Patina vs Rust

[PayneTrain]

I understand they are two different things, but can/do they coexist on a blade? Does a good patina include some actual red rust, or is that strictly damage which should be removed? Will a patina form over rust?

Basically what I'm asking is what I should do with my knife here. It's a Spyderco Gayle Bradley, M4 blade that was soaked in sweat for a good few hours. The resulting spots are obviously rust, albeit very fine spots of rust compared to other steels I've damaged. I do ultimately want to get a nice patina to form on it naturally, but I'm worried that now I have to start over and polish the rust spots off and possibly damage the laser markings. Thoughts?

Click on the pic to go to photobucket and get the full size original to really see what I've done here.

 

[RevDevil]

Yes, they can coexist on a blade, if the initial patina is not set in enough. Remember, patina a light surface oxidation or carbonization, and rust is full on corrosion which can vary in severity. If you want to add a patina to your blade, I would recommend the following to force it:
Onion, vinegar, lemons, lime, mustard, basically any fruit/vegetable with a high acidic content or vinegar based condiments will work.

If you want to reverse it, you would need to use some sort of micro-abrasive polish (Flitz, toothpaste, sandpaper, automobile rubbing compound, etc). The best way to avoid it is to clean your blade thoroughly after use, and make sure the blade is dry and then oiled. You do not want water resting on the blade or other corrosive materials.

 

[PayneTrain]

I do want a patina, but obviously I don't want rust. So even as fine as the rust spots are on this blade, they should be removed in order for a proper patina to set? A patina can't "take over" the rust? I just want to be sure before trying to polish it away, again because I don't want to damage the markings.

 

[Obsessed with Edges]

Red rust can be scrubbed off relatively easily with WD-40 and some very fine steel wool (#0000). The red rust is much softer and not firmly bound to the steel, so it comes off pretty easily. The black oxide 'patina' left behind, after the red rust comes off, is much more durable. The scrubbing with WD-40 and steel wool will not remove much, if any of that. That's not a bad thing; especially if you're already considering letting the blade patinate on it's own. If you want to remove the patina entirely, some metal polish made for hardened steel will do the job, with some 'elbow grease'. Flitz or Simichrome will work for this.

A good way to 'test' the spots on your blade for the presence of red rust is to rub a clean, white pencil eraser on the spots. If there's any red rust present, the white eraser will be stained red/brown by it. Very easy to see it, like this.


David

 

[hiker45acp]

I have had good luck with using Mustard to force an artificial patina. Whats neat about it is, you can aquire some interesting paterns that look pretty nice. You can dab it on with a brush, or rag, or streak it on with a brush. Let it sit for at least 4 hours, wash it off with some dish soap rinse, dry and oil. Repeat the application process several times only rinsing in between and aquire a realy nice pattern. Then to make things interesting, take some gun blue paste and wipe it very lightly, let it sit no more than 10-15 seconds and rinse with water. Then oil well. You will get a deeper color and still show a great pattern and protect the blade pretty good for future use. Experiment, if you don't like it you can always remove with the methods mentioned above. Good Luck.

 

[PayneTrain]

Red rust can be scrubbed off relatively easily with WD-40 and some very fine steel wool (#0000). The red rust is much softer and not firmly bound to the steel, so it comes off pretty easily. The black oxide 'patina' left behind, after the red rust comes off, is much more durable. The scrubbing with WD-40 and steel wool will not remove much, if any of that.

This is very cool to know. I thought they were both just as easily removed. I only have PB Blaster around, and I assume it works the same being a super light oil, unless there's something else in WD-40 that does the magic. So this should be safe for the markings if it's safe for a patina, right? And this is less abrasive than using toothpaste?

Also, I'm wondering if the process of forcing a patina with an acid will lift away any rust currently there. I've been reading the chemistry behind a patina and rust on another forum, and it's pretty interesting stuff. Well, to me at least.

I have had good luck with using Mustard to force an artificial patina.

Yup, I know a lot of acidic foods will do the trick. I'm kinda planning to just use it as normal and let the patina form as it does rather than purposefully applying one. I think it may look more interesting that way.

