Zinc Plating Shortfall: Time, Current Density or Bath Chemistry?
A plating shop zinc-plates steel brackets in an acid chloride bath. One batch carries 0.20 m² of surface to be coated with 20 micrometre of zinc. The rectifier is set at 20 A and the bath datasheet quotes 92 per cent cathode efficiency for zinc, atomic mass 65.4, density 7140 kg per cubic metre, one faraday = 96500 C. Parts are degreased, pickled and rinsed before loading. After a 40 minute run the magnetic thickness gauge reads 7.5 micrometre at five points, far below the 20 micrometre minimum on the drawing. The operator reacts by raising the current to 45 A; within 10 minutes the edges are dark, rough and powdery, and the bath pH has drifted from 5.2 to 5.8. Inspection rejects the batch as burnt. The shop has no Hull cell and no bath analysis sheet for the last two months.
1. Calculate the charge and time needed for 20 micrometre at 92 per cent efficiency, and use the measured 7.5 micrometre to decide whether the problem is time, current or bath chemistry.
Read the worked answer
Volume of zinc = 0.20 m² × 20 × 10⁻⁶ m = 4.0 × 10⁻⁶ m³. Mass = 4.0 × 10⁻⁶ × 7140 = 0.02856 kg = 28.56 g. Moles = 28.56/65.4 = 0.4367 mol. Since Zn²⁺ + 2e⁻ → Zn, charge = 2 × 0.4367 × 96 500 = 84 280 C.
At 20 A: 84 280/20 = 4214 s = 70.2 min at 100 per cent, and 70.2/0.92 = 76.3 min (4580 s) at 92 per cent.
A 40 min run at 92 per cent should have produced 20 × 40/76.3 = 10.5 micrometre, but the gauge reads 7.5. Implied efficiency = 92 × 7.5/10.5 ≈ 66 per cent.
Judgement: the clock is not the only shortfall. Even at the correct 40 minutes the bath should have given 10.5 micrometre, so roughly a quarter of the expected metal never arrived. That points to bath chemistry or electrical contact - low Zn²⁺, pH drift, a cold bath, or poor jig contact - not simply to a short plating time. Adding time to a deficient bath wastes labour and produces a coarse deposit.
2. Compute the cathode current density at 20 A and at 45 A, and explain the burnt edges and the pH rise.
Read the worked answer
Area = 0.20 m². At 20 A: 20/0.20 = 100 A/m² = 10 mA/cm². At 45 A: 45/0.20 = 225 A/m² = 22.5 mA/cm², which is 1.5 times the datasheet maximum of 15 mA/cm².
At high current density the zinc ions near the cathode cannot be replenished by diffusion fast enough, so hydrogen evolution takes over. Three consequences follow. First, the local pH at the cathode climbs steeply as H⁺ is consumed, precipitating zinc hydroxide and oxide that codeposit as a dark, powdery, non-adherent layer. Second, the effect is worst where current density is highest - edges and corners - which is exactly where the burn appeared. Third, hydrogen uptake and pitting risk on the steel rise.
Raising the current cannot cure a bath that delivered 7.5 micrometre instead of 10.5: past the limiting current density it makes the deposit worse, not thicker, and roughly doubles sludge and drag-out losses. A pH of 5.8 in an acid chloride bath (5.2 was already at the top of the normal band) says the bath is out of balance, with zinc depleted and acid consumed. It needs analysis, not more amperes.
3. Set a corrected schedule at 15 mA/cm² and 80 per cent efficiency, and state the process checks you would impose before restarting production.
Read the worked answer
At 15 mA/cm² the current is 0.20 m² × 150 A/m² = 30 A. Required charge at 80 per cent efficiency = 84 280/0.80 = 105 350 C. Time = 105 350/30 = 3512 s = 58.5 min, so schedule a 60 minute run and pull a sample at 55 minutes for thickness.
Checks before the clock restarts:
- Analyse Zn²⁺ and total chloride, top up to the datasheet range, and bring pH 5.8 back to about 5.0 with dilute acid added slowly to a cool, agitated bath.
- Check bath temperature and agitation; without cathode movement the limiting current density at edges is far below the average.
- Verify jig contact and current distribution; burnt edges often mean the nearest part is drawing more than its share.
- Plate a test coupon and measure five points before loading the production batch.
Judgement: 60 minutes at 30 A (15 mA/cm²) with 80 per cent efficiency is the defensible schedule. The 45 A setting is outside the process window and will keep producing rejects faster than the original 20 A setting did.