Stainless Steel Knives In A Dishwasher
Why You Should Avoid It

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

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.