CPM M4 Corrosion Resistance

Gaspumps · May 14, 2012

Just got a Benchmade 520 in CPM M4 with coated blade. Very nice, VERY sharp and very "aggressive" ribs on the scales.
My question- how corrosion resistant is CPM M4? I have had blades in 154CM and ATS34 show small spots so I know that no steel is rust proof. Just wondering what I can expect when compared to ATS34, S30V and D2.
Thanks

res1cue · May 14, 2012

I don't have any non-coated CPM-M4 blades, but looking at the chemistry, I would guess it would be more prone to corrosion than any of the steels you listed

1.24-1.4% carbon
3.75-4.75% chromium

it's a tool steel I believe, definitely not stainless. I would treat it like you would treat D2. Bit of flitz + dry it off after you use it and I'm sure it'll be fine

clockman · May 14, 2012

I have a Spyderco Gayle Bradley and cpm 4 Millie. I forced a a patina on the GB and left the juice from cutting an orange on the blade longer than I

should have on purpose.

I like the look of a patnina on cpm 4. With a light/medium patina you don't have to worry about corrosion IMO. I still wipe/wash off the blade after I use it tho.

res1cue · May 14, 2012

clockman said:
I have a Spyderco Gayle Bradley and cpm 4 Millie. I forced a a patina on the GB and left the juice from cutting an orange on the blade longer than I

should have on purpose.

I like the look of a patnina on cpm 4. With a light/medium patina you don't have to worry about corrosion IMO. I still wipe/wash off the blade after I use it tho.

How bout some pictures?

SPNKr · May 14, 2012

M4 will stand up to most things you cut pretty well, even fruits and stuff, without even developing much of a patina. I've seen year-old Spyderco GBs on this forum that still look pristine.

However, if you live in the tropics like I do, the sheer humidity of the air will cause it to develop spots of surface rust, even after you force a patina on it. My rust spots could be just because there were gaps in the patina though (it's more than 90% humidity here most of the time). With a patina you could pretty much treat it as a stainless steel, and keeping it dry without one should do it fine. Here's a pic of a forced patina.

This is after 6 hours in Heinz cider vinegar, I opted for a gray instead of a black finish. I believe you can get pitch black, which offers better protection, if you leave it in there for more than 12 hours.

Twindog · May 14, 2012

I live in a rainforest -- literally in the forest -- on the edge salt water. M4 likes to rust on me, even with good care (TufCloth). But I don't like coated blades, as a rule. What works for me is to sand the blade with 600-grit wet/dry sandpaper before each sharpening. Then I finish with TufCloth. The rust/patina never gets a chance to get well started.

ericvar101 · May 14, 2012

I would have gone with S30V if I lived in the rainforest, M4 and D2 not the best choices for your enviorment

Twindog · May 14, 2012

ericvar101 said:
I would have gone with S30V if I lived in the rainforest, M4 and D2 not the best choices for your enviorment

Well, my favorite stainless steel is currently M390. I have no trouble with it. But like most people here, I like to try and use different steels. I've backed off S30V, except for special knives like the original Manix. And in any case, I'd rather have a blade in M4 than S30V, even if the M4 develops a heavy patina. With care, I have no trouble with D2, but I prefer other steels.

I even use 52100, which is my favorite all-around steel, but I either I don't use it in the rain or I compensate.

I've been carrying a small fixed-blade made by Larry Withrow in 1095. That's a really nice steel too, and Larry did a great job on the heat treat. Very simple steel. Very easy to put a nice edge on it.

There are really only a few circumstances where we are limited in the steel we use, but there are many circumstances where we choose the appropriate steel for the situation or compensate for the situation.

michaelm466 · May 14, 2012

ATS-34 and S30V are both stainless steels, D2 is a tool steel like CPM-M4, however D2 is a semi-stainless, which means its hovers around or right under the 12% chromium cut-off for stainless steels, M4 will be more prone to staining than D2, however in my experience it will be more stain resistant than carbon steels, I haven't had any rust on the M4 knives I have (2 uncoated, 2 coated), however I haven't had any staining on any of my stainless steels either like you have. Just watch/protect it a little more than D2 and you should be good.