STUDY OF ANODE PROCESSES DURING DEVELOPMENT OF THE NEW COMPLEX THIOCARBAMIDE-CITRATE COPPER PLATING ELECTROLYTE

Copper plating is one of the most common processes for applying coatings in electroplating engineering. Excellent levelling and covering properties make copper an ideal sublayer when other metals are applied. To apply coatings to steel products, as well as to parts with complex shape, complex electrolytes of copper plating are used, the most common of which are the cyanide electrolytes. Disadvantages of cyanide electrolytes (instability of composition and high toxicity) stimulate the development of other complex electrolytes for copper plating. Such electrolytes must meet the following requirements: 1) to ensure application of high-quality coatings with durable adhesion to the base; 2) to have a stable composition; 3) to be suitable for operational use and to be non-toxic; 4) preparation of the electrolyte must include widespread and cheap components. There are known complex electrolytes for copper plating containing ammonia, potassium pyrophosphate, ethylenediamine, trilon B, carboxylic acids. The most common of these electrolytes is the pyrophosphate electrolyte, which has two significant drawbacks: 1) high content of components predetermines their great losses as the solution is taken out of bath with parts; 2) complexity of purification of washing water from pyrophosphate ions, which when ingested in water cause intense algal blooms. Other complex electrolytes are not applied in industry. Thus, it is an important task to develop a new electrolyte for copper plating. The electrolyte should ensure acceptable technological parameters for a copper plating process and remain stable under operation conditions. Electrolyte components should be affordable and harmless for the environment.


Introduction
Copper plating is one of the most common processes for applying coatings in electroplating engineering.Excellent levelling and covering properties make copper an ideal sublayer when other metals are applied.
To apply coatings to steel products, as well as to parts with complex shape, complex electrolytes of copper plating are used, the most common of which are the cyanide electrolytes.
Disadvantages of cyanide electrolytes (instability of composition and high toxicity) stimulate the development of other complex electrolytes for copper plating.Such electrolytes must meet the following requirements: 1) to ensure application of high-quality coatings with durable adhesion to the base; 2) to have a stable composition; 3) to be suitable for operational use and to be non-toxic; 4) preparation of the electrolyte must include widespread and cheap components.
There are known complex electrolytes for copper plating containing ammonia, potassium pyrophosphate, ethylenedi-amine, trilon B, carboxylic acids.The most common of these electrolytes is the pyrophosphate electrolyte, which has two significant drawbacks: 1) high content of components predetermines their great losses as the solution is taken out of bath with parts; 2) complexity of purification of washing water from pyrophosphate ions, which when ingested in water cause intense algal blooms.
Other complex electrolytes are not applied in industry.Thus, it is an important task to develop a new electrolyte for copper plating.The electrolyte should ensure acceptable technological parameters for a copper plating process and remain stable under operation conditions.Electrolyte components should be affordable and harmless for the environment.

Literature review and problem statement
Complex copper plating electrolytes with organic ligands are the insufficiently explored area of technical electrochemistry.Studying the kinetics of charge and the ionization of
Paper [2] addresses influence of the concentration of thiourea on the electrochemical deposition of copper.Increasing the concentration of CS(NH 2 ) 2 leads to a growth of polarization of the reduction process and contributes to obtaining fine-grained coatings.The process of complex formation at anode dissolution of copper in a given electrolyte was not shown.
The results of study [3] reveal that the introduction to electrolyte of CS(NH 2 ) 2 enables the formation of shiny compact copper coatings.The authors did not explore the anodic processes on a copper electrode.
Effect of additives of chloride ions and thiourea on the structure of cathode copper deposition was studied in article [4].It was established that thiourea predetermines formation of compact copper deposition through the inhibition of growth of the most active places in the cathode.Influence of the anodic processes on the stability of electrolyte was not investigated in the article.
Authors of paper [5] investigated deposition of copper from baths containing sodium citrate and triethanolamine.It is shown that such electrolytes contain copper in monoand divalent form.Effect of triethanolamine on the process of complex formation in the solution was not tackled.The anode process a given electrolyte was not addressed.
Study [6] aims to develop the electrolyte for copper deposition on microelectronics products.It was established that electrodeposition leads to the formation of plastic coatings.Influence of citrate ions on the course of anode dissolution of copper anode was not studied.
Authors of paper [7] investigated the morphology of copper coating from the proposed citrate electrolyte.The result of deposition process was the formation of compact coatings, which were characterized by the presence of small grain of the metal.Influence of the patterns of anode dissolution of copper on electrolyte stability was not considered.
Thus, the purpose of studies performed in this field was to explore the cathode process of copper deposition.There are almost no data on the kinetics of dissolution of copper in thiocarbamide-citrate electrolytes.It is thus impossible to assess the effect of anodic processes on the electrolyte stability and the constancy of composition of the plating bath.However, development of such electrolytes is a relevant task.Acid thiocarbamide-citrate electrolytes could enable rapid build-up of copper coatings by reducing a copper (I) complex.

