DLC and rust.

[Surfingringo]

I'd think that if DLC makes steels impervious to rust, it'd work on any steel.

Maybe. But we also have to entertain the idea that maybe we don’t fully understand the “how” of the improved corrosion resistance that DLC offers. Maybe the bond with the underling metal offers some protection but not in the same way as a completely impermeable coating. I don’t know. I’m definitely not arguing because I don’t know enough to argue one side or the other...just saying that DLC may work differently than we imagine.

[Evil D]

Rust is a fairly hard thing to stop if given enough time and conditions to form. There are lots of things that are coated for rust prevention that ultimately fail over time, like rebar inside concrete structures or underbody coatings on cars. What I'm curious about is how fast this 4V knife seemed to rust in fairly mild conditions. To me it suggests a defect in the coating itself and not so much a debate about how effective DLC is overall. Each person's body sweat may be different but it's odd to me that someone like Vivi who is known to be able to rust even well known rust resistant steels just by carrying them yet has no problem carrying a DLC knife and it doesn't rust, but then the next guy has such a major issue with a DLC knife rusting. I don't believe there's THAT big of a difference between these two people's sweat to go from totally effective to totally ineffective. It seems more logical to me that the coating wasn't applied thick enough or the blade was already contaminated before it was applied.

[vivi]

I'd think that if DLC makes steels impervious to rust, it'd work on any steel.

Maybe. But we also have to entertain the idea that maybe we don’t fully understand the “how” of the improved corrosion resistance that DLC offers. Maybe the bond with the underling metal offers some protection but not in the same way as a completely impermeable coating. I don’t know. I’m definitely not arguing because I don’t know enough to argue one side or the other...just saying that DLC may work differently than we imagine.

Absolutely. All most of us can really do is relate our experiences. I don't have the equipment to test this issue as much as I'd like to.

[emanuel]

Here is my humble opinion on that subject.

Rust is iron oxide. Rust is prevented in stainless steels by forming chromium oxide on the surface. In both cases there is a metal reacting with the oxygen in the air.

DLC is almost pure carbon. It's reaction with the oxygen in the air will Form most likely carbondioxide. But that's not the case because the carbon is more stable in the dlc coating. There are DLC coatings with ppm of metals induced to enhance wear resistance. These metals Form carbides with the carbon in the coating. And carbides are really stable and don't "rust".

So my guess is it looks like there is a metal abrasion in the coating surface which contains iron and formed rust.

Or the coating is so thin that there are iron atoms diffunding through it. But that would mean we could see the grindlines and steelcolor because dlc can be applied quite thin.

But I guess maybe Larrin can say a word or two on this matter.

Greetings.

Carbon doesn't oxidize in air at normal temperature just because iron does, it needs 1300+ degrees F in order for that to happen. DLC is as stable as graphite/diamond at room temperature. By itself, it's chemically inert in our day to day life, and theoretically it should last millions of years by itself against normal atmospheric weathering, just like the other 2 materials mentioned above.

The reason why it doesn't rust-proof metals is because the deposit method we use cannot create a uniform enough layer so imperfections/pores will occur, so all those imperfections allow oxygen, salt or other catalysts to reach the metal beneath. On the other hand, the porosity helps with absorbing and holding oil which is why DLC is known to reduce the need to oil carbon blades simply because its harder for the oil to be wiped away by use. So yes, the rust you see is the steel beneath it, not something on the DLC. If someone has a good microscope to look at those rusty zones after they're cleaned up, I'm sure it would be clear for everyone what actually happened to the coating during the rusting process.

[Surfingringo]

Here is my humble opinion on that subject.

Rust is iron oxide. Rust is prevented in stainless steels by forming chromium oxide on the surface. In both cases there is a metal reacting with the oxygen in the air.

DLC is almost pure carbon. It's reaction with the oxygen in the air will Form most likely carbondioxide. But that's not the case because the carbon is more stable in the dlc coating. There are DLC coatings with ppm of metals induced to enhance wear resistance. These metals Form carbides with the carbon in the coating. And carbides are really stable and don't "rust".

So my guess is it looks like there is a metal abrasion in the coating surface which contains iron and formed rust.

Or the coating is so thin that there are iron atoms diffunding through it. But that would mean we could see the grindlines and steelcolor because dlc can be applied quite thin.

But I guess maybe Larrin can say a word or two on this matter.

Greetings.

Carbon doesn't oxidize in air at normal temperature just because iron does, it needs 1300+ degrees F in order for that to happen. DLC is as stable as graphite/diamond at room temperature. By itself, it's chemically inert in our day to day life, and theoretically it should last millions of years by itself against normal atmospheric weathering, just like the other 2 materials mentioned above.

