Stainless 316 & 316L Parts

What the molybdenum buys

The difference between 316 and 304 is molybdenum: 316L is listed with 2-3% (source 1), and the 304 listing shows none (source 2). Chlorides break down the protective film on a stainless steel and start pits (source 11), and the molybdenum gives 316 more resistance to that localized attack than 304 has (source 1). 304 is considered resistant to pitting in water with up to about 400 mg/L chlorides at ambient temperature, and where that is too little, 316 is the usual replacement (source 2). The stainless steel grade guide sets 303, 304, 316 and 17-4 side by side.

Both are austenitic stainless steels (source 3), and austenitic steels cannot be hardened by heating and cooling (source 3). So 316 is a corrosion choice, not a strength choice. When a part has to be both corrosion-resistant and strong, that is the job of 17-4 PH instead.

316 vs 316L, and when the L matters

316L is the low-carbon version of 316 (source 1), and the place the low carbon matters is at a weld. Austenitic stainless held around 700 °C for too long, as it can be during welding, precipitates chromium carbide at the grain boundaries and leaves chromium-depleted zones beside them, a condition called sensitization (source 4). Keeping carbon below 0.03 percent leaves too little carbon to form those carbides (source 4), which is what the L is for. The carbon limit is 0.08% maximum for 316 and 0.030% maximum for 316L (source 13).

So if the part will be welded, put the L on the print. If the print does not say which, say in the quote notes whether the part gets welded, and the grade gets confirmed before the quote goes out.

What gets made from it

316 and 316L are used in the chemical and petrochemical industry, in food processing, in pharmaceutical equipment and medical devices, and in marine and seashore applications (source 1). In machined parts that means marine and coastal hardware, fittings and fasteners, pump and valve parts, and chemical and process components. Turned parts like fittings and shafts, and milled parts like flanges, brackets, and housings, all come in 316 when the service calls for it. Milled features are quoted from the print, and ±0.001 in is typical on critical turned diameters. Other alloys and plastics quoted on request. Name the grade on the print.

One trade to keep in mind: 316 is harder on tooling than 304. In a 2017 turning comparison, machining AISI 316 produced larger wear on the cutting inserts, higher cutting forces and rougher surfaces than AISI 304 (source 7), and 303 is easier to machine than either because sulfur is added to it (source 6). A turned part that will never see a chloride is a candidate for 303 or 304, so reserve 316 for the parts that need it. Describe the environment in the quote notes and the grade gets flagged.

Passivation, finishes, and the magnet question

On stainless, passivation removes exogenous iron from the surface and restores the passive oxide layer that stops further rust; the processes are controlled by standards, the most prevalent being ASTM A 967 and AMS 2700 (source 5). A finish called out on the print is quoted with the part, passivation included; the surface finishes guide explains how a print calls a finish out. Drawings for sterile applications sometimes call out electropolishing, a process used in food, medical and pharmaceutical work (source 8).

And the magnet test proves nothing here. Austenitic stainless steels are essentially non-magnetic (source 3), but work hardening can make them slightly magnetic (source 9). A faint pull on a magnet is therefore normal on a machined part and not a sign of the wrong grade. If a true non-magnetic requirement applies, call it out on the print so the material and process suit it.

Sources

All accessed 4 October 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, “SAE 316L stainless steel”: en.wikipedia.org (chromium 16-18%, nickel 10-12%, molybdenum 2-3%; molybdenum gives greater resistance than 304 to localized attack by chlorides; 316L is the low-carbon version of 316; uses in the chemical and petrochemical industry, food processing, pharmaceutical equipment, medical devices and marine and seashore applications).

2. Wikipedia, “SAE 304 stainless steel”: en.wikipedia.org (304 composition table, chromium 18.0-20.0%, nickel 8.0-11.0%, no molybdenum listed; resistant to pitting in water with up to about 400 mg/L chlorides at ambient temperature; commonly replaced by 316 when too sensitive to pitting or crevice corrosion by chlorides, or to general corrosion in acidic applications; used in food handling and processing equipment).

3. Wikipedia, “Austenitic stainless steel”: en.wikipedia.org (austenitic steels not thermally hardenable and essentially non-magnetic; cold working forms martensite; type 316 contains molybdenum for resistance to pitting and crevice corrosion).

4. Wikipedia, “Intergranular corrosion”, the weld decay passage: en.wikipedia.org (heating around 700 °C for too long, as in welding, precipitates chromium carbide at grain boundaries and leaves chromium-depleted zones, called sensitization; carbon content reduced below 0.03 percent so insufficient carbon is available for carbide formation).

5. Wikipedia, “Passivation (chemistry)”: en.wikipedia.org (passivation of stainless steel removes exogenous iron and restores a passive oxide layer; controlled by industry standards, most prevalent ASTM A 967 and AMS 2700).

6. Wikipedia, “Machinability”: en.wikipedia.org (AISI grades 303 and 416 are easier to machine because of added sulfur and phosphorus).

7. Ozbek, N. A., Cicek, A., Gulesin, M. and Ozbek, O., “Evaluation of Machinability of AISI 304 and AISI 316 Austenitic Stainless Steels”, Journal of Polytechnic (Politeknik Dergisi), 2017, record on Gazi University’s research portal: avesis.gazi.edu.tr (larger wear on inserts, higher cutting forces and higher surface roughness machining AISI 316 than AISI 304; the abstract concludes 316 had harder machinability).

8. Wikipedia, “Electropolishing”: en.wikipedia.org (a clean, smooth surface that is easier to sterilise; an essential process in the food, medical and pharmaceutical industries).

9. Wikipedia, “Stainless steel”: en.wikipedia.org (austenitic stainless steels, usually non-magnetic, can be made slightly magnetic through work hardening, and magnetism can occur along a cut or bent edge; cleanability, strength and corrosion resistance have prompted use in pharmaceutical and food processing plants).

10. R. M. Kain, A. W. Zeuthen and J. R. Maurer, “Localized corrosion resistance of stainless type materials in aerated, deaerated and stagnant sulfide bearing seawater”, December 1997, U.S. Department of Energy Office of Scientific and Technical Information record: osti.gov (UNS S31603 was most susceptible in slowly moving, refreshed, aerated seawater).

11. Wikipedia, “Pitting corrosion”: en.wikipedia.org (pitting occurs in alloys protected by a passivating oxide film, such as stainless steels, in environments containing aggressive species such as chlorides).

12. Wikipedia, “Crevice corrosion”: en.wikipedia.org (the oxide film on stainless steel can break down in halide solutions; crevice corrosion occurs in occluded spaces where a stagnant solution is trapped and not renewed).

13. British Stainless Steel Association, “Comparison of 304 or 316 and 304L or 316L type compositions and effect on corrosion resistance”: bssa.org.uk/bssa_articles/2-comparison-of-304-or-316-and-304l-or-316l-type-compositions-and-effect-on-corrosion-resistance/ (carbon ranges 0.08% maximum for 304 and 316 and 0.030% maximum for the 304L and 316L types; below 0.030% carbon the intercrystalline corrosion does not take place after exposure to weld temperatures).