What a grain-silo anti-corrosion coating is
A grain-silo coating is the industrial protective system applied to the inner and outer walls of a steel grain bin, guarding the steel or galvanised substrate under grain storage, chemical fumigation and bulk handling—while the interior must also meet food-contact rules.
Key data (with units)
- FAO PH3 residue tolerance: 0.1 ppm (as PH3) in traded grain.
- Common fumigation dose: 0.2-0.3 g/m3 held 3-7 days, or 1-2 g/m3 for 2-3 days (industry practice).
- Hot-dip galvanising typical 275 g/m2 zinc (Z275); PH3 forms zinc phosphide Zn3P2, destroying the layer.
- Humid, ammonia-bearing agricultural air is classified C4-C5 under ISO 12944-2.
- Food-contact coating: GB 4806.10-2016 total migration ≤ 60 mg/kg (aligned with EU 10/2011).
- Film build: outer system 160-240 um; inner food-grade epoxy 2 coats 150-250 um, 300-500 um total.
Why phosphine "eats" the galvanised layer
Aluminium/magnesium phosphide tablets release PH3 with moisture to kill stored-grain insects. PH3 is aggressive to many metals—zinc, copper, silver and brass especially.
- Zinc converts to Zn3P2; both barrier and sacrificial protection fail and rust spreads from defects.
- The reaction consumes PH3, so the fumigation concentration drops below the killing target.
- Copper valves, gauges and earth straps corrode in the same space.
Inner vs outer systems: two contexts
The outer wall faces weather and needs durability; the inner wall faces grain and fumigation and needs chemical resistance plus food-contact compliance. They cannot share the same resin or film logic.
| Item | Outer system | Inner system |
|---|---|---|
| Driver | UV, wet/dry cycling, ammonia-bearing air | PH3, condensation, dust abrasion, wash water |
| Typical build | Epoxy primer + epoxy MIO + PU or fluorocarbon topcoat | Food-grade solvent-free / waterborne epoxy, 2-3 coats |
| Total DFT | 160-240 um | 300-500 um |
| Core need | Weathering, adhesion, salt spray | PH3/water/abrasion resistance, migration, cleanability |
| Reference | ISO 12944-2/-5, GB/T 30790 | GB 4806.10-2016, EU 10/2011, FDA 21 CFR 175.300 |
Food-contact compliance is a hard gate
Compliance is about the finished, fully-cured lining, not just "food-grade" marketing.
- China: GB 4806.10-2016 (total migration, KMnO4 consumption, heavy metals as Pb, colouration) with GB 9685 raw materials.
- EU: (EC) 1935/2004 framework + (EU) 10/2011 migration incl. specific limits.
- US: 21 CFR 175.300 extraction limits for organic coatings.
Application and acceptance window
- Surface prep Sa 2 1/2 (ISO 8501-1) with anchor profile ~Rz 50-80 um; galvanised surfaces need dedicated keying.
- Substrate 3 C above dew point; ventilate and control ignition when spraying inside a closed bin.
- Airless-spray the solvent-free epoxy in controlled passes; respect recoat windows.
- Accept with DFT (ISO 19840), pull-off adhesion, and holiday/pinhole testing (rule of thumb ~100 V per 0.1 mm)—then fully cure and air out before loading grain.
FAQ
Do galvanised silos still need a coating?
If the bin is fumigated, yes: bare zinc is consumed by PH3. Cover gas-facing metal with a dense barrier coat.
Can I use a PU or fluorocarbon topcoat inside?
Not on the grain-contact face—these target weathering, not migration. Use a food-grade epoxy lining inside; keep weathering topcoats outside.
Where does PH3 damage actually start?
At "humidity + PH3 + coating defect": vapour tracks pinholes straight to zinc or steel. Continuity and holiday testing matter more than thickness.
Can I load grain right after coating?
No. Reach full cure and air out until migration/odour pass, per the TDS post-cure time.
Same film thickness inside and out?
Generally not. Outer systems run 160-240 um; the inner chemical barrier is usually thicker (300-500 um) because the failure modes differ.
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