The most common mistake on chemical projects is treating 316L as a 'chemical-duty steel'. It is not. 316L is only corrosion-resistant under specific combinations of media; 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 assessment
First determine whether the medium is oxidising or reducing, then look at concentration and temperature, and finally at 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 be used with 316L piping.
Empirical boundaries by medium
- Nitric acid (oxidising): 316L is suitable; from ambient to moderate temperature and up to around 65% concentration is the common range; fuming nitric acid or high-temperature, high-concentration service requires re-evaluation.
- Sulphuric acid (reducing; concentration decides everything): dilute sulphuric acid (below about 20%) is usable at ambient temperature; concentrated sulphuric acid (above about 90%) is also usable at ambient temperature (passivation); intermediate concentration plus 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 required, use high-molybdenum, high-nickel grades such as 904L or 254SMO, or a non-metallic lining.
- 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 impurities — the chlorine in industrial-grade acetic acid is often the real cause of corrosion.
The table above reflects industry practice, not mandatory provisions of a standard. Actual material selection must be based on the medium analysis report (including chloride, oxygen content and impurities) and measured data from comparable service conditions; the same 'sulphuric 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 destroy the passive film, and erosion-corrosion becomes pronounced when the flow rate is sustained above about 1.5 m/s; pump outlets, elbows and tees are the first places to fail. Upsize the pipe diameter where it should be upsized — do not gamble on 'thicker wall thickness'.
② Crevices and deposits. Under flange gaskets, in dead-leg sections of branch pipes, and at weld undercut are chloride concentration 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%), so solution treatment after welding is generally not required, but 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 certainly start at the weld. When 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 containment is required, go straight to duplex steel (2205 has a pitting resistance equivalent 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) — leaving no basis for judgement in a dispute;
- Substituting 316 for 316L: higher carbon content means a risk of intergranular corrosion in the heat-affected zone after welding;
- Hydrostatic test water containing chlorine and left standing in the pipe — after testing, the pipe must be drained and blown dry; this is a common cause of perforation in new piping within the first year;
- Treating 'corrosion-resistant' as 'maintenance-free': regular flushing to remove deposits is the cheapest corrosion protection measure.
Send us your medium composition (including chlorides), operating temperature, concentration and governing standard, and we will give material recommendations and corresponding quotations based on the service conditions, explaining which criterion the recommendation is based on — rather than 'let's try 316L first'.
