Do unused knives get dull?

Jeez! That's quite a set of ideas, but just to throw in my 2 cents:

Chromium, when exposed to air, forms an inert clear oxide that seals the surface of the metal. That's why the trim on your car stays shiny and why high chromium steels are called stainless. A freshly sharpened stainless knife will develop this thin oxide layer over time, taking away the fine edge. It's really thin, and not real hard, so it only takes a few strokes on a strop to put things right.

Carbon steels will rust and corrode, but interestingly, can actually get sharper when the corrosion flakes off! There really is a technique involving an acid etch that will sharpen steel.
 
I have a bunch of knives that will stay sharp while stored for extended periods of time. I have one particular one that will not. Sharpen it up shaving sharp and put it away. A few weeks later the shaving edge is gone. I always assumed it was a corrosion effect particular to that knife.
 
Rust really isn't the issue. It's the slight surface corrosion that naturally occurs on the surface of any steel, even stainless.

On carbon steel, an edge should keep indefinately in a dry place. Living in the East, all my handguns are stainless; however, if I lived in a broiling desert, I wouldn't need it. Steel just doesn't tend to rust in a dry environ. On stainless steel, the chromium content of a gun or knife will create a protective layer on it (as LedSled noted), and that gives it its resistance to corrosion. If you wear a stainless part, the protective layer will form almost immediately. And this happens even more quickly if the steel is polished or sharpened. Chromium is tough, but it's not sharp, thus the carbon and other elements in most modern knives. On a satin finished stainless knife or gun, corrosion is more likely to form than on polished surfaces. Salts and moisture get trapped in those microscopic nooks and crannies, and speaking of salt, fingering a blade will leave minute deposits of oil and salt that will degrade the sharpness of a knife. If you wash your hands with soap and dry thoroughly, you can handle steels with less chance of corrosion. And some people have different amounts and kinds of oil and salts. I've known people who can't handle a blued gun without running the chance of leaving rust prints.

Oil does the same thing chromium does with stainless steels. Keeping an edge on a stainless knife depends on how stainless it is. Tough stainless steels like 410 and 420 will dull just by staring at them. Carbon and premium steel knives depends of where they're kept, how long they're kept there, how dry or moist the environ and how much oil and salt might be left on the blade. (Fingering a blade will dull a wicked sharp knife, but we all know that. Yet we can't help testing the sharpness again and again.) So everything above affects blade sharpness. Even sheaths dull knives.

That said, you can put a Cold Steel Voyager with a serrated blade in a drawer and it will always be wicked sharp, which is one reason I like that particular pattern of serration. Premium steels are more likely to keep an edge, but they aren't immune. On carbon steel, sharpen the damn blade and keep your fingers off it. Then put it in a dry safe. AUS8, VG-1, 440A/C and the like will pretty much lose their wicked sharp edges no matter what you do. At least that's been my experience.


bolt.jpg


Left alone unprotected, carbon steel tends
to begin corroding very quickly and eventually
slows, the rust itself forming a protective barrier.
Bluing is simply a controlled rust that is
attractively finished.
 
This is expected behavior for straight razors so it should be true for very fine edges honed at a very acute angle. The surface of a metal is much less stable than the interior. When you hone or machine the material you remove atoms from the surface and expose atoms that previously were surrounded by others of their kinds. Previously they were nestled in a reasonably uniform lattice of similar atoms (in a balanced state) now they are only connected on one side. This is an unstable (aka high energy) state. This surface will try and capture dirt and water molecules and is even inclined to move a little (relax) to a more balanced condition. The apex of an acute edge is the most unstable part of a honed surface. It is almost all surface and it is exposed all around and supported very little. It will try and relax through a lot of different mechanisms.

We think of edges as simple and ideal and the honing process as clean and without creating undo stresses. In reality there is always some degree of roughness to a honed surface and the honing process removes material through local overstress of the bonds between the metal atoms. We have left a high energy surface with minor damage to the underlaying structure. We have done something similar sanding wood and we can expect the grain structure to rebound somewhat. It can particularly shift in spots where there are peaks in the surface structure that are adjacent to overstressed bonds in the underlying lattice.

So what is the common wisdom of centuries of straight razor users? After honing or using a razor it is expected that the edge will be damaged. There will be microscopic dings in the edge. These dings will exhibit ragged burrs and spurs that don't align with the intended edge. They will also be somewhat brittle due to work hardening (lattice dislocations) during the honing or use that created the dings. You are advised not to use or strop your razor at this time. Stropping or working the edge at this time will cause the edge spurs to break off. You are supposed to let your razor rest for a couple of days (you should have two razors) and then you strop just before you use it. The wisdom is that the edge will have relaxed, somewhat realigned itself and will have grown a trifle less brittle, during the two days. When you strop then the burrs and spurs will not break off and the edge can be reformed close to true. You are supposed to use the razor before it moves again.

I think that when you put away a freshly honed acute knife edge it will relax somewhat randomly over time. Remnant stresses of honing will get relieved through a mixture of movement, oxidation, and absorption of moisture through the air. For example stainless steel resists oxidation by forming a thin oxide coating (when fresh it does oxidize) to prevent open-ended oxidation (rusting). After honing, the first thing a stainless edge does is to rebuild an oxide layer. It will also absorb a mono molecular layer of water on its surface. It will want to absorb any other chemical that you have handy.

Frequent light honing may help you create a more stable edge. Another trick is to lightly strop your blade periodically starting a couple days after honing. So I do believe in edges doing funny things when you aren't looking.
 
Yay! I'm not crazy! (knife nut, sure... but not crazy)

Thanks for all the great info, I know a hell of a lot more now than when I started.
 
So how does one prevent a knife from getting dull? Pardon my noobishness. Can you coat the blade with something that will prevent oxidation of the blade? This would be for an EDC knife of d2 steel that hardly ever gets used...it's EDC in the sense that it's for self defense.
 
I would just do a few very light repeat honings, just two or four strokes per side, every few days for a week. I bet that would stabilize the situation. On the other hand you may never observe this effect. I have only noticed it on a few knives over the years. It is not what I would call prevalent or conspicuous in general. I would rate it as a small effect, seldom noticed.
 
Jeff knows. After my brief little post, I decided to shut up and leave the floor to him. Watch out for scientific folks, though. Ask us the time, and we'll tell you how to build a watch.
 
Hard H2O said:
I would surmise that your own idea of a sharp edge is maturing. You are expecting blades that met your approval months ago to meet your now raised expectations.
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I completely relate to this. I recently found a knife I stashed in the ashtray of my car about a year ago. I remember at the time thinking it was really sharp but now it ranks right at the bottom compared to all my other knives and the bevel is a mess. I think I need to go out and get that puppy and fix it up right.
 
DRider said:
Thanks for the info, guys.

No acid, no use, steels like s30v or vg10... I get them to that sharp where you just wave it past your arm and the hair leaps off... then I pull them out months later and, sure they still shave hair, but its not that amazing scary sharp like it was, its only... real sharp. You know what I mean, I hope.
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Memories about pleasant things tend to grow as time pases by.

Sorry, couldnt resist the penaltykick.

Bosse
 
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