EFFECT OF ANIONS, CATIONS AND IONIC STRENGTH IN THE BATH ON THE cr (III) ELECTRODEPOSITION ẢNH HƯỞNG của ION DƯƠNG, ION âm và NỒNG độ ION TRONG bể mạ đến QUÁ TRÌNH mạ điện cr (III)

Nguyen Van Cuong1c 1College of Engineering Technology, Can Tho University, Viet Nam 2Nation Central University, Taoyuan City, Taiwan a nvtai@ctu.edu.vn; b jclin4046@gmail.com, c nvcuong@

EFFECT OF ANIONS, CATIONS AND IONIC STRENGTH IN THE BATH ON

THE Cr (III) ELECTRODEPOSITION ẢNH HƯỞNG CỦA ION DƯƠNG, ION ÂM VÀ NỒNG ĐỘ ION TRONG BỂ

MẠ ĐẾN QUÁ TRÌNH MẠ ĐIỆN Cr (III)

Eng Nguyen Van Tai1a, Prof Jing - Chie Lin2b, Dr Nguyen Van Cuong1c

1College of Engineering Technology, Can Tho University, Viet Nam

2Nation Central University, Taoyuan City, Taiwan

a nvtai@ctu.edu.vn; b jclin4046@gmail.com, c nvcuong@ctu.edu.vn

ABSTRACT

In this study, the effect of background electrolyte and the ionic strength on electrochemical deposition of chromium using trivalent chromium baths is considered Candidate baths were first chosen after a series of testing in Hull cell, then they were conducted by using direct current electroplating potentiostat in the bath The working potentials of this process and quality of the chromium deposits were considerably depended

on the ionic strength of the baths as well as different source of anions and cations Sequences

of electrochemical plating were performed to investigate the optimal range of ionic strength, and to get the most suitable kind of anions and cations for the popular baths containing trivalent chromium The results showed that the conditions in which the ionic strength within

a range of 7.75 to 10.75 (sodium sulfate in the range from 0.25 M to 1.25 M, magnesium sulfate in the range from 0.19 M to 0.94 M), the pH of the bath from 1.5 to 2.0, bath’s temperature from 30°C to 35°C, current density within the range from 4.5 to 7.0 A/dm2, the satisfying Cr-coatings were obtained with a thickness of several tens of micrometers, revealed

a good appearance with a smooth and good luster surface

Keyword: ionic strength, sodium sulfate, electrodeposition, trivalent chromium,

corrosion resistance

TÓM TẮT

Nghiên cứu này trình bày ảnh hưởng của tính chất dung dịch mạ và nồng độ của các ion trong dung dịch lên quá trình mạ điện hóa Crôm trong bể mạ chứa ion Cr3+ Trước tiên, những

bể mạ sẽ được lựa chọn sau loạt thí nghiệm sử dụng thiết bị Hull Cell (gồm 2 điện cực: điện cực âm và điện cực dương), sau đó những bể mạ phù hợp sẽ được thực hiện với việc sử dụng dòng điện trực tiếp trong thiết bị điện hóa potentiostat Điện thế (DC) được sử dụng cho quá trình mạ điện hóa và chất lượng của lớp mạ Crôm phụ thuộc đáng kể vào nồng độ của ion trong dung dịch, những loại ion dương và ion âm khác nhau Quá trình mạ điện hóa được tiến hành riêng biệt và theo trình tự để xác định khoảng nồng độ tối ưu của các ion, từ đó chọn ra được những ion âm và ion dương phù hợp cho quá trình mạ điện hóa với bể mạ chứa ion Cr3+ Kết quả chỉ ra rằng, ở điều kiện nồng độ của các ion trong dung dịch từ 7.75 đến 10.75 (tương ứng với nồng độ natri sulphate từ 0.25 M đến 1.25 M, nồng độ magnesium sulphate từ 0.19 M đến 0.94 M), độ pH của bể mạ từ 1.5 đến 2.0, nhiệt độ bể mạ từ 30°C đến 35°C, mật độ dòng điện trong khoảng từ 4.5 đến 7 A/dm2, lớp mạ Crôm thu được đạt chất lượng với bề dày lớp

