304 and 316 are the two most widely used stainless steels in plants. They look alike and are welded the same way, but they do not resist corrosion equally well. Choosing the wrong one can mean redoing a part after a few months.

The difference: molybdenum

304 contains about 18 to 20% chromium and 8 to 10.5% nickel. 316 contains about 16 to 18% chromium, 10 to 14% nickel and, above all, 2 to 3% molybdenum (ranges from the ASTM A240 standard). Molybdenum is what gives 316 better resistance to pitting corrosion caused by chlorides: salt, brine, seawater and certain chlorinated cleaning products.

The two are often compared using the PREN index (%Cr + 3.3 × %Mo + 16 × %N). 304 sits at around 18 to 20, and 316 at around 24 to 26. The higher the index, the better the pitting resistance.

When 304 is enough

• Counters, work tables, guards and supports in dry environments.
• Food equipment with no salt or brine, washed with non-chlorinated products.
• Indoor structures and guards with no contact with corrosive products.

When to choose 316

• Salted products, brines, marinades, cheese plants and meat processing.
• Frequent washing with chlorinated products or at high temperature.
• Pharmaceutical equipment: the ASME BPE standard is generally applied with 316L.
• Outdoor or seaside environments, and chemical plants.

And the "L" in 304L and 316L?

The L stands for low carbon (0.03% maximum). During welding, a stainless steel with normal carbon content can form chromium carbides near the joint and lose its corrosion resistance at that spot. The L version reduces this risk: it is the choice for welded parts. The matching filler metal is then used, ER308L for 304L and ER316L for 316L (AWS A5.9 standard).

Price

316 costs more than 304, because molybdenum and nickel are expensive and their prices vary with the market. On a simple part, the difference is often small compared to the cost of labour. If the part is exposed to salt or chlorine, 316 generally saves you from having to redo it.

After welding: clean and passivate

Whatever the grade, the heat tint left by welding and iron particles from ordinary steel tools can start rust. Clean the joint, then passivate the part following the practices of the ASTM A380 and A967 standards.

Unsure which one to use for a specific part? Send us a photo and the intended use, or call 438-357-8800.

Sources

1. ASTM A240/A240M — Chromium and chromium-nickel stainless steel plate, sheet and strip
2. AWS A5.9/A5.9M — Filler metals for welding stainless steels
3. ASTM A380/A380M and ASTM A967/A967M — Cleaning, descaling and passivation of stainless steel
4. ASME BPE — Bioprocessing Equipment (sanitary and pharmaceutical equipment)

Photo: Unsplash. This article provides general information; it does not replace the manufacturer's requirements or your facility's risk assessment.