The reason why it doesn't rust-proof metals is because the deposit method we use cannot create a uniform enough layer so imperfections/pores will occur, so all those imperfections allow oxygen, salt or other catalysts to reach the metal beneath. On the other hand, the porosity helps with absorbing and holding oil which is why DLC is known to reduce the need to oil carbon blades simply because its harder for the oil to be wiped away by use. So yes, the rust you see is the steel beneath it, not something on the DLC. If someone has a good microscope to look at those rusty zones after they're cleaned up, I'm sure it would be clear for everyone what actually happened to the coating during the rusting process.

Excellent info. Makes sense. Thank you. This would offer an explanation of why dlc can help with corrosion without making a steel rust proof. I would guess too that some applications would come out with more/less porosity than others, which could explain some of the different results we see. Good info on the oiling of dlc too!

[Chumango]

It is better to have no coating than to have a coating with imperfections, at least in terms of structural failure. A good paint layer will protect carbon steel, but a paint layer with pinhole imperfections can actually lead to premature failure of the substrate (for example a chemical tank with coated steel walls, pinholes in the paint can lead to rapid wall perforation at the pinholes). The imperfections set up a very unfavorable area effect for galvanic corrosion. The inert areas that receive electrons are large (cathode), while the active areas that supply the electrons are small (anode - and corrosion occurs at the anode). The result can be accelerated corrosion of the small anode areas. For a coated knife blade this is limited somewhat by the need for a conductive path for the electrons, which could be provided by a combination of moisture and salt.

[The Meat man]

Share your experiences! St Nick's exclusive owners especially, have you seen any rust? What do you feel about DLC?

I just pulled out my St Nick's Lil Native exclusive and sprinkled some water and salt on the blade. I'll report back tomorrow morning.

Here are the results after about 10 hours:



Looks like some rust developed. I dare say not nearly as much as would have formed on uncoated CPM 4V though, and I was able to easily wipe off all the orange spots with my finger:



Then I cleaned the blade with a paper towel and some mineral oil. Looks good as new...


...however at just the right light some spots are still visible:


They're not rust spots though and it takes a close look to see them at all.

[northmanscall]

The reason why it doesn't rust-proof metals is because the deposit method we use cannot create a uniform enough layer so imperfections/pores will occur, so all those imperfections allow oxygen, salt or other catalysts to reach the metal beneath.

Thank you that wasn't clear to me. Just found a paper in German where someone investigated this exact topic to get his PhD.

Apparently certain layer thicknesses (and other parameters) made by PVD, physical vapor deposition, form different amounts of porosity and even micro cracks in the coating.

So thank you again for making me curious and learning.

Greetings

[blues]

DLC = Displaying Limited Corrosion

[emanuel]

I'm glad to be of help Lance and Stefan.

It is better to have no coating than to have a coating with imperfections, at least in terms of structural failure. A good paint layer will protect carbon steel, but a paint layer with pinhole imperfections can actually lead to premature failure of the substrate (for example a chemical tank with coated steel walls, pinholes in the paint can lead to rapid wall perforation at the pinholes). The imperfections set up a very unfavorable area effect for galvanic corrosion. The inert areas that receive electrons are large (cathode), while the active areas that supply the electrons are small (anode - and corrosion occurs at the anode). The result can be accelerated corrosion of the small anode areas. For a coated knife blade this is limited somewhat by the need for a conductive path for the electrons, which could be provided by a combination of moisture and salt.

If you take the DLC by itself only, yes, you are correct. But thanks to surface tension, oil will infiltrate and bond to do sides of those "pinholes", cracks or any other microscopic imperfections, sealing them (of course, not forever, the oil can be displaced with other materials, react chemically with the material being cut or simply evaporate). Also, the analogy with a hole isn't exactly correct, you need to understand that at a microscopic level that layer of DLC is actually surprisingly thick in regard to its molecular constituents, and no imperfection goes straight down towards the metal, but instead its more like a 3D labyrinth of small cracks and cavities, explaining why oil is so effective at transforming a flawed coating into something that can withstand substantially more corrosive exposure than just an oiled satin blade, without the annoying friction from paint or other rubbery coatings.

[Pelagic]

So basically DLC is like a hardened sponge.

Good info. Should've oiled it!

Share your experiences! St Nick's exclusive owners especially, have you seen any rust? What do you feel about DLC?

I just pulled out my St Nick's Lil Native exclusive and sprinkled some water and salt on the blade. I'll report back tomorrow morning.

Here are the results after about 10 hours:



Looks like some rust developed. I dare say not nearly as much as would have formed on uncoated CPM 4V though, and I was able to easily wipe off all the orange spots with my finger:



Then I cleaned the blade with a paper towel and some mineral oil. Looks good as new...


...however at just the right light some spots are still visible:


They're not rust spots though and it takes a close look to see them at all.

What dedication, thanks! Looks like those spots are definitely the first sign of rust.