The aim and objectives of the study
The aim of present study was to examine the anodic processes on a copper electrode in the acid thiocarbamide-citrate electrolyte.This would make it possible to optimize the composition of a copper plating electrolyte and the mode of electrolysis.
To accomplish the aim, the following tasks have been set: -to investigate the effect of electrolyte composition on the behavior of copper under conditions of anode polarization in solutions containing organic ligands; -to determine the kinetic indicators and the nature of limiting stage of copper ionization in acid thiocarbamide-citrate solutions.

Procedure for conducting experimental research
To prepare the electrolyte, we used as a starting salt the copper (II) citrate, obtained by neutralizing citric acid with base copper carbonate.We used thiourea and citric acid as organic ligands.These reagents of qualification "pure for analysis" were dissolved in distilled water at a temperature of 25-35 °C.The electrolyte was filtered upon complete dissolution of components.
We used copper M00 as the anode during research.The surface of copper was degreased in an alkaline solution, activated in dilute H 2 SO 4 and washed with warm and cold water.
Polarization measurements were performed using the potentiostat PI 50-1-1.In the research, we used a three-electrode cell with separated cathode and anode spaces.In the course of the experiment, the cell was thermostated.A smooth rod made of platinum, brand Pl 99.9, was applied as an auxiliary electrode.Reference electrode was a silver chloride electrode, brand EVL-1M1.

Results of the study into kinetics of anodic processes on a copper electrode in thiocarbamide-citrate solutions
Kinetics of the copper anodic dissolution was examined by registering voltammograms.Voltammograms make it possible to establish the effect of electrolysis conditions through a graphical representation of the current density dependence on electrode potential in a given medium.Polarization dependences are acquired at a linear shift of the electrode potential from its value of stationary potential E st .
Fig. 1 shows anodic polarization dependences of the dissolution of a copper electrode in the solutions of thiocarbamide, citric acid, and in a mixed solution.The dependences were acquired under conditions of the electrode potential sweep toward the region of positive values at a speed of 10 mV•s -1 .The results obtained show that the anodic polarization of the copper electrode in 0.1 M solution of CS(NH 2 ) 2 is characterized by small current densities of copper dissolution (Fig. 1, Curve 1).The ionization of copper in a solution occurs with considerable inhibition.In 0.1 M solution of C 6 H 8 O 7, copper ionization proceeds without difficulties (Fig. 1, Curve 2).
Adding citric acid to the solution of thiocarbamide dramatically changes the character of anodic behavior of copper.Citric acid stimulates active dissolution of copper in the range of potentials (-0.25)-(+0.4)V. Analysis of dependence 3 allows us to isolate three typical sections on it.These sections correspond to the region of active dissolution, current density limit, and to the region of further electrochemical oxidation of copper to the ions of Cu 2+ .The dependences shown are consistent with earlier findings for the anodic behavior of copper in CS(NH 2 ) 2 solutions and a mixture of CS(NH 2 ) 2 and C 6 H 8 O 7 [8].The results are supplemented by the dependence acquired in a solution of citric acid.
The composition of a solution affects the value of stationary potential and the potential of copper electrode activation (Table 1).The difference between the values of potentials (Δ) between E st and Е act in the solution containing only CS(NH 2 ) 2 or C 6 H 8 O 7 is 0.095-0.1 V.In solutions containing both components, Δ=0.005-0.01V. Combined presence of CS(NH 2 ) 2 and C 6 H 8 O 7 in a solution leads to a significant activation of the copper electrode.Increasing the concentration of thiocarbamide results in the offset of copper potentials towards a negative region.Influence of C 6 H 8 O 7 is insignificant.
Table 1 Values of stationary potential (E st ) and activation potential (Е act ) of copper Potentials of copper that are given in Table 1 are consistent with those obtained by authors in [9].They have the same sign, but differ slightly in absolute magnitude.
To determine the limiting stage in the process of copper dissolution, the dependences obtained were represented in the semi-logarithmic coordinates.This allowed us to establish the stage of an electrode reaction whose rate determines the total speed of the process.
Results of graphical processing are shown in Fig. 2. According to them, the electrode reaction of copper dissolution proceeds at diffusion control.This is indicated by the fact that the greatest linearization of polarization dependence is observed at its mapping in coordinates η-lg(1-j/j d ), where η is the overvoltage of anodic reaction, η=E j -E st , V; j is the anodic current density, А•cm -2 ; j d is the diffusion current density, corresponding to its mean magnitude on the section of current limit of curve 3 (Fig. 1), А•cm -2 .In this case, there is a slow progress of the stage of removal of the anode reac- The anodic reaction overvoltage is 0.6-1 V at j а =0.5-3 mА•cm -2 .The calculated values of certain kinetic characteristics: exchange current density j 0 =6.73•10 -6 А•cm -2 ; charge transfer coefficient of anodic reaction β=0.12, the number of electrons participating in the ionization reaction z=1.This means that the complexes of univalent copper are formed in the solution: ( At a potential of +0.8 V, a current maximum is reached for a given reaction; in this case, j а =3.6 А•cm -2 .A further rise in the current along curve 3 points to the progress of another oxidative reaction.Since the potential of oxygen isolation is not reached yet, this process implies the formation of copper (II) complexes: The presence of C 6 H 8 O 7 in a thiocarbamide solution contributes to the formation of a complex compound of the mixed type: Effect of thiocarbamide concentration on the rate of copper dissolution in citrate solutions was determined by establishing the order of reaction for СS(NH 2 ) 2 .The results are shown in Fig. 3 in the form of a logarithmic dependence j d of copper dissolution on the concentration of thiocarbamide.The resulting dependence is rectilinear.The order of reaction for thiocarbamide is close to unity (P=1).Consequently, the reaction participates one molecule of thiocarbamide participates in the reaction with the formation of cation [Cu(SC(NH 2 ) 2 )] + .The formation of copper complexes is possible in the volume of electrolyte.with a large coordination number, for example, [Cu(SC(NH 2 ) 2 ) 2 ] + , [Cu(SC(NH 2 ) 2 ) 3 ] + and Cu(SC(NH 2 ) 2 ) 4 ] + .A composition of the complex depends on the concentration of ligand.Increased concentration of СS(NH 2 ) 2 leads to an increase in the number of molecules, coordinated by the copper ion.

