Scope of this guide
Intermediate version: symptom-to-cause triage with the confirm-by column — find the variable, run one change, verify.
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
| Symptom | Likely root | Confirm by | First move |
|---|---|---|---|
| Film wipes off, powdery | Bath too hot / too long | Time-temperature log | Cut dwell; check heater |
| Iridescent patchy colors | Uneven zinc surface from rinse drag | Pre-chromate rinse check | Fix counter-flow rinse |
| Bare spots after chromate | Oil/silicone on zinc | Water-break test before chromate | Pre-clean audit |
| Weak color, fades fast | Bath exhausted / pH drift | Titration + pH | Rebalance per TDS |
| Color varies by rack row | Drag-in concentration gradient | Position sampling | Dip discipline; agitation |
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: hexavalent chromate is a known carcinogen and sensitizer. Gloves, apron, face shield, exhaust on; trivalent lines preferred wherever spec allows.
FAQ
Patchy iridescence — bath or rinse?
Rinse first: drag-in contaminating the film shows as patches; check the pre-chromate rinse.
How do I time trivalent vs hexavalent?
Trivalent 30–90 s, hexavalent 15–45 s by color target — set a timer, not a habit.
Why does color drift through the shift?
Drag-in concentrates the bath and pH drifts; titrate and log.