RU2292408C1 - Pyrophosphate containing electrolyte for depositing tin-zinc alloy - Google Patents

Abstract

FIELD: electroplating processes for deposition of tin-zinc alloy coatings.

SUBSTANCE: electrolyte for deposition of coatings of tin-zinc alloy including 70 - 80% of tin contains, g/l: tin (II) chloride with 2 molecules H₂O (on conversion to metal), 17 - 19; zinc sulfate with 7 molecules H₂O (on conversion to metal) 3.5 - 7.5; sodium pyrophosphate with 10 molecules H₂O, 130 - 155; gelatin, 1 - 2; methylene blue (m. m. 373.9), (0.05 - 1.0)x10^-3 mol/l.

EFFECT: possibility for depositing semi-bright tin-zinc alloy coatings having good cohesion with base, increased electric current yield, improved stability of electrolyte.

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Description

The invention relates to the galvanic production of tin-zinc alloy coatings with a tin content of 70-80%.

Pyrophosphate, alkaline cyanide electrolytes based on divalent tin salts are known [1-2].

The disadvantages of all electrolytes containing tin salts with an oxidation state of +2 are their instability in operation, since tin (II) is oxidized, which leads to a change in the composition of the alloy and the quality of the coating.

Of the known electrolytes, the closest in composition and technological characteristics is an electrolyte containing (g / l): tin chloride dihydrate 34; zinc oxide 3-8; ammonium chloride 100; potassium pyrophosphate 144; gelatin 0.5; hydrazine sulfate 5-10 [3]. However, the introduced antioxidants (reducing agents), such as hydrazine sulfate, formalin, glucose, ascorbic acid, etc., cannot fully solve this problem, since they are oxidized during operation.

The technical result of the proposed electrolyte is to obtain semi-shiny, well adhered to the base coatings of the tin-zinc alloy, with a high current efficiency. The electrolyte is stable in operation.

This is achieved by the fact that the pyrophosphate electrolyte for applying the tin-zinc alloy containing (g / l): tin (II) chloride is two-water (in terms of metal) 17-19, zinc sulfate is seven-water (in terms of metal) 3,5- 7.5, sodium pyrophosphate decahydrate 130-155, gelatin 1-2 additionally contains a redox active additive - phenothiazine dye methylene blue (methylene blau) (0.05-1.0) · 10 ^-3 mol / L.

The mechanism of action of the redox active additive (methylene blue) is that at optimal pH values, the cathode and anode current densities and temperatures, the redox active additive (methylene blue) is restored at the cathode and is an electron donor for tin (IV) located in the electrolyte, restoring the latter to tin (II).

The dynamic equilibrium of the oxidized and reduced forms of the redox active additive and tin (II) and tin (IV) ions is established in the solution, while tin (IV) ions are practically absent in the electrolyte.

The mechanism of action of a redox active additive (methylene blue) can be described by the following reaction equations:

1. The recovery of the redox active additives at the cathode according to equation (1).

2. The recovery of tin (IV) ions in the electrolyte volume of a redox active additive according to equation (2).

Reaction 1

Reaction 2

He identified solutions with signs of the claimed electrolyte.

To study the effect of the concentration of the redox active additive on the change in the concentration of tin (II), tin (IV) in the electrolyte, the current efficiency of the alloy, and the tin content in the alloy, an aqueous electrolyte was prepared, the composition of which is shown in Table 1.

Table 1. The composition of the electrolyte. Electrolyte No. 1 Electrolyte No. 2 Electrolyte No. 3 Tin chloride dihydrate (in terms of metal), g / l 19 17 eighteen Zinc sulfate seven-water (in terms of metal), g / l 5 3,5 7.5 Sodium pyrophosphate decahydrate, g / l 145 130 155 Gelatin, g / l 1-2 1-2 1-2 Methylene blue, · 10 ^-3 mol / l 0.25-0.5 0.05-0.25 0.5-1.0

The electrolyte was prepared as follows.

In a separate container, dissolved, according to the composition of the electrolyte, salts of tin and zinc. Sodium pyrophosphate was dissolved in another vessel. Then, a solution of sodium pyrophosphate was poured into a container with a solution of tin (II) salts and zinc. The precipitate of tin and zinc pyrophosphates formed was filtered off, washed with water, and then dissolved in the remaining sodium pyrophosphate solution. A solution of gelatin and a solution of methylene blue were introduced into the solution, and then the electrolyte volume was adjusted to a predetermined value.

The electrolysis was carried out in a bath at an electrolyte temperature of 30 ° C, a cathode current density of 1.5 A / dm ² , with a ratio of the working surface of the cathodes and anodes 1: 2.

Electrolysis was carried out to achieve 14 A · h / l.

During the electrolysis, changes in the concentration of tin (II), tin (IV) were analyzed, and the current efficiency of the alloy and the tin content in the alloy were investigated.

