Common Misconceptions About Stainless Steel: 304, 316, Rust, Magnetism and Galling

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓There are hundreds of stainless steels, not just 304 and 316. The austenitic 300 series is the most common, and 316 was developed from 304 by adding molybdenum for the seaside.
• ✓316 is not a seawater grade. Seawater has about 20 times the chloride 316 can withstand, and it rusts in seawater, especially in crevices such as nuts and bolts. "Marine grade" only means it stays bright with a good finish and rain washing or maintenance; it can visibly corrode where salt spray builds up.
• ✓Rust on stainless is usually carbon steel contamination (if it appears within days or weeks) or the wrong grade. Rough surfaces, aggressive environments and no washing cause rust over a longer time.
• ✓Magnetism has nothing to do with corrosion resistance, and nickel affects the microstructure, not corrosion resistance.
• ✓Stainless still needs maintenance: cleaning to remove stuck-on deposits left after rain washing. A high polish makes cleaning easier.
• ✓A 316 nut on a 304 bolt is not a reliable cure for galling (cold welding of threads under load). Galling is worse with fine threads, tight clearances, poor thread profiles, no lubrication, dirt and over-tightening.

Who it helps: Anyone choosing stainless fasteners for outdoor, coastal or wet jobs.