Surface Rust on DLC Blade

[senorsquare]

An interesting phenomenon occurred last weekend. While out on a ride I had the DLC M4 Para 3 clipped in my shorts pocket. During the ride I was caught out in a light, steady rain. Not a heavy rain, but enough for me to get nice and soaked. After the ride I noticed some surface rust on the DLC, but only on the side of the blade that was against my leg. The clip side seemed to be clean.

This morning I removed the blade and cleaned with some Bar Keepers Friend. I don't have a loupe or microscope at my disposal, but it appears that surface rust is gone and the DLC does not appear to have suffered damage from corrosion. There was a faint amount of rust on the liner and stop pin, and that cleaned up easily as well.

I found this interesting in light of the larger discussion of DLC as a protective coating on tool steel blades. Not sure that I have any real conclusions to draw, as there was visible rust on the blade, but at the same time the DLC seems unfazed. See before and after pics below.

[Para3]

Sorry to hear about your bad luck. Can't be sure without seeing in person, but it looks to me like some of the rusted areas are pitted. If I had to guess, I would say this is going to occur again. Like you, I am surprised that it happened in the first place. Anxious to hear what others have to say.

[500Nitro]

Salty sweat ?

Or something that the clothes were washed in ?

Some chemical residue left over in the clothes ?

[sal]

Hi Senorsquare,

Very interesting. Thanx for sharing. It does present more questions than answers. Did the rust travel through the DLC barrier by some sort of osmosis? Is there rust under the DLC? We have a smsple sent to us by a customer that had rust on the surface of an H1 blade that was coated with ti-carb-nit, for which we have not been able to solve.

sal

[Rutger]

Thin DLC coatings have microporosity and won't fully stop corrosion. Not all coatings are created equal and some do way better.
I think it is best to coat the blade in some oil which could stay in these pores.

[vivi]

Thanks for the detailed photos. I wonder which part started rusting first?

Salty sweat ?

Is there any other kind? :p

[A.S.O.K.A]

Hi Senorsquare,

Very interesting. Thanx for sharing. It does present more questions than answers. Did the rust travel through the DLC barrier by some sort of osmosis? Is there rust under the DLC? We have a smsple sent to us by a customer that had rust on the surface of an H1 blade that was coated with ti-carb-nit, for which we have not been able to solve.

sal

I specialize in plating and coating (primarily black oxide) and if there is one thing that I have to be constantly aware of on the job, it is the variables of exposure. Every steel coating has an exposure variable. Steel is porous, even powdered steel. Normally, it is advised to customers after coating is complete that they choose one of the following options: have the parts dipped in oil, wax, or sealer. The customer can opt for none of the above after coating is complete, but are advised on exposure variables. Coatings are not always able to get into every nook and pore of the steel, thicker coating can be achieved and will increase protection levels, but not without obvious changes to the dimension of the part, which can be critical if the piece is part of a precise fitment or is to be precisely fitted to a structure. This being said, coating should always be considered as protection to the steel as apposed to thinking it makes the steel impervious to rust. DLC and PVD coating is the best option for knives because they are rust protection with high abrasive and heat resistance, but should not be thought of as making the steel completely rust proof.

[sal]

Hi A.S.O.K.A.

Thanx for sharing your experience. I've got a fair amount of plating experience as I owned and operated a custom prototype circuit board manufacturing company for the aerospace industry back in the 60's. But coatings on steel are still a learning curve for me. I've never been a big fan of coatings as I don't feel they offer as much as they're promoted to do, eg; corrosion resistance. We coat because our customers want coatings for a variety of reasons.

Jason (on another forum) thinks that many of the coatings are also porous and he suggests that ceramic coatings as less porous.

sal

[ugaarguy]

...I owned and operated a custom prototype circuit board manufacturing company for the aerospace industry back in the 60's. ...

sal

So, circuit boards for aerospace, then a knife sharpener company that became one of the most iconic knife manufacturers in the world, and now you have a boat company. What haven't you done? :cool:

[sal]

Hi Ugaarguy,

Lots I haven't done. Only have one life and there's lots to do. I was a beekeeper. :eek: Bought a book on beekeeping. Built my own hives and frames, caught my own swarms and built an apiary. Sold it to help build Spyderco. Beekeeping is a really great hobby.

sal

[brainfriction]

Thin DLC coatings have microporosity and won't fully stop corrosion. Not all coatings are created equal and some do way better.
I think it is best to coat the blade in some oil which could stay in these pores.

