Scope of this guide
Senior version: all six fixes A–F with parameter windows, the rack-position diagnostic method, and the 8%→0.5% line case.
Pitting is the most-asked bright nickel defect on finishing forums — and most answers online jump straight to anti-pitting additive. This series (four levels) starts from classification instead.
The triage table: six pits, six causes
Magnifier pass first. Every pit type has a signature; match before you mix anything.
| Pit type (magnifier) | Signature look | Root cause | First fix |
|---|---|---|---|
| Gas pit | Teardrop tail, shiny bottom | Hydrogen sticking to the part | Raise surface tension to 35–45 dyn/cm |
| Solids pit | Gritty rough bottom | Particles in bath or on part | Filter finer; improve pre-dip |
| Burn pit | Dendritic, on high-CD zones | Local current density too high | Lower CD / reposition anodes |
| Base-metal pit | Pit in substrate visible under deposit | Porosity or scale on casting | Bake-test; fix upstream casting |
| Oil pit | Doughnut ring, wet look | Oil droplet pinned to surface | Skim, degrease, check cleaner |
| Organic pit | Irregular, random spread | Decomposed brightener | Hull cell test; carbon treat |
Fix window A–F
| Fix A — gas pits | Surface tension 35–45 dyn/cm (stalgmometer); wetter 0.5–1.5 mL/L; cathode rod strokes 20–30/min full dip travel |
|---|---|
| Fix B — solids | Filtration 2–5 turns/hour, 5 µm nominal; anode bags intact; transfer racks through cleaner, not over it |
| Fix C — burning | Cut CD 0.5 A/dm² steps; check anode-to-cathode gap ≥ 150 mm; nickel anode length minus 10 cm if bottom parts pit |
| Fix D — base metal | Bake zinc diecast 100–110 °C × 1–2 h before plating; reject lots that blister in bake test |
| Fix E — oil | Skim daily; alkaline soak 55–70 °C; electro-clean 3–5 A/dm²; check for hydraulic oil leaks at line |
| Fix F — organics | Hull cell 267 mL / 2 A standard; carbon pack 3–5 g/L 40–50 °C; re-dose brightener by Amp-hour curve |
The 10-minute routine
- Photograph the part before touching it — pit pattern is evidence
- Magnifier pass: classify each pit by the table (tail = gas, grit = solids, ring = oil)
- Position test: mark rack row of each pit cluster (bottom row = agitation dead zone)
- Swap test: move suspect rack position to the middle for the next load
- Run one corrective action only; Hull cell before and after
- Log dose, readings and outcome on the tank card
Line case: 8% to 0.5%
A decorative nickel line fought bottom-row pitting for months. Fix C plus agitation: nickel anodes shortened 10 cm, cathode rod stroke widened to full travel, surface tension brought into 35–45 dyn/cm with a wetter titrated by stalgmometer. No new chemistry purchases. Pitting fell from 8% to 0.5% in two weeks and held.
Engineer notes
- Most pitting calls are gas pits misdiagnosed — measure surface tension before adding anything
- Bottom-row pitting with a clean bath means agitation dead zone, not chemistry
- Boric acid below 40 g/L burns before it pits; keep 40–50 g/L in Watts
- Air agitation louder is not better: coarse bubbles pin to surfaces
- Pits that show substrate metal are not a plating problem — fix casting first
Hydrogen from bright nickel baths is explosive in enclosed air at 4–75% volume. Keep tank lids open, ventilate, no flames. Nickel salts are skin sensitisers — nitrile gloves, apron, eyewash at the tank.
FAQ
Pitting only on bottom rows for tall racks — anode or agitation?
Both interact: shorten nickel anodes 10 cm and raise cathode rod stroke; expect bottom-row pitting to clear first.
How do I set the filter cut without losing brightness?
Step 5 µm nominal with 2–5 turns/hour; watch Hull cell brightness after each step before going finer.
When do I carbon-treat the whole tank?
When three consecutive Hull cells show organic banding and pits persist after filtration and wetter corrections.