Types of Electrolytes

Tin can be deposited from either alkaline or acid solutions. Electrolyte compositions and process operating details are provided in Ref 1, 2, and 3, as well as in publications of the International Tin Research Institute. Table 2 gives the basic details of electrolyte composition and operating conditions for alkaline solutions, and Tables 3 and 4 provide this information for acid solutions. Tin ions in the alkaline electrolytes have a valence of +4, whereas those in the acid electrolytes have a valence of +2. Consequently, the alkaline systems require the passage of twice as much current to deposit one gram-molecule of tin at the cathode.

Table 2 Composition and operating conditions for stannate (alkaline) tin plating electrolytes

Values of composition are for electrolyte startup; operating limits for the electrolyte composition are approximately -10 to + 10% of startup values

Table 2 Composition and operating conditions for stannate (alkaline) tin plating electrolytes

Values of composition are for electrolyte startup; operating limits for the electrolyte composition are approximately -10 to + 10% of startup values

Solution

Composition

Operating conditions

Potassium stannate

Sodium stannate

Potassium hydroxide

Sodium hydroxide

Tin metal(a)

Temperature

Cathode current density

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

°C

°F

A/dm2

A/ft2

A

105

14

15(b)

2(b)

40

5.3

6688

150190

3-10

30100

B

210

28

22

3

80

10.6

7788

170190

0-16

0160

C

420

56

22

3

160

21.2

7788

170190

0-40

0400

D

105(c)

14

10(b)

1.3(b)

42

5.6

60-

140-

0.5-3

(b) Free alkali may need to be higher for barrel plating.

(c) Na2SnO3 ■ 3H2O; solubility in water is 61.3 g/L (8.2 oz/gal) at 16 °C (60 °F) and 50 g/L (6.6 oz/gal) at 100 °C (212 °F)

Table 3 Composition and operating conditions for sulfate (acidic) tin plating electrolyte

Constituent

Amount

Operating limits

g/L

oz/gal

g/L

oz/gal

Stannous sulfate

80

10.6

60-100

8-13

Tin metal, as sulfate

40

5.3

30-50

4-6.5

Free sulfuric acid

50

6.7

40-70

5.3-9.3

Phenolsulfonic acid(a)

40

5.3

30-60

4-8

P-naphthol

1

0.13

1

0.13

Gelatin

2

0.27

2

0.27

Note: Temperature range for sulfate electrolytes is 21 to 38 °C (70 to 100 °F), and they do not require heating. Cooling can be considered if temperature rises to reduce adverse effects of heat on the electrolyte constituents. Cathode current density is 1 to 10

(a) Phenolsulfonic acid is most often used. Cresolsulfonic acid performs equally well and is a constituent of some proprietary solutions.

Table 4 Composition and operating conditions for fluoborate tin (acidic) plating electrolyte

Constituent or condition

Standard

High-speed

High throwing power

Electrolyte, g/L (oz/gal)

Stannous fluoborate

200 (26.7)

300 (39.7)

75 (9.9)

Tin metal(a)

80 (10.8)

120 (16.1)

30 (4.0)

Free fluoboric acid

100 (13.4)

200 (26.8)

300 (40.2)

Free boric acid

25 (3.35)

25 (3.35)

25 (3.35)

Peptone(b)

5 (0.67)

5 (0.67)

5 (0.67)

ß-naphthol

1 (0.13)

1 (0.13)

1 (0.13)

Hydroquinone

1 (0.13)

1 (0.13)

1 (0.13)

Temperature, °C (°F)

16-38(c) (60-100)(c)

16-38 (60-100)

16-38 (60-100)

Cathode current density, A/dm2 (A/ft2)

2-20 (20-200)

2-20 (20-200)

2-20 (20-200)

Note: The standard electrolyte composition is generally used for rack or still plating, the high-speed composition for applications like wire plating, and the high-throwing-power composition for barrel plating or applications where a great variance exists in cathode current density as a result of cathode configuration.

Note: The standard electrolyte composition is generally used for rack or still plating, the high-speed composition for applications like wire plating, and the high-throwing-power composition for barrel plating or applications where a great variance exists in cathode current density as a result of cathode configuration.

(a) As fluoborate.

(c) Electrolytes do not require heating. Cooling may be considered if temperature rises to reduce adverse effects of heat on the electrolyte constituents.

Alkaline electrolytes usually contain only a metal stannate and the applicable hydroxide to obtain satisfactory coatings. Unlined mild steel tanks are satisfactory. These can be heated by electrical immersion heaters, steam coils, or external gas burners. If steam coils are used, they should be supported 5 to 10 cm (2 to 4 in.) above the bottom of the tank to allow sediment to remain undisturbed. It is not necessary to filter still baths of this type, except at infrequent intervals. The electrical equipment is the same as that used in other plating operations. A rectifier for converting alternating current to direct current or a pulse-plating rectifier, which allows more precise control of electrical parameters, can be used. Factors such as operating temperature, solution constituent concentration, and operating current density all affect the efficiency and plating rate of the system and must be properly balanced and controlled.

Unusual operating conditions of the alkaline electrolytes involve:

• Tin anode control and electrochemical solution mode (discussed below)

• Cathodic deposition occurring from Sn+4

• Solubility of the alkaline stannate in water

Ninety percent of the problems encountered in alkaline tin plating result from improper anode control. Conversely, operating the alkaline electrolytes is simple if one understands anode behavior, because there are no electrolyte constituents except the applicable stannate and hydroxide.

Tin anodes must be properly filmed, or polarized, in alkaline solutions to dissolve with the tin in the Sn+4 state. Once established, the anode film continues to provide the tin as Sn+4. The anodes can be filmed either by subjecting them for about 1 min to a current density considerably above that normally used, or by lowering them slowly into the bath with the current already flowing.

Three reactions are possible at tin anodes in alkaline solutions:

Equation 1 represents the overall process occurring at the anodes when the film is intact and the tin is dissolving as stannate ion, with tin in the Sn+4 state. Film formation is confirmed by a sudden increase in the electrolyte cell voltage, a drop in the amperage passing through the cell, and the observation of a yellow-green film for pure tin anodes. High-speed anodes (containing 1% Al), used for tinplate production, turn darker. Because the anodes do not function at 100% efficiency when filmed, moderate gassing occurs as the result of the generation of oxygen, as in Eq 3.

Equation 2 is the process occurring if there is no film and the tin is dissolving as stannite ion, with tin in the Sn+2 state. The presence of stannite in the electrolyte produces unsatisfactory plating conditions, and the deposit becomes bulky, rough, porous, and nonadherent. The addition of hydrogen peroxide to the electrolyte oxidizes the Sn+2 to Sn+4, returning it to a usable condition. If this remedy is required frequently, it indicates other problems that must be addressed. The concentration of caustic may be too high. This can be remedied with the addition of acetic acid.

Equation 3 shows the decomposition of hydroxyl ion with the formation of oxygen. While this is a normal reaction at the anode, it should not be permitted to become the dominant reaction, as occurs when the anode current density is too high. Under this condition, no tin dissolves and the anodes take on a brown or black oxide film. The anode current density should be reduced until the normal film color returns. If this is allowed to become thick enough, it is removable only by the action of strong mineral acids. Stannate baths normally appear colorless to straw colored, and clear to milky, depending on the quantity of colloidal material present. If an appreciable quantity of stannite builds up in the bath, it will appear light to dark gray, depending on the quantity of stannite that has formed. The gray color is caused by the precipitation of colloidal tin as a result of the disproportionation of stannite:

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