mạ khoảng vài chục micrometer, cùng với lớp bề mặt mạ sáng bóng và đồng đều

Từ khóa: nồng độ ion, natri sulphate, mạ điện hóa, Cr 3+ , sự chống ăn mòn

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

1 INTRODUCTION

Hard chromium coatings were widely applied in various industries to improve hardness, wear ability, corrosion resistance and decorative appearance of mechanical parts and instruments Chromium coatings were usually deposited using conventional highly toxic hexavalent chromium process Although the traditional hexavalent chromium electroplating process is simple and low-cost with small amount of H2SO4 or fluoride as catalysts, its serious health and environmental problems making the hexavalent chromium deposition process faces

to environmental strictly considerations through-out the world, resulting hexavalent chromium electroplating can be possible extinction [1-2] In the past decades, continuous investigations have been done to find out a new technology that replaces the for the conventional hexavalent bath for their less toxic properties The attempts have been pointed out trivalent chromium bath is a suitable alternative with a good quality deposited coating However, trivalent chromium process stills has problems that need to be solved to extend range of applications for this technology Firstly, it is difficult to get the high current efficiency because the rate of Cr electrodeposition was reported to be rapidly diminished with deposition time and after several tens of minutes of electrolysis no mass gain [3] Secondly, trivalent chromium plating process is unable to get the thick chrome coatings, making the deposits unsuitable for hard chromium, wear-resistance and other functional applications [4] Moreover, amount of pinholes and cracks on coating surface are main channels to chloride ions penetrating into surface and corrode the coating These problems restrict the application and expansion of the Cr3+ plating process [5]

In electrodeposition process, organic additives have played important roles to improve characteristics of coating surface such as roughness surface, cracks, brightness and packing factor There are many various organic and inorganic additives, such as formaldehyde, methyl alcohol, form amide, were added into chromium electrolytes to examine the effect of additives

to coatings The content of carbon reduction and the organization of carbon particles in the deposits depend on the organic additive nature The electro-chemical behavior was significantly controlled by the formation of chromium coatings from different organic additives The behavior of various inorganic compounds was investigated, he pointed out the inorganic additives increase the solution conductivity, decrease deposition time, low throwing power, and give a better roughness surface and brightness [6 - 9] Trivalent chromium electroplating occurred two continuous reduction steps, Cr3+ ion → Cr2+ ion (at E0 = - 0.41 V

vs SCE) and then from a Cr2+ ion to Cr(s) (E0 = - 0.91V vs SCE)

During the trivalent chromium deposition process, the formation of Cr2+ ions as known

as cathodic film (CF) was very important and was directly controlled by diffusion process The formation of the CF is based on adsorbed oxo-hydroxo complexes (OHC) such as {[CrOp(OH)3-2p]q xH2O}ads, where parameters p, q, and x depend on the electrolysis and its duration Cr2+ ions are formed and adsorbed onto cathode surface by equation (2):

[𝐶𝑟(𝐼𝐼𝐼)𝐻𝐶𝑂𝑂𝑚(𝐻2𝑂)𝑛]3−𝑚+ 𝑒 → [𝐶𝑟(𝐼𝐼)𝐻𝐶𝑂𝑂𝑚(𝐻2𝑂)𝑛]𝑎𝑑𝑠2−𝑚 (2) After discharge of chromium ions, the released ions (HCOO−, H+) and H2O molecules were diffused to the bulk solution via the adsorption layer The formation of the new adsorptive layer were slower than the diffusion of ions from CF to solution This reduces adsorption on the cathode surface and CF acts a barrier that hinders the penetration of ions into the cathode surface So, the thickness of adsorption layer is an important factor for the diffusion of release ions and re-absorption of ions in trivalent chromium process

In this study, we try to invest and explain the effect of various kind of anions and cations as well as ionic strength on trivalent chromium electrodeposition to overcome the

stilling problems (current efficiency, anticorrosion, ) and find out the suitable inorganic additives for Cr (III) deposition process with varying the ionic strength

2 EXPERIMENTAL METHODS

The chemical compositions of the baths were summarized in Table 1 The solutions were heated to 700C in 2 hours to promote equilibrium state and kept in room temperature 24 hours to reach the thermodynamically stable form with completely Cr(III) complex ions formation before deposition The pH = 1.7 of solution in bath was controlled by adding of NaOH or HCl

Table 1 Composition of the baths containing the different inorganic additives

Firstly, candidate baths were conducted under a series of testing in Hull cell of 250 ml with the platinized titanium (50x65) as anode, Cu sheets (65x100) as cathode, current of 2 A

at 30°C in 10 min The range of ionic strength was determined based on the length of bright coating The current densities were preliminary calculated by equation (3)

