CC BY
2252
TECHNICAL SCIENCE / «ШУУШШУМ-ЛШШаИ» #1И34), 20222
TECHNICAL SCIENCE
UDC: 621.35
Гаврилова А.А.
Майзелис А.А.
Национальный технический университет «Харьковский политехнический институт»
DOI: 10.24412/2520-6990-2022-11134-52-56 ЭЛЕКТРОДНЫЕ ПРОЦЕССЫ В ПИРОФОСФАТНОМ ЭЛЕКТРОЛ1ИТЕ ЦИНКОВАНИЯ
Havrilova A.A.
Maizelis A.O.
National Technical University "Kharkiv Polytechnic Institute" ELECTRODE PROCESSES IN ZINC PYROPHOSPHATE ELECTROLYTE
Аннотация
Покрытия сплавами, основным компонентом которых является цинк, используют для антикоррозионной и механической защиты стальных деталей, а также придания поверхности различных функциональных свойств. Условия выделения цинка, который является в большинстве сплавов более отрицательным компонентом, в значительной степени определяют его химический и фазовый состав, выход по току и другие характеристики. А условия его химического и анодного растворения необходимо выяснять не только для оптимизации работы анодов, но и, например, оценки подтравливания слоев сплавов, содержащих фазу свободного цинка, при формировании мультислойных покрытий. Целью исследований было определение особенностей электрохимического поведения цинка как основного компонента сплавов на его основе в пирофосфатном электролите в зависимости от рН. Исследования проводили в пирофосфатном электролите цинкования, имеющем рН 7,5 и 8,5. Циклические вольтамперные зависимости, хронопотен-циограммы и вольтамперограммы получали в трехэлектродной ячейке с помощью потенциостата MTech PGP-550s с насыщенным хлорид-серебряным электродом сравнения, по отношению к которому приведены значения потенциалов на графиках. Выход по току определяли по отношению количества электричества на растворение осадка цинка к количеству электричества на его осаждение. В результате анализа циклических поляризационных зависимостей в пирофосфатном электролите для осаждения цинка выявлено, что превышение скорости выделения цинка над скоростью его растворения начинается вблизи потенциала -1,3 В. Активное растворение цинка происходит в пике анодной ветви ЦВА при потенциале -1,2 В, затем поверхность начинает пассивироваться и растворяться в полупассивном состоянии в широком диапазоне потенциалов, что предусматривает возможность достижения общего потенциала растворения комбинированных анодов при электроосаждении различных сплавов цинка. Выход по току цинка в электролите, имеющем рН 8,5, максимален при 5-6 мА/см2, рост значений с плотностью тока в электролите, имеющем рН 7,5, происходит более медленно, чем в случае электролита, имеющего рН 8,5.
Abstract
Alloy coatings, the main component of which is zinc, are used for anticorrosive and mechanical protection of steel parts, as well as imparting various functional properties to the surface. The conditions for the deposition of zinc, which is a more negative component in most alloys, largely determine its chemical and phase composition, current efficiency, and other characteristics. And the conditions of its chemical and anodic dissolution must be clarified not only to optimize the operation of the anodes, but also, for example, to evaluate the etching of alloy layers containing the free zinc phase during the formation of multilayer coatings. The aim of the research was to determine the features of the electrochemical behavior of zinc as the main component of alloys based on it in a pyrophosphate electrolyte, depending on pH. The studies were carried out in a pyrophosphate electrolyte having a pH of 7.5 and 8.5. Cyclic voltammetric dependences, chronopotentiograms and voltammograms were obtained in a three-electrode cell using an MTech PGP-550s potentiostat with a saturated silver chloride reference electrode, with respect to which the potentials are shown in the graphs. The current efficiency was determined as the ratio of the charge for dissolving the zinc deposit to the charge for its deposition. As a result of the analysis of the cyclic polarization dependences in the pyrophosphate electrolyte for zinc deposition, it was revealed that the excess of the zinc evolution rate over the rate of its dissolution begins near the potential -1.3 V. Active dissolution of zinc occurs at the peak of the anodic CVA branch at a potential of -1.2 V, then the surface begins to passivate and dissolve in a semi-passive state in a wide range ofpotentials, which provides for the possibility of achieving the total dissolution potential of combined anodes during the electrodeposition of various zinc alloys. The current efficiency of zinc in an electrolyte with a pH of 8.5 is maximum at 5-6 mA/cm2, the growth of values with a current density in an electrolyte with a pH of 7.5 occurs more slowly than in the case of an electrolyte with a pH of 8.5. Key words: electroplating; zinc; zinc alloys; pyrophosphate electrolyte; cyclic voltammetry; current efficiency.
