The most common mistake on chemical projects is treating 316L as a 'steel dedicated to chemical service'. It is not. 316L is corrosion-resistant only under specific medium combinations; switch to another medium and it may perforate within days — yet both service conditions are written on the purchase order as '316L piping'.
One-sentence judgment
First check whether the medium is oxidizing or reducing, then check concentration and temperature, and finally check flow rate. 316L contains 2–3% molybdenum, giving it markedly better resistance to chloride pitting than 304, but it has almost no resistance to reducing acids (especially hydrochloric acid). Hydrochloric acid at any concentration should not use 316L for piping.
Empirical boundaries by medium type
- Nitric acid (oxidizing): 316L is usable; from ambient to medium temperature and up to around 65% concentration is the common range; fuming nitric acid or high-temperature high-concentration service requires re-evaluation.
- Sulfuric acid (reducing; concentration decides everything): dilute sulfuric acid (below about 20%) is usable at ambient temperature; concentrated sulfuric acid (above about 90%) is also usable at ambient temperature (passivation); intermediate concentration + elevated temperature is the most dangerous combination, and 316L will corrode rapidly.
- Hydrochloric acid: not recommended at any concentration or any temperature. Where hydrochloric acid resistance is needed, use high-molybdenum high-nickel grades such as 904L or 254SMO, or use non-metallic lining.
- Caustic soda (sodium hydroxide): usable at ambient temperature and low-to-medium concentration; at high temperature and high concentration, guard against caustic stress corrosion cracking (caustic embrittlement), and stress relief after welding is mandatory.
- Organic acids (acetic acid, citric acid, etc.): generally usable, but watch for chloride ion impurities — the chlorine in industrial-grade acetic acid is often the real cause of corrosion.
The table above reflects industry practice, not mandatory standard clauses. Actual material selection must be based on the medium analysis report (including chloride ions, oxygen content, impurities) and measured data from comparable service conditions; the same 'sulfuric acid tank' in different plants may differ in corrosion rate by an order of magnitude.
Three hard constraints
① Flow rate. Media containing solids or gas bubbles destroy the passive film; when flow rate is persistently above about 1.5 m/s, erosion-corrosion becomes significant; pump outlets, elbows and tees are the first places to fail. Upsize the pipe diameter where it should be upsized — do not gamble on 'thickening the wall' instead.
② Crevices and deposits. Under flange gaskets, in dead-leg sections of branch pipes, and at weld undercut are chloride enrichment zones. A 316L pipe with crevices has far lower corrosion resistance than an open vessel of the same material — this is also the common reason why 'the same batch of pipe is fine in the tank but perforates on the line'.
③ Welding and passivation. 316L is ultra-low carbon (C ≤ 0.03%), and post-weld solution treatment is generally not required, but acid pickling and passivation are mandatory: if the oxide scale on the weld and heat-affected zone is not removed, the passive film cannot regrow, and corrosion will definitely start from the weld. When acid pickling the entire piping system, remove instruments and valve internals.
How it compares with 304 and duplex steel
Where chloride concentration is low and temperature is mild, 304 is sufficient; where chlorides are present and temperature is on the high side, 316L is the lower limit; where chlorides are high and pressure must be contained, go straight to duplex steel (2205 has a pitting resistance equivalent number PREN of about 34–36, while 316L is only 24–26). Ranking by PREN is more reliable than ranking by price: the steel cost saved is often not enough to pay for one shutdown repair.
Common pitfalls
- Specifying only '316L' without the governing standard (ASTM A312 / EN 10216-5 / GB/T 14976) — in a dispute you have no basis for judgment;
- Substituting 316 for 316L: higher carbon content means risk of intergranular corrosion in the post-weld heat-affected zone;
- Hydrostatic test water contains chlorine and remains stagnant in the pipe for a long time — after testing it must be drained and blown dry; this is a common cause of perforation in the first year of new piping;
- Treating 'corrosion-resistant' as 'maintenance-free': regular flushing of deposits is the cheapest anti-corrosion measure.
Send us your medium composition (including chloride ions), operating temperature, concentration and governing standard, and we will give material recommendations and corresponding quotations based on the service conditions, and explain which criterion the recommendation is based on — rather than 'let's try 316L first'.