Where: Ic - cathode current density [A/dm2]; I - total current in the cell [A]; xc - coordinate along the cathode [cm]

Secondly, the electrodeposition was conducted by using direct current electroplating potentiostat in the bath with the platinized titanium, Cu foils as anode and cathode, respectively

To evaluate the effect of anions and cations, the electroplating was carried out under potentiodynamic polarization with potential from -0.2 V to -2.5 V and chronopotentiometry with current density of 5 A/dm2, pH = 1.7 in bath at 30°C for 15 min

To examine effect of current density on current efficiency, the current density was set from 4 to 7 A/dm2 for the best source of anion, cation and ionic strength in the same conditions After electrodepostion, the coatings were clearned / rinsed by distilled water and dried by N2 gas at room temperature

To access anti-corrosion of the coatings, all coatings were examined using potentio-dynamic polarization in a standard three-electrode cell using platinized titanium electrode and

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

Saturated Calomel Electrode (SCE) as auxiliary and reference electrodes, respectively The potentiodynamic polarization tests were carried out using an EG&G potentiostat / galvanostat (model 273A) with Powersuite software, at a scan rate of 1 mV·s−1 and applied potential range from OCP to −600 and +400 mV in the cathodic and anodic directions, respectively

The current efficiency were calculated by comparing the weight of specimen before and after experiments (𝜂% =∆𝑚𝑚𝑒𝑎𝑠𝑢𝑟𝑟𝑒

𝑚𝑡ℎ𝑒𝑜𝑟𝑦 ∗ 100).The average values of current efficiency were recorded from more than 3 independent measurements

3 RESULTS AND DISCUSSION

3.1 Effect of Ionic Strength on Bright Range Examination

The bright range of coatings under Hull cell test for different ionic strength solution are shown in Fig 1 It reveals that ionic strength clearly influences to bright and current density range For free solution at ionic strength 7.00, bright range of coating is seemly footling and shows a dull coating It can be attributed to the formation of thick adsorption layer on electrode surface that acts as steric hindrance of diffusion process of released ions and re-adsorption layer on surface leading to the reduction of Cr3+ ion cannot occur continuously When ionic strength increases gradually from 7.75 to 11.50, the range of bright coating and current density are significantly improved comparing with free solution However, the narrow bright and large burning coating appear on surface at ionic strength 11.50 due to the increase

of solution’s conductivity at high ionic strength Thus, ionic strength from 7.75 to 10.75 is suggested as suitable values for trivalent chromium electrodeposition process to obtain a bright coating and optimal current density range

3.2 Effect of Anions and Cations on Potentiodynamic Polarization

Figure 2 shows the polarization curves from -0.2 V to -2.5V depended on various electrolyte with different sources of anions and cations at the same ionic strength 10.75 As mentioned above, the reduction of Cr3+ ion occurred via 2 steps, this is evidently confirmed in Fig 2 (a, b, c, d, e and f) During scanning, there are 3 regions (A, B, C) are observed for Cr (III) electrodeposition process, exception the electrolyte contain NO3- anion

In region A (Fig 2b), from - 0.2 to 0.71 V, there is a slight increase of cathodic current reflecting to no phenomena occurs on the cathode surface In region B, from - 0.72 to - 1.34 V, the current begins to increase significantly in this potential range, this indicates the reduction reaction of Cr3+ ion has happened An electroplating experiment using Cr3+ - Na+ solution at 30°C, pH 1.7 in 15 min with a constant potential of -1.30 V was conducted to examine the deposition reaction The result showed that there was no deposit at this potential The result could be ascribed to the reduction of Cr3+ to Cr2+, the first step of trivalent

Figure 1: Bright and current density range

of the copper after “Hull cell” experiments

in different ionic strengths in conditions of

pH = 1.7; T = 30 ° C and current = 2 A (a) 7.00; (b) 7.75; (c) 8.50; (d) 9.25; (e) 10.00;

(f) 0.75; (g) 11.50

chromium reduction, indicating that the reduction of Cr2+to Cr0 did not occur The reduction

of Cr3+ to Cr2+ was controlled by a diffusion process via the absorption layer In region C, when the potential increased to be more negative than -1.34 (3.8 A/dm2), the cathodic current increased rapidly This showed that another reduction reaction had occurred and the mass increased on the cathode surface Another electroplating experiment with the same conditions

at -1.5 V was performed Chromium coating was deposited on substrate at this potential, which could be assigned to the reduction of Cr2+ to Cr0, the second step of trivalent chromium reduction For cases of free solution, Cr3+ - Mg+2 solution, Cr3+ - Al+3 solution and Cr3+ -

