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Nickel is a very common electroplate that is typically applied as a preliminary to other electroplates. Numerous small articles (eg. key blanks ) that are to be electroplated with nickel (a preliminary to many other electroplates) are typically barrel plated. The small parts are slowly tumbled in a barrel shaped cage made of non‑conducting material which is submerged in a tank of the electrolyte. Cathodic electrical contact with the mass of small metallic articles that are to be plated is maintained with flexible metal wires (danglers) that enter the barrel through the hubs at each end. Stationary anodes line the inside of the tank and in this way surround the barrel and parts within. Effectively the average area of the cathode is thus about 10 times that of the anode; the boundary layer at the cathode is 0.01 cm whereas that at the anode (where the electrolyte is less well stirred) is about 0.03 cm. The Watts nickel bath that is used in this process can be described as follows:
280 g L-1 NiSO4.6H2O, 40 g L-1 NiCl2.6H2O, enough boric acid to buffer the pH at about 4 (note, this compound does not introduce a significant population of ions). The density of nickel is 8.9 g cm-3 and the current efficiency for plating is virtually 100%.
The
ionic mobilities are as follows for the operating temperature of 325 K.
|
Ion |
Mobility |
|
(cm2 s-1
V-1) |
|
|
Ni2+ |
6x10-4 |
|
Cl- |
8.5x10-4 |
|
SO42- |
9.0x10-4 |
Determine the minimum time required to deposit an average
of 2.5x10-3 cm of nickel on the parts, realizing that you first have
to determine whether the maximum current density is limited by:
anode blocking which occurs when the nickel concentration adjacent to the anode reaches 2.8 molar or by
depletion of nickel ions in the electrolyte contacting the cathode surface
What recommendations would you make to the electroplating
firm to increase the production rate of a given barrel?
Constant |
F (C mol (e)-1)
= |
96485 |
|
Data |
Surface (cm2) = |
1 |
|
Ni |
MW (g mol-1)= |
58.69 |
|
n (e) = |
2 |
|
|
density (g cm-3)
= |
8.9 |
|
|
thickness (mm) = |
25 |
|
|
NiCl2.6H2O |
MW (g mol-1)= |
237.69 |
|
NiSO4.6H2O |
MW (g mol-1)= |
262.85 |
|
Diffuse layer |
cathode (cm ) = |
0.01 |
|
anode (cm) = |
0.03 |
|
|
Temperature |
(K) = 325 |
This problem is solved by referring to the Nernst diffusion layer for cases when reactants are depleted at the working electrode or for cases when the reactants buildup in the Nernst diffusion layer eventually renders an electrode blocking.