Ключевые слова: электроосаждение; цинк; сплавы цинка; пирофосфатный электролит; циклическая вольтамперометрия; выход по току.
Key words: electroplating; zinc; zinc alloys; pyrophosphate electrolyte; cyclic voltammetry; current efficiency.
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Introduction
Zinc is the most common metal for sacrificial coating of steel parts due to the more negative potential in many aggressive environments as compared with many brands of steel. Depending on the characteristics of the coated parts, operating conditions and coating functions a large number of electrolytes can be used, both acidic and alkaline, complex electrolytes, the basic compositions of which, in turn, are supplemented by a wide range of additives [1, 2]. However, the coating itself corrodes quickly due to the large difference in potential. Therefore, to increase the service life of products, it is necessary to increase the thickness of the coating, which reduces e.g., weldability and ductility of steel products [3]. Zinc alloys with various metals are used to increase the anticorrosive properties of coatings, their hardness, as well as various functional properties.
Cu-Zn alloy coatings are known as decorative coatings, they are used to increase protection against aggressive environments [4], as a sublayer before the deposition of metal, ceramic or plastic coatings [5], battery electrodes [1] and to promote adhesion of rubber to steel tires. Thin film alloys with shape memory are promising for use in microelectromechanical systems and other micro-dimensional applications [1, 4]. Sn-Zn alloy coatings combine functional and protective properties of both metals: high corrosion resistance, soldering ability, good conductivity, non-toxicity. They are used in the aerospace, automotive, microelectronics, in the production of goods for marine climate [6]. Sn-Zn alloy is proposed to replace tin-lead solders, Sn-Ag alloys, cadmium coatings [7]. Sn-Zn alloy coatings combine high chemical resistance with the anodic nature of protecting the steel substrate from corrosion. Zn-Ni alloy coatings are used as coatings with high protective properties, for substrate made of high-strength steels in compliance with special requirements for mechanical properties, as a sublayer on steel parts before applying chrome-nickel coatings. Ni-Zn alloy coatings ("black nickel") are used to increase the ability of the surface to absorb light in the optical industry. To replace cadmium coatings, an alloy containing from 10 to 15 wt. % of nickel [8] is used. The y-phase has the greatest corrosion resistance [9] having more negative potential then steel.
Complex electrolytes are used to obtain alloy coatings due to the large difference between the standard potentials of zinc and most common metals. Among the electrolytes not containing cyanides, the electrolytes based on EDTA, tartrate, gluconate and glycine are proposed for electrodeposition of brass. Pyrophosphate [4, 5] and citrate [10] electrolytes are mostly used. Among the electrolytes for the Zn-Sn alloy deposition to replace cyanide, a number of both acidic and alkaline complex electrolytes have been developed. Tetra-fluoroborate and silicon fluoride electrolytes, citrate, sulfate-gluconate, sulfate-tartrate, gluconate-citrate and pyrophosphate [11] electrolytes are proposed. Coatings with Zn-Ni alloys are deposited both from electrolytes based on both simple and complex metal ions: ammonia, sulfate, chloride, citrate, amines, glycine and ammonia, pyrophosphate, as well as from electrolytes with sulfamic acid and acetates [8, 12, 13].
Conditions for the deposition of electronegative metal from the electrolyte, which in some alloys is zinc, largely determine not only the chemical and phase composition of the alloy, but also the current efficiency of the alloy, as it deposition occurs together with hydrogen evolution. Additionally, it is necessary to determine the parameters of both chemical and electrochemical dissolution of zinc in the electrolytes, because e.g., the presence of free electronegative metal in alloy layers containing more positive phases during the formation of multilayer coatings by alternating deposition of thin layers of alloys of different composition.
