Chemistry help: Black oxide using vinegar? 

"Old-Busted-Hotness" (old-busted-hotness) 02/13/2016 at 10:40 • Filed to: None

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I had some hardware soaking in vinegar to remove rust. Been in there maybe a week. Last time I saw this bolt it was almost rust-free, so I let it go another couple days. Then it got cold. It’s been around 15F for the last couple days. When I pulled my bolt out of the vinegar, I had this:

This has gone from clean steel to black oxide(?) in just a couple days. The coating doesn’t rub off or clean off with any chemicals at my disposal (carb cleaner, brake cleaner, POR-15 Metal Prep, mineral spirits and lacquer thinner have failed to budge it), so I assume some sort of chemical change has happened. Can anyone enlighten me?

I’m going to assume (again) that once the weather warms up, I can dunk it back in the vinegar and it’ll start going back to clean steel.

Mattbob > Old-Busted-Hotness 02/13/2016 at 10:51

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weird. Depending on what the bolt is for, I would roll with the black oxide. People pay to have that done.

Rusty Vandura - www.tinyurl.com/keepoppo > Old-Busted-Hotness 02/13/2016 at 10:54

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15F? You need to move. My cousin is a chemical engineer. I’ll see if he has any ideas about the black coating.

Phyrxes once again has a wagon! > Old-Busted-Hotness 02/13/2016 at 11:08

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I believe the vinegar is stripping the outer layer, which includes the rust off and exposes the steel underneath which is then oxidizing. Functionally this is a “forced” patina that is then preventing the material underneath from rusting.

1111111111111111111111 > Old-Busted-Hotness 02/13/2016 at 11:18

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Was that galvanized? Cause that’s what it looks like to me.

Also it was never clean steel. To get that off you’ll have to tumble or sandblast it.

jariten1781 > Old-Busted-Hotness 02/13/2016 at 11:48

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Vinegar is essentially just acetic acid and water. I bet you can find the reactions searching ‘acetic acid and steel’ or ‘acetic acid and iron oxide’. I’m alergic to chemistry so I wouldn’t know what I was reading haha.

I’ve never done the vinegar thing but I’ve always heard you need to immediately/thoroughly spray it off with very hot water when removing it from the bath to prevent the blackening. Don’t know how true that is. Also heard it’s not an issue on plated parts.

1111111111111111111111 > jariten1781 02/13/2016 at 11:56

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The black the zinc coating oxidizing. It’s because it was plated that it turned black. It it were steel it would have been the usual red rust.

Old-Busted-Hotness > 1111111111111111111111 02/13/2016 at 12:13

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Then why does it only blacken when it’s cold? This thing was clean steel and nearly rust-free three days ago.

Old-Busted-Hotness > 1111111111111111111111 02/13/2016 at 12:14

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Might have been zinc-plated when it was new.

Old-Busted-Hotness > Phyrxes once again has a wagon! 02/13/2016 at 12:16

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In warm weather, vinegar removes the rust and doesn’t blacken it.

1111111111111111111111 > Old-Busted-Hotness 02/13/2016 at 12:29

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The cold doesn’t matter. It will take longer in the cold actually. I use hot vinegar to remove zinc. The reason its black is because its not steel, its the zinc coating that blackens. It’s not coming off, so use it as is or put it in a rock tumbler with dry media to shine it again. It will be more prone to rust now that the coating is gone or thinner though.