Material selection errors cause more tank failures than welding defects. The right answer is never "stainless is best"; it is "what matches the ion, the temperature and the cleaning cycle."
Carbon steel Q235B / Q345R
Best for crude oil, diesel, fuel oil, fire water, asphalt (heated), molasses. Add internal coating for water: epoxy coal tar for raw water, pure epoxy for potable, epoxy phenolic for food oil. Do not use bare carbon steel for anything acidic or chlorinated.
SUS304 stainless
Good for refined vegetable oil, milk powder slurry, clean process water, organic solvents without halides, nitric acid. Avoid if chloride > 200 ppm in liquid or cleaning water, or if service temperature > 60 °C with stagnant spots-pitting and SCC risk.
SUS316L stainless
The default for seawater-exposed tanks, palm oil export, sulfate/chloride cleaning, dilute phosphoric acid, lactic acid. Low carbon avoids weld sensitization. Cost premium 25–35 % over 304, justified when recall risk is high.
FRP (vinyl ester or isophthalic)
Excellent for HCl up to 35 %, sodium hypochlorite, tertiary effluent, some organic acids. Not for aromatic solvents (benzene, toluene) above ambient unless novolac vinyl ester specified. External UV needs pigmented gel coat.
Rubber-lined carbon steel
For dilute sulfuric acid 10–30 %, fluoride solutions, abrasive slurries. Natural rubber, chlorobutyl or bromobutyl chosen by temperature. Lining must be spark-tested; a 2 mm pinhole is a 2 year failure.
Common traps
Specifying 304 for coastal fire water → chloride pitting within 3 years; use epoxy-coated CS or 316L
Using carbon steel for 98 % sulfuric acid is fine, but 70 % H2SO4 eats it → switch to 316L or rubber lining
Edible oil in carbon steel without lining → oxidation off-flavor, customer rejection
FRP tank for hot palm oil at 90 °C → vinyl ester Tg exceeded, softens
Food-grade finish rules
Internal Ra ≤ 0.4 μm on stainless, ≤ 0.8 μm on coated carbon steel after cure; no threaded dead ends; all nozzles sloped; passivation certificate; borescope photo of manhole neck.
Zhongneng Huajian issues a Material Approval Sheet with ASTM–GB equivalence (e.g., Q345R ≈ A516 Gr.70, SUS316L ≈ 316L/S31603) so overseas reviewers do not stall on nomenclature.
