Statistic Evaluation of Cysteine and Allyl Alcohol as Additives for Cu-Zn Coatings from Citrate Baths

Departamento de Química Analítica, Universidade do Estado do Rio de Janeiro – UERJ, Rua São Francisco Xavier, 524, Sala 427, Pavilhão Haroldo Lisboa da Cunha, Maracanã, CEP 20559-013, Rio de Janeiro, RJ, Brasil Departamento de Química Inorgânica, Universidade Federal do Rio de Janeiro – UFRJ, Av. Athos da Silveira Ramos, 149, Bloco A, Sala 634A, Ilha do Fundão, CEP 21941-909, Rio de Janeiro, RJ, Brasil


Introduction
Alloy coatings can be obtained by electrodeposition, producing materials whose properties are dependent on the chemical composition of the deposited layers.However, each coating deposition system must be exhaustively studied before its commercially use because the simultaneous discharge of different ions on the cathode surface is a complex process that is influenced by several parameters, such as the substrate surface and the activity of each metallic ion on the double layer 1-4 .This last feature can influence the process to a great extent when the electrodeposition is performed with electrolytes that contain ligands and/or additives.
Additives (brightening and stress release compounds) are usually added to the electrodeposition bath to improve the properties of the coatings.These substances are generally organic compounds whose function is to refine the grains of the coatings, maintain the surface morphological components of the electrodeposits in the same plane, and produce coatings that reproduce the shine of the original metals 3,5 .The addition of these compounds causes a decrease in the internal stress release of the coating/substrate system, which generally favors the production of coatings without defects, pores or cracks.Therefore, coatings with improved shine, leveling, adherence, grain refinement, and appearance are obtained from baths containing additives.Amino acids can be applied as additives for electroplating.The interaction of these compounds with metal surfaces may occur via either the amino or carboxylate groups, depending on the conditions of interaction and the substrate 6 .In an electrodeposition process, amino acids can act as ligands for metallic ions; they can also enhance or decrease the intensity of parallel reactions (for example, the hydrogen evolution reaction, HER) 7,8 .The side chain of the amino acid can also play an important role.For example, with a sulfur-containing amino acid like cysteine (HS-CH 2 -CH(NH 2 )-COOH), the strong affinity of sulfur for different metals is a key factor 9 .Cysteine presents a thiol or sulfhydryl (S-H) group that is the most chemically reactive site on a protein under physiological conditions 10 ; it is also a preferred ligand for the metallic cations.However, these thiol groups may suffer oxidation, causing the dimerization of the cysteine molecule.Cysteine can also adsorb onto the surfaces of different metals, which has been reported in the literature using several surface science and electrochemical techniques 6 .
Allyl alcohol is a leveling agent additive that can also be added to an electroplating bath to increase the brightening of the produced coatings.However, few data are available in the literature concerning the role of this additive on the electrodeposition process.It is probable that its metallic complexes must be linked by the allyl radical, mainly in an acidic medium.Vagarlyuk et al. 11,12 indicated that a copper (I)-allyl alcohol complex may exist in an acidic medium.
Senna et al. 13 suggested that the presence of allyl alcohol in a Cu-Zn pyrophosphate bath could decrease copper activity in the solution as a result of the formation of a new and more stable Cu-complex, in which allyl alcohol and pyrophosphate ions would act as a mixed ligand.Such additive complex formed with the metal ions may increase their adsorption at the electrode surface, thus increasing their reduction rate (induced adsorption) 5 .In addition, its stress relieving properties may also prevent coating embitterment caused by the simultaneous HER, producing refined compact grains and bright Cu-Zn coatings 14 .Lustrous coatings were also obtained in the presence of allyl alcohol in certain modes of electrolysis and under stirring when Sn coatings were produced from a complex bath 15 .
Previously, we have shown that sodium citrate can produce adherent and corrosion resistant Cu-Zn alloys 16 .However, only a few coatings presented a lustrous appearance.In the present work, a 2 3 composite design was used to verify the influence of the addition of additives (cysteine and allyl alcohol) to the base citrate bath, the applied current density (I), and the mechanical stirring speed (S) on the metal content of the alloy (%m/m Cu and %m/m Zn), the corrosion current density (I corr ), the microstructure, and the morphology of the Cu-Zn alloys produced on mild steel AISI 1020.