Data on the effect of the concentration of the redox active additive (methylene blue) on the change in the concentration of tin (II), tin (IV) in the electrolyte, the current efficiency of the alloy, and the tin content in the alloy are shown in table 2.

It should be noted that in the presence of a redox-active additive (methylene blue) in the electrolyte, the coatings turn out to be more shiny. So, if at C _add = 0 the coatings are matte with a gray tint, with the introduction of the additive C _add = 5 · 10 ^-5 ... 2.5 · 10 ^-4 mol / l, the coatings are semi-shiny. In the absence of an additive in the electrolyte, from 0 to 14 A · h / L, the tin content in the alloy decreases from 60 to 34.5%, and in the presence of a redox active additive with C _add = 5 · 10 ^-4 mol / L tin in the alloy does not even increase significantly, which is associated with a constant concentration of Sn (II) in the electrolyte.

As follows from table 2, the optimal concentration of redox-active additives (methylene blue) is 2.5 · 10 ^-4 -5.0 · 10 ^-4 mol / l. At these concentrations, electrolyte stability and good coating quality are ensured. At additive concentrations greater than 1 · 10 ^-3 mol / L, the current efficiency of the alloy decreases, which is obviously associated with excessive adsorption of methylene blue on the surface of coated parts.

The redox active additive (methylene blue) is introduced into the electrolyte in oxidized form. Electrolysis showed that in the cathode space the redox-active additive (methylene blue) is completely restored, while in the anode space it remains in the oxidized form. In the electrolyte volume, a redox-active additive (methylene blue) is present in both forms. The reduced form of the redox active additive diffuses into the electrolyte volume and stabilizes the concentration of Sn (II). The oxidized form of the redox-active additive diffuses into the cathode space, where it is reduced. Thus, the redox-active additive (methylene blue) does not allow Sn (II) to transfer to Sn (IV).

The alloy composition and current efficiency are influenced by the concentration of metal ions discharged at the cathode, current density, temperature and pH of the electrolyte.

According to table 3, the concentration of zinc in the electrolyte has a significant effect on the composition of the alloy. With an increase in the zinc concentration in the electrolyte from 0.025 mol / L to 0.1 mol / L, the tin content in the alloy decreases from 86 to 69%, the current efficiency of the alloy increases from 64 to 71%. In the investigated concentration range of zinc and tin ions, semi-shiny coatings are deposited by the alloy on the cathode.

The mathematical dependence of the tin content in the alloy on the concentration of zinc in the electrolyte obeys polynomial, logarithmic and exponential equations with high correlation coefficients:

y = 0.1875x ² -4.725x + 94.75 R ² = 0.9997; y = -12.271lgx + 95.005 R ² = 0.9909; y = 92.021e ^-0.037x R ² = 0.9921.

Current density affects alloy composition and current efficiency. According to table 4, with an increase in current density from 1 to 2 A / dm ^2, the tin content in the alloy decreases from 79 to 60%, the current efficiency of the alloy also decreases from 74 to 62%. In a given range of current densities, semi-brilliant alloy coatings are deposited. At current densities above 2.5 A / dm ^2, matte coatings are deposited.

The mathematical dependence of the tin content in the alloy on the cathode current density also obeys polynomial, logarithmic and exponential equations with high correlation coefficients:

y = 2.5714x ² -19.686x + 96 R ² = 0.9995; y = -17.359lgx + 79.01 R ² = 0.9999; y = 89.224e ^-0.1367x R ² = 0.9852.

As follows from the data of table 5, the temperature of the electrolyte affects the composition of the alloy and the current efficiency of the alloy. With increasing temperature of the electrolyte from 22 ° C to 52 ° C, the tin content in the alloy increases from 65 to 78%, the current efficiency of the alloy also increases from 63 to 82%. In the temperature range of 20-40 ° C, semi-shiny coatings are deposited with the alloy, and at higher temperatures matte coatings are deposited.

The mathematical dependence of the tin content in the alloy on the temperature of the electrolyte obeys the polynomial, logarithmic and exponential equations:

y = -0.0131x ² + 1.359x + 42.224 R ² = 0.9624; y = 14.015lgx + 23.091 R ² = 0.9461; y = 59.724e ^0.0054x R ² = 0.8737.

According to table 6, the pH of the electrolyte affects the composition of the alloy and the current efficiency. With an increase in pH from 7.5 to 9.0, the tin content in the alloy increases from 67 to 75%, the current efficiency decreases from 76 to 70%. In the pH range of 8–9.5, semi-shiny coatings are deposited by the alloy, and at pH <7.5, dark coatings are deposited.

The mathematical dependence of the tin content in the alloy on the pH of the electrolyte obeys the polynomial, logarithmic and exponential equations:

y = -3x ² + 54.5x-172.75 R ² = 0.964; y = 41,422lgx-15,563 R ² = 0.912; y = 40.094e ^0.0704x R ² = 0.8908.

