Materials Science, Vol. 46, No. 5, March, 2011 (Ukrainian Original No. 5, September–October, 2010)
ELECTRODEPOSITION OF CHROMIUM COATINGS FROM SULFATE–CARBAMIDE ELECTROLYTES BASED ON CR(III) COMPOUNDS V. . Hordienko,1 V. S. Protsenko,1 S. C. Kwon,2 J.-Y. Lee,2 and F. I. Danilov1,3
UDC 669.268.7
We propose an electrolyte based on chromium sulfate (1 mole/liter Cr(III)) and containing both formic acid and carbamide (urea). This electrolyte enables one to get Cr coatings with a thickness of several micrometers. It is shown that the current yield and deposition rate increase as the current density and pH value increase and temperature decreases. We select the optimal conditions of electrolysis under which bright high-quality chromium deposits are obtained. In this case, the deposition rate of the metal varies from 0.5 to 1.5 μm/min. It is shown that the optimal concentration of both formic acid and carbamide is equal to 0.5 mole/liter. The necessity of using certain surface-active substances to prevent the formation of pitting on the surface of the deposit is demonstrated. Moreover, it is discovered that the microhardness of Cr deposits attains its highest values (950–980 kg/mm 2 ) for currents with densities of 30– 35 Adm –2 and decreases as the pH value and temperature increase. Electrolysis is realized by using titanium–manganese-dioxide anodes and, hence, it is not necessary to separate the cathodic and anodic spaces. Keywords: trivalent chromium, electrodeposition of solid chromium, formic acid, carbamide, microhardness.
The electrodeposition of chromium coatings plays an important role in contemporary industry because the presence of galvanic deposits of chromium guarantees high corrosion and wear resistance, high microhardness, and good outward appearance of the surfaces. At present, hard chromium coatings (with a thickness from several tenths to several hundreds of micrometers) are obtained from highly toxic solutions of hexavalent chromium (chromate electrolytes) extremely harmful for the environment. The development of new advanced technologies of chromium deposition from ecologically safe solutions of trivalent chromium compounds is an urgent problem of contemporary electrochemistry. However, the deposition of high-quality thick-layer chromium coatings from solutions of Cr(III) salts is an extremely complicated problem. The main cause of the impossibility of deposition of thick-layer coatings from “trivalent” electrolytes is connected with a continuous decrease in the rate of deposition of the metal with time [1–4]. In recent years, it was indicated that this effect can be avoided by using carbamide–formate electrolytes based on the compounds of trivalent chromium [1, 5–7]. In what follows, we study the influence of the chemical composition of electrolytes and the modes of electrolysis on the current yield, deposition rate of chromium, and some other properties of the coatings from Cr(III) sulfate electrolytes containing both carbamide (urea) and formic acid. 1
“Ukrainian State Chemicotechnological University” State Higher School, Dnipropetrovs’k, Ukraine.
2
Korea Institute of Machinery and Materials, Changwon, South Korea.
3
Corresponding author; e-mail:
[email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 46, No. 5, pp. 71–75, September–October, 2010. Original article submitted January 22, 2010. 1068-820X/11/4605–0647
© 2011
Springer Science+Business Media, Inc.
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AND
F. I. DANILOV
Fig. 1. Dependences of the current yield of the reaction of electrodeposition of chromium on the concentration of SAS: (1) AD-1, (2) AD-2, (3) AD-3. The duration of deposition is equal to 15 min for a current density of 35Adm – 2 , pH 1.5, and a temperature of 35°C.
Fig. 2. Dependences of the current yield of the reaction of electrodeposition of chromium on the current density for various concentrations of formic acid (M): (1) 0.3, (2) 0.5, (3) 0.7 [and a constant concentration of CO(NH 2 ) 2 equal to 0.5 ]. The duration of deposition is equal to 15 min; pH 1.5; 35°C.
