Metallurgists/Chemists?

[lxskllr]

I got a new carbon steel knife last week, and since it was a cheap blade, I thought I'd give it the hot vinegar treatment which is supposed to prevent corrosion. I first tried cold vinegar, but all that did was tarnish the blade, and caused rust when left wet in the air. I then boiled some vinegar, poured it in a bottle, then put the knife in the bottle. When I did that, a heavy stream of bubbles started from the knife tip, and became almost violent as it reached the top. I figured about 15 minutes would be a good amount of time, and took a shower while the knife was bubbling away.

I got back ~20 minutes later, and it didn't look like the bubbling subsided at all. Pulled the knife out, and the blade was covered in black 'soot'. After wiping that off, I was left with a nice grey patina, and no pitting, or any other visible defects.

So... What exactly happened here? What caused the bubbles in hot vinegar that wasn't there in cold, and what was the black substance?

 

[EliteRetard]

Just like when you dissolve a body in acid, there's lots of carbon left behind.

 

[Paperdoc]

I doubt the black was carbon. What happened in both cases of placing the steel blade in vinegar (a mild acid) is that it attacked the steel (mostly the iron component thereof), oxidizing it from metal to positive ions that can dissolve in the water solution and be washed away. But this action also produces some oxides of iron and of other metals in the steel alloy that remain as solid tiny particles on the surface. Such a coating of tiny particles is very good at scattering light in random directions. The effect to your eye is that the reflectivity of the (previously clean and polished) blade surface is reduced significantly, giving a dark grey appearance. Often such a thin layer of metal oxides is poorly bonded to the underlying metal and can be removed at least partially with mild scrubbing.

I say this happens in both cases. However, in the very hot vinegar solution the rate of the reactions is MUCH faster, so in the space of 20 minutes the results were quite obvious. When it was just room-temperature vinegar the reaction was much slower. If you leave it for a few days you'd start to see the same results.

 

[EliteRetard]

I doubt the black was carbon. *snip*

While you did a good job of explaining the "anodizing", I have experience with dissolving "stuff" in acid...and I think it quite likely the soot was carbon residue.

 

[JEDIYoda]

actually the soot was formed at the atomic nucleus level of carbonization! Then the oblifishiation of the nitrosiphite stsrted to exude gaseous omniferous nucleai! That's your s]hort explanation in simple terms!

 

[dave_the_nerd]

While you did a good job of explaining the "anodizing", I have experience with dissolving "stuff" in acid...and I think it quite likely the soot was carbon residue.

Doesn't that depend on what it is you're dissolving and in what type of acid?

It's been decades since I had to balance a chemistry equation... but I remember they do have to come out even.