Am I worried about M4 for nothing?

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Nephron44

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Aug 20, 2015
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I have always stayed away from M4 steel because of its lack of corrosion resistance…until yesterday!

The Benchmade Auto Immunity is essentially my dream knife, exactly how I would have designed it, apart from the M4.

How worried do I REALLY need to be about corrosion on my M4 blade?
 
If you're worried it will rust if you neglect it, yes your worries have merit.

If you're worried it will disintegrate if you lightly care for it, your worries are unfounded.

With a little care you have nothing to worry about. M4 is awesome.
 
Very worried. I wouldn't take it swimming in the ocean with me, that's for sure. I certainly wouldn't take it for the weekly car battery stabathon either.

*end snarkiness

What types of uses are you using it for and are you going to take the time to clean it off afterwards? Also, BM usually coats their M4, so only the edge corrodes.

I don't even worry about carbon steel these days as I enjoy seeing patina buildup, so just wipe and dry it when done. I don't oil my knife blades routinely and rarely see rust. If I find rust, it's always because I didn't dry the knife, usually from being distracted while cleaning.

IMO, most people's worries are unwarranted, but if you work outside and sweat all over your knife all day everyday, then maybe it's not for you.
 
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YMMV but this is a PM2 with CPM-M4. My wife has been using it to take up the weed barrier in our back yard, which full of grit and is wet from being sprinkled. She has been doing that with it for a few weeks and other than wiping a couple of dirt clods off before I took the photo, it hasn't received any care.

That amount of rust doesn't affect its functionality. There is a little on the edge bevel but it is (at this point) shallow enough that it comes off on first use and I can't tell it was there. I am working on the other side of the yard but using a cheap Fishers knife, some kind of stainless, and it looks better but cuts worse.

WETPM2.png
 
Citori said:
YMMV but this is a PM2 with CPM-M4. My wife has been using it to take up the weed barrier in our back yard, which full of grit and is wet from being sprinkled. She has been doing that with it for a few weeks and other than wiping a couple of dirt clods off before I took the photo, it hasn't received any care.

That amount of rust doesn't affect its functionality. There is a little on the edge bevel but it is (at this point) shallow enough that it comes off on first use and I can't tell it was there. I am working on the other side of the yard but using a cheap Fishers knife, some kind of stainless, and it looks better but cuts worse.

WETPM2.png
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It should be noted that, that blade is uncoated and BM's M4 is coated so much of that rust would not be present on a BM blade. It does give a good idea on what can cause M4 to rust.

Edit: just saw you noted a coated blade your next post.

Wiping the blade with an acid, like vinegar, will help clear up the stains while also creating a protective oxidative layer to limit rust later on (forced patina).
 
pholder said:
You will be so impressed with m4 and using it so much that you will have forgotten about your worries and will be kicking yourself for not getting one sooner.
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Exactly. I have had my Spyderco GB1 in CPM-M4 for ten years, and once in a while it gets a little spotting if I don't clean it after cutting messy stuff. It sat in my knife case unused for a year while I was in the hospital, no rust.
 
Only time m4 ever rusted on me is when I brought my manix 2 on a camping trip and we had heavy thunderstorms 2 days in a row. High humidity in the tent over night made some spots appear all over the blade. They were light enough that a twig was all I needed to scrape off all the rust. Hasn’t rusted since.

I love m4, holds an edge long enough and easy to touch up. Used my manix for cooking a lot and never really developed a noticeable patina. Still nice and clean
 
I have a native chief in m4 and have had zero issues. I keep a bottle of food grade mineral oil laying around and will give it a rubdown once every couple of months... more like biannual.... maybe.

Run it, love it
 
irsh said:
With all of the great stainless steels available, why bother? ;)
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CPM-M4 has good edge retention while maintaining pretty good toughness.

Magnacut has slightly better toughness, better corrosion resistance, but much less edge retention.

It depends on what you are doing. CPM-M4 will cut well for a long time and resist chipping, whereas something like S110V or M390 will be more susceptible to chipping if say you are cutting cardboard and hit a staple.

It's a nice balance of toughness and edge retention.
 
M4 is pretty rust and stain resistant. My GB2 has accumulated only a few minor blemishes over a couple years of cutting everything from veggies to meat to tape/zipties/you-name-it, which don't affect its cutting performance at all. I wouldn't worry about it.
 
bikerector said:
IMO, most people's worries are unwarranted, but if you work outside and sweat all over your knife all day everyday, then maybe it's not for you.
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Not all day, but for a couple hours a day, that's what I do in the summer, and D2 (which Larrin rates at 4.5 for corrosion resistance) rusts so much for me that I quit buying it. Larrin rated M4 at 4 for corrosion resistance, so I would never buy that. I do use 3V (5.5 for corrosion resistance) in the winter when we have occasional rain, wipe it off carefully, no problems so far.

It may be that some people have saltier sweat than others, and that could make a difference in how much a knife rusts.
 
I have M4 with Benchmade's cerakote in a Bailout, Super Freek and Mini Super Freek (very new) and I've had no issues. The cerakote works well but I do wipe them with gun oil about once a month or so. If I'm carrying them in the summer I'll give them a quick wipe every day or two. The tangs are uncoated and they're still spot free. You don't want to neglect M4 but you don't have to baby it either in my experience.
 
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