Yet Another Thread About Rust And Corrosion

VermontEdge

VermontEdge

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Jun 10, 2015
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So, I've been into knives for a while now. At first I would only buy stainless steel knives as I was really worried about rust.

About a year ago I got my first carbon steel blade and am now up to 3. I love them. Period. Especially my knife in A2.

Anyway, I've been wondering lately (and this may sound like a dumb question so...forgive me) what actually happens when a knife "rusts", or develops red rust to be more specific.

I mean, we're all clear that a patina is a non corrosive and good form of rust and that red rust is corrosive and "bad rust". But when that bad rust develops, what happens?

I've read a lot of threads where people say "just scrape it off with your fingernail" or something to that effect. Some people really get worked up when they find rust on a knife and some people are terrified of "pitting".

So I mean...what's the deal, exactly? What does rust really DO to a knife? I know that years and years and years of red rust developing is bad news but what if I'm carrying a fixed blade with a layer of red rust on it? What's going to happen in my lifetime? And I suppose that I mean "happen" in a practical sense rather than a scientific sense.

Again, I apologize if this is a dumb topic or anything, but I just realized today that for all the carbon steel maintenance and patina forcing and worrying I don't actually know what would happen if I just let my knives rust.

Anyway, curious and interested to hear any of your thoughts on the matter.

Best,
-Taylor
 
one of the main problems would be that the thinnest parts would be most damaged first, so while the side might pit a bit, the edge might be fully removed. Rust is also self-perpetuating as it can hold more moisture and keep the chemical reaction going. Different steels are going to act differently of course, but some can have fissures formed deep into the structure by the corrosion. This could be very bad say, along the bolster which could lead to a failure of the knife, or along the edge which would severely reduce its usability. I have a Case medium stockman that got some rather bad pitting after just 12 hours, when I took it out of my pocket, all of the exposed metal was bright red. it had been a very hot 30-34C day, with heavy work. When I cleaned it all up, I was not able to remove all of the pitting. In this case not a huge deal, everything looks fine to me, but it wouldn't have taken much longer to rust shut, and it wouldn't have survived years of that treatment.

There is also a bit of a difference between surface staining rust, and pitting rust. generally you start with with just a surface stain, and then get the aggressive stuff.
 
In a practical sense, taking off from what gadgetgeek said, you won't have much of a sharp edge. You won't be able to sharpen it. You won't be able to cut with it relatively speaking. It will be less ductile and more brittle.

Forced patinas are a form of corrosion but it's a stable corrosion in that it forms a layer over the steel protecting it from further oxidation by becoming a barrier between the uncorroded steel and oxygen. Regular uncontrolled corrosion do not form a barrier and so the steel underneath continues to be exposed to oxidation.
 
Rey HRH said:
In a practical sense, taking off from what gadgetgeek said, you won't have much of a sharp edge. You won't be able to sharpen it. You won't be able to cut with it relatively speaking. It will be less ductile and more brittle.
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What?!
I have many carbon steel kitchen knives from my grandparents that have seen red rust dozens of times. They have pit marks all over them, some fairly deep. They still sharpen up beautifully and cut like a razor.
If you don't want your blade to even patina, wipe it with breakfree clp.
BTW, tool steels do not corrode (pit) near as fast as simple 10xx carbon steels.
 
jpm2 said:
What?!
I have many carbon steel kitchen knives from my grandparents that have seen red rust dozens of times. They have pit marks all over them, some fairly deep. They still sharpen up beautifully and cut like a razor.
If you don't want your blade to even patina, wipe it with breakfree clp.
BTW, tool steels do not corrode (pit) near as fast as simple 10xx carbon steels.
Click to expand...

I was taking off his statement of "but what if I'm carrying a fixed blade with a layer of red rust on it?"

He's going to have to go down to good steel that can be sharpened. In that case, that area won't have a layer of red rust on it. I was taking what he said literally.
 
Sorry for the confusion!

I never, ever neglect my edges. I keep them all razor sharp with a strop and will give my knives a few passes any time I notice that they've lost their razor edge.

To clarify, I think what I meant was:
If I'm maintaining a clean, polished and sharpened edge but just not doing anything to prevent rust on the rest of the blade, what's the harm? Within my lifetime, what kind of performance detriment will be caused by a rusty blade with a maintained edge?

Thanks for the responses so far!
 
VermontEdge said:
Sorry for the confusion!

I never, ever neglect my edges. I keep them all razor sharp with a strop and will give my knives a few passes any time I notice that they've lost their razor edge.

To clarify, I think what I meant was:
If I'm maintaining a clean, polished and sharpened edge but just not doing anything to prevent rust on the rest of the blade, what's the harm? Within my lifetime, what kind of performance detriment will be caused by a rusty blade with a maintained edge?

Thanks for the responses so far!
Click to expand...

Depending on where you are (how humid/wet or not) and how you use the knife (exposure to caustic/acidic materials, constantly wet conditions or storage, or NOT), it may ruin the knife very quickly (months or a few years), or it may not matter much at all. It would be foolish to deliberately ignore obvious rusting, as it's very easy to control and/or minimize it anyway, just by keeping the knife clean & dry after each use. If or when some visible rust shows up on the surface, it just takes a little scrubbing with some steel wool & oil (or WD-40), or some baking soda will scrub it off as well.

As mentioned, red rust that's ignored will foster conditions which enable and accelerate more rusting (because rust is porous, which traps stuff that makes more rust), therefore more pitting and complete destruction of the rusted steel. The deep pitting left behind will make the blade structurally weaker (more prone to breakage under stress, and the rusted, weakened steel behind the sharpened edge won't be able to support the edge as robustly), as well as trap potentially harmful stuff like bacteria in the pores/pits. Using the knife for food prep? This is why stainless blades are mandated in many commercial kitchens, BTW. And the Tetanus bacteria propagates and lives in the dirt trapped in rusty metal as well. In a nutshell, all kinds of problematic things can happen, and deliberately letting it rust is just tempting fate in any number of ways.

If it's really let go behind the sharpened edge, it could eventually get to the point where you won't have any healthy steel left behind your sharpened edge anyway; in which case, you might actually reach a point where you won't be able to sharpen it anymore, without your edge just folding or crumbling away because the underlying 'healthy' steel is too thin to support it.

Another obvious downside to using & carrying an extremely rusted blade (or any other rusted tool) is, it'll leave rust-colored smudges on everything it touches: your hands, your clothes, anything you cut with it, etc, etc. Heavy rust is just plain messy. :grumpy:


David
 
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Everything makes sense so far. Thanks for the responses!

I think my main concern is some rust spots that have formed on my exposed tang. I really want to just not worry about it and not fuss about it, scrape them off when I use it otherwise just think that it'll be fine.
 
VermontEdge said:
Everything makes sense so far. Thanks for the responses!

I think my main concern is some rust spots that have formed on my exposed tang. I really want to just not worry about it and not fuss about it, scrape them off when I use it otherwise just think that it'll be fine.
Click to expand...

Sometimes just a pencil eraser is enough to rub off the light rust, if you catch it before it gets too deep into the steel. If that doesn't quite get it, soak the spots with some WD-40 and wait 15-30 minutes, then scrub it off. If the rust is hiding in slight pits that make it difficult to to access, that's where the baking soda works well, mixed with a little water (to a coarse paste consistency) and an old toothbrush. A little bit of metal polish can be used the same way.


David
 
Thanks David. Totally makes sense. I think part of me just kind of feels like working my way back to stainless. My blade is fine but with so many issues with the exposed tang I'm not sure it's worth it!
 
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