PO43- solution, the results showed the same tendency with solution containing Na+ ion with 3 region of this potential, but the different potential range and current density for each range were found In region C, the beginning potential for Cr(III) deposition was -1.38 V (4.7 A/dm2), -1.33 V (3.3 A/dm2), -1.32 V (5.5 A/dm2) and -1.63 V (13.1 A/dm2) for free solution,

Cr3+ - Mg2+ solution, Cr3+ - Al3+ solution and Cr3+ - PO43- solution, respectively This can be attributed the various kind of source of anions and cations that were added into solution and the different configuration of adsorption layer were formed on electrode surface leading to change of potential and current density For Cr3+ - NO3- solution (Fig 2 e), in region A’, nitrate reduction occurs at cathode (-0.61 V) to form nitrite and hydroxide ions that shows in Reaction (4) It causes a significant increase in pH value at cathode leading to in the formation

of olation, polymerization and precipitation processes of Cr3+ ions that prevent strictly the Cr (III) deposition process

Figure 2: Potentiodynamic polarization curves for various electrolytes at conditions:

scan rate = 1 mV/s, pH = 1.7, T = 30 0 C 3.3 Effect of Anions, Cations and Ionic Strength on Current Efficiency

Figure 3 shows that the current efficiency depends sharply on ionic strength as well as source of anions and cations It is easily recognized that Na+ and Mg2+ ion are appropriate series of cations for Cr (III) electro-deposition process For solutions containing Al3+ ion, the current efficiency is very low and change insignificantly in increase of ionic strength It can

be explained that a co-deposition (Al3+ and Cr3+ ion) occurs at cathode when potential is more negative than -1.35 V The reduction of Al3+ ion was measured at suitable voltage bias lies

A

C

B

C

C

A

B

C

B

A

A

B

A’

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

between -1.0 V and -1.25 V versus SCE and then Al metal combined with oxygen to form aluminum oxide thin film This film has a high stability and acts as an excellent surface passivation leading to limit deposition process Thus, the current efficiency is so worse in Cr3+

- Al3+ solution This diagram also indicates that the current efficiency with Cr3+ - Na+ solution

is higher than that of Cr3+ - Mg+ solution at the same ionic strength The Na+ ion is believed as the best cation to get the high current efficiency in trivalent chromium deposition process This might be attributed the competition between ions at adsorption layer, resulting the different compositions of adsorption layer that amount of Cr3+ ions decrease with higher valence of added cations That caused a reduction of the current efficiency

To evaluate the effect of anions on current efficiency, the solution of NN5 and NP5 were prepared and performed to compare with SO42- ion at the same conditions with solution of N5

(current density – 5A/dm2, pH in bath – 1.7, temperature – 300C and time – 15 min) The results shown that no coatings were found on surface for both solution contain of NO3- and

PO43- ion As discussed in Fig 2, NO3- ion was added into electrolyte causing increase of pH value at cathode during deposition process The olation, polymerization and precipitation processes have happened for Cr3+ ion instead of deposition For Cr3+ - PO43- solution, the viscosity of solution rises dramatically comparing with solution of N5 and insoluble deposits would be produced [24] Therefore, SO42- ion is a good anion for Cr (III) electroplating to obtain a homogeneous and smoother surface with high current efficiency

Fig 3 also confirms that the suitable range of ionic strength is from 7.75 to 10.75 to get

a good coating with acceptable value of current efficiency The highest current efficiency obtains at ionic strength 10.75 (for Na+ and Mg2+ cations) with a remarkable value comparing

to free solution It can be interpreted that ionic strength increases via rise of the total concentration of ions in the bath, resulting in increase of the solution conductivity and throwing power

