For me it was always obvious, based on observations of other people's knives and basic chemistry knowledge that knives and a dishwasher don't go well together. I have seen two types of damage caused by dishwasher, impact damage caused by physical contact of the knife with other hard objects in the dishwasher, such as other knives, utensils, dishes, whatever else is in the dishwasher at that time, and chemical damage, which translates into discoloration or rust. Lately, with the appearance of the new dishwashers which have a dedicated tray for knives and other small utensils, a lot of people, at least the ones I have communicated with, feel that it is 100% safe to wash the stainless steel knives in the dishwasher. To be honest, logically, that belief has some merit to it. Modern dishwashers use less water, less detergent and the presence of the dedicated tray for the utensils eliminates one of the problems completely, the impact of a knife with another knife. Although, the corrosion question remained open, at least for me. I have seen reports that there is no corrosion if you use a tray, and reports that contradicted that, i.e. corrosion is still a threat even with the tray. Obviously, a lot depends on the environment, as in how hard the water is, humidity in the area, although inside a dishwasher that isn't of any importance, but outside of it, humidity does play significant role.
Long story short, I have decided to conduct a simple test myself. Since I am very cautious about my own knives, and I don't feel like getting rust spots on 500$ knives, plus, those knives are hardly the representative of average kitchen knives out there, I had to get a sacrificial knife so to speak. Few months ago, I was given Wusthof boning knife, which is made of X50CrMoV15 stainless steel. Perfect sample of an average mainstream kitchen knife. Better yet, X50CrMoV15 is highly corrosion resistant, much more so than the high end stainless steels, such as Takefu VG-10 stainless steel, Hitachi ZDP-189 steel and many other exotic stainless steels.
As to why those exotic steels are less stain resistant compared to good old X50CrMoV15, which isn't very intuitive if you look just at chromium content, 20% in ZDP-189 vs. 14% in X50CrMoV15, that is really visible visually if you compare the compositions of those steels using atom counts, or molar masses - X50CrMoV15 vs. VG-10 vs. ZDP-189 steel composition comparison. As you can see on the graph, in atom count mode, the difference between Carbon and Chromium is far greater for X50CrMoV15 than for ZDP-189 or VG-10 steels. In other words, the amount of free chromium, or number of atoms of chromium that are not bound in carbides with Carbon is considerably smaller for ZDP-189 and VG-10, because both have considerably higher carbon atom count. To be more exact, for every 1000 atoms in ZDP-189 alloy we have 250 atoms of Carbon, and 385 atoms of chromium. Bear in mind that a pair of carbon atoms bonds with 3 atoms of chromium - Cr3C2 is the formula. Luckily, ZDP-189 ally has other elements to form carbides, Molybdenum, Vanadium and Tungsten(Wolfram), otherwise, corrosion resistance of the ZDP-189 would've been even lower. Free chromium is what makes a steel stain resistant, and more is better for stain resistance, but not for edge holding and toughness. Same old story, no perfection in this world, it's always about compromises. Well, I guess that's enough of the alloy science :)
Test equipment and conditions
- I guess it's a little bit of a stretch to call a boning knife and a dishwasher test equipment, but considering that I was attempting to test real life use/damage/corrosion scenario, that is a very good approximation. So, the test was conducted using the following equipment: Wusthof Classic Flexible Boning Knife, and Miele Diamante Plus model dishwasher. The test knife was placed in the tray, and the normal washing cycle was executed. Currently, the dishwasher gets filled up sufficiently enough to run the washing cycle about twice a week. At least that was a pattern for last 3 weeks. The only exception was the last cycle, when the dishwasher was run inthe Pots And Pans mode. I'm being specific about the modes, since the mode selection affects the length of the cycle and in pots and pans more water is hotter, more detergent, in other words chemicals are used. For all cycles, standard dishwasher dry procedure was also used. After the dishwasher completed the dry cycle, it was left open for several hours, and whenever I unloaded it, the knife was removed and placed in the knife block.
Prior to dishwasher testing, the Wusthof flexible boning knife was in the same knife block for about 6 months, never used. As you can see on the attached image there was no rust on may 14th, 2010 when the testing started. Here is the other side of the Wusthof boning knife blade. The 6 month waiting period wasn't intentional, but in the end, served the purpose of confirming that in my environment, unused stainless steel knives in that knife block do not rust. This may seem funny to some, however given the example of the same ZDP-189 steel in a humid environment, to be precise on Hawaii, it was not excessive to test that, ok may be not 6 months, but I didn't have time ;) Basically, unused knives in humid environments can rust more than the ones being in use. Kershaw 1840CBZDP Shallot ZDP-189 which was used got less rust on the blade compared to another one, which was sitting in the backpack all the time (about 10 days). You can see two Kershaw ZDDP-189 Shallots on the linked image here, the user knife on the top sustained much less damage compared to the unused one on the bottom. Now, given the fact that I live in SF South Bay area, which is quite dry, there was no real need for testing knife corrosion in unused condition, but that doesn't hurt either. Nobody can say, the tested Wusthof would've suffered the same corrosion just by sitting in the block. It was there for 6 month and there was no rust whatsoever.
