I'm supposing the "migration" is :
the iron portion of the steel on the outer most rusts away
the carbon is left alone on the surface.
The carbon didn't actually move it just got uncovered .
Sounds good huh ?
Carbon is such a SMALL percentage of the total make up though .
Magic .
We'll call it Magic.
was cleaning some different old folders' springs using evaporust and, like many before me, accidentally discovered the phenomenon of carbon migration...
Thanks for starting this thread! Have always puzzled about some steels coming out of the EvapORust tank darker than others. Must've been the higher carbon steels. Often I've been able to buff the darker color (mostly) off. Other times I just leave it, because it is rather attractive.
And I really enjoyed your rebuttal above...was chuckling all the way through it, thinking, "Now THAT's how you rebut!"
I asked Chat GPT your question, and it said:
I disagree that it’s “carbon migration.” That explanation shows up in forums, but it doesn’t match the metallurgy or chemistry involved. Evapo-Rust is a chelating rust remover. It uses EDTA and similar agents to bind iron oxides and pull them off the surface. It does not heat the metal, so it cannot cause carbon migration, which is a high-temperature metallurgical process (hundreds to thousands of degrees Fahrenheit). Ambient-temperature chemicals don’t transport carbon atoms through steel.
Why the surface turns darker: Evapo-Rust doesn’t just remove rust — it can also convert certain residual iron oxides into a stable black iron phosphate or iron tannate layer. This dark film is thin and actually protective.
It’s similar to a light parkerizing effect. The darker the surface you started with (pitting, deep oxide), the darker the final color. High-carbon steels appear to darken more. This is probably where the “carbon migration” myth comes from. Higher-carbon steels can react slightly differently with the solution, often leaving a darker phosphate/tannate film. But this is not carbon moving — just the surface chemistry interacting with the steel’s alloy composition.
What Evapo-Rust themselves say: The manufacturer has stated (multiple times) that the darkening is normal, caused by oxide conversion and surface micro-topography — not carbon movement. (If you’d like, I can pull exact language or source quotes.)
At this point I asked for the quotes, and Chat gave me these quotes that it attributed to EOR's makers, who have responded in various forums:
“Evapo-Rust can leave a dark gray or black film on some steels.
This is a conversion of remaining oxides on the metal surface.
It is not harmful and can be removed if desired.”
“High-carbon steels sometimes darken more due to how they interact with the solution.”
“Carbon is not brought to the surface. The solution only removes iron oxides.”
“The black layer is normal. If you want a bright finish, it can be removed with light abrasion or acid.”
Last edited by Bolster on Thu Nov 13, 2025 11:20 pm, edited 4 times in total.
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I’m struggling to conceive how carbon in steel can migrate at room temperature. It’s in a crystalline structure isn’t it?
The evaporust writeup might be referring to what it’s doing to the rust layers on the metal. I’m not really sure.
I think you are correct, Zhyla. The chelation removes the iron oxide, but maybe not the carbon that was bound with the iron when it was steel (iron plus carbon) before the iron oxidized into rust. At low temperatures the carbon won’t burn like the iron does (rust is iron ash, if you will) so it stays behind and demonstrates its affinity for iron by clinging to the steel after the rust has been chelated away into solution. I’ve seen this in my own restoration projects using Evapo-Rust.
I think you are correct, Zhyla. The chelation removes the iron oxide, but maybe not the carbon that was bound with the iron when it was steel (iron plus carbon) before the iron oxidized into rust. At low temperatures the carbon won’t burn like the iron does (rust is iron ash, if you will) so it stays behind and demonstrates its affinity for iron by clinging to the steel after the rust has been chelated away into solution. I’ve seen this in my own restoration projects using Evapo-Rust.
Yeah, that's what I'm thinking. I'm hesitant to say the Evapo Rust people are wrong since they likely know their chemistry a lot better than I do. But in any case I'm pretty sure this "carbon migration" they're describing is a surface effect, very different from the carbon migration you see in steel during heat treatment.
As far as advantages, which was the actual question of this thread... I don't think carbon itself is protective as it will chip off or wear easily.
I’ve never seen the term “carbon migration” used in this context before and the explanation from that website is strange. I would avoid using it to avoid confusion.
Larrin! I would love to know what your preferred language on the matter is. I would generally discuss it under the term of diffusion, or even precipitation depending on the conditions causing the segregation. What would your preferences be?
FWIW, I'm a PhD materials scientist (amorphous metals patents) with 10 years at an R1, and now another 5 in railroad. I've been making Japanese style cutlery (swords and kitchen knives) for a little over a decade. Usually people only care about carbon's movement when it leaves your steel (decarb).
It is kind of a fun discussion though, as blister steel and its derivatives kind of relate to the idea.
I’m struggling to conceive how carbon in steel can migrate at room temperature. It’s in a crystalline structure isn’t it?
The evaporust writeup might be referring to what it’s doing to the rust layers on the metal. I’m not really sure.
i had a similar question; part of me wonders if they're just using a shorthand for 'when you remove iron oxide, the carbon is left behind in higher proportion.' but their wording suggests it actually moves to the surface vs is just what remains. is it a both/and situation?
will say i've liked the finish it left on some of my older traditional knives using unnamed, probably very low alloy carbon steel (i'm guessing not even 1095; talking old camillus and robeson). it came out smoky dark but slightly iridescent, its cool
