Corrosion resistance of AUS-8A, VG-10 and D2 on hemp (Deerhunter)

Cliff Stamp

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To examine the difference in corrosion resistance, the knives were dipped in lemon juice for a quarter inch back from the edge. The blades were tested for sharpness before the soaking, after two hours (the juice was applied again at the one hour mark), and then after some rope (3/8" hemp) cutting. The results :

Sharpness before the soaking, as measured on 1/4" poly (length of blade needed to cut the cord) :

D2 -> 1.10 +/- 0.16 cm
VG -> 1.14 +/- 0.22
A8 -> 1.10 +/- 0.09

All blades have equally sharp edges at the start. The knives were sharpened using a DMT 600 grit rod set at 22 degrees. After a two hour soak the AUS-8A (A8) and VG-10 blade looked no different, no sign of corrosion by eye or under magnification. The edge bevel grinds lines were still clear under the scope. The D2 blade however had a distinct patina in all the contact area and parts of the edge bevel were solid black with oxidation (~one cm lengths). Checking the edge under magnification on the D2 knife, the grind lines were blurred out, and the edge bevel showed frequent large irregularities where corrosion had removed significant amounts of metal (~10x greater than the micro teeth left by the DMT rod).

Repeating the sharpness testing :

D2 -> 1.31 +/- 0.18 cm
VG -> 1.20 +/- 0.10
A8 -> 1.13 +/- 0.19

The D2 one shows the greatest increase but it is too small to say definitely that it isn't just a random increase. In retrospect this would have been a good occasion to use thread or as Jeff suggest ribbon cutting as it would have been more sensitive to the blunting than the poly which could actually be enhanced by edge corrosion as it can actually enhance the saw like effect of coarse edges. However since such edges contain iron oxide which is very weak, they are prone to excessive blunting in use.

To examine how the blades edges wore, the knives were used to cut sixteen pieces of 3/8" hemp with a two inch draw. The D2 knife felt very different on the rope, it acted more aggressive, but was much harder to pull through the rope (this would have been a good time to measure the force I was using on the hemp as it was obviously larger with the D2 blade). The blades were then again tested for sharpness :

D2 -> 2.30 +/- 0.24 cm
VG -> 1.66 +/- 0.17
A8 -> 1.70 +/- 0.28

If you compare this to how the blades had done in the last bout of rope cutting after a similar amount of cuts [no corrosion factor in that rope cutting], the results are not significantly different for the VG-10 and AUS-8A blades. Those results :


D2 -> 1.30 +/- 0.08 cm
VG -> 1.35 +/- 0.09
A8 -> 1.55 +/- 0.15

Thus the VG-10 and AUS-8A blade were not significantly effected by the lemon juice and performed very similar as if they had not been exposed. However there was a huge difference in the D2 performance. It wore down much faster in use after being exposed to lemon juice. It was now blunting the fastest out of all three steels.

It would be interesting to repeated the hemp rope cutting trials completely, with exposing the blades in salt water during the cuts. How does blunting correlate to the soak time? If you just dipped the blades in while doing the cutting would rust ever have a chance to form [probably not as Cougar frequently points out]?


Ref :

http://www.bladeforums.com/forums/showthread.php?s=&threadid=236308

http://www.bladeforums.com/forums/showthread.php?s=&threadid=241331

-Cliff
 
These results are pretty dramatic when you consider the brief corrosion time (2 hours) and consider that D2 is considered a "semi-stainless". D2 has 11%-13% chrome which is not much less than the other two alloys (VG10 14.5%-15.5% and AUS-8 13%-14.5%). Chrome carbides are not supposed to contribute to corrosion resistance, so the much higher carbon content in D2 (1.5% vs 1% in VG10 and .7% in AUS-8) must be a major factor. I would have thought that the 1.1% of vanadium in D2 would have helped soak up enough of the carbon to have evened the odds some. I'm guessing that the 1.4% of cobalt in VG10 helped it's corrosion resistance to compensate for having more carbon than AUS-8.

So D2 holds an edge twice as long as VG10 when not subject to corrosion and holds an edge 70% as long when subject to significant corrosion. So keep your D2 blade wiped clean and dry.
 
On further thought I guess that citric acid is a particularly challenging solution for testing chromium alloys. It is frequently used in solutions for removing oxides from stainless. The corrosion resistance of chromium alloys is provided by non-conductive oxides. It might be more representative (but slower) to do a salt water spray test.
 
Another very informative experiment Cliff. I have seen no fairer comparison of steels than your Deerhunter experiments. Thanks a lot for sharing your experience with us all.
 
I have used other D2 blades and the corrosion resistance was very low there as well, custom D2 from Mel Sorg. That one rusted actually in the shipping to me, and pitted very badly when exposed to salt water. The carbon content in D2 is far too high for good corrosion resistance. Still though, it does a lot better than steels like L6 which can take a visible patina in minutes even cutting up an apple.

Yes it was a rather extreme enviroment. I was thinking what was the worst way these could be treated in normal (sloppy) use. So I figured cutting up acidic foods before supper and leaving them until after to clean up. A plain water soak as well as salt water would be informative as well.

The difference in edge retention from a viewpoint of how much material needed to be cut is rather extreme. The stainless blades would need to cut another ~16 pieces of cord to get to a similar stage of blunting as the D2 blade. Thus it was now blunting twice as fast as the VG-10 blade, whereas it was holding its edge for twice as long without any corrosive effects.

You can really notice the difference in sharpening. The edge appears to clean up without any problems, but it refuses to take a clean finish. If you check under magnification you see the edge breaking away due to pits becoming exposed to the edge. You have to grind away all the corroded steel before the edge will grind clean.

phwl, thanks, appreciation to A.G. Russell should be noted as well for donating the knives.

-Cliff
 
Thanks for the excellent reports. Your Deer Hunter reports are real eye openers. And I also want to thank A.G. for providing the knives. He solicited alloy suggestions on Bladeforums when he selected to add VG10 and D2 to his Deer Hunter offering. He gave you an opportunity to run them through comparisons. He really knows how to provide service and value.
 
Jeff Clark :

He really knows how to provide service and value.

Indeed. I have worked with and interacted with many knife makers / manufacturers, it is rare to see them respond in a positive manner to such work let alone actually ask for it to be done. It speaks of confidence, and a desire to increase knowledge and not simply promotion.

As you noted in another thread, the threads combined together show how all the blades steels are solid choices, and all stand out over the others in certain characteristics. Which would be preferred just depends on how the individual values each performance attributes.

-Cliff
 
Perhaps the nickel in the AUS-8A ha something to do with the lack of corrosion, as nickel like chromium resists corrosion.
 
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