3.4 Effect of Current Density on Current Efficiency

In Cr(III) deposition process, the current efficiency as function of current density is indicated in Fig 4 The current efficiency increases rapidly when cathodic current density is more than 4 A/dm2 and increase continually in rise of current density However, the current density continues to accrete over 7A/dm2, the surface of coatings appear burning at edge of samples and become worse Thus, the optimal values of current density is suggested between

to be 4.5 and 7 A/dm2

Figure 4: Effect of current density on the current efficiency of chromium electrodeposition using solution of N 5 Conditions: pH = 1.7; T = 30 0 C,

time = 15 min

Figure 3: Effect of cations and ionic strength on the current efficiency of chromium electrodeposition in

different baths

Conditions: pH = 1.7; T= 30 0 C, current density = 5 A/dm 2 , time= 15 min

3.5 Corrosion Behavior

The polarization curves of coatings in free solution and baths containing Na+ and Mg2+ ions at different ionic strength are shown in Fig 5a, b and c, respectively Fig 5a shows the deposited Cr coatings in the baths containing Na+ or Mg2+ ion have a lower corrosion current comparing with Cr coating was deposited by free solution The corrosion resistance of the coating has been improved by addition of Na+ or Mg2+ ion The nano-sized grain structure of the Cr coating with reducing amount of pinholes or cracks may be contributed to the improved corrosion resistance

Figure 5 Comparison of polarization curve of different Cr coatings which electrodeposited from free solution and solution contain of Na + ; Mg 2+ ions and varying

of ionic strength

Using Tafel extrapolation method, the corresponding electrochemical parameters were extracted from polarization curves for Fig 5 which were summarized in Table 2 As observed

in Table 2, the Ecorr for all the electrodeposited coatings is approximately from – 0.35 to −0.27

V The coatings with ionic strength from 7.75 to 10.75 shows the no far difference of icorr

value for both of baths containing Na+ and Mg2+ ions It can be explained that the electrolytes contain of constant of formic acid concentration, which give a constant C content in deposits because the percentage of C atom depends directly on concentration of formic acid At a higher formic acid concentration, the corrosion resistance will be remarkable enhanced due to the high carbon content

Anodic polarization curve shows the coating dissolution process As it can be seen in Fig 5, a decrease in the chromium dissolution current for Cr coatings at Epass (from – 0.059 to – 0.075 V) corresponds to the formation of passive film on the coating But, as potential is scanned toward positive direction, the anodic current begins to rise which indicates the dissolution of the passive film It indicated that this film is not stable which could be due to the presence of micro-cracks or pinholes on the coating surface, they play as crucial path for chlorine ions to penetrate and corrode leading to the relatively large current density in the passivity region due to dissolution of the brass substrate rather than to dissolution of the deposits Moreover, the ipass is always larger than the icorr regardless of the concentration of

Na+ or Mg2+ ion This is different from the metallurgical high-purity Cr, which displays an

ipass markedly lower than the icorr

Kỷ yếu hội nghị khoa học và công nghệ toàn quốc về cơ khí - Lần thứ IV

Table 2 Corrosion parameters extracted from polarization plots with different composition baths responding to ionic strength (IS) from 7.75 to 10.75

Electrochemical corrosion tests show that Cr coatings at ionic strength 10.75 exhibit better corrosion resistance in 3.5 wt % NaCl solution which is characterized by lower icorr This ionic strength also reveals that the higher current efficiency can be obtained at 10.75, especially the bath contains Na+ ion as conducting salt

4 CONCLUSION

Potentiodynamic polarization has confirmed that trivalent chromium electrodeposition process occur via 2 steps of reduction: 𝐶𝑟3+ +𝑒− → 𝐶𝑟2+ +2𝑒− → 𝐶𝑟(0) The configuration of adsorption layer plays an important role and is significantly influenced by source of anions and cations Hull cell tests point out the ionic strength from 7.75 to 10.75 is the suitable range for deposition process The current efficiency was clearly studied under effect of different kind of anion, cation, ionic strength and current density The results show that the electrolyte contain of Na+ cation and SO2- anion at ionic strength 10.75 give the highest current efficiency The current efficiency increase rapidly in increase of current density The coatings were deposited in the bath containing added Na+ and Mg2+ ions had to be higher corrosion resistance than free solution However, the anti – corrosion of coatings are seemly similar although they were conducted with different ionic strength

ACKNOWLEDGEMENT

The authors acknowledge the financial support from the Ministry of Science and Technology of Taiwan under contrast No MOST 103-3113-E-008-003

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AUTHOR’s INFORMATION

1 Eng Nguyen Van Tai, College of Engineering Technology, Can Tho University, Viet Nam

Email: nvtai@ctu.edu.vn, nvtai87@gmail.com

2 Prof Jing - Chie Lin, Nation Central University, Taoyuan City, Taiwan

Email: jclin4046@gmail.com

3 Dr Nguyen Van Cuong, College of Engineering Technology, Can Tho University

Email: nvcuong@ctu.edu.vn, Tel.: 0989909034