My thoughts as well, iirc DLC coating is usually only 1-2 microns thick which is not optimal for corrosion resistance. I think where DLC really shines is its hardness and wear resistance.

[A.S.O.K.A]

Hi A.S.O.K.A.

Thanx for sharing your experience. I've got a fair amount of plating experience as I owned and operated a custom prototype circuit board manufacturing company for the aerospace industry back in the 60's. But coatings on steel are still a learning curve for me. I've never been a big fan of coatings as I don't feel they offer as much as they're promoted to do, eg; corrosion resistance. We coat because our customers want coatings for a variety of reasons.

Jason (on another forum) thinks that many of the coatings are also porous and he suggests that ceramic coatings as less porous.

sal

I agree with you, Sal. I myself, primarily like coatings on blades for aesthetic purposes, like on my para3 with cpm m4 and para3 with cpm 4v. Those two are dlc coated and still get a oil treatment from me. Glad to get a chance to have a chat with you.

[Rutger]

Maybe Spyderco could oil DLC blades with food safe oil from the factory. I think this would stop a lot of these DLC rust complaints.

[MMGRenovations]

I remember a while back I think I posted I was having the same thing happen. I was working in a attic and had my M4 DLC Manix. I forgot to take it out of my pocket and I sweated like crazy that day. It developed rust spots that almost wiped away. It did leave a slight differences finish on the DLC. It made it shiny kinda. My Manix still as those shiny spots. I tried to take pictures but it was really hard to pick up. I will say that the DLC coating does make the knife a lot more corrosion resistant. My uncoated M4 rusts way more easily in my pocket. I carry a uncoated and coated M4 blade almost everyday. The days that the uncoated blade has rust the coated one doesn’t even have a spot. I don’t oil either one. I also believe I have corrosive sweat. Frankly I’m sold on the coating of tool steels. I don’t have a 52100 Manix yet but that one would be another one to test since in my experience 52100 rust way more easily than M4.

[500Nitro]

I used to have very corrosive salty sweat,
I just had to look at a firearm and it rusted, especially old M60's, M16s etc without any bluing left !

When I cut down my salt intake much less corrosive.

[ThePeacent]

Hi Ugaarguy,

Lots I haven't done. Only have one life and there's lots to do. I was a beekeeper. :eek: Bought a book on beekeeping. Built my own hives and frames, caught my own swarms and built an apiary. Sold it to help build Spyderco. Beekeeping is a really great hobby.

sal

well, we have the Caribbean's scales so that's something :p
On topic: I agree with those who say that DLC helps with corrosion, in my experience, my DLC coated folders don't rust, discolor or pit nearly as much as my uncoated blades from the same company using the same steel, under the same conditions
:o

[curlyhairedboy]

I'm by no means an expert in corrosion mechanics, but perhaps there is some kind of Fe ion transport occurring through the conformal DLC layer?

The chief destructive property of conventional rust is the fact that once formed, it flakes off and exposes the next layer underneath. I'm not certain we're seeing that happen here.

This is merely a guess:

If the rust layer forms on the DLC and not directly on the underlying steel, then while there is likely some kind of pitting loss underneath, perhaps the rust formation is rate-limited by Fe transport through the DLC layer.

That might explain the large differences seen in corrosion rates between uncoated M4 and DLC-coated M4.

[BigGrove]

Could just as likely be that dissolved iron was transported via the rain water.

[Joey]

Could just as likely be that dissolved iron was transported via the rain water.

That’s doubtful simply because there is some evidence of the coating integrity being compromised. You can see the pictures of the cleaned blade shows damage to the coating.

[Bill1170]

Could just as likely be that dissolved iron was transported via the rain water.

That’s doubtful simply because there is some evidence of the coating integrity being compromised. You can see the pictures of the cleaned blade shows damage to the coating.

It makes sense that rust forming under the DLC will bulge up and damage the coating, or at least remove some of the steel substrate the coating is bonded to. To be safe, stick with DLC over H-1. :D