Test Log
Weeks 1 05/14/10-05/23/10
- To be exact, leftover of the week and a full week. Nothing interesting happened. Run dishwasher on normal cycle, followed by dry. Total of 3 cycles. No visible results of any kind.Week 2 05/24/10-05/30/10
- Run the cycle twice. Visible discoloration appeared on the blade. Both sides. No photos were taken at that time.
Week 3 05/31/10-06/06/10
- First cycle, but unlike the previous cycles, washing cycle was set to Pots And Pans, then dry. Examined the blade next day, discoloration became slightly more pronounced, and a rust spot appeared. Second cycle commenced on Sunday, Jun 6th, 2010. Discoloration is more pronounced on both sides of the blade. Attempted to clean the rust spot using cloth. Discoloration around the rust spot was removed, but the rust didn't. That proves the rust formed on the blade, and wasn't simply an iron deposit from the water which oxidized later.Week 4 06/07/10-06/13/10
- One cycle this week, Sat. 06/12. Dishwasher was set on normal program. Examined the blade after washing and drying cycle. Strange results. Discoloration is gone. I've cleaned one side myself last week, when I was checking if the rust spot was a deposit of the oxidized iron from the tap water or it was a rusting blade, which proved ot be the later. Anyway, I am not sure why the dark discoloration which was steadily building up for 3 weeks all of the sudden is gone. The knife is sitting in the same spot in the cutlery tray, same dishwasher detergent and sme tap water after all. However, there are two more rust spots forming near the first one.Week 5 06/14/10-06/20/10
- Two cycles, both on normal. No changes.Week 6 06/21/10-06/27/10
- Two cycles, both on normal. No changes.Week 7 06/28/10-07/04/10
- Two cycles, normal and express. No changes.Week 8 07/05/10-07/11/10
- Two cycles, both on normal. Discoloration reappeared.Week 9 07/12/10-07/18/10
- Two cycles, both on normal. After second cycle, new rust spot, next to the old one. Strange, it took so long to develop, after the first one.Week 10 07/19/10-07/25/10
- Two cycles, both on normal. No changes after first wash, but after second one, seversl rust spots near the bolster.
Week 11 07/26/10-08/01/10
- Two cycles, both on normal. Rust became more pronounced on the old spots. New one appeared aftr cycle #1, mid blade, right side.Week 12 08/02/10-08/08/10
- Two cycles, both on normal. Rust became more pronounced on the old spots. No new spots.Week 13 08/02/10-08/08/10
- Two cycles, both on normal. Existing spots become darker. new spot appeared mid blade, on the left side.
Week 14 08/09/10-08/16/10
- Two cycles, both on normal. Rust became more pronounced on the old spots. No new spots. Examination under microscope showed rust pitting. The photo attached here shows the very first rust spot, which appeared on the week 3, on week 14, at the end of the experiment. As you can see, single spot became two and pitting developed quite severe for 14 weeks. Actually it is 11 weeks of rust development, since the spot appeared on week 3. You can also see the wider rusted area arund the pits and layering steel in the area. More micrographs of the affected areas are in the X50CrMoV15 Dishwasher Experiment album.Summary
- That concluded the experiment, at least run #1. In 12 weeks, with 2 washes per week and unused knife I got rust pitting. As far as rust is concerned, pitting is as bad as it gets, after that it's just a matter of the depth and the size of the affected area. So, there wasn't really much left to investigate, as far as rust formation was concerned. May be it'd accelerate later on, may be it'd decelerate, but that's outside of the scope of this experiment. The goal was to see how long it'd take for the rust to develop. Originally I've planned just a single run of this test, but later during the discussions on knife and blade forums, the importance of tap water pH factor, in other words water hardness. At that point I've checked the dishwasher and found out that the water softener salt out. I've decided to continue the experiment as it was, and at least I'd know what un-softened tap water did to the knife.So, based on all the photos and logs, conclusion is very simple, dishwasher does induce rust, even on highly rust resistant alloy such as X50CrMoV15 steel. The knife wasn't used in the kitchen, which on one side could've helped rust development, I've already seen unused knives developing rust quicker than the used ones, but those were not kitchen knives, which have to deal with acidic ingredients, stay wet for prolonged times, I don't let my knives stay wet if they're not in the process of cutting, but most of the people are not really pampering their knives, especially stainless ones. So, in a retrospect, it would be interesting to see how fast working knife would rust under the same conditions, but there are too many variables, what to cut, how long, leave it dirty or wipe it dry, etc. May be when I figure out what is some generic pattern of use amongst average users, I'll do that test as well.