The aim of the study is to determine the characteristics of the electrochemical behavior of zinc as the main component zinc alloys in pyrophosphate electrolytes depending on pH.
Methods
Cyclic polarization dependences, chronopotenti-ograms and voltammograms were obtained using the MTech PGP-550s potentiostat. The potential scan rate was 10 mV s-1. Platinum electrodes with a surface area of 1 cm2 were used. They were treated with boric acid and cathodically reduced in sodium sulfate solution before experiment. A platinum plate was used as the auxiliary electrode. A saturated chloride-silver electrode was used as the reference electrode.
The current efficiency of thin (about tens of nm) coatings deposition was calculated as a ratio of the charge spent on dissolving the deposit to the charge spent on its deposition. charge for zinc dissolution was determined by integrating the anode branch of the CVA or anode voltammograms (in the case of its deposition under galvanostatic conditions).
The study was performed in the electrolyte containing 0.2 mol dm3 ZnSO4; 0.6 mol dm3 K4P2O7; 0.2 mol dm3 KCl; pH was 7.5 and 8.5.
Experimental results
Figure 1 shows the CVA in a pyrophosphate electrolyte having pH of 8.5 (Fig. 1a) and pH 7.5 (Fig. 1b) with different range of cathode polarization. The insignificant current value at the first section of the cathode branch of CVA corresponds to the reduction of oxides on the surface of the platinum electrode and dissolved oxygen in the electrolyte. In the range of potentials of the wave, starting from -0.65 V, hydrogen evaluation occurs. And near the potential of -1.3 V zinc is reduced. The reverse branch of the CVA with a cathode limit of 1 mA cm-2, has no peak dissolution of zinc (Fig. 5, insert). It appears only when the cathode branch is reversed with a scan limit of 3 mA cm-2 (Fig. 4, insert). In the cathodic process with peaks at -1.4 V and -1.5 V, zinc is deposited from different complex compounds or by different discharge mechanism.
The main part of the zinc coating dissolves in the first anode peak at a potential of -1.2 V. Dissolution of more zinc coating at this potential scan rate occurs under conditions of partial passivation of the surface by dissolution products. The anode branches of CVA indicate the possibility of zinc dissolution in a semi-passive state in a wide range of potential values, which provides the ability to achieve a common dissolution potential of combined anodes in the electrodeposition of different zinc alloys.
54 TECHNICAL SCIENCE / «©OUUOqUIUM-JOUrMaL» #1W34), 2022
a b
Figure 1 - CVA on Pt in pyrophosphate electrolyte pH 8.5 (a) and pH 7.5 (b) for zinc precipitation. Limit of the potential scan is current density of 20 mA cm'2 (1); 6 mA cm'2 (2); 5 mA cm'2 (3); 3 mA cm'2 (4); 1 mA cm'
2 (5). The potential scan is 10 mV s'1.
In a less alkaline medium (pH 7.5, Fig. 1b), the ratio of peak heights on both the cathode and anode branches of the CVA changes.
The peak of zinc dissolution in electrolyte with pH 8.5 occurs at the reverse course of the cathode branch with a scan limit of 3 mA cm-2 (curve 1, Fig. 2a). In the electrolyte with a pH of 7.5 (curve 2, Fig. 2a), in contrast to the more alkaline electrolyte (pH 8.5, curve 1),
there is a much greater contribution to the primary reduction of hydrogen, so that even at the scanning limit of 3 mA cm-2 the beginning of zinc deposition is not achieved. The lower charge corresponding to the anode branch of CVA in the electrolyte having a pH of 8.5 with increasing the scan limit to the area of zinc deposition (curve 2, Fig. 2b), also indicates less zinc deposit as compared to the more alkaline electrolyte (curve 1).