[The Meat man]

What dedication, thanks! Looks like those spots are definitely the first sign of rust.

No problem. Your experience piqued my interest, especially because there are such disparate theories here about the effects DLC has on enhancing corrosion resistance.
It seems the most accurate answer is that DLC on a blade does provide some degree of protection, but does not necessarily make it rust-proof.

I take it the rust on your Shaman is not easily rubbed off?

[Bloke]

Here are the results after about 10 hours:

Greatly appreciated, Connor!

Hope you haven’t started a reaction that will lead to further corrosion problems down the track on a knife not easily replaced.

[Pelagic]

What dedication, thanks! Looks like those spots are definitely the first sign of rust.

No problem. Your experience piqued my interest, especially because there are such disparate theories here about the effects DLC has on enhancing corrosion resistance.
It seems the most accurate answer is that DLC on a blade does provide some degree of protection, but does not necessarily make it rust-proof.

I take it the rust on your Shaman is not easily rubbed off?

Definitely not. I'll try polish in 2 days when I'm off and back home.

[The Meat man]

Here are the results after about 10 hours:

Greatly appreciated, Connor!

Hope you haven’t started a reaction that will lead to further corrosion problems down the track on a knife not easily replaced.

I hope not either. I'm not too worried about it.

If it does though that will be another interesting data point.

[Chris_P_Bacon]

Sal himself stated in a post (that I cannot be bothered to hunt down) that DLC was NOT to prevent rust, that in fact could actually hide it, till it became a problem (or something to that effect).

I was always under the impression from various posts, that DLC's benefit was to help hold oil in place, and that any added corrosion resistance (verses naked steel) was simply from the oil itself.

I'd been meaning to ask you Pelagic, had you oiled that 4v DLC blade, ever since you posted about it rusting a week or so ago.

I'm glad emanual brought that up here, and reminded everyone.

But Meatman, just throwing this out there, you down to repeat that exact same test, this time with an oiled blade so we can see if it actually helps, and to what extent?

[The Meat man]

But Meatman, just throwing this out there, you down to repeat that exact same test, this time with an oiled blade so we can see if it actually helps, and to what extent?

Sure I can try that.

[Chris_P_Bacon]

Meatman

[The Meat man]

Lol

[Bill1170]

I'm glad to be of help Lance and Stefan.

It is better to have no coating than to have a coating with imperfections, at least in terms of structural failure. A good paint layer will protect carbon steel, but a paint layer with pinhole imperfections can actually lead to premature failure of the substrate (for example a chemical tank with coated steel walls, pinholes in the paint can lead to rapid wall perforation at the pinholes). The imperfections set up a very unfavorable area effect for galvanic corrosion. The inert areas that receive electrons are large (cathode), while the active areas that supply the electrons are small (anode - and corrosion occurs at the anode). The result can be accelerated corrosion of the small anode areas. For a coated knife blade this is limited somewhat by the need for a conductive path for the electrons, which could be provided by a combination of moisture and salt.

If you take the DLC by itself only, yes, you are correct. But thanks to surface tension, oil will infiltrate and bond to do sides of those "pinholes", cracks or any other microscopic imperfections, sealing them (of course, not forever, the oil can be displaced with other materials, react chemically with the material being cut or simply evaporate). Also, the analogy with a hole isn't exactly correct, you need to understand that at a microscopic level that layer of DLC is actually surprisingly thick in regard to its molecular constituents, and no imperfection goes straight down towards the metal, but instead its more like a 3D labyrinth of small cracks and cavities, explaining why oil is so effective at transforming a flawed coating into something that can withstand substantially more corrosive exposure than just an oiled satin blade, without the annoying friction from paint or other rubbery coatings.

Thanks, Emanuel. I have long wondered about this question. Not owning any DLC coated blades I always wondered about this. Vivi’s very positive experience with DLC over S30V combined with other users seeing massive rust growing up through their DLC coatings had caused me to conclude that this is not some simple binary thing like 1095 vs H-1 rust resistance.

The tip about DLC plus oil is excellent and if I ever get a DLC’d knife I will oil it ASAP! A rigid sponge, yeah.

Part of why I don’t own any DLC is because I like to modify my users by polishing or regrinding the blades to be more what I want. A coating would be ruined by this, and only increase the difficulty of the mod.

Edited to add the following:

I suggest treating the rusted blade in Evapo-Rust, which will chelate our all the iron oxide. Rinse really well in distilled water, then rinse again with alcohol or acetone to pull out all the remaining water. Once dry, oil the blade, wiping off the excess surface oil. Should be like new, only better due to the retained oil. If the porosity is extremely fine it might be good to oil with a mixture of mineral oil and mineral spirits. The solvent thins the oil so it can penetrate better, then evaporates, leaving the oil behind.