What's the difference between patina and rust?

K

kubelik

Joined
Oct 16, 2013
Messages
240
So I got my first traditional knife (a GEC #15 which I believe uses 1095 carbon steel), which is not my first 1095 blade but it is my first un-coated 1095 blade. my other knives are typically some form of stainless. I was carrying this knife as my regular EDC'er for a couple of weeks straight, and then I oiled it up (using cutting board oil since I do food prep with all my EDC knives) and put it away for a few weeks. when I took it back out to EDC again, this is what it looks like:

17321069501_ef3db0ea50_b.jpg


now, I'm not one of those guys that freaks out when he sees a spot of rust. I don't buy my knives to flip them or in hopes that they will appreciate in value. but I also don't want to be an abuser of knives, I think that is wasteful and disrespectful. so I'm just hoping to get a better understanding of the difference between patina vs rust, and if there's anything I should be doing differently to maintain my traditional knife. also, if I were to want to clean up the blade back to an as-new look (which I probably won't, but again, out of educational curiosity), how should I do that?

Thanks!
 
Once in a while I'll clean the patina from a blade using A German made product GHE called "Semichrome Polish." It will also polish bone and other non metal surfaces.

I generally like patina and have experimented with forcing the process with boiling vinegar. Still, naturally occurring looks the best to me. Patina will inhibit rust but not prevent it. The key in my opinion is usage. I only oil the knives that are NOT in my weekly EDC rotation. IOW, the ones sitting on display or stored away.

Here's two of my favorite users with patina.
 
kubelik said:
if I were to want to clean up the blade back to an as-new look (which I probably won't, but again, out of educational curiosity), how should I do that?
Click to expand...

I use Mother's Mag Wheel polish, which is found in the auto section of Wal-mart, Target, etc. Its a very mild polishing paste that is a lot like Flitz, but at about 1/10th the cost. I apply it with a Q-tip for a couple of minutes, and then wipe it off. No biggy. I use Mother's on my knives, as well as the chrome bumpers of my sweet, restored 1969 Chevy CST-10 truck. If you keep applying Mother's for a few minutes with a light rubbing action, it will produce a semi-mirror shine, so don't overdo it if you want to keep the original brushed finish or whatever it is.

As for patina vs. rust, its sort of the same as flowers vs. weeds. If you like what you see, its patina (or flowers); if you don't like what you see, its rust (or weeds).
 
It looks like the sheep's foot has patina and the pen blade rust. Generally, rust is orange and looks like - rust (for lack of a better description). Rust continues and eats away at the metal, and needs to be addressed. Patina is self limiting, and it actually helps to prevent rust. Some love it, some hate it. I love it. YMMV.

If you want to put on a fantastic patina and get rid of rust, go to a hardware store and buy some naval jelly. It's pink and has the consistency of jelly. It will completely dissolve all your rust and put a great dark patina (very dark) on your blade. It's my favorite, but there are many other ways to put a patina on. I would recommend using the NJ on something you don't care about first to see if the patina is ok for you, it's a love it or hate it kind of thing.

Nobody in the store will probably have ever heard of Naval Jelly...

http://www.loctiteproducts.com/p/s_trmt_naval/overview/Loctite-Naval-Jelly-Rust-Dissolver.htm
 
'Patina', a.k.a. 'black' iron oxide (Fe[SUB]3[/SUB]O[SUB]4[/SUB]), is non-destructive to steel. It forms when iron reacts with oxygen.

'Rust', a.k.a. 'red' iron oxide (Fe[SUB]2[/SUB]O[SUB]3[/SUB]), is destructive to steel, because the compound occupies more space than the steel from which it's created; that causes it to break/slough away, which causes pitting. It forms when iron reacts to oxygen in the presence of water. This differs from black iron oxide, which doesn't occupy more space than the steel it was created from, and therefore won't break/slough off and cause pitting.

Under many circumstances, both will form on a piece of steel simultaneously; but the 'red' rust is the only thing to be concerned about and minimized. Due to the loose & porous nature of red rust, it can also absorb moisture and hold onto stuff that creates more rusting, if it isn't cleaned from the surface of the steel.

Some will describe both black iron oxide ('patina') and red iron oxide as 'rust'. But to me, it's a mis-characterization, due to the extreme differences in the physical properties of each and what each will or won't do to steel, in terms of damage.


