Edge decay

D

Dragotan

Joined
Sep 2, 2005
Messages
8
A question to all metallurgists and philosophers.
For how long will an edge stay sharp if you never use a knife? It depends on the type of steel used for the blade I suppose, but I am thinking of a ballpark figure. A year, five years?

There should be some edge decay on a microscopic level due to oxidation etc. Hey, maybe I should try to find out what the half life of steel is, then we'll know...

I bought a CS Night Force about nine months ago and it doesn't feel quite as sharp as when I received it. I haven't cut anything with it yet, I use it as an EDC for self defence purposes.

Any thoughts?
 
I would think there are a lot of factors that go into that. Type of steel, edge geometry, local weather/humidity conditions. Tendency of some folder blades to strike the inside of the handle.

Also, it's hard to tell just from memory if it's really not as sharp.
 
I've seen this happen quite often actually. Leave a knife alone for a long time, and it can lose its biting edge. I left one of my Tachyons (Microtech balisong) alone for several months, and it's once hair-popping, tissue slicing sharp edge felt dull, would tear regular paper, and wouldn't shave. However, all it took was 2 passes on each side with a ultrafine Sharpmaker rod, and it was back. I noticed this on my Spyderco Calypso Jr. (grey/VG-10) and my BM43MC balisong.

Unless you polish the very edge of the knife (like with 3000 or 6000 grit tape, or a strop), the edge of a knife is basically tiny tiny saw serrations and scratches caused by the grit of whatever you sharpened it with. Like beadblasting, moisture likes to hide in these microscopic pits and crevices, which can cause oxidation (rust). Leaving a knife alone for a long time (especially if it's not treated with a protectant) can certainly "dull" the edge. But unless the corrosion is rather severe, it'll only take the slightest effort to remove that tiny microscopic bit of corrosion and give the knife back its edge.
 
If the knife was stored properly in a cool dry spot and lubed before storing it should not oxidize. What you might get is some form of electrolysis but usually the iron will steal atoms from the aluminum, brass, and lower metals.

Also the type of steel has alot to do with oxidation. For instance 440-C is one of the highest chromium steels used in knife making and resists oxidation well.

I have never noticed stored knives getting dull. If that happened I would think I would see a red spot from the rust on the rag when I wipe them down.
 
You can get rust in the form of the black oxidization which will not come off on a rag, even the orange form which will come off can be so slight you can't see it and the edge can be mangled. Carbon steel knives with a protectant can last a long time, I have taken blades back into rotation over two years out and they are just as sharp as new.

-Cliff
 
There are more microscopic processes that occur besides oxidation. For example there is a certain amount of what could be considered "hydration" that takes place. When you cut a new surface onto metal, such as by honing or machining, you expose fresh metal surfaces which have not previously been exposed to the air. What is now a surface was previously metalically bonded to neighboring metal atoms. You have sort of an unstable surface since there are bonding sites that miss their connections to their now-missing neighbors. This is sort of a high potential energy surface that wants to reduce its stressed state by bonding to things. It will quickly pick up water molecules and develop a monomolecular hydrated layer. This surface will also oxidize and collect contaminants of various types very quickly. All this added stuff lets the surface adjust a little. You are supposed to sand metals before you paint them so that the paint can grab onto that fresh high energy surface.

Edges are very thin and accute surfaces. As you approach the apex of the edge you have very little core material relative to all the surface area. This means that you have sort of weak support to resist surface processes. Things happen that you don't see in the macroscopic world. Unfortunately I don't have the equipment to watch an edge relax. The lore about this stuff comes out of the world of straight razors. When you sharpen a straight razor and strop it you form what is described as a "fin" on the edge. The fin is conceptualized as being fragile, ductile, and sort of self mobile. It is described as "growing" between shaves. It also gets dinged up to some degree during the shaving process. The fin straightens itself up partially between shaves and with gentle stropping can be groomed into a straight shaving edge. You are supposed to let your razor rest for a couple days between shaves (you need two razors) and only strop before shaves (not after). If you strop after shaving the dinged-up fin will snag on the strop, get ripped up, and leave metal burrs in the strop surface.

Knife blades are pretty coarse and obtuse compared to a straight razor so the mechanism isn't quite the same. Regardless there are ways that the edge changes over the period of days or weeks. I would suspect that a lot of those subtle changes could be moderated by a little gentle stropping. In some cases gentle stropping that is done a few days or weeks after sharpening might give you a better edge than you first saw after honing. This is speculation, but it seems plausible to me.
 
It's amazing the knowledge on this site! Much appreciated.

Related to Jeff's explaination, I have also read about very fine-edged (less than 15 degrees per side) chef's knives having their microscopic edge (or "burr"?) "relax" over time....hence the use of a smooth steel to straighten the edge out, just before using the knife.
 
I noticed my BM 42 (with 440c) became noticably dull over a few months even though i have never cut anything with it. I was wondering how come that happened...very interesting stuff.
 
Aha - good question and good answers. :thumbup: I noticed the same thing dniice87 did with my knives and never really wondered about it - just happy to be fussing with my knives
 
Yeah, I notice this sometimes also after my knives have been stored for a long time.
 
this is a phenomenon I have see quite frequently over the last 23 years of making knives............I have had people ask me about it......

I really dont have an answer, but something does happen..........I am sure it must be oxidation, what else could it be?

Knives that will easily shave when finished........two weeks later at a show just dont feel as sharp.

I have had knifemakers from Alaska ask me if there was something in the air that made knives get dull in Hawaii!?

It would be a good topic to research.......if you had the proper equipement and it interested you enough.

Please note that this happens on HIGHLY polished edges!!!! That is NOT the solution.
 
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