of 10-4 to 10-5. Considering fitting criteria such as the "number of model parameters" and "chi-square values",39 the NOVA software version 2.1.5 was utilized to develop the electric equivalent circuit (EEC) model and determine the values of its components, resulting in a very good approximation to the experimental data. EEC parameter values obtained by modeling the experimental data of Sn, Sn37Sb63, and Sn77Sb23 coatings after 1 h of immersion are presented in Table 4. EEC parameter values for the Sn77Sb23 coating, analyzed for 24 h of immersion, are presented in Figure 8.
Figure 9 illustrates a decrease in RCT2 with increasing immersion time. This trend may be attributed to the release of Sn2+ ions into the solution, which reduces the corrosion resistance of the coating. A slight increase in RCT1 value accompanies this decline in resistance. The action of the chloride ion results, to some extent, in the dissolution of the passivation film of the coating. Concurrently the passivation film is repaired due to the corrosive processes of the coating. This behavior corroborates the values of the first-time constant (RCT1-CPE1) couple, as they exhibited little variation with immersion time. For SnSb electrodeposited alloys (Figures 2b and 2c), the SEM images reveal that the electrodeposited coating is granular, and it is possible to observe the occurrence of smaller hexagonal and circular grains in the presence of Sb, making the coatings more homogeneous and with a larger surface area.
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This result aligns with the capacity parameter (Yo) observed in the Sn37Sb63 and Sn77Sb23 coatings, as they exhibited higher values. The α values obtained for the Sn37Sb63 and Sn77Sb23 coatings after 1 h of immersion in 0.1 mol dm-3 NaCl support diffusion phenomena, with values close to 0.5. The diffusion phenomena observed at low frequencies suggest rapid coating corrosion, likely due to the inadequate formation of an efficient protective film. RCT2 values for the Sn and Sn77Sb23 coatings were 12.6 and 10.4 kΩ cm2, respectively, while the Sn37Sb63 coating exhibited the lowest value at 2.7 kΩ cm2. Consistent with the polarization curves, the results obtained from EIS suggest a correlation between corrosion resistance and the percentage of Sn, indicating an increase in corrosion resistance with higher Sn content. The XRD data reveals a single Sn matrix, with Sb appearing in intermetallic forms such as SnSb dispersed within the Sn matrix. The proportion of intermetallics increases with Sb content, potentially forming multiple galvanic couples that can affect the resulting corrosion resistance.36,46,47 The formation of Sn-based oxides and hydroxides has been extensively reviewed in the literature. Kapusta and Hackerman48 elucidate the pathways leading to the formation of Sn(OH)2 and SnO, corresponding to Equations 5 and 6, respectively. The redox potentials of these half-reactions exhibit proximate values and are contingent upon pH variations.49 ![]() Since Sn(OH)2 can be considered as an H2O-containing SnO. Dehydration of Sn(OH)2 can indeed result in the production of SnO, as indicated by Equation 7.36,39 This transformation occurs specifically when the Sn-based surface is exposed to a dry atmosphere.50 However, the experimental conditions in the present study do not involve a dry atmosphere. The formation of SnO, validated by the XRD spectrum presented in Figure 5, is attributed to reaction 7. ![]() The literature suggests the formation of Sn(OH)4 and SnO2 according to Equations 8 and 9, respectively.36,39-42 However, these compounds were not observed in the XRD spectrum provided in Figure 5. Kapusta and Hackerman48 indicate that these reactions exhibit slow kinetics, and an increase in the anodic potential may promote the formation of these compounds. ![]() The experimental investigation conducted in 0.1 mol dm-3 NaCl was performed under open circuit potential conditions. The absence of Sn(OH)4 and SnO2 as corrosion products on the Sn77Sb23 coatings can be attributed to this specific experimental setup.
CONCLUSIONS SEM and EDS analyses demonstrated that the morphology of the electrodeposits exhibited a uniform distribution of dendritic clusters, and the occurrence of grains with different sizes was evident with increasing Sb content (63%). Characterization by XRD revealed the presence of crystalline phases of Sn, Sb, SbSn and intermetallic phases Cu2Sb, Cu6Sn5 and Cu6(Sn,Sb)5. Polarization tests in 0.1 mol L-1 NaCl medium determined Rp values equal to 4597, 14692 and 1805 kΩ cm2. Immersion tests were carried out for 24 and 48 h, and it was observed that the morphology of Sn77Sb23 did not change with the immersion time. XRD analyses for the coatings showed the presence of SnO, SnSb and Sb2O4 for both immersion times. When polarizing the coatings in NaCl for 1 h, SnO2, SnSb and SnO phases were observed for −0.1 V and SnO, Cu2O, Sb2O3, SnSb, Sb2O3 for 0.2 V. By EIS, the RCT1 values for the Sn, Sn77Sb23 and Sn37Sb63 coatings were 1.15; 10.5 and 1.46 kΩ cm2, respectively. RCT2 values were 12.5; 7.43 and 2.7 kΩ cm2. Consistent with the Rp values, the results suggested a correlation between corrosion resistance and the percentage of Sb, indicating increased corrosion resistance for Sn77Sb23.
SUPPLEMENTARY MATERIAL Complementary material for this work is available at http://quimicanova.sbq.org.br/, as a PDF file, with free access.
ACKNOWLEDGMENTS This study was financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológica (FUNCAP). A. N. C. gratefully acknowledges the funding provided by CNPq (proc. 305103/2022-9). P. L. N. thanks the financial support received from CNPq (proc. 302825/2022-3). N. G. S. thanks CNPq (proc. 141171/2021-9). A. A. C. A. thanks CAPES for her grants. The authors thank the Central Analítica-UFC/CT-INFRA/MCTI-SISANO/Pró-Equipamentos CAPES and Laboratório de Raios-X (UFC) for their support.
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Associate Editor handled this article: Lucia Mascaro |
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