 

[Obsessed with Edges]

This is very cool to know. I thought they were both just as easily removed. I only have PB Blaster around, and I assume it works the same being a super light oil, unless there's something else in WD-40 that does the magic. So this should be safe for the markings if it's safe for a patina, right? And this is less abrasive than using toothpaste?

Toothpaste usually isn't very abrasive anyway, on steel. I doubt it would affect the black oxide patina much, if at all (or blade markings/etches). If it were abrasive enough to scrub these off, I'd likely go shopping for something different to brush my teeth with, else I'd kiss my tooth enamel goodbye. It might work for scrubbing the lightly-bound red rust, though. Not sure about PB Blaster; don't know about the composition of it, so I can't comment on that. WD-40 has some light mineral oil, thinned with some mild solvent that allows better penetration of the oil into nooks & crannies. The mineral oil scrubbed with steel wool is what does most of the work on red rust, anyway (some use straight mineral oil in this same manner).

Also, I'm wondering if the process of forcing a patina with an acid will lift away any rust currently there. I've been reading the chemistry behind a patina and rust on another forum, and it's pretty interesting stuff. Well, to me at least.

Forcing a patina with anything acidic will alter and/or remove some existing oxides (both rust and previous patina), but will also likely create some new corrosion and patina in the process. Can still do this, but finish by rinsing & scrubbing the blade with baking soda, which will scrub away any rust formed, along with some of the new & lightly-bound black oxide. Baking soda is somewhat alkaline, which means it'll also neutralize the acidic reaction (good thing), which will keep the corrosion under control. More durable black oxide will be underneath the really dark stuff that forms, and will remain (likely as a light grey patina).


David

 

[Bill DeShivs]

Boiling the red rust will convert it to black oxide.

 

[PayneTrain]

I'm pretty sure at least whitening toothpaste is mildly abrasive. That's how it whitens, by sanding off surface stains while being soft enough to not damage enamel. Either way, I don't think I'll try it. I think I'll try the WD-40 idea to just remove the rust, and be a little more careful from now on until I can get the patina to form naturally. I don't think it will form quickly enough to count on it overtaking the existing rust. I really don't want to force a patina, though I would like to see what the process would do to rust. I don't know, I'm torn now.

Boiling the red rust will convert it to black oxide.

I don't think I'll do this though. Somehow that doesn't sound right...

 

[Obsessed with Edges]

Toothpaste IS abrasive, correct. It's just that the abrasive in toothpaste (it's called 'hydrated silica') generally isn't hard enough to abrade hardened steel, or the black iron oxide on it. Tooth enamel is softer than steel (much), and decent toothpastes will be abrasive enough to remove food stains/debris, but not abrasive enough to damage the enamel. In other words, the abrasive is softer than the enamel, and the enamel is much softer than steel.

{ Edit: It's easy to test the abrasiveness of toothpaste vs. steel. Just spread some on some clean, white paper and 'strop' your blade on it. The complete lack of black residue (steel swarf) seen on the paper is the visual cue that the toothpaste isn't doing anything at all to the steel. Compare this to doing the same with some Flitz or Simichrome polishing paste, which both use much-harder aluminum oxide abrasive; the paper will blacken quickly, showing the presence of steel swarf removed from the blade. }

The best one might expect in using toothpaste on a knife blade is that it'll do essentially the same for it, as for teeth. It'll 'clean' it, if there's any lightly-stuck crud on a blade, but not much else (this is why it might remove some rust, because red iron oxide is very loosely-bound to the steel). I'd go nowhere near a toothpaste that's actually hard enough to abrade steel; that'll wreck your teeth quick.


David

 

[PayneTrain]

I'd go nowhere near a toothpaste that's actually hard enough to abrade steel; that'll wreck your teeth quick.

Hah good point. Yeah I think I'll give it a good proper polishing and be a little more careful how I treat it. Start from scratch. Thank you sirs for all your help and knowledge!

