Report on m4 corrosion resistance

[Surfingringo]

Hi all, I know that m4 is a popular steel with many around here and I thought some might be interested to hear what I have learned about corrosion resistance while working around salt water, blood and meat. The knife I have been using is not a Spyderco but since Spyderco makes a couple of knives in m4 and it seems that folks on here are always asking for more I thought it would be relevant data. I also know there is talk of a run of the frn knives in hap40. I don't know the specs on that steel but I've heard it likened to m4 so maybe this has relevancy to that too?

The knife is one of Phil Wilsons customs and is m4 hardened to 64/65. I forced a medium patina on the knife when it was new with boiling vinegar to help prevent rust and pitting. Opinions vary on the value of a patina. I cannot definitively say that it helps prevent rust but it's just something I do that seems to work for me.

I have used the knife every day for the last 6 weeks as a fish cleaning tool. Typically I will come in and spend 15-45 minutes at the cleaning table depending on the number of fish I have. During this time, the knife blade is covered with blood, fish goo and residual saltwater from the kayak and fish. The only effect I have seen on the steel is a gradual and even darkening of the patina. There has been zero rust, pitting or red/orange residue. It should be noted that I always rinse the knife within a half hour of finishing. I usually dry it too but sometimes I just leave it out wet and have seen no ill effects from this. For the first few weeks I would spray it with a shot of wd40 after cleaning it but recently I have stopped bothering because it doesn't seem to be necessary.

As far as ocean use its a little less capable. I took it on the kayak a couple of times. I have a small mesh enclosed cavity in the center where I kept the knife in its sheath. The cavity constantly has about a half inch of saltwater in it so the knife was basically bathing in ocean water for the entire 5 hours. When I got in there was light orange swirls on both side of the blade. No pitting at all but the rust process had definitely begun. I was able to remove all signs of corrosion in 30 seconds of light rubbing with one of those green and yellow kitchen pads. Not horrible results by any means considering the usage but enough to make it obvious that this wouldn't be the best choice for a saltwater knife. Could you use it with care? Yes, but there are better choices.

As far as the results I am getting from cleaning fish, I can confidently say at this point that m4 has plenty of corrosion resistance to make a perfectly functional hunter/skinner. Yes, you want to pay a bit more attention and you can't be completely cavalier about just throwing it in the sink bloody all night, but given even the most basic maintenance the steel exhibits no rusting issues.

[Gravedigger]

My Gayle Bradley is my go to knife for gutting, skinning out deer and cutting into quarters so it can be hung in side by side refrigerator. The m4 in the Gayle Bradley has had no issues with corrosion BUT I have an Manix 2 in m4 which shows signs of corrosion very easily. The Gayle Bradley makes me a big fan of m4 but if I owned the Manix first I would not buy anymore m4. M4 is an impressive steel and will purchase other knifes in m4. The SPRIG would be nice in m4 but I know it's going to be s90v and one will be mine as soon as it is released.

[Philo Beddoe]

M4 is a decent steel but I'll take K390 over M4 any day of the week..

[Gravedigger]

M4 is a decent steel but I'll take K390 over M4 any day of the week..

How is the corrosion resistance of k390?

Haven't used my k390 mule yet because I use mules in the kitchen mainly . The s110v mule doesn't do the work of the Gayle Bradley m4.

[bearfacedkiller]

My M4 mule has been icefishing quite a few times and has done some kitchen duty and has experience minimal rusting, just what could be wiped off. It is often wet most of the day when on the ice and used to gut fish and cut bait and just washed when I get home. I don't mind maintaining carbon steels and have used 1095 in that same role so M4 works just fine for me.

[The Mastiff]

Surfgringo, thanks for the update. I was wondering how it was turning out. Many people feel knives must be stainless for such uses but you prove that with just basic care one can enjoy the attributes of a high speed steel around the ocean and cleaning fish. CPM M4 is a pretty amazing steel with better performance in things like corrosion resistance than one would expect. Toughness as well considering it's carbide fraction and high wear values.

Joe

[xavierdoc]

Thanks, Surfgringo.

My M4 Manix stained easily but now has a good patina.
My Gayle Bradley shows no signs of corrosion.
My M4 mule has some pitting.

