The most common mistake in chemical projects is treating 316L as a "steel dedicated to chemical applications." It is not. 316L is corrosion-resistant only under specific medium combinations; switching to another medium may cause perforation within days — yet both operating conditions are listed as "316L piping" on the purchase order.
One-sentence judgment
First determine whether the medium is oxidizing or reducing, then consider concentration and temperature, and finally flow rate. 316L contains 2–3% molybdenum, giving it significantly 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
- Nitric acid (oxidizing): 316L is usable; common range is from ambient to moderate temperature and concentration up to around 65%; fuming nitric acid or high-temperature high-concentration conditions require re-evaluation.
- Sulfuric acid (reducing, concentration determines 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. For hydrochloric acid resistance, use high-molybdenum high-nickel grades such as 904L or 254SMO, or non-metallic linings.
- Caustic soda (sodium hydroxide): usable at ambient temperature and low-to-moderate 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 above table 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 similar operating conditions; the same "sulfuric acid tank" in different plants may show corrosion rates differing by an order of magnitude.
Three hard constraints
① Flow rate. Media containing solids or gas bubbles will damage the passive film; when flow rate exceeds about 1.5 m/s for extended periods, erosion-corrosion becomes significant; pump outlets, elbows, and tees are the first to fail. Increase pipe diameter when necessary — do not gamble by "adding wall thickness."
② Crevices and deposits. Under flange gaskets, in dead legs of branch pipes, and at weld undercuts are chloride ion enrichment zones. 316L piping with crevices has far lower corrosion resistance than open vessels of the same material — this is also a common reason why "the same batch of pipes is fine in the tank but perforates in the pipeline."
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 welds and heat-affected zones is not removed, the passive film cannot regrow, and corrosion will inevitably start at the welds. When pickling the entire piping system, remove instruments and valve internals.
How to distinguish from 304 and duplex steel
For low chloride concentration and mild temperature, 304 is sufficient; for chloride-containing and relatively high temperature, 316L is the lower limit; for high chloride and pressure-bearing applications, go directly 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 insufficient to pay for one shutdown maintenance.
Common pitfalls
- Specifying only "316L" without the governing standard (ASTM A312 / EN 10216-5 / GB/T 14976) — in disputes, there is not even a basis for judgment;
- Substituting 316 for 316L: higher carbon content, risk of intergranular corrosion in the heat-affected zone after welding;
- Hydrostatic test water contains chlorine and remains 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 resistance" as "maintenance-free": regular flushing to remove deposits is the cheapest anti-corrosion measure.
Send us your medium composition (including chloride ions), operating temperature, concentration, and governing standard; we will provide material recommendations and corresponding quotations based on the operating conditions, and explain which criterion the recommendation is based on — rather than "let's try 316L first."
