Corrosion Resistance, Steel and Knife Properties

Corrosion Resistance Testing of Stainless Knife Steels

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Corrosion Resistance of Steel

At its most basic, chromium in steel leads to the formation of a chromium oxide layer on the surface which protects the steel from rusting. Molybdenum and nitrogen are known to help with “pitting resistance” and can support the chromium oxide layer. Chromium, molybdenum, and nitrogen can form carbides or nitrides which leads to less of the alloying element available for contributing to corrosion resistance. So the overall composition and the heat treatment are significant parts of corrosion resistance not just the “bulk” composition. For example, D2 has about 12% Cr which would be enough to be stainless but with its high carbon content there is insufficient chromium “in solution” to be stainless. Read more about D2 corrosion resistance here.

Previous Corrosion Resistance Article

I have previously written other articles about the corrosion resistance of steel, including this article where I talked about what gives steel its corrosion resistance and used JMatPro calculations and reported experiments to rate each steel. The amount of chromium and molybdenum in solution at the recommended hardening temperature for each steel was estimated with the software. I used some reported experiments on knife steel corrosion resistance to develop a simple equation to determine the relative contribution of Cr and Mo and then ranked each steel accordingly. However, there were a couple limitations to that previous article which I hope to improve here:

  1. The corrosion resistance experiments reported were either a mass loss after a set amount of time in acid or a voltage measurement of “pitting resistance” in salt water. Neither was a measure of rust formation itself. It may be that one of these tests correlates well with rust formation but I don’t know which of them it would be. Rust would be the main issue people are concerned about with knives so that is the basis on which we want to compare corrosion resistance.
  2. I used JMatPro at the time rather than ThermoCalc because JMatPro is faster to use and I was not sure which had the better estimate of alloy in solution. I found cases where one was closer to experimentally reported values than the other so it seemed overall to basically be a tie. However, I have recently learned that ThermoCalc previously updated their database based on tool steels with high Cr and V content that conflicted with JMatPro and earlier versions of ThermoCalc [1]. Old databases showed steels like Vanadis 6 and the original Vanadis 4 to have no chromium carbide in them at typical hardening temperatures while in reality they have a significant amount of it. This means that stainless steels with high vanadium content like S30V, M390, Elmax, S35VN, S90V, etc. would have estimated Cr in solution which is too high. However, ThermoCalc is now better at calculating Cr in solution for these steels. ThermoCalc seems to regularly update their database while looking at several versions of JMatPro the database seems never to change. Therefore, I have more confidence in ThermoCalc in general. The differences between the two software packages varies somewhat, in general they are not radically different, but in some cases the differences are significant.

New Experiment

For each of the corrosion resistance experiments that I performed I started with ~1/8″ starting stock and cut out 1 x 1.5 inch pieces. Each was wrapped in heat treating foil and austenitized in my Evenheat furnace for 15-30 minutes depending on the steel. The pieces were quenched between aluminum plates and given a cryo treatment in liquid ntirogen, then tempered twice for two hours at 400°F. The austenitizing temperature selected for each steel is shown below along with the ThermoCalc-estimated Cr, Mo, and N in solution. The exception is CPM-154 which uses the Cr solution measured experimentally in this article. Each piece was then ground and finished to 400 grit. With corrosion testing of materials the steel is often mirror polished because scratches can reduce corrosion resistance. That helps with making it a test of the material only rather than the finish. However, I decided to use a more practical finish that would better represent the state of consumer knives.

You can see that Cr in solution ranges from 9.4 to 14.4%, Mo from 0 to 2.7%, and N from 0 to 0.31%. These steels were selected to have these wide ranges so that the effect of the different alloy elements could be evaluated.

Water Spray Test

I first started by testing S35VN, AEB-L, CPM-154, 14C28N, LC200N, and XHP. I sprayed each with distilled water every 8 hours and continued the test for 4 days. Only XHP saw any rust, and it had significant rusting after only 10-12 hours. I was concerned about potential contamination of steel dust so I re-finished the sample and repeated the experiment but it rusted after about 10 hours the second time as well. In my previous article about XHP I speculated that XHP had insufficient Cr in solution to be stainless and this experiment appears to have confirmed that speculation. Interestingly, CPM-154 with a similar Cr in solution did not have rust, which is an indication that the high Mo is making an improvement to corrosion resistance apart from pitting resistance in saltwater solutions only, which is where Mo has the biggest effect.

