Nickel plating line misses a thickness audit after a rack fault
A job-shop plating line coats steel brackets with nickel from a Watts bath (NiSO4·6H2O, NiCl2, boric acid, pH 4.0, 50 °C). Production runs 12 brackets per rack, each bracket exposing 0.0045 m² of surface. The rectifier is set to 8.0 A for 45 min, and the bath is claimed to run at 94% current efficiency. Nickel density is 8.90 g cm⁻³ and M(Ni) = 58.69 g mol⁻¹. The customer specification is a minimum 6.0 µm nickel thickness, with a target of 8.0 µm to allow for later grinding.
Quality control finds three racks: rack A averages 11.8 µm, rack B averages 6.2 µm, and rack C averages 5.1 µm. On rack C, the rectifier still displays 8.0 A, but the tank voltage is 1.1 V higher than usual and one end of the rack feels warm. Scattered pits appear along the high-current edges of several parts. The production manager proposes simply doubling the plating time to 90 min for all racks.
1. Calculate the theoretical average thickness for one rack at 94% efficiency, 8.0 A and 45 min. Then decide whether doubling the time to 90 min is a sound fix for rack C. Support the decision with numbers.
Read the worked answer
Charge passed per rack
Q = It = 8.0 A × (45 × 60 s) = 21,600 C
Nickel deposited
Ni²⁺ + 2e⁻ → Ni, so n(Ni) = Q/(2F) = 21,600/(2 × 96,485) = 0.1119 mol
m(Ni) = 0.1119 × 58.69 = 6.57 g (100% efficiency)
At 94%: m = 6.57 × 0.94 = 6.18 g
Thickness
V = m/ρ = 6.18/8.90 = 0.694 cm³
Total area = 12 × 0.0045 m² = 0.054 m² = 540 cm²
t = V/A = 0.694/540 = 1.285 × 10⁻³ cm = 12.85 µm
So a healthy rack should average about 12.9 µm, comfortably above the 6.0 µm minimum and the 8.0 µm target. Racks B and C are far below this, so the problem is not insufficient plating time. Rack C at 5.1 µm corresponds to an effective efficiency of only 0.94 × 5.1/12.85 ≈ 0.37, i.e. about 37%. At 94% efficiency, 8.0 µm would need only t = 45 × 8.0/12.85 ≈ 28 min.
Doubling to 90 min would deposit about 25.7 µm on a healthy rack, wasting nickel and risking edge buildup, while rack C would reach only about 10.2 µm if the fault persists. That nominally passes 6.0 µm but misses the 8.0 µm target and gives more hydrogen evolution and pitting. The correct action is to diagnose rack C, not to double the time globally.
2. Identify the most likely cause of the warm rack end and the low thickness on rack C. Propose two changes that improve thickness uniformity without increasing the total charge passed.
Read the worked answer
The warm end points to a high-resistance contact at the rack hook, busbar or jig. A poor contact dissipates I²R heat and can divert current through a parallel path such as a wet rack, a stray contact or the tank wall. The rectifier display may then show total current while the brackets receive much less, which explains why rack C is thin even though the meter reads 8.0 A. The pitting at high-current edges is consistent with locally excessive current density and hydrogen bubble masking elsewhere.
Two charge-neutral improvements:
Strip, clean and re-torque all rack contacts, then verify current per rack with a clamp meter rather than trusting the rectifier display. This removes the contact resistance without changing the set charge.
Improve current distribution geometrically: add thieving shields or robbers at the high-current edges, reposition the rack in the bath, and increase solution agitation. Lowering the current density and extending the time proportionally keeps the same coulombs but gives a more uniform deposit and less edge burning.
A leveling additive or pulse plating at the same average current can also help, but the contact fault must be fixed first.
3. The bath pH rises near the cathode during plating. Write the relevant cathode reactions and explain why boric acid and agitation matter, and how a pH rise can produce pitting.
Read the worked answer
The main cathode reaction is:
Ni²⁺ + 2e⁻ → Ni
A competing side reaction is hydrogen evolution:
2H⁺ + 2e⁻ → H2
and, once H⁺ is depleted near the surface:
2H2O + 2e⁻ → H2 + 2OH⁻
This generates hydroxide at the cathode and raises the local pH. If the pH rises too far, Ni(OH)2 can precipitate in the diffusion layer and become incorporated in the deposit, producing pits, nodules or poor adhesion. Hydrogen bubbles adhering to the surface also mask areas from nickel deposition, leaving pits.
Boric acid is a weak acid buffer that holds the cathode layer near pH 4 and reduces Ni(OH)2 precipitation. Agitation replenishes Ni²⁺ and H⁺ at the surface, sweeps away hydrogen bubbles and reduces the thickness of the diffusion layer, which improves both deposit quality and uniformity. Without adequate agitation, a high local current density makes the pH rise and pitting worse.