Discussion of results of studying the kinetics of anodic processes
Study of the kinetics of anodic process showed that active copper dissolution in the electrolyte occurs at a potential more positive than-0.36V.The process is predetermined by the presence of two componentsthiocarbamide and citric acid.Copper forms stable complexes with thiocarbamide, in which it has an oxidation degree of +1.In this case, the largest contribution to the total overvoltage of anodic reaction is provided by the diffusion overvoltage.
A confirmation of the diffusion control over copper dissolution is the influence of potential sweep rate υ р on polarization dependences (Fg.4).A change in υ р from 5 to 100 mV•s -1 has no effect on Е st of copper electrode, but it leads to an increased current density due to the elimination of diffusional limitations.The range of potential values in which copper actively dissolves also extends.This indicates that the progress of reaction obeys the laws of diffusion kinetics.
Anodic process is characterized by uniform dissolution of copper anodes with no transition into the passive state.Anode current output is close to 100 %.This is explained by the fact that copper forms with thiocarbamide and citric acid stable complex compounds [10][11][12]  The evidence of stability of anodic process is the chronopotentiograms acquired on copper in the current density range of 2-8 mА•cm -2 [13].There are no abrupt changes in potential on dependences, indicating copper dissolution with a uniform etching of boundaries and the volume of grain.Metal dissolution occurs evenly, without the formation of sludge.Anode passivation does not occur, which contributes to maintaining anodes in an active state.Anode current output is close to 100 %, there is no oxygen evolution.Stable work of copper anodes enables a constant replenishment of the electrolyte with copper ions.

Conclusions
1. We have examined influence of the composition of a complex electrolyte on copper dissolution under conditions of anodic polarization.It was established that thiocarbamide contributes to the formation of stable complexes of copper (I) in a solution.Citric acid ensures acid pH value (2-4) of the solution, but it also affects active copper dissolution in the region of potentials (-0.25)-(+0.4)V. Composition of the solution affects the values of E st and E act of the copper electrode.The difference in values between E st and E act in the solution containing only CS(NH 2 ) 2 or C 6 H 8 O 7 is 0.095-0.1 V.For solutions that contain both substances, Δ=0.005-0.01V. Joint presence of CS(NH 2 ) 2 and C 6 H 8 O 7 in the solution contributes to the copper electrode activation under conditions of anodic polarization.Increasing the concentration of thiocarbamide leads to a drastic shift in copper dissolution potentials towards the region of negative values.
2. Analysis of polarization dependences allowed us to establish that the dissolution of copper in a thiocarbamide-citrate electrolyte is controlled by the diffusion phase.This is confirmed by the results of graphical processing of polarization dependences in coordinates η-lg(1-j a /j d ).An increase in potential υ р within 5-100 mV•s -1 causes an increase in j d from 2.2 to 12.0 mА•cm -2 , which indicates diffusion control over the process.The anodic reaction overvoltage is 0.6-1.0V at j а =0.5-3 mА•cm -2 .Processing the dependences obtained made it possible to calculate the number of electrons involved in the reaction of ionization and the order of reaction for CS(NH 2 ) 2 .The results obtained show that copper forms cationic complexes of the type [Cu(SC(NH 2 ) 2 )] + in a thiocarbamide solution.

Fig. 3 .
Fig. 3. Dependence of the rate of copper dissolution on the thiocarbamide concentration in 0.1 M solution of C 6 H 8 O 7 .The range of change in the concentration of СS(NH 2 ) 2 is 0.1-1 М