XHP 10 hours after being sprayed with water

Salt Water Spray Test

I did not continue to test XHP because it did so poorly with water. I next tested the same steels (apart from XHP) with a 3.5% saltwater solution, which is a similar salt content to seawater. However, differentiating between the different steels was somewhat difficult. LC200N did the best, 14C28N was second best, and AEB-L was last but differentiating between S35VN and CPM-154 was difficult. Significant rusting was seen after only 12 hours apart from LC200N which saw no corrosion. So I decided to reduce the salt to 1% and perform the experiment with a few more steels (now the full matrix shown above) to better differentiate between the different grades. Every 8 hours I cleaned the samples, photographed them, and then sprayed them with the 1% saltwater spray again. Here are the specimens, after 24 hours on the left and 48 hours on the right:

LC200N

S110V

204P

14C28N

S35VN

CPM-154

S90V

AEB-L

Comparisons Between Steels

The Mo-free AEB-L had relatively poor corrosion resistance when compared with these other steels. 14C28N had much better corrosion resistance than AEB-L. 14C28N was designed by Sandvik to have improved corrosion resistance vs their 13C26 which is identical to AEB-L. The 14C28N was designed to have much more Cr in solution than AEB-L which is why its corrosion resistance is so much better. S110V was also found to have improved corrosion resistance when compared to its predecessor, S90V. No corrosion was observed with LC200N due to its high Cr in solution in combination with Mo and N. 204P and presumably also M390/20CV has very good corrosion resistance due to its high Cr in solution in combination with Mo and W. CPM-154 has decent corrosion resistance due to the high Mo in solution despite its relatively low Cr in solution.

Relative Effects of Cr and Mo

Similar to the method used by Carpenter in development of XHP [2], I assigned a rating from 1 to 10 of the surface of each specimen, where 10 means no visible rust, 9 means small light spots, 8 means easily visible rust spots, 7 means the rust spots have grown to be “medium-small,” 6 means 5-10% covered, 5 means 10-20% covered, 4 means 20-40% covered, 3 means 40-60% covered, 2 means 60-80% covered, and 1 means 80%+ covered with rust. In some cases I gave a 0.5 addition if I had difficulty assigning one of the values. I then plotted the result vs chromium in solution and got the following result:

There was a strong effect of both chromium and molybdenum on corrosion resistance. Whether there is an effect of nitrogen is somewhat less clear. The 14C28N steel seems to be closer to the 0.6-1.0% Mo steels than AEB-L and that may be from the ntirogen addition. But LC200N has such high Cr in solution so it is hard to separate out the nitrogen effect leaving 14C28N as our only datapoint. CPM-154 is on the same trend line as S110V and S35VN despite its significantly higher Mo in solution (~2.7%), which may mean either the Mo effect is not as strong with low Cr in solution or perhaps that the effect of Mo is saturated at some amount greater than 1%. This makes sense as the chromium oxide layer is most important, and the Mo in solution supports the existing Cr-oxide layer. I calculated a regression for the effect of Cr and Mo in solution for the 24 hour data which was Rating = -13.6 + 1.528*Cr + 2.419*Mo, which provides a strong correlation:

Dividing the Mo coefficient by Cr (so that Cr is equal to 1) gives 1.6 for the 24 hour and 48 hour data. The 1.6 coefficient for Mo is much lower than the PREN equation, which is Cr + 3.3*Mo + 16*N. The 1.6 value for Mo is relatively similar to the coefficient I calculated to in the previous article. The PREN equation was derived for austenitic stainless steels that have very high Cr in solution (16%+) so the chromium oxide layer was relatively strong in all of them. So it makes sense that the behavior would be somewhat different for these knife steels where 15% Cr in solution is a very high number. Also PREN is from pitting resistance specifically, not necessarily for rust formation alone.

Tungsten is reported to have half of the contribution to molybdenum so it would have a coefficient of 0.8. And if we are generous and assume that the contribution of nitrogen is significant to 14C28N, that would give a coefficient of approximately 6 to nitrogen. So our final equation for predicting the effect of each alloying element in solution is Cr + 1.6*Mo + 0.8*W + 6*N.

Rating other Steels

I next used ThermoCalc to estimate the Cr, Mo, W, and N in solution for each steel and plugged those into the equation above. I assumed that the contribution of Mo, W, and N saturates at an overall contribution of 2.6%, corresponding to 1.6% Mo (1.6*1.6 = 2.6). Each of those elements supports the chromium oxide layer rather than replacing it so I grouped them together with this contribution. Using that equation I then normalized to a 10-point scale to provide a rating for each of the steels below. I don’t think these ratings are perfect but I think it is a reasonable approximation and is an improvement over the table in the previously published article. I have a solid line which is where I think the rough border is between “stainless” and “non-stainless” steel. Apart from XHP, ZDP-189 is also below the line.

