Rust on AISI 316L

A. Two solutions, passivation or electropolish. Your problem is not the 316SS, but is the traces of iron smeared on the surface from your machining equipment. This rusts and then spreads. Stainless does not mean stain "proof" or rust proof. It is only "LESS" likely to rust than steel.

A. You also need to look at what is the 316L being exposed to. If it is being exposed to bleach or other chlorides even 316L can rust.

A. We see 316L from many sources used in our boat building industry. I have had 25 years of experience with stainless steel products in this field. European milled metal is very unlikely to "bleed" a stain (rust) without a ferrous catalyst, but eastern sources are more likely to bleed for no apparent reason, sometimes because the 316L standard is loosely interpreted. Installation practices can also be a problem - the use of ferrous tools, contamination with iron filings, fittings left in installer's toolbox for example. Passivation or electropolishing can be very helpful in preventing no-catalyst corrosion in poor quality metals, and also prevent good quality metals from going dull.