Basic Process Considerations

Before describing decorative, engineering, and electroforming plating processes, some basic facts are reviewed that make it possible to control the nickel plating process, predict the amount of nickel deposited, and estimate nickel coating thickness.

The Basic Process. Nickel plating is similar to other electroplating processes that employ soluble metal anodes. It requires the passage of direct current between two electrodes that are immersed in a conductive, aqueous solution of nickel salts. The flow of direct current causes one of the electrodes (the anode) to dissolve and the other electrode (the cathode) to become covered with nickel. The nickel in solution is present in the form of divalent positively charged ions (Ni++). When current flows, the positive ions react with two electrons (2e~) and are converted to metallic nickel (Ni0) at the cathode surface. The reverse occurs at the anode, where metallic nickel is dissolved to form divalent positively charged ions, which enter the solution. The nickel ions discharged at the cathode are replenished by those formed at the anode.

Hydrogen Evolution and Cathode Efficiency. The discharge of nickel ions is not the only reaction that can occur at the cathode; a small percentage of the current is consumed in the discharge of hydrogen ions from water. This reduces the cathode efficiency for nickel deposition from 100% to 92 to 97%, depending on the nature of the electrolyte. The discharged hydrogen atoms form bubbles of hydrogen gas at the cathode surface.

Anode Efficiency. Under normal conditions the efficiency of dissolution at the anode is 100% and no hydroxyl ions are discharged from the water. If the pH of the solution is too high, however, hydroxyl ions may be discharged in preference to the dissolution of nickel, and oxygen will be evolved. Under those conditions, the nickel anode becomes passive and ceases to dissolve nickel. Activated nickel anode materials are available commercially that resist the onset of passivity and replenish the solution with nickel ions over a wide range of plating conditions.

Nickel Ion and pH Changes. Under normal operating conditions, the nickel ion concentration and the pH of the solution will slowly increase as plating proceeds. The rate of increase in nickel ion concentration depends on the difference between cathode and anode efficiencies. Because cathode efficiencies may vary from 92 to 97%, whereas anode efficiency is always 100%, the rate of increase in nickel ion concentration depends on the nature of the plating solution and not on the type of soluble nickel anode material that is used.

Faraday's Law for Nickel. The amount of nickel deposited at the cathode and the amount dissolved at the anode are directly proportional to the product of the current and time (Faraday's Law). The proportionality constant is equal to M divided by nF, where M is the molecular weight, n is the number of electrons involved in the electrochemical reaction, and F is Faraday's constant, equal to 96,500 coulombs (ampere-seconds). For nickel, the constant is 1.095 g/A • h. The constant for nickel deposition is calculated assuming that cathode efficiency is 100%; because a small part of the current goes to discharge hydrogen, the constant must be adjusted by multiplying by the cathode efficiency (for example, 1.095 * 0.955 = 1.046).

Faraday's Law for nickel may be expressed as m = 1.095 (a) (l) (t), where m is the amount of nickel deposited at the cathode (or dissolved at the anode), in grams; l is the current that flows through the plating tank, in amperes; t is the time that the current flows, in hours; and a is the current efficiency ratio for the reaction of interest. In almost all cases, the anode efficiency is 100% (a = 1). The cathode efficiency may vary from 92 to 97% and accordingly, a will vary from 0.92 to 0.97.

Average Nickel Thickness. The nickel electrodeposition data compiled in Table 1 have been calculated on the assumption that cathode efficiency is 95.5%, which approximates the case for most nickel plating solutions. From the table, one can estimate the time required to deposit a specified thickness of nickel at a specified current density. If the plating process is operated at 5 A/dm2, for example, it takes about 20 min to deposit a nickel coating with an average thickness of 20 ^m.

Table 1 Nickel electrodeposition data

Deposit thickness, ^m

Weight per unit area, g/dm2

Amp hours per unit, A • h/dm2

Time (min) required to obtain deposit at current density (A/dm2) of:

0.5

1

1.5

2

3

4

5

6

8

10

2

0.18

0.17

20

10

6.8

5.1

3.4

2.6

2.0

1.7

1.3

1

4

0.36

0.34

41

20

14

10

6.8

5.1

4.1

3.4

2.6

2

6

0.53

0.51

61

31

20

15

10

7.7

6.1

5.1

3.8

3.1

S

0.71

0.68

82

41

27

20

13

10

8.2

6.8

5.1

4.1

10

0.89

0.85

100

51

34

26

17

13

10

8.5

6.4

5.1

12

1.1

1.0

120

61

41

31

20

15

12

10

7.7

6.1

14

1.2

1.2

140

71

48

36

24

18

14

12

8.9

7.1

16

1.4

1.4

160

82

54

41

27

20

16

14

10

8.2

18

1.6

1.5

180

92

61

46

31

23

18

15

11

9.2

20

1.8

1.7

200

100

68

51

34

26

20

17

13

10

40

3.6

3.4

410

200

140

100

68

51

41

34

26

20

Note: Values are based on 95.5% cathode efficiency.

The data in Table 1 provide a means of estimating the average coating thickness. The actual thickness on an individual part depends on the uniformity of current density distribution. Under practical plating conditions, the thickness of the nickel on a batch of parts is measured in one or more trials, and adjustments are made, if necessary, as to how the parts are placed in the tank relative to the anode and how they are positioned on the plating racks. In some cases, shields and auxiliary anodes may be required to obtain acceptable thickness uniformity. Shields are made of nonconductive materials and may be placed on the anode, on the cathode, or between electrodes to block or control current flow. Auxiliary anodes may be either soluble or insoluble, and they are placed closer to the cathode than principal anodes so as to direct current to a recessed or relatively small area on the cathode. With care, current density distribution and coating thickness can be made reasonably uniform and predictable.

0 0

Post a comment