Cathodic polarization curves
Galvanostatic polarization curves were obtained at 25 °C for current densities between 0.1 and 120 A m -2 using a potentiostat/galvanostat developed for this purpose 16 .The mechanical stirring speed was varied from 0 to 400 rpm.The experiments were performed in the baths presented in Table 1.All chemicals used were pure grade.A threeelectrode system was applied in which the working electrode was a mild steel AISI 1020 disk with 4.9 × 10 -4 m 2 of area, previously polished with sand paper and degreased with sodium lauril sulfate solution, while the counter electrode was a brass slab (67% m/m Cu and 33% m/m Zn).The potential was measured versus a saturated sulfate reference electrode (SSE).

Electrodeposition experiments
To verify the best conditions for the production of a coating/substrate system with elevated corrosion resistance, the Cu-Zn alloy electrodeposition was performed by employing a 2 3 composite design.The influence of the electrodeposition parameters (independent variables) composition bath (B), applied current density (I), and mechanical stirring speed (S), on the process responses, which included the content of the metals in the alloy (%m/m Cu and %m/m Zn), corrosion current density (I corr ), microstructure, and morphology of the Cu-Zn alloys produced on AISI 1020 mild steel, were then evaluated based on the design matrix shown in Table 2.The same table also presents the codified and real values for the independent variables.The I and S values were chosen from the cathodic polarization curves and the earlier results 17 .The Cu-Zn coatings were obtained using the same apparatus and the same three-electrode cell mentioned previously.A commercial software package (STATISTICA for Windows, release 7.0) was used for the experimental data regression analysis.

Chemical analysis
Flame atomic absorption spectrometry (Perkin-Elmer AAnalyst 300) was used to determine the content of copper and zinc in the alloy coatings produced from the baths shown in Table 1 after the dissolution of the coatings in HNO 3 20% v/v.

Electrochemical analysis
The coating/substrate systems obtained from the solutions shown in Table 1 were electrochemically evaluated by their potentiostatic polarization curves performed with a potentiostat/galvanostat PG29 (Omnimetra Instruments).The potential was varied from -1 to +1 V SCE at a scan rate of 5 mV s -1 .The experiments were performed in a 0.5 mol L -1 NaCl solution (pH = 6.5) at 25 °C.The counter electrode was a platinum net, and the reference electrode was saturated calomel (SCE).I corr was obtained from the Tafel slopes of the polarization curves.

Morphological characterization
The morphologies of the coatings were evaluated using a scanning electron microscope (JEOL JSM 6460LV).The coatings were ultrasonically cleaned for 15 minutes, dried and attached to the stub with conductive tape.The analyses were conducted in high vacuum using the secondary electron mode.

Microstructure characterization
X-rays diffractograms of the coatings were obtained using a diffractometer (Siemens D-6000) with a copper source (CuKα = 1.5406Å).The 2θ range extended from 10° to 90°, which was scanned at a rate of 0.020° s -1 .The more prominent diffractogram lines from the Cu-Zn alloy deposit were fitted by a Gaussian equation (using Microcal Origin  , release 8.0), and the values of 2θ obtained were used to calculate the d (h k l) values of the diffraction lines.The phase identification was conducted using JCPDS-PDF standard data 18 .The full width at half maximum (FWHM), estimated grain size (G S ), and height of the diffraction lines (H) of the most intense diffraction line (100%) were also evaluated.The grain size was related to the particle diameter using Equation 1 19 : where d p is the particle diameter (Å), k is a constant related to the crystal microstructure, λ is the wavelength of the incident radiation (Å), and β is the enlargement of the diffraction line, which can be calculated from Equation 2 19 : where B M is the measured FWHM value and B S is the FWHM of a standard material.

