We purchased some 304 SS nuts. They are used in a potable water application and we have noticed some corrosion resembling rust. I tested the water for total dissolved solids and it came up a little high (880 mg/L NaCl). I thought at first that the chlorides were causing the corrosion. Then we checked some of the nuts that had yet to be used and some were slightly magnetic (they are stored in a warehouse in south florida on the coast so humidity may play a part). The nuts in storage do not look corroded in any way to me and I am wondering if we got a bad batch of SS nuts or the salt content in the air is causing some weakness of the oxide layer that we can't observe (but why wouldn't this happen to all the nuts?). In short, is it possible for 304 to become magnetic over time, yet show no observable rust or corrosion? Should we be switching to 316 SS if our water TDS remains this high?

If you would have placed this question a few days ago, my answer would have been "no". However, yesterday we were investigating a problem on a yacht with various SS around and the captain came up with a magnet and I was surprised to see that the magnet would attract ALL stainless steel I could see around me, slightly. Curious, I put the magnet against ferrite material and it was attracted very forcefully. I asked about the magnet, and it comes from a computer hard disc drive was the answer. I organized a "normal" magnet from a loudspeaker and I could then classify all stainless metals around me as "non-magnetic"! So it depends on the magnet and 304 - 316 series stainless steel IS INDEED slightly magnetic, I believe this is not a question of time.