-0.5 --1.0 -
-2.0 --2.5 -
-3.0 -'--'-1-'-1-'-
-1.5 -1.0 -0.5 0.0 0.5
E, V
E, V a b
Figure 2 - CVA on Pt in pyrophosphate electrolyte pH 8.5 (1) and pH 7.5 (2) for zinc deposition. Limit of the potential scan is current density of (a) 3 mA cm'2 and (b) 20 mA cm'2. The potential scan rate is 10 mVs'1.
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In the case of galvanostatic deposition of zinc (Fig. 3), the deposition potential shifts to the region of more negative potential values within 1 min; at pH 8.5 this process is faster (Figs. 3a and 3b). In contrast to the accumulation of deposit in the conditions of CVA (curve 1, Fig. 2a), zinc accumulates in sufficient quantities at a current density of 2 mA cm-2 in galvanostatic conditions (curve 2, Fig. 3c). In the electrolyte with pH 7.5,
the values of zinc deposition potential is not reached at a current density of 1 mA cm-2 (curve 1b). According to the faster achievement of the zinc deposition potential, the area under the anode curve of dissolution of the deposit obtained in electrolyte at pH 8.5 is larger than in the case of the electrolyte at pH 7.5 (compare curves 3c and 3d).
<
1 1 1 1 1 1 l< 11 11 i < 1 « 1 < 1 < 1 < 1 « 1 1 i i i | i
-1-'-1-rA /
| 1 , l — ,, ^ *
- 1 \2 i--. * y .
-1.5
-1.0
-0.5
0.0
E, V
0.5
1.0
d
Figure 3 - Chronopotentiograms of deposition (a, b) and voltammograms of dissolution (c, d) of zinc coatings. pH of pyrophosphate electrolytes is (a, b) 8.5 and (b, d) 7.5. Deposition current density is 1 mA cm'2 (1); 2 mA cm'2 (2); 6 mA cm'2 (3); 10 mA cm'2 (4).
Figure 4 shows the dependences of the zinc current efficiency on the current density of cathode limit of the potential scan (Figs. 4a, based on Figs. 1 and 2) and in galvanostatic mode (Figs. 4b, based on Fig. 3). In both cases zinc is not deposited at low current density (3 mA cm-2 for CVA and 1 mA cm-2 for longer dep-
osition under galvanostatic conditions). The current efficiency in the electrolyte with pH 8.5 (curve 1) has maximum at 5-6 mA cm-2. When the electrolyte is acidified, the current efficiency decreases (Fig. 2), and its growth with current density occurs more slowly than in the case of an electrolyte with pH 8.5.
b
a
c
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TECHNICAL SCIENCE / «<g©LL©(MUM~JOUTMaL» #1W34), 2022
a b
Figure 4 - Dependences of zinc current efficiency on (a) the limit ofpotential scan in the conditions of CVA and (b) the density of the deposition current in the galvanostatic mode from the pyrophosphate electrolyte at pH 8.5 (1) and pH 7.5 (2)
Conclusions
The analysis of cyclic polarization dependences in the pyrophosphate electrolyte for zinc deposition revealed that:
(1) exceeding the rate of zinc deposition over the rate of its dissolution begins near the potential of -1.3 V (vs. saturated silver chloride electrode). It corresponds to the cathode limit of CVA 3 mA cm-2 in the electrolyte with pH 8.5;
(2) in the electrolyte with pH 7.5 the beginning of zinc dissolution is not yet achieved at the scanning limit of 3 mA cm-2 due to the greater contribution of the primary hydrogen evolution;
(3) active zinc dissolution occurs at the peak at a potential of -1.2 V, then the surface begins to passive and dissolve in a semi-passive state in a wide range of potentials providing the ability to achieve a common dissolution potential of combined anodes by electro-deposition of different zinc alloys;
In the case of galvanostatic, longer-term deposition of zinc, the potential shifts to the region of negative values corresponding to the region of zinc deposition potentials in the case of electrolyte with pH 8.5 faster than at pH 7.5. The current efficiency in electrolyte with pH 8.5 has maximum at 5-6 mA cm-2, the increase in values with the current density in the electrolyte with pH 7.5 is slower than in the case of an electrolyte with pH 8.5.
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