What Does Rust Do To The Steel?

Gondor

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I'm not a metallurgist and have no idea what rust does chemically to the steel but I know that once it appears heavily you can throw away your knife.
 

wvbreamfisherman

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Well aside from the obvious (oxidation of the iron to various iron oxides, some of which are very weak mechanically and flake off), removing iron from the matrix of the steel weakens it and makes it more prone to cracking.

Surface rust is usually not a big problem unless it causes deep pits.

Remove the surface rust with steel wool or a wire brush and oil the blade to keep it from rusting further, and it will likely be fine.
 

Gondor

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Yes, aside from the obvious. Some say that it messes with steel quality and that it destroys the effect of heat treatment which is, as we all know, one of the most important features of the steel.
 

Theo

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Yes, aside from the obvious. Some say that it messes with steel quality and that it destroys the effect of heat treatment which is, as we all know, one of the most important features of the steel.
A lot of heat treatments are surface treatments. Damaging the surface of these types of metals will result in sub standard performance.
 

Judy Ann

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I read somewhere that if you have a rusty knife you can stick the blade in an onion for a period of time and the rust will be gone. I take care of my stuff so I haven't anything to try that out with.
 

mamabear

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Like Judy Ann said, it's important to take care of your "stuff". I have been know to use a little sand paper to take the rust away if it's not too bad and then be sure the knife is well oiled afterwards.
 

dinosaur

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The breamfisherman is right. Rust is iron oxide. Scrape a bunch of it off and blend it with aluminum powder, easily produced by melting down an aluminum can, and you have a great fire starter (thermite).
 

Barney

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The breamfisherman is right. Rust is iron oxide. Scrape a bunch of it off and blend it with aluminum powder, easily produced by melting down an aluminum can, and you have a great fire starter (thermite).
Excellent thinking! I have never heard let alone tried that before - live and learn. As soon as rust appears clean it with steel wool.
 

Gondor

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A lot of heat treatments are surface treatments. Damaging the surface of these types of metals will result in sub standard performance.
Thank you, that is the answer I've been looking for. I didn't know that such treatments include just the surface. I always wondered about that.
 

wvbreamfisherman

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Thank you, that is the answer I've been looking for. I didn't know that such treatments include just the surface. I always wondered about that.
Well, to be more precise SOME heat treatments only effect the surface.

Tempering and stress relieving affect the entire blade.

Some heat threatments for the surface result in hardening (like laser heating and fast air quenching of an edge). Case hardening only affects the surface, as well, although I can't quite imagine why you'd case harden a knife blade.

Also, there are various Iron Oxides (Fe2O3, FeO2, Fe3O5, etc) they all have different mechanical and chemical properties.

The classic blued finish on a gun is produced by oxidizing the surface in a controlled way to make a hard protective layer. Browning (used on old firearms like the "Brown Bess" musket is also a controlled oxidation process.

Parkerizing, used on military weapons is creating an Iron Phosphate layer, which is rust resistant, and also resistant to wear. It is prepared using a phosphoric acid base and manganese or zinc salts .

Naval jelly is a phosphoric acid-based preparation which removes rust by converting it to a phosphate compound.
 

Newanderthal

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Treatment option for rusted blades

1: scour the surface with sandpaper to remove most of the rust and leave a smooth surface to work with.

2: brush on a chemical rust converter (Rustaid, Rust Converter by Eastwood, etc. available from your local hardware store). The milky white liquid will slowly turn brown.

3: after about 24 hours, flip the blade and paint rust converter on the other side.

4: once the rust converter has done its work (rust should now be hard and black), sharpen the edge

the blade should now be virtually rustproof (converted rust turns black and is almost impervious to rusting) and should function quite well.

I once found a Case brand pocket knife buried in the mud while riding. It had probably been there for months. It was rusted so bad I had to take the entire knife apart to treat the blade because it wouldn't open. After sanding and treating, I used the knife for quite a few years before losing it on a canoe trip.





This can also be used as a sort of poor-man's-powdercoating. If you have a knife/hatchet/ax that is prone to rusting, brush on salt water and let it sit out for 48 hours, then smooth it with steel wool and treat with rust converter. This will prevent future rust.

However, the best way is to clean, dry, and oil your high carbon blades after each use.
 
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oldsarge

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I never seen this stuff before, but I think it's a lot like POR15 (paint over rust)POR15, Inc. - Stop Rust Permanently - Repair Gas & Fuel Tanks . You apply it like a paint, it's real watery and doesn't seem like it would do the job. But once you see what it does to rust, it's amazing, it's almost like a hard epoxy type finish and really thin not heavy like normal paint. I'm going to look for this stuff, POR15 is expensive.
 

Newanderthal

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Sarge, that POR15 sounds similar to the stuff I use. Rust Converter is also very thin and it doesn't take much to treat a knife, ax or machete. I used the same 1 quart jug for years and eventually lost it. It was still more than half full... and I had inherited the jug from a friend.

And it's great on lawn mower blades.
 

oldsarge

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It's great stuff, but you have to be careful with it. You get it on your hands and you're not washing it off anytime soon. It has to wear off. I was torching some metal that was coated with POR15 and it was really making it difficult controlling the cutting.
 

wvbreamfisherman

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It probably leaves a layer that won't oxidize well (since it converts/prevents further rust. That's going to make cutting it with a torch a lot tougher. A plasma cutter would probably work fine, since it doesn't depend on oxidation (that's why it will cut Stainless just fine).
 

oldsarge

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It probably leaves a layer that won't oxidize well (since it converts/prevents further rust. That's going to make cutting it with a torch a lot tougher. A plasma cutter would probably work fine, since it doesn't depend on oxidation (that's why it will cut Stainless just fine).
I'll have to remember that, thanks!
 

CaverGroupie

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/quote Rust is iron oxide. Scrape a bunch of it off and blend it with aluminum powder, easily produced by melting down an aluminum can, and you have a great fire starter (thermite). (/quote)

I had no idea you could make something as dangerous as thermite in the kitchen, basically. I am not going to tell my sons, the pyros. But it would be a really good and cheap fire starter. What about storing it?

And I'm going to try the onion thing on my pocket knife, so I'll let you know how it goes.
 
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ppine

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Keep an eye on your stuff and maintain it. Do something at the first sign of rust. It is one of the benefits of a dry climate- little if any rust or mildew.
 

wvbreamfisherman

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/quote Rust is iron oxide. Scrape a bunch of it off and blend it with aluminum powder, easily produced by melting down an aluminum can, and you have a great fire starter (thermite). (/quote)

I had no idea you could make something as dangerous as thermite in the kitchen, basically. I am not going to tell my sons, the pyros. But it would be a really good and cheap fire starter. What about storing it?

And I'm going to try the onion thing on my pocket knife, so I'll let you know how it goes.
Thermite is potent stuff, but not particularly dangerous, since it takes a pretty high temperature to ignite it. Something like magnesium ribbon, a sparkler or something like a propane or oxyacetyene torch. You're not going to start it by accident.

Once it goes- it generally goes pretty fast, but not explosively. It gets white-hot and will melt practically any normal metal. If you set it off on concrete, it can be quite dangerous, as the high temperature will cause the moisture in the concrete to form steam, and cause spalling (the surface of the concrete flies off explosively) which can spread white hot burning metal around and start fires and/or cause really nasty burns.

Here's a good article : Thermite - Wikipedia, the free encyclopedia
 
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