I have not listed alternate names for each steel. For example, LC200N is also called Cronidur 30 and Z-Finit. M390 is also called 20CV and 204P. S90V is also called 420CV or 20CP. I do not differentiate between CPM and conventional steels with identical composition like 154CM and CPM-154. In some cases PM technology may make the alloy more evenly distributed for better corrosion resistance but this is not factored into a ThermoCalc estimate.

Effect of Heat Treatment

The heat treatment of the steel matters. In the table above I used the recommended “austenitizing,” or hardening, temperature for each steel. If a single recommend temperature was not given then I took the average of the recommended range. With higher austenitizing temperatures more carbide is dissolved putting more Cr, Mo, etc. in solution. You can read more about austenitizing here: Part 1, Part 2, Part 3. There are limits to how high the temperature can be, whether because hardness drops from excessive retained austenite or because the steel melts. For example, no austenitizing temperature is sufficient to make D2 stainless.

The tempering temperature also affects corrosion resistance, as high temperature tempering (>750°F) leads to precipitation of chromium carbides and therefore a reduction in chromium in solution. Some steels are relatively commonly tempered in that high temperature range and therefore the results of the testing would be somewhat different. Non-stainless steels and high speed steels like M2, M4, Maxamet, Vanadis 4 Extra, 10V, 3V, CruWear, etc. are often heat treated with a high temperature temper. And certain stainless steels like CPM-154 and BG-42 are relatively commonly tempered at a high temperature. In general I recommend using a low tempering temperature instead for better corrosion resistance and in some cases a low temperature temper gives better toughness as well.

Effect of Surface Finish and Contamination

As noted previously, a rough finish leads to faster corrosion, in general the finer the finish the better the resistance to corrosion. Contamination of steel particles, particularly those of a non-stainless steel, also can create sites for corrosion. Therefore, when two knives are compared to each other it is not necessarily a pure comparison of the steel type because there is a contribution from heat treatment and surface finish, assuming no contamination.

Corrosion of Edges

Corrosion is not only a cosmetic issue, it can also affect edge retention as shown in this study.

Future Studies

I would like to use the less extreme water spray test to compare more non-stainless steels to find the effects of a couple variables. One thing to test is whether the high Mo and W contents in high speed steels are sufficient to improve corrosion resistance when using low tempering temperatures or if the low Cr content means that those steels behave similarly to low Mo steels like A2. This would also help to test whether the contribution of Mo/W/N does indeed saturate at some value as assumed above. I would also like to see if a patina makes any difference with corrosion in water.

Conclusion and Summary

Higher chromium in solution leads to better corrosion resistance, as expected. ThermoCalc was found to be better than JMatPro for predicting Cr in solution. Molybdenum was found to improve corrosion resistance whether in water or saltwater. The contribution of Mo appears to saturate at some amount of Mo around 1.6%, and this is likely true of the contribution of W and N as well. XHP was found to have poor corrosion resistance and I don’t consider it to be a “stainless” steel. ZDP-189 also likely has poor corrosion resistance. Using a regression derived for the contribution of each element in solution, a rating system was devised for a range of knife steels. This rating system should be improved when compared with the previous attempt at ranking knife steels in terms of corrosion resistance. The rating system may be updated using information from future corrosion resistance experiments.


[1] Bratberg, Johan, and Karin Frisk. “An experimental and theoretical analysis of the phase equilibria in the Fe-Cr-VC system.” Metallurgical and Materials Transactions A 35, no. 12 (2004): 3649-3663.

[2] Novotny, Paul M., Thomas J. McCaffrey, and Raymond M. Hemphill. “Corrosion resistant, martensitic steel alloy.” U.S. Patent 5,370,750, issued December 6, 1994.

21 thoughts on “Corrosion Resistance Testing of Stainless Knife Steels”

  1. Thank you for the really nice work on corrosion; it’s terrific to have analysis like this available.
    Your final table seems quite in line with anecdotal experience and help explain precicly why tool steels often seem more rust resistant than plain steel. As a knife enthusiast it also is a nice guide to see what level of rust resistance one might be willing to tolerate to see what materials are worth considering.

    If you still have your samples would you consider testing them with a mirror polish to see if the trends are identical ? To keep it easier maybe just keep them all together in a damp area and check every few days.