Experimental Procedure For the electrodeposition, we used degreased and pickled specimens of copper foil and sheet steel (St3) in a temperature-controlled glass cell with nonseparated electrode spaces. We also used low-wear titanium–manganese-dioxide anodes (MD) [8]. The galvanostatic mode of electrolysis was maintained with the help of a B547 current source. The uniform distribution of the current over the surface of cathode was realized by using Teflon cartridges. The deposition rate and current yield were measured for the total three-electrode reaction of chromium deposition according to the data on the weight increments of cathodes in a cell for chromium plating and in a coulometer connected in series. The outward appearance of the coatings was estimated both visually and with the help of an MBS-9 microscope. The microhardness of chromium deposits was measured by using a PMT-3 device (under a load of 100 g; the thickness of deposits was equal to 20 μm). Results and Discussion It is shown that the electrolyte required to get bright high-quality chromium deposits must contain not only
ELECTRODEPOSITION OF CHROMIUM COATINGS FROM SULFATE–CARBAMIDE ELECTROLYTES BASED ON CR(III) COMPOUNDS
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chromium sulfate (a source of Cr(III) ions), formic acid, and carbamide but also aluminum sulfate and boric acid (as buffering admixtures), and a special surface-active substance (SAS) (wetting agent). In the absence of Al 2 (SO 4 ) 3 18H 2 O or H 3BO 3 in the electrolyte, we get dull rough coatings. If electrolysis is performed from a solution without SAS, then the surface of the deposit is damaged by pitting. For a concentration of organic detergents in the solution of about 0.05–0.1 g/liter, we can completely prevent the formation of pitting on the surface. Note that all these admixtures are anionic SAS containing atoms of sulfur. It was established (Fig. 1) that, as a rule, the SAS decrease the current yield of the reaction of electrodeposition of chromium whose values depend on the nature of admixtures. It is clear that it is reasonable to use wetting agents for which the coatings are deposited with the highest values of the current yield of chromium (D-1 and D-2 admixtures). The current yield of the reaction of electrodeposition of chromium and the outward appearance of the coatings depend on the concentrations of formic acid and carbamide in the electrolytes. If a constant concentration of carbamide is maintained in the electrolyte and the content of formic acid varies from 0.3 to 0.7 , then the current yield undergoes extreme changes and its minimum value corresponds to a concentration of HCOOH equal to 0.5 M (Fig. 2). It is worth noting that, for low contents of formic acid (0.3 mole/liter), the coatings readily exfoliate from the base. For higher concentrations of HCOOH ( 0.5 mole/liter), chromium deposits are uniform, bright, and characterized by a high level of adhesion to the substrate. If a constant concentration of formic acid is kept in the electrolyte for chromium electroplating and the content of carbamide varies, then we get a similar dependence, namely, the current yield is minimum for a certain intermediate concentration of CO(NH 2 )2 (equal to 0.5 ) and increases both with the decrease and increase in the concentration of carbamide (Fig. 3). Note that, for relatively low contents of carbamide (0.3 ), the deposits are bright but very stressed (characterized by the presence of tensile stresses) and contain large numbers of cracks on the surfaces. However, for a concentration of CO(NH 2 )2 equal to 0.7 , the coatings lose their brightness. Thus, the optimal concentrations of both formic acid and carbamide in electrolytes are 0.5 mole/liter. It should also be emphasized that, it is impossible to get high-quality bright deposits in the absence of formic acid and carbamide in the electrolyte. Thus, deposits of the highest quality are formed for a certain “optimal” ratio of the concentrations of carbamide and formic acid in the solution, which agrees with the assumption made in [1] that a complex ion
[Cr(carbamid)n (H 2 O)6– n ]3+ is formed in these electrolytes; here, the ligand (carbamide) is a product of the chemical interaction of CO(NH 2 )2 with HCOOH. This complex prevents the formation of low-soluble oligomeric particles in the near-electrode layer, which leads to the deposition of high-quality chromium coatings [1]. Clearly, the formation of the maximum number of complexes of the indicated type should be expected for a certain optimal ratio of the concentrations of CO(NH2)2 and HCOOH. It is shown that the current yield and, hence, the rate of electrodeposition of chromium increase with the pH value of the electrolyte. However, for > 1.5, we observe the formation of dark lusterless striated coatings with weak adhesion to the base. This is why, for the deposition of high-quality coatings with sufficiently high current yield, the pH values of the electrolyte should be kept at a level not higher than 1.5. As follows from the obtained data (see Table 1), the current yield increases both with the increase in the cathodic current density and with the decrease in temperature. Note that, as the cathodic current density increases to 40Adm – 2 , the outward appearance of the deposits deteriorates. As temperature becomes higher, the coatings become brighter and more uniform (although, in this case, the rate of electrodeposition decreases). In view of these facts, we conclude that the optimal conditions for electrolysis are attained for a current density of 30– 35Adm – 2 at a temperature of about 35°.
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AND
F. I. DANILOV
Fig. 3. Dependences of the current yield of the reaction of electrodeposition of chromium on the current density for different concentrations of carbamide (M): (1) 0.3, (2) 0.5, (3) 0.7 (for a constant concentration of HCOOH equal to 0.5 ). The deposition time is equal to 15 min; pH 1.5; 35°C.
Fig. 4. Dependences of the current yield of the reaction of deposition of chromium and the thickness of deposits on the time of deposition for a current density of 35A dm – 2 and a temperature of 35°C; pH 1.5.
Table 1. Influence of the Current Density and Temperature of Electrolyte on the Current Yield of Chromium Current yield of the reaction of chromium deposition, % Temperature, °C
current densities
25Adm – 2
30Adm – 2
35Adm – 2
30
29.6
40.6
41.9
35
17.5
27.3
33.5
40
13.9
17.4
24.7
Comment: Time of deposition is equal to 15 min; pH 1.5.