David
 
Obsessed with Edges said:
'Patina', a.k.a. 'black' iron oxide (Fe[SUB]3[/SUB]O[SUB]4[/SUB]), is non-destructive to steel. It forms when iron reacts with oxygen.

'Rust', a.k.a. 'red' iron oxide (Fe[SUB]2[/SUB]O[SUB]3[/SUB]), is destructive to steel, because the compound occupies more space than the steel from which it's created; that causes it to break/slough away, which causes pitting...
Click to expand...

Best description I've seen thus far explaining the difference. Thanks!

I've been cutting apples and strawberries with an old slip joint jack knife I've got and the results look pretty sweet.
 
The red iron oxide (Fe2O3) referred to above is an 'active' rust (corrosion). Exposed to moisture and oxygen, it will rust and rust and rust until there is no steel left.

The black iron oxide (Fe3O4, aka magnetite) is a 'passive' rust. Once formed, it seals the underlying steel from oxygen and further rusting, aka passivation. It is very desirable on carbon steels. In commercial/industrial boilers, it is formed on purpose so that that the tubes do not rust away, particularly at the high temperatures seen.

Frank
 
Obsessed with Edges said:
'Patina', a.k.a. 'black' iron oxide (Fe[SUB]3[/SUB]O[SUB]4[/SUB]), is non-destructive to steel. It forms when iron reacts with oxygen.

'Rust', a.k.a. 'red' iron oxide (Fe[SUB]2[/SUB]O[SUB]3[/SUB]), is destructive to steel, because the compound occupies more space than the steel from which it's created; that causes it to break/slough away, which causes pitting. It forms when iron reacts to oxygen in the presence of water. This differs from black iron oxide, which doesn't occupy more space than the steel it was created from, and therefore won't break/slough off and cause pitting.
Click to expand...

The black steel patina is reduced iron oxide, aka ferrous oxide, aka iron(II) oxide. Its formula is FeO.

The red steel rust is oxidized iron oxide, aka ferric oxide, aka iron(III) oxide. Its formula is as give above, two iron atoms ionically bonded with three oxygen atoms.

Iron(II) is in the +2 valence state (its ion is deficient two electrons), while iron(III) is in +3. Oxygen is virtually always -2 (i.e., an oxygen ion generally has two excess electrons), which is why Fe(II) bonds with only one O to create a charge-balanced molecule of reduced ferrous oxide, while Fe(III) bonds with three atoms of O to create a charge-balanced molecule of oxidized ferric oxide.
 
Frank_Zuccarini said:
The black iron oxide (Fe3O4, aka magnetite) is a 'passive' rust. Once formed, it seals the underlying steel from oxygen and further rusting, aka passivation. It is very desirable on carbon steels. In commercial/industrial boilers, it is formed on purpose so that that the tubes do not rust away, particularly at the high temperatures seen.
Click to expand...

A molecule of magnetite has one ferrous atom and two ferric atoms. I may be wrong, but I doubt a blade patina contains much magnetite. Gun bluing is magnetite, and the process to achieve it takes much more energy and controlled conditions than those generating a patina on a knife blade (unless you use a gun bluing method to force a patina).
 
powernoodle said:
I use Mother's Mag Wheel polish, which is found in the auto section of Wal-mart, Target, etc. Its a very mild polishing paste that is a lot like Flitz, but at about 1/10th the cost. I apply it with a Q-tip for a couple of minutes, and then wipe it off. No biggy. I use Mother's on my knives, as well as the chrome bumpers of my sweet, restored 1969 Chevy CST-10 truck. If you keep applying Mother's for a few minutes with a light rubbing action, it will produce a semi-mirror shine, so don't overdo it if you want to keep the original brushed finish or whatever it is.

As for patina vs. rust, its sort of the same as flowers vs. weeds. If you like what you see, its patina (or flowers); if you don't like what you see, its rust (or weeds).
Click to expand...

Patina is a less corrosive metal oxide covering, than a more corrosive metal oxide, that's usually represented by a colored protective coloring.

While patinas usually slow the more aggressive type of corrosive chemical oxidative reactions that degrade metallic alloys, patinas don't usually prevent the more destructive type oxidation from eventually breaking down the alloy, entirely.
 
You must log in or register to reply here.
Back
Top