rusty stainless steel self tapping screws

[Theo]

These ones, I see:

now 4* rating is due to your 1* review)

Looking at the pic, I wonder if they're actually stainless. They look more like BZP to me, although it's hard to tell from photos like that.

If they were BZP and they were in electrical contact with stainless, the zinc plating might have sacrificially corroded since it's more reactive. I assume there wasn't very much of it, so the zinc oxide then falls off and leaves you with mild steel which then rusts.

Maybe you could try putting one in salt water and seeing what happens?

SourcingMap is random Aliexpress-level junk offered via the FBA system. Sometimes it's the only way to get a thing at short notice, but they're expensive for the quality you (don't) get.

I'd always go to a reputable supplier if I had an option. Farnell/CPC is usually where I go for fasteners, but then I'm often putting in orders anyway. It's often vastly cheaper to buy 100x of something, you might pay roughly the same as of buying a dozen from Amazon.

For small quantities of stuff ebay is often ok - I got some bolt kits from Falcon Workshop Supplies (BZP, SS) which I've been pleased with.

(ebay sellers often repackage things to fit in 'large letter' or 'small parcel' which makes the postage cheaper than buying from a regular retailer)

Theo

[Colin Bignell]

I used to use several stainless steels to make surgical instruments. However, I don't know of any common stainless steel that won't rust, given the combination of damp, to cause the rust, and a reduced supply of oxygen, which is critical to maintaining the protective surface coating of chromium oxide.

The only one I can think of that is guaranteed to avoid corrosion in the most severe conditions is 316 stainless steel that has been twice refined under vacuum. That removes the impurities that can cause the chromium oxide coating to fail and it is used for certain bone implants.

[Brian Howie]

Thanks, I'll drop them an enquiry.

Brian

[Brian Gaff]

You need to specify what form of stainless you want.I assume these will pick up with a magnet? The alloy mixed with a ferrous metal has some uses and if protected is good, but you should not make self tapping screws that are not just going into plastic in an indoor application using it. They all used to have different bs standard classifications, I guess this was put into those cecc ones in the end. Brian

[tony sayer]

Don't know quite what grade is used but we have taken aerials used for Two way radio and broadcast transmission down that have been up for years, never seen any rust at all!...

[Colin Bignell]

I think one thing an aerial would get plenty of would be air, so low oxygen levels are not something I would expect to be a problem. A close fitting hole underwater, say a keel bolt on a boat, that is another matter entirely.