Alkaline clean → rinse → phosphate → seal → dry. Five baths that decide everything that comes after, sequence, chemistry, and dwell time all matter.
55 °C alkaline bath strips drawing oils and machining lubricants. Skip this and phosphate crystals form on top of grease, no anchor, no paint life.
Zinc phosphate at 50 °C for 90 sec deposits a 2–4 g/m² crystalline layer. This is the actual mechanical key for the paint. Time × temperature drives crystal size.
Hot DI rinse after phosphate stops the reaction. Bad rinse leaves white salt deposits that lift paint locally. Spec rinse conductivity below 30 µS/cm.
Above 140 °C the phosphate dehydrates and loses adhesion. Dry-off removes moisture only. Powder cure is a separate oven, downstream.
The 7-zone phosphate line that sits between blast and powder oven.
| Zone | Chemistry · temp | Purpose |
|---|---|---|
| 1, Degrease | Alkaline · 55°C | Oils, grease, drawing compound |
| 2, Rinse 1 | Tap water · ambient | Carryover removal |
| 3, Etch / derust | Acidic · 35°C | Light rust, mill scale |
| 4, Rinse 2 | Tap water · ambient | Acid removal |
| 5, Phosphate | Zn/Fe phosphate · 45°C | Conversion coating |
| 6, Rinse 3 | Tap water · ambient | Salt removal |
| 7, Passivation / DI | Cr-free seal · ambient | Final corrosion seal |
Salt-spray failure starts in pre-treatment, not in the coat.
Iron phosphate gives 200-400 hours salt-spray. Zinc phosphate gives 500-1000+ hours under the same paint. For Jeddah and Dammam projects, zinc phosphate is non-negotiable.
Tap water in Saudi contains chlorides. The final rinse must be deionised (≤ 30 μS/cm conductivity) or all the salt-removal work upstream is wasted. Test conductivity weekly.
Free alkalinity, total acid, phosphate concentration, every bath has a titration window. Outside the window, conversion fails silently. Plant chemists titrate every shift; smaller shops should titrate at least daily.
Counter-flow rinse water from zone 6 back to zone 4. Last rinse is freshest; first rinse drops to drain. Halves water use and doubles part cleanliness vs same-tank rinsing.
Immersion is gentler on fine geometry, slower throughput. Spray is faster, harder to control on complex enclosures. Modern lines use a hybrid: spray for the heavy stages (degrease, phosphate), immersion for rinses.
Zinc-phosphate primary, Cr-free final seal, DI rinse, conductivity logged per shift. Salt-spray validation: 800 hours minimum for outdoor systems, 1000+ for coastal.
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