Cathodic polarization curves
The polarization curves were used to select the current density values used for the electrodeposition experiments and to verify if this technique is able to show the effects of the additives on the Cu-Zn electrodeposition process.Figures 1a, b present the polarization curves for steel substrates immersed, respectively, in Baths 2 and 3 of Table 1, at stirring speeds (S) ranging from 0 to 400 rpm.In all cases, substrate depolarization was observed as S increased.Strong polarization had been noted for the deposition of copper and copper alloys in a citrate electrolyte without stirring because of the effects of blocking species (mainly Cu-citrate complexes) 14,[18][19][20] .This influence usually decreased at higher values of S 14,15,18 due to the increase in the mass transport.This effect was less pronounced for the curves of Bath 2 (Figure 1a), for which cysteine was added to the citrate bath.Cysteine can also be adsorbed to the substrate surface, and it could have probably acted as another blocking species at low S values.
To avoid any kind of blocking effect, the polarization curves of the substrates immersed in the baths of Table 1 at 300 and 400 rpm were compared (Figure 2) to verify the influence of low amounts of cysteine and allyl alcohol added to Bath 1, which contains only Cu (II), Zn(II) and citrate ions.There was no difference among the polarization curves, independent of the S and bath B used.Because cysteine increases the overpotential for HER, zinc deposition was expected to take place at less negative potentials.However, Figure 1.Cathodic polarization curves of carbon steel AISI 1020 in Baths 2 and 3 of Table 1, using mechanical stirring speeds ranging from 0 to 400 rpm.this effect was not observed with the applied electrochemical technique.Additionally, the influence of allyl alcohol as a surface leveling agent in the reduction process of Cu-Zn alloys could not be verified by the curves of Figure 2.These results show the limitations of the applied technique for the description of the effects of the mentioned variables on the electrodeposition process.This desired result can be probably achieved by the electrodeposition experiments, as shown below.
Based on these results and the earlier ones published by Senna et al. 13 and Ferreira et al. 16 , two I and two S levels were selected (Table 2) for the electrodeposition experiments.

Electrodeposition experiments
The surface model obtained from the relationship between the responses and the varied parameters of the electrodeposition process using a 2 3 composite design is represented by Equation 3.
where ŷ is the studied response variable (%m/m Cu, %m/m Zn, I corr , 2θ, FWHM, and H); x 1 , x 2 , and x 3 correspond to the electrodeposition parameters I, S, and B, respectively; x i x j represents the interaction between two of the parameters; and x i x j x k is the interaction among the three parameters.This equation was applied to all variables based on the t test, and only the effects that present significant influence (p < 0.05) will be shown.It is important to point out that even though some individuals trends could be observed in how I, S, and B influenced the response variables, the final results concerning the influence of the studied parameters were obtained using the response surfaces, which take in account all the responses observed for each effect.These surface responses are represented as threedimensional diagrams, in which the entry variables (studied electrodeposition parameters) are presented in their codified values (Table 2) in the x and y axes, while the response variables (%m/m Cu, %m/m Zn, I corr , 2θ, FWHM, and H) are presented in the z axis, with their real values 21 .In all cases, the particular bath used is a qualitative parameter that is only considered to be -1 (base bath) or +1 (bath with additives) in the following results.Therefore, this parameter cannot assume any other values because other values would not have a physical meaning.All of the numerical results obtained with this model are shown in Table 3.

