Decarburisation/carbon migration in evapo rust.

[AESamuel]

Hey there,

I recently bought some evapo rust after hearing good things and it's worked a treat on rust.
On some of the tools it's left a black coating which evapo rust says is a layer of carbon that has settled in the pores of the metal due to carbon migration. I know that decarburisation can have a bad effect on tool steel during the heat treatment process, but evapo rust claim it won't have any effect in this instance.

I'm not too worried about it as it's easy to remove, but out of pure curiosity, does anyone else think this is carbon migration and could it be of a level that would actually be noticeable on the steel? I've uploaded an image of some of the blades, these ones in particular have a very clear line where the black coating starts, looks like a hardening line.

 

[chaoticbob]

Well, Evapo Rust should know what they're talking about I suppose, but sure as heck it isn't carbon migration. The carbon in tool steel is pretty happy where it is, and the only way to shift it is with heat as far as I know. My money would be on be on some sort of tannic complex with the iron/rust - de-rusters often have 'tannic acids' in their formulas.
Bob.

 

[Ozi]

It will be the carbon you had already lost, when the steel rusted and iron became iron oxide the carbon was still in the mix but not doing anything useful. The oxygen has been removed giving you iron not steel in a thin surface layer, a galvanic action in the process bonds this to the steel below effectively migrating the carbon to the surface. The black carbon containing layer can act as a surface protection to some extent and should only be removed for cosmetic reasons.

If you want to watch migration within an alloy happen find an old 2p piece from around 1980. Later coins don't seem to work. The surface needs to be really clean, then heat it in the absence of oxygen. The easiest way to do that is on top of the burner on a gas stove, not in the flame but resting right in the middle. If you have the correct age of coin nickle will migrate to the surface and the two pence piece looks silver.

My materials science lecturer didn't believe this was possible till we showed him. Then it started a massive argument between a group of people who all had Phds that went so far over my head that I cannot explain the process. I take comfort in not being the one daft enough to pick up the coin while it was still hot.