It should be emphasized that the current yield and the rate of electrodeposition in the proposed electrolyte remain almost constant as functions of time (Fig. 4). This is why the thick-layer chromium coatings (20–30 μm and more) can be obtained for the processes with rates of up to 1–1.5 μm/min.
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Fig. 5. Dependences of the microhardness of Cr deposits on the current density: (1) without SAS, (2) 0.1 g/liter AD-2, (3) 0.1 g/liter AD-1; pH 1.5; 35°C.
Fig. 6. Dependences of the microhardness of Cr deposits on temperature (1) and the pH value (2) for a current density of 35A dm – 2 in the presence of 0.1 g/liter AD-2: (1) pH 1.5, (2) 35°C.
By the method of small-angle X-ray scattering (SAXS), it is shown that the obtained coatings have nanocrystalline structures. The microhardness is one of the most important properties of electrodeposited chromium coatings. The dependences of microhardness of the investigated chromium deposits on the cathodic current density possess an extremum, and the maximum values of microhardness are attained within the range 30
35Adm – 2 (Fig. 5). By adding organic SAS to the electrolyte, we make an attempt to somewhat increase the level of microhardness for i < 35Adm – 2 and, on the contrary, decrease it for i 35Adm – 2 . The level of microhardness strongly decreases as temperature increases to 40°. At the same time, the dependences of microhardness on the value of the electrolyte are less pronounced (Fig. 6). Thus, the maximum values of microhardness are attained for the parameters of electrolysis determined as optimal for the highest rate of deposition and the best outward appearance of the chromium coatings. CONCLUSIONS For the deposition of high-quality chromium coatings with a thickness of several tens micrometers, we propose to use an electrolyte based on trivalent chromium sulfate and containing 1 Cr(III), 0.5 M HCOOH, 0.5 M CO(NH 2 )2 , 0.5 M H 3BO 3 , 0.3 M Na 2 SO 4 , 0.15 M Al 2 (SO 4 ) 3 18H 2 O , and 0.05–0.1 g/liter organic
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SAS. It is shown that the current yield and the rate of electrodeposition of chromium increase as the cathodic current density and temperature increase and as the pH value of the electrolyte decreases. The optimal modes of electrolysis are determined. It is also discovered that, in an electrolyte of the proposed composition, the rate of electrodeposition almost does not decrease in the process of electrolysis and, hence, it is possible to obtain thicklayer coatings (20–30 μm and more) at rates of up to 1–1.5 μm/min. The maximum microhardness of the chromium coatings is attained for cathodic current densities of about 30– 35Adm – 2 . The increase in the value and temperature of the electrolyte leads to a decrease in the microhardness of chromium deposits. The authors express their gratitude to the Ukrainian–Korean Industrial-Technological Center of Cooperation for financing the present work (Contract No. GL 2009-5). REFERENCES 1. S. Survilene, O. Nivinskiene, A. Cesuniene, and A. Selskis, “Effect of Cr(III) solution chemistry on electrodeposition of chromium,” J. Appl. Electrochem., 36, 649–654 (2006). 2. F. I. Danilov, V. S. Protsenko, T. E. Butyrina, et al., “Electroplating of chromium coatings from Cr(III)-based electrolytes containing water soluble polymer,” Zashch. Met., 42, No. 6, 603–612 (2006). 3. I. Drela, J. Szynkarczuk, and J. Kubicki, “Electrodeposition of chromium from Cr(III) electrolytes in the presence of formic acid,” J. Appl. Electrochem., 19, 933–936 (1989). 4. V. Protsenko and F. Danilov, “Kinetics and mechanism of chromium electrodeposition from formate and oxalate solutions of Cr(III) compounds,” Electrochim. Acta, 54, No. 24, 5666–5672 (2009). 5. V. N. Kuznetsov, E. G. Vinokurov, and V. V. Kuznetsov, “Thin-layer chromium plating from electrolytes based on chromium sulfate,” Gal’vanotekhn. Obrab. Poverkh., 6, No. 1, 24–30 (1998). 6. V. V. Kuznetsov, E. G. Vinokurov, and V. N. Kuznetsov, “Kinetics of electroreduction of trivalent chromium ions in sulfate solutions,” Élektrokhimiya, 37, No. 7, 821–825 (2001). 7. X. He, G. Qiu, B. Chen, et al., “Process of pulse electrodeposition of nanocrystalline chromium from trivalent chromium bath,” Trans. Nonferrous Met. Soc. China, 17, s685–s691 (2007). 8. F. I. Danilov, A. B. Velichenko, S. M. Loboda, S. E. Kalinovskaya, “Anodic processes in a sulfate electrolyte based on trivalent chromium salts,” Élektrokhimiya, 23, No. 7, 988–991 (1987).
