Staining? Patina?

HikingMano

HikingMano

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Sep 2, 2008
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What exactly is a "stain" on a carbon steel blade?

What specifically differentiates a patina from staining? Are both just different types or degrees of the same oxidative processes?
 
If I remember correctly there are only 2 types of rust, FeO and Fe2O3. Whether a patina and red rust are different or the same I am not sure. But I would say both terms are referring to the same thing, just varying amounts. A patina for the most part covers the blade. Staining is the same thing but possibly unwanted and in smaller 'patches.' I don't think there is any difference between the two.
 
Thanks FlaMtnBkr. My understanding is that the iron oxides formed in a patina or as a product of passivation (FeO, or push past Fe2O3 to Fe3O4) is different from that formed in rust (Fe2O3).


Anyone know for certain if a patina is really just the sum of localized staining?
 
Well even if they are different or the same you are relying on what someone calls something when they don't know what it really is. If you see a discoloration on your knife that you don't want you are likely to call it staining. Also, if it is not red rust you are likely to call it staining. I'm sure all those people with carbon blades that used to be nice and shiny slowly had staining until the whole knife was covered and uniform. I think you are referring more to descriptive terms than a chemical composition. And people who use those terms are just trying to describe something with no clue as to what it actually is.

Some metals like M4 won't really get red rust in my experience but you might get a couple small spots of discoloration. If I didn't polish them off they would eventually cover the blade in a patina. But I don't know what they are for certain.

Any particular reason you are curious about this?
 
Right, that's the reason for my first question:


rksoon said:
What exactly is a "stain" on a carbon steel blade?
Click to expand...
Physically/chemically, what specifically is happening when your knife goes from uniform grey to splotches of discoloration? As you point out, people use the term without precisely defining it. Stainless steels are described as corroding or rusting less than steels with lower chromium content, so I've always assumed that a stain is simply a form of oxidation or corrosion. But I don't know for certain.

rksoon said:
What specifically differentiates a patina from staining? Are both just different types or degrees of the same oxidative processes?
Click to expand...

The second question follows. Is there a difference between a patina and staining? I've always assumed what you stated, that a patina is just the sums of stains over time. But again, don't know for certain.


No particular reason for the curiosity, just self-edification. I always hear the term "patina" thrown around, but since I've never forced a uniform layer of oxidation by soaking in weak acid (or whatever other method), I've always wondered if the random "stains" I accumulate on my carbon blades are really the beginnings of a patina. I've always assumed so, but don't know for certain. I know it's making a mountain out of a mole hill, but I like to have the concrete facts so I can explain it to others when needed. :o:)
 
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