Copper content in the coating (% m/m Cu)
Table 3 presents the % m/m Cu results for the coatings produced with the baths shown in Table 1.The values of % m/m Cu in the coatings vary from approximately 63% m/m (for Bath 2 at 20 A m -2 and 400 rpm) to 99% m/m (also for Bath 2, at 15 A m -2 and 300 rpm).Therefore, different deposition conditions seem to influence the copper concentration, producing coatings that can be rich or poor in copper.
Based on the composite design described in Table 2, only the average coefficient (b 0 ) showed significant statistic meaning concerning the influence of the deposition parameters on the % m/m Cu values, at a confidence level of 95%.This result is shown in Equations 4 and 5 for Baths 2 and 3, respectively.Although the adjusted R 2 was low in both cases (approximately 65%), Figure 3 still shows the main effects (in this case, only b 0 ) of the presence of cysteine and allyl alcohol on the studied parameters in the base citrate bath.Ferreira et al. 16 observed an elevation in the % m/m Cu values with increasing S and decreasing I for the base citrate bath (Bath 1).This behavior was related to the following two properties of the system: first, copper is easily reduced mainly at low values of the current density (where other parallel reduction reactions, such as Zn ion reduction and HER, cannot compete with this main reaction); second, ion transport is favored by S, thus increasing the Cu content in the coating.Additionally, when pH variation and citrate concentration are considered, the proposed mechanisms for Cu deposition in a citrate medium 22,23 established that several Cu-citrate complexes are directly reduced without previous dissociation.After reduction, the resulting species can be either directly incorporated into the deposit or undergo decomplexation, yielding molecules of free complexing agents.Therefore, an increase in S can also enhance the transport of any free ligand from the electrode surface to the solution bulk after discharge.
Concerning the coatings produced from Bath 2, which contains cysteine, Figure 3a shows that an increase in % m/m Cu tends to occurs at small values of both I and S. The cause of the I influence can be considered to be similar to what has been observed by Ferreira et al. 16 for Bath 1.In fact, Silva et al. 17 have shown that, for the smallest I value used, the substrate potential for the deposition of Cu-Zn alloy from a citrate bath containing cysteine was shifted to more positive values when compared to that obtained from the base citrate bath, enhancing the copper content in the coating.However, the S effect was in an opposite direction to what was projected.As cysteine can adsorb on the copper surface 6 , it would be expected that a decrease in S should facilitate this adsorption, increasing the polarization and making the deposition of copper more difficult.In Bath 2, the bulk electrolytic solution is at pH = 6.77, which is higher than the first pK a of cysteine (pK a1CYS = 1.71) and smaller than its second pK a (pK a2CYS = 8.33), suggesting that the possible ionic forms of cysteine in solution at this pH range were NH 3 + /SH/COO -(zwitterionic form) and NH 2 /SH/COO -(basic form) 6 .It is known, however, that the pH in the substrate/solution interface may increase due to the HER reduction 13,14 , which can favor the presence of the basic form of cysteine.Using infrared analysis to study the adsorption of cysteine on copper surface, Marti et al. 6 have shown that, at intermediate values of pH, there was a higher fraction of NH 2 groups adsorbed, which was reflected by the intense signal at 1080 cm -1 .Additionally, Sukava et al. 24 have shown that the adsorption of cysteine onto copper electrodes during copper electrodeposition was sufficiently dynamic and the adsorbate phase was sufficiently mobile to enable the copper lattice to grow without covering the additive Table 3. Numerical results obtained with the proposed composite design 2 3 ; B1 -results obtained using Bath 1; B2 -results obtained using Bath 2; B3 -results obtained using Bath 3.  molecules.Therefore, the adsorption of cysteine on the basic form might have been favored at the present experimental conditions, probably changing the cathodic processes kinetic and enhancing the copper reduction, resulting in a higher copper content in the coating.Similar to the observations of Ferreira et al. 16 for the base-citrate bath, an increase in % m/m Cu tends to occur at small values of I for the coatings produced from Bath 3, which contains AA (Figure 3b).This behavior was observed for all S values, although a small increase in % m/m Cu was noted for high S values.Allyl alcohol can act as a ligand for Cu (I) in acidic media 12 .Moreover, because this additive is a leveling agent, it was expected that its adsorption on the substrate surface could affect the copper deposition process.An increase in S may remove the additive, as well as any blocking agent, from the substrate surface, enhancing copper deposition.Nonetheless, under the conditions of this study, the effect of S was more pronounced for the bath without allyl alcohol.