The use of three equations eliminates a random error in the automated control of the technological process of electrodeposition of a tin-zinc alloy.

Thus, the use of a redox active additive (methylene blue) makes it possible to stabilize the electrolyte composition with tin (II) ions and precipitate semi-shiny coatings with an alloy with a tin content of 65-75%.

The advantages of industrial use of the inventive electrolyte:

1. The lack of oxidation of divalent tin, and therefore, a stable electrolyte in operation.

2. Obtaining a tin-zinc alloy without changing the composition of the alloy during electrolysis and reducing the quality of the coating.

table 2 The dependence of the dynamics of changes in the concentration of Sn (II) - Sn (IV), VT _spl , the percentage of tin in the alloy on the concentration of the additive in the electrolyte. The concentration of additives, mol / l Q, A · h / l 0 one 5 10 fourteen 0 (no supplementation) C _{Sn (II)} , mol / L 0.141 0.144 0.151 0.152 0.151 C _{Sn (IV)} , mol / L 0.02 0.02 0.06 0.09 0.1 VT _spl 56.8 fifty 43 35 32 ω _Sn ,% 60 53 42 37 34.5 5 · 10 ^-5 C _{Sn (II)} , mol / L 0.142 0,120 0.106 0,100 0,101 C _{Sn (IV)} , mol / L 0 0.012 0,035 0,038 0,038 VT _spl 56.7 57 53 fifty fifty ω _Sn ,% 60 58 54 55 55 2.5 · 10 ^-4 C _{Sn (II)} , mol / L 0.141 0.131 0.122 0.123 0.122 C _{Sn (IV)} , mol / L 0 0.005 0.0076 0.0076 0.0075 VT _spl 56.7 57.7 57.1 57.1 57.2 ω _Sn ,% 60,2 59 56 56 56 5 · 10 ^-4 C _{Sn (II)} , mol / L 0.141 0.146 0.149 0.150 0.149 C _{Sn (IV)} , mol / L 0 0.002 0 0 0.002 VT _spl 56.8 62 63 62 63 ω _Sn ,% 60 63 65 64 65 1 · 10 ^-3 C _{Sn (II)} , mol / L 0.135 0.146 0.150 0.151 0.150 C _{Sn (IV)} , mol / L 0 0 0 0 0 VT _spl 55 62 62 61 62 ω _Sn ,% 60 63 65 67.2 67

Table 3 The dependence of the composition of the alloy on the concentration of zinc in the electrolyte. Czno, g / l 2 four 6 8 ω _Sn ,% 86 79 73 69 BT,% 64 68 70 71 ω _Zn ,% fourteen 21 27 31 Table 4 Dependence of alloy composition and current efficiency of the alloy on current density ic, A / dm ² 1,0 1,5 2.0 2,5 3.0 ω _Sn ,% 79 72 67 63 60 BT,% 74 69 66 64 62 Table 5 The dependence of the composition of the electrolyte, the composition of the alloy and the current efficiency of the alloy on temperature. t ° C 22 27 32 42 52 CSn ²⁺ , _{mol / L} 0.150 0.151 0.150 0.152 0.150 CSn ⁴⁺ , _{mol / L} 0 0 0 0 0 ω _Sn ,% 65 70 73 75 78 BT,% 63 65 70 77 82 Table 6 The dependence of the composition of the alloy and the current efficiency of the alloy on pH. pH 7.5 8.0 8.5 9.0 ω _Sn ,% 67 72 73 75 BT,% 76 72 70 70

Information sources

1. Yampolsky A.M., Ilyin V.A. Quick reference electroplating. - 3rd ed., Revised. and add. - L .: Engineering, Leningrad. Separation. 1981, p. 124.

2. Larin I.O., Maksimenko S.A., Tyutina K.M., Kudryavtsev V.N. The influence of some organic substances on the process of tin oxidation in acidic electrolytes for the deposition of tin and its alloys. Progressive technology and environmental issues in electroplating and PCB manufacturing: Conference proceedings. Penza, 1996, p. 6.

3. Vahramyan T.A., Odeosama B.N. Some features of the electrodeposition process of zinc-tin alloy. Replacing and reducing the cost of scarce metals in electroplating. Seminar materials. M., 1983, p. 116-119.

Claims (1)

Pyrophosphate electrolyte for the deposition of tin-zinc alloy containing tin (II) chloride two-water, zinc sulfate seven-water, sodium pyrophosphate ten-water, gelatin, characterized in that it additionally contains a redox active additive methylene blue in the following ratio, g / l: Tin (II) chloride, two-water (in terms of metal) 17-19 Zinc sulfate seven-water (in terms of metal) 3,5-7,5 Sodium pyrophosphate decahydrate 130-155 Gelatin 1-2 Methylene blue, mol / l (0.05-1.0) · 10 ^-3