    Your patina thoughts are interesting as well, I have an opinel I keep polished on one side and it definitly does patina significantly but I’ve never thought if that layer might resist corrosion beyond the polish itself. (It would seem likly, but small )

  2. Love your work on this site!

    I too would LOVE to see a comparison of the main Stainless Steels with a mirror finish on them. Including 440C, if you don’t mind. THANK YOU!!

  3. Hi Larrin – This bit is confusing: “14C28N had much better corrosion resistance than AEB-L which was designed by Sandvik to have improved corrosion resistance vs their 13C26 which is identical to AEB-L.”

    If AEB-L was designed to improve on 13C26, how can it be identical to AEB-L? Is that a typo?

    Also, you have a typo in the next sentence where you say “soltuion”.

  4. Hi Larrin! Thank you so much for sharing all that information!

    I have a question regarding foil wrapping stainless steel…
    I do most of the grinding after HT for AEB-L and MagnaCut (thin kitchen knives). Soak times are between 15 and 20 minutes. I wrap in foil, but I’m still not sure if it makes a difference. What is the advantage of foil wrapping in such a case? Does it affect corrosion resistance or other properties?

      1. Sure, I understand that, but decarburization is very shallow, right? When bevels are ground after HT and scale does not matter, would it have any negative effect doing HT without foil?

        1. I would like to know the answer to this as well. In my experience, holding a piece of stainless steel (with mill scale, not flat ground) at 2050 for 30 minutes produces a sizable amount of scale. So much that if the steel is held edge-up in a grooved fixture, the piece can grow .025-.050″ in width and become stuck in the gap. This is because of the loose, expanding texture of scale, I think. The cooled and cleaned steel may only have lost 0.005″ or less due to scaling. To completely remove pitting and decarburization, a further 0.010″ might have to be taken off.
          .
          I have tried a commercial “clay” slip made by ATP to attempt to mitigate scaling, but it is useless at this temperature, compared to the 1500F range for which it is designed and in which it works well.
          .
          For me, I would prefer to not have to deeply surface grind my blades, and loose valuable thickness in order to hardness test them, so a protective layer is valuable. I refuse to mess with foil at this point, so I will try graphite next run.
          .
          Other concerns are that lots of scale may interfere with heat transfer in plate quenching, and that scale may accelerate wear on belts.
          .
          The consensus on the internet has been that high and long holds will ruin your knife–that there will be nothing left. The only way that is true is if you put it in the oven with expectation to loose no thickness and come out blemish-free. It was to my surprise, at first, that something like 10V at .065″ would survive–(questionably) protected by only a few coats of ATP slip– for 20 minutes at 1950F. The finish is as-forged and rough, but it works.

  5. Great article! Do you know what type of stainless steel was used years back for making knives? It was almost rust proof, and would not rust even when kept in its leather scabbard.

    1. The most common stainless steels in production knives back in the day were 420 and 440A. When custom knifemaking was growing in the 1960s and 1970s it was 440C but 154CM was also being used in the 1970s.

  6. I loved the article! Can you speculate on the corrosion resistance of LC200N vs BG-42? I suspect that BG-42 would be similar to 154CM with the amounts of alloying elements in solution and thus perform worse than LC200N but I was curious to hear your thoughts.

    1. Yes I would expect BG42 and 154CM to be similar. Though when either is given the high temperature tempering treatment their corrosion resistance is significantly reduced. LC200N is much better than either of them when it comes to corrosion resistance.

  7. I am not sure if you are still monitoring this article for comments. I have a OKC SPL pack knife in magnacut. And observed some corrosion from putting it in the sheath wet from fresh water. I am thinking this is related to the oxide layer being interrupted from contact with the sheath and this being some form of crevices corrosion which stainless steels are susceptible to. I am wondering if the high corrosion resistant steels like magnacut and LC200 can be further differentiated by forcing crevice corrosion by putting some sort of intert material plastic or ceramic in contact with the steel in the test. As per above this has applications for knives in sheaths and also folding knives and knife handles. Cheers David

      1. I did some more corrosion testing on the knife with some other steels 3.5% salt water sprayed on for 8 hr then wiped off and then another spray for 3 hr before I stopped the test. The OKC SPL pack knife in magnacut was actually one of the worst stainless steels for corrosion. So I am thinking OKC have done something wrong with the heat treatment. I can send you some photos if you like. For reference the magnacut on this knife did about the same as 8cr14mov. S35vn only had one spot.

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