Zinc Content in the coating (% m/m Zn)
The % m/m Zn results for the coatings produced with the baths shown in Table 1 are also presented in Table 3.The smallest values of % m/m Zn were observed in the coatings produced from Bath 2, which indicates that the presence of cysteine might have affected this variable.This result could be related to the stability of copper and zinc complexes with citrate and to the formation of a stable zinccysteine complex 25,26 .The stability constants for Cu-Citrate and Zn-Citrate complexes are 1.62 × 10 14 and 4.45 × 10 7 , respectively 25 .It means that in a base citrate bath, all Cu (II) ions are complexed with citrate and most of the Zn (II) ions are not complexed with this ligand.The stability constant for the zinc-cysteine complex is 9.33 × 10 8 , when small amounts of cysteine are present in the bath 26 (as in Bath 2), which is almost the same order of magnitude of the Zn-Citrate constant.It suggests that part of the Zn (II) ions may also be complexed with cysteine, which could interfere in their deposition process.However, it is important to point out that the influences of the deposition parameters on the % m/m Zn values of coatings obtained from Bath 2 were not statistically significant, at a confidence level of 95%.Therefore, further experiments are still needed to reach a final conclusion on this topic.
On the other hand, for coatings produced from Bath 3, only the I parameter affected the % m/m Zn values significantly, as shown in Equation 6.This result means that high values of I enhance the % m/m Zn in the coatings.Because this effect has also been verified for coatings produced from citrate baths without additives 16 , the presence of allyl alcohol in the bath cannot be said to influence the % m/m Zn values significantly, as observed in Table 3.; that is, % m/m Zn increased for increasing I and decreasing S. Large current densities are needed to promote the reduction of zinc ions to metallic zinc.On the other hand, forced convection may improve copper deposition and decrease the zinc content in the coating, as observed earlier.In agreement with Equation 6, Figure 4 shows that an increase in % m/m Zn can be obtained at higher values of I, regardless of the S value used.The produced coatings were electrochemically evaluated by their polarization curves in a 0.5 mol L -1 NaCl solution.Figure 5 present an example of these curves, obtained from the coatings produced at 15 A m -2 and 300 rpm.There were almost no differences among the curves obtained for coatings produced from the same baths, at other deposition conditions.The steel substrate was submitted to the same  E corr for the coating/substrate systems produced from Baths 1 and 2 are at more negative values than the steel (-0.58 V SCE and -0.75 V SCE , for Baths 1 and 2, respectively).The coatings produced from Baths 1 and 3 present only active dissolution in their anodic branches, while a small passivation region is shown in the coating obtained from Bath 2. Only for Bath 3 the cathodic reaction observed seemed to be the oxygen reduction, controlled by mass transport (diffusion), and identified by a bending in the cathodic Tafel curve at approximately -0.60 V SCE .
The I corr values for the substrate/coating systems are shown in Table 3, as well as the I corr value for the uncoated steel substrate is presented (0.30A m -2 ), as comparison.The coatings produced from Bath 2 always presented the highest I corr values, regardless of the conditions used, while the lowest values were obtained for the coatings produced from Bath 1.There is a strong, positive, and statistically significant influence of parameter B (the kind of bath used) in the comparison of Baths 1 and 2, while no statistical effect of the deposition parameters could be noted in the comparison between Baths 1 and 3.This result indicates  experiment and its polarization curve is also shown.It is interesting to note that the coating/substrate system produced from Bath 3 presents E corr value more positive than the uncoated substrate (-0.38 V SCE and -0.55 V SCE for the coated and the uncoated substrate, respectively), while the that the coatings produced from baths containing cysteine would probably show poorer anticorrosive performance in an aggressive medium.This result is shown in Equation 7, and the adjusted R 2 was found to be approximately 96% in this case.The presence of allyl alcohol in the bath, however, may not have significantly affected the anticorrosive features of the Cu-Zn coatings produced from the citrate bath.% 0.17 0.09 Figure 6 shows the response surfaces relevant to the effects of the studied deposition parameters on I corr of the coatings produced from the bath containing cysteine.In agreement with Equation 7, the bath effect can be easily noted (as shown in Figures 6a, b), regardless of the I and S values studied.However, I corr trends (non statistically) to increase at low values of both I and S (Figure 6c).These conditions are the same as those used to grow coatings with high copper contents from Bath 2 (Figure 3a), indicating that this kind of coating may not protect the substrate efficiently.

