Scope of this guide
Intermediate version: the 10-minute magnifier triage — classify the pit, find the variable, apply the matching fix with data.
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 |
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
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
Two pit types on one part — which do I fix first?
Fix the one with the larger area share; but if base-metal pits exist, quarantine the lot — that one is upstream.
How do I confirm gas pits without a stalgmometer?
Swap-test a rack position and raise wetter slightly; if the pattern moves with position, it is agitation/CD, not chemistry.
When is carbon treatment needed?
When Hull cell shows banding and pits are irregular across all positions — that is organic contamination.