All see similar use and after care. I don't know if variations in heat treat would explain this. Difference in use is just as likely.

[Doc Dan]

Thanks for reporting to us on that. I would have thought it would show rust a little easier.

How was the edge retention using it on fish?

[Surfingringo]

Thanks, Surfgringo.

My M4 Manix stained easily but now has a good patina.
My Gayle Bradley shows no signs of corrosion.
My M4 mule has some pitting.

All see similar use and after care. I don't know if variations in heat treat would explain this. Difference in use is just as likely.

I tend to believe that ht is going to have a huge effect on the corrosion resistance of a steel like this.

Thanks for reporting to us on that. I would have thought it would show rust a little easier.

How was the edge retention using it on fish?

Edge retention has been excellent. I don't have many high performance "super" steels but this m4 outperforms anything I have used. It outperforms my s90v southfork in edge retention. Honestly, "working edge" edge retention is pretty low on my list of steel attributes. I like to use my cleaning knife with a razor sharp medium grit finish. It gets a light touchup after every session so a knife steel that will hold a semi sharp working edge for hours of use just isn't that big of a deal to me. I like a steel that will hold a VERY high level of sharpness as long as possible. The m4 seems to be performing well in that regard. I cleaned a half dozen red snapper the other day and used the m4 blade to clean and cut the heads off of all of them. I was push cutting through heavy skin, scales and bone and it required so much pressure that I had to put my left hand on the spine and lean a good bit of my body weight into the cuts. When I was done there was no visible chipping or deformation and the knife would still shave arm hair easily without scraping. Good enough for me. :)

[Surfingringo]

Btw, I remember hearing Sal say recently that Eric had a few projects going in m4. I'm curious if those are new designs, sprint runs of existing knives or both. Maybe "someone" will chime in and give us a hint!

[Evil D]

It's interesting how different results can be with this steel. I've read a lot of "horror stories" that end up with no rust and no pitting, while my own experience was that just carrying it in my pocket caused pretty significant rust spotting, and I had never used the knife in wet environments so it was just moisture from my body. That was also with a vinegar patina AND spraying the whole thing down with Tuff-Glide. The general opinion on mine was that the rust had to have started forming before the patina/TG, but I dunno. I would still buy the steel again though.

[Surfingringo]

It's interesting how different results can be with this steel. I've read a lot of "horror stories" that end up with no rust and no pitting, while my own experience was that just carrying it in my pocket caused pretty significant rust spotting, and I had never used the knife in wet environments so it was just moisture from my body. That was also with a vinegar patina AND spraying the whole thing down with Tuff-Glide. The general opinion on mine was that the rust had to have started forming before the patina/TG, but I dunno. I would still buy the steel again though.

Wow, that is interesting! I have been using mine around salt and blood everyday in the tropics where I live 100 yards from the ocean with no rust. Not doubting your experience at all...just confirming your point that different iterations of this steel seem to behave quite differently in their resistance to corrosion. Which knife are you talking about? The Manix or the GB or something different? Seems like there is a pattern emerging about folks having more corrosion issues with the manix?

[Phil Wilson]

I have seen better than expected corrosion resistance on CPM M4 but Lance's use in a wet and salty environment kind of surprised me. I think his attention to keeping the gunk and salt off after use it has a lot to do with it. A couple of observations: The chrome in M4 starts out as a chrome carbide in the annealed steel, but with the higher austenitizing temps for high speed steel it mostly dissolves in the matrix. Depending on the tempering cycle some can precipitate back but overall you end up with a high percentage of it in the matrix. Therefore since it is not tied up as carbide there is more chrome in the blade to resist corrosion. CPM m4 seems very clean so the lack of contamination from other elements eliminates the "battery" effect that can cause pitting. A finer finish also contributes since there are less spots to capture salt and moisture. Lance's testing really tells the story and this feed back is appreciated. Phil

[Evil D]

It's interesting how different results can be with this steel. I've read a lot of "horror stories" that end up with no rust and no pitting, while my own experience was that just carrying it in my pocket caused pretty significant rust spotting, and I had never used the knife in wet environments so it was just moisture from my body. That was also with a vinegar patina AND spraying the whole thing down with Tuff-Glide. The general opinion on mine was that the rust had to have started forming before the patina/TG, but I dunno. I would still buy the steel again though.