Coating microstructure and morphology
Figure 7 presents a diffractogram of the coatings produced from Bath 1 (Table 1) at 300 rpm as an example of the coating microstructure.The same diffraction lines are present in all of the diffractograms, regardless of the conditions used to produce the coatings.Most of the peaks are sharp and well defined, indicating that crystalline coatings were obtained.It is possible to see the diffraction lines of the substrate (Fe), as well as those corresponding to the Cu and Cu 5 Zn 8 phases (datasheets no.04-0836, 06-0696, and 25-1228, for the Cu, Fe, and Cu 5 Zn 8 phases, respectively) 18 .The crystallographic parameters of the marked diffraction lines are presented in Table 4.The coatings produced from Bath 3 presented microstructures similar to those observed for Bath 1, although the coatings from Bath 3 were always shiny and red-yellowish, while those obtained from Bath 1 were always reddish.
None of the studied deposition parameters exhibited statistically significant influence on the variables FWHM, d p , or H of the more intense diffraction line under the conditions employed in this work, although the adjusted R 2 values were always higher than 83% for all responses.There are also some trends that can be observed concerning the effects of I and A on these variables for the baths containing additives, as observed in Figures 8 and 9 for Baths 2 and 3, respectively.
Additionally, the morphological images of the coatings are presented in Figure 10 for comparison.
For Bath 2, higher values of I and lower values of S indicated a decrease in the FWHM (Figure 8a), with a parallel increase in d p (Figure 8b) and in H (Figure 8c), which could be related to an increase in the crystalline characteristics of the coatings.In Figure 10 (experiments 5A to 8A) I seems to have a greater effect on the increase of G S .Senna et al. 14 have shown a similar result concerning the effect of I on the production of Cu-Zn alloys from pyrophosphate baths containing saccharine or butyl alcohol as additives.This effect was related to an increase of the % m/m Zn in the coatings at higher values of I, which could also increase the residual stress and the microhardness of the coatings 2,3,14 .Although no significant influence of I on the value of % m/m Zn was observed for the coatings produced from Bath 2 in the present work, this influence can still provide a possible explanation for the obtained results.Additionally, it is possible to observe cracks on the surfaces of the coatings produced from this bath, which could be correlated to the influence of cysteine on the decrease of the anticorrosive protection, as shown in Equation 7.

ŷ
b b x b x b x b x x b x x b x x b x x x

Figure 2 .
Figure 2. Cathodic polarization curves of carbon steel AISI 1020 in the baths of Table 1 at a) 300 rpm and b) 400 rpm.

Figure 3 .
Figure 3. Fitted response surface of standardized effects of I and S for % m/m Cu: a) Bath 2; b) Bath 3.

Figure 4
Figure 4 presents the surface responses showing the effects of the studied deposition parameters on the % m/m Zn values.As no statistical influences on the studied variables were observed for the coatings of Bath 2, only the trends verified for the coatings produced from Bath 3 are shown.As a comparison, it is interesting to know that, for

Figure 4 .
Figure 4. Fitted response surface of standardized effects of I and S for % m/m Zn: Bath 3.

Figure 5 .
Figure 5. Polarization curves of the coated substrates produced from the baths of Table1in NaCl 0.5 mol L -1 .Polarization curves of uncoated substrate is also shown, as comparison.

Figure 6 .
Figure 6.Fitted response surface of standardized effects of a) I and B; b) S and B and c) I and S for I corr : Bath 2.

Figure 7 .
Figure 7. X-rays diffraction of the Cu-Zn coatings produced at 300 rpm and 15 A m -2 from Bath 1 ofTable 1.

Figure 9 .
Figure 9. Fitted response surface of standardized effects of I and S on: a) FWHM; b) d p and c) H: Bath 3.

Table 2 .
Composite design 2 3 experimental matrix, showing codified and normal values of the studied variables.

Table 4 .
d (h k l) values for the Cu-Zn coatings produced from the baths of Table1.
Figure 8. Fitted response surface of standardized effects of I and S on: a) FWHM; b) d p and c) H: Bath 2.