stainles steel turned black

Postby lari » Sat Nov 19, 2016 18:17

hi all,
i have a ss ball valve which i tried to clean thoroughly with some bar keepers friend. long story short i left parts of ss valve soaking in bkf solution for a few days and its now almost black, dar greyish colour and smells a bit ‘metaly’. i assume its some kind of iron oxidation or something. any ideas?
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Re: stainles steel turned black

Postby beer buddah » Sat Nov 19, 2016 18:24

I'm guessing it isn't stainless as no stainless I have has ever reacted to caustic or acid based chemicals in that way.
Is it stamped stainless ?
When you scrub it does it become shiny again ?

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Re: stainles steel turned black

Postby lari » Sat Nov 19, 2016 18:44

its a chinese made ss valve supposedly 316. magnet doesn’t stick to it so it could well be ss.
it doesn’t come off with a hard brush or even ss scrubby but sandpaper does seem to work in restoring the shiny. I’m reading people using various acids to etch stainless steel.
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Re: stainles steel turned black

Postby HTH1975 » Sat Nov 19, 2016 20:17

Even SS will pit if left in contact with caustic or acid long enough.

2016: 330L brewed (72 gallons, over 8 firkins)
2017: 105L brewed
Drinking: store-bought beer as my bar is dry
Conditioning: choc-coffee oatmeal wheat stout, various ciders, cherry 'brett' brown ale, imperial Pilsner.
Fermenting: Bock (6.5%), IPA (5.5%), Pale Ale (4.5)
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Re: stainles steel turned black

Postby lari » Sat Nov 19, 2016 20:23

HTH1975 wrote:Even SS will pit if left in contact with caustic or acid long enough.


question is would it still be ok to use it?
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Re: stainles steel turned black

Postby beer buddah » Sat Nov 19, 2016 23:34

If SS pits with caustic or acid you must be talking months @ high strength. I have not experienced it and I'm around it a bit. But Murphy's law and all.
If it's Chinese and looks like that it ain't stainless. I guess it depends on what metal it is, as to if it's OK or not food wise.
I'd find a use for it but for my hobby home brewing I'd bin it buy quality valve. Then again it won't kill Ye [SMILING FACE WITH OPEN MOUTH AND TIGHTLY-CLOSED EYES]

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Re: stainles steel turned black

Postby sledgehammer » Mon Nov 28, 2016 14:15

resistance to acids will depend on the grade of stinless steel and exposure time
https://en.wikipedia.org/wiki/Stainless_steel
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Re: stainles steel turned black

Postby PeeBee » Tue Nov 29, 2016 22:05

lari wrote:hi all,
i have a ss ball valve which i tried to clean thoroughly with some bar keepers friend. long story short i left parts of ss valve soaking in bkf solution for a few days and its now almost black, dar greyish colour and smells a bit ‘metaly’. i assume its some kind of iron oxidation or something...

Oh aye, far from indestructible is "stainless steel".

I left a bit of VWP solution (not an exceptionally strong caustic) in a brand new 65L conical fermenter and it went black where it was in contact (and dull, and even rusty), and the liquid was a foul oily, scummy, nastiness. Just 4 or 5 days contact.

I attacked this with stuff used by bikers to polish up their bike exhausts (Autosol Bluing Remover - careful, very strong hydrochloric acid base). It came up alright. The black scars are still visible but it got back the metallic lustre. Fortunately I avoided the scary sounding "pitting".

Lesson learnt. I still use VWP on it but never leave it soaking more than an hour.
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Re: stainles steel turned black

Postby Kev888 » Wed Nov 30, 2016 00:40

I remember that BarKeepers Friend contains oxalic acid, which can be used in such potions to passivate stainless. It eats away surface iron which otherwise could compromise the passive barrier and cause rust. Possibly in the extended contact time though, it could also have attached the chromium (and/or re-deposited dissolved stuff on the surface).

Unfortunately I'm no metallurgist, I 'think' this may mean chromium oxalate was formed, but whether whats left on the surface poses any health concerns I've no idea, though I would have expected it needs to re-dissolve to affect your beer. Perhaps its more a question for a scientific forum?

If the container you soaked it in was metallic, that could also be relevant.

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