Wow, that is interesting! I have been using mine around salt and blood everyday in the tropics where I live 100 yards from the ocean with no rust. Not doubting your experience at all...just confirming your point that different iterations of this steel seem to behave quite differently in their resistance to corrosion. Which knife are you talking about? The Manix or the GB or something different? Seems like there is a pattern emerging about folks having more corrosion issues with the manix?

It was the Manix 2 sprint. I have also had rust issues with ZDP, which is known to be "stain resistant" but not totally stainless. And again, the knife in question has never been used wet and only lives in my pocket (ZDP Ladybug on my keys, clipped to a belt loop and tucked into my back pocket). I can only assume that body sweat/moisture is very acidic stuff and I must sweat more than I realize.

[Surfingringo]

Another thing that I believe helps with this particular knife is how much it gets used. I'm not sure I can give a logical explanation but it often seems like the more something gets used, the less it decays. This seems to hold true for all manner of things. A car sitting on blocks will rust faster than the same car being driven everyday. An abandoned house will succumb to the elements much more quickly than one that is being lived in. And a knife that sits unused will rust away much faster than one that is being used everyday. It even seems like this holds true regardless of whether more care is given or not. For example, the car that is being driven will rust slower than the one on blocks even if it is never washed or cared for. It's almost as if just the use is enough. It's a weird observation that doesn't make a lot of sense logically but seems to hold true in a lot of cases.

[Evil D]

Another thing that I believe helps with this particular knife is how much it gets used. I'm not sure I can give a logical explanation but it often seems like the more something gets used, the less it decays. This seems to hold true for all manner of things. A car sitting on blocks will rust faster than the same car being driven everyday. An abandoned house will succumb to the elements much more quickly than one that is being lived in. And a knife that sits unused will rust away much faster than one that is being used everyday. It even seems like this holds true regardless of whether more care is given or not. For example, the car that is being driven will rust slower than the one on blocks even if it is never washed or cared for. It's almost as if just the use is enough. It's a weird observation that doesn't make a lot of sense logically but seems to hold true in a lot of cases.

Funny you mention this. Me and the family were out driving around the town we lived in about 5 years ago and a lot has changed. There are fast food chain places that have closed, and the buildings look 100 years old. We were talking about how just daily life in and out of a place like that can keep it looking alive along with regular maintenance. The fact that you use your knife on a daily basis could well be the reason it's holding up so well. We all forget that it wasn't long ago that most pocket knives were made of non stainless steels and people didn't walk around with chunks of rust in their pockets. I'd imagine that even if you did have any rust fears, a little mineral oil wipe down between uses would do the trick.

[Cliff Stamp]

It would be interesting to know where Spyderco tempers their M4 as the corrosion resistance of HSS is strongly dependent on the tempering temperature.

The reactions in tempering are fairly complex as there are a number of things happens at the same time, but critical to corrosion resistance is that if you temper between 575 °C and 700 °C then there is dissolution of M4C3 and M23C6 carbides which are rich in chromium. If you temper above or below this range then the carbides form vs dissolve so the corrosion resistance is significantly different.

[Brock O Lee]

I have the M4 Military, Manix and GB, and use all of them for cutting food and fruit. I wash and dry them properly soon after use, and blow out the pivots (lung power) to get rid of moisture.

The GB seems to be the most resistant to corrosion, and have almost no patina. I did cover it with Tuff Glide when I received it initially.

The Manix and Military have some patina, but I've not seen any pitting or red rust. No Tuff Glide. The Military has a dark spot on the tang where moisture sometimes collect despite my best efforts, but no rust. You can see it in this recent pic



I find M4 less prone to rust compared to solid Superblue. I also get less of that metallic taste than with the Superblue Caly 3.

[Cliff Stamp]

Brock, have you used Benchmades M4,I would be curious if you notice a difference.

[Brock O Lee]

Sorry Cliff unfortunately I have no experience with Benchmade's M4