Scope of this guide
Senior version: the fix set with parameter windows, boundary cases and the discipline that keeps them fixed.
Series: finishing.com recurring questions answered with platingclub line practice. Parameters are industry-standard starting ranges — confirm against your supplier TDS.
Triage: symptom → confirm → first move
| Observation | Likely cause | Confirm by | First move |
|---|---|---|---|
| Rate drops late in bath life | Orthophosphite build-up | Turnover count + rate log | Plan rebuild at 5–6 turnovers |
| P content unknown, spec dispute | No assay routine | Lab assay per rebuild | Set assay schedule |
| Dark deposits, slow plate | pH low / stabilizer drift | pH + Hull cell | Hold pH 4.4–4.8 per TDS |
| Plate stops on recesses | Gas sticking, agitation dead | Position pattern | Work the agitation, not the bath |
| Bath decomposes (black rain) | Overload or palladium contamination | Load history | Strip load; discipline on make-up |
The fix set
- Fix 1 — pH discipline — Hold per TDS (typically 4.4–4.8): rate, phosphorus and stability all track pH
- Fix 2 — turnover ledger — Count metal turnovers; plan rebuild at supplier limit, usually 5–6
- Fix 3 — loading discipline — Keep loading inside the TDS band; over-loading invites decomposition
- Fix 4 — phosphorus by spec — Low-P (1–3%) hard, mid-P (4–9%) general, high-P (10–13%) corrosion; pick by use, not habit
- Fix 5 — assay gate — Assay Ni and P per rebuild; adjust make-up chemistry from data
Case method
A shop ran EN until it died: dark decomposition, full dump, twice a year. A turnover ledger moved dumps to planned rebuilds at the supplier limit — chemistry spend smoothed out and decomposition incidents stopped. (Method illustration: your own turnover records.)
Engineer notes
- One variable per corrective action; verify with the confirm-by column.
- Records beat memory: log every trial on the tank card.
- Two failed actions in a row is a senior call — escalate with your log.
Safety: EN baths run hot and acidic with nickel salts and reducing agents. Heat gloves, face shield, ventilation; never add chemistry to a decomposing bath.
FAQ
High-P or mid-P for corrosion spec?
High-P (10–13%) for corrosion barrier; mid-P for general; confirm the assay after each rebuild.
How do I stabilize a marginal bath?
Loading band discipline, filtration polish, pH hold — and plan the rebuild early.
What causes black rain?
Overload, overheating or catalytic contamination; strip the load, cool, and follow the supplier recovery procedure.