 

[salmonkiller]

Thank you guys for the education.This is a great thread!
salmonkiller

 

[PayneTrain]

Just a small update:

So I was thinking some more about it, and thought if I'm going to polish it, I might as well see what foods really do to the M4. First was a red onion. I let the juice sit on it for a few minutes, then rubbed it off with a dry paper towel. Then I had a pickle. I let that juice sit on it till it dried, then again just wiped it down with a paper towel. No patina formed that I could see, but what I did notice is that the rust spots are all but gone. From the right angle I can see them very faintly, but I'm wondering if the rust has been removed and/or replaced. I'll have to find my microscope.

 

[Obsessed with Edges]

Just a small update:

So I was thinking some more about it, and thought if I'm going to polish it, I might as well see what foods really do to the M4. First was a red onion. I let the juice sit on it for a few minutes, then rubbed it off with a dry paper towel. Then I had a pickle. I let that juice sit on it till it dried, then again just wiped it down with a paper towel. No patina formed that I could see, but what I did notice is that the rust spots are all but gone. From the right angle I can see them very faintly, but I'm wondering if the rust has been removed and/or replaced. I'll have to find my microscope.

The acids in the fruits/foods can & will remove or dislodge existing rust, which is pretty lightly-bound to the underlying steel anyway. It's a handy way to 'clean up' some red rust, spur-of-the-moment, on a blade, so long as the acid used is quickly cleaned up and/or neutralized afterwards (scrubbing with baking soda works well for this). If left on the blade, it's still capable of making more rust.


David

 

[Bill DeShivs]

Whether boiling "sounds right" or not, it's how gun bluing is done. It's called "rust bluing" andis one of the most durable forms of bluing. A rusting solution is applied, rust is carded off with steel wool, and the piece is boiled. This process is done several times until the proper shade of bluing is obtained.
Boiling won't hurt your blade.

 

[PayneTrain]

Whether boiling "sounds right" or not, it's how gun bluing is done. It's called "rust bluing" andis one of the most durable forms of bluing. A rusting solution is applied, rust is carded off with steel wool, and the piece is boiled. This process is done several times until the proper shade of bluing is obtained.
Boiling won't hurt your blade.

But does the boiling actually turn the red oxide into black oxide, or is it the solution on the metal that forms the black oxide in the heat of boiling water? That's pretty interesting, I'll have to read more into it.

 

[Bill DeShivs]

The boiling does it. The solution only causes the rusting.

 

[Obsessed with Edges]

But does the boiling actually turn the red oxide into black oxide, or is it the solution on the metal that forms the black oxide in the heat of boiling water? That's pretty interesting, I'll have to read more into it.

This is a description of 'Rust bluing" from a Wiki article, which more closely hints at what actually takes place:

""Rust Bluing" and "Fume Bluing" provide the best rust and corrosion resistance as the process continually converts any metal that is capable of rusting into magnetite (Fe3O4)."

Magnetite (Fe[SUB]3[/SUB]O[SUB]4[/SUB]) is the black oxide of iron. The above quote would seem to make more sense, in that the conversion by boiling is directly from unoxidized clean steel ("metal that is capable of rusting") to black iron oxide. In other words, forming the black oxide before the steel can truly rust in the first place (to red oxide: Fe[SUB]2[/SUB]O[SUB]3[/SUB]). Converting red oxide to black oxide wouldn't make much sense, because the red oxide is already ruined metal (truly 'rusted'), and wouldn't re-bond to the steel anyway; it's lost metal, no matter what. Some new red rust will form in the process of boiling the steel (because of contact of steel with water + oxygen), but the red rust is a by-product, and would be cleaned away as Bill described.

This sounds essentially similar to what I've seen when using hot water + vinegar to patina 1095 steel. The blade turns very black, but there will be some residual red rust formed where the steel intersects the surface of the hot liquid, where the air (oxygen) is in simultaneous contact with the liquid and the steel. The red rust is then scrubbed off, along with some of the black, leaving a grey patina of firmly-bound black oxide behind. I'd assume the additional repetitions of boiling would take the black oxide deeper into the pores of the steel, minimizing the exposure of any other clean steel, which could otherwise rust on it's own.


David