Everybody keeps telling me don't put knives in dishwashers. And if I ask them why, nobody seems to have an answer or an argument to support his/her advice. So I thought I'll shoot the question to stackexchange:
Do knives get dull in dishwashers?
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Yes, knives get dull in the dishwasher, through rubbing against other items.
When I bought a set of nice Wüsthof-Trident kitchen knives, I heard this rumour too. I also heard that cutting onto a plate can dull them. As this was about 25 years ago, in the days before I had heard of the web, let alone a StackExchange, I wrote to my knives' manufacturer, Wüsthof, asking for advice.
They kindly sent me a brochure which has this section (my emphasis):
Caring for Fine Knives
- The best of edges will quickly dull if it strikes metal, glass or formica. A soft wood or plastic cutting board makes the best surface. And if a slip occurs, a proper cutting board is safer for the user.
- Knives should be used only for the purpose intended. Never use good cutlery to cut string or paper - it is an outrage to cut bones or metal with a good blade.
- Blades should never be heated in a flame in an oven. Elevated temperatures will destroy the temper of the steel.
- After use, knives should not be allowed to soak in water. The best practice is to hand wash and dry them immediately. This is especially true if they have been used on fruit or salty foods, which may cause some staining, even of stainless steel blades. Although Wusthof-Trident knives can be cycled in a dishwasher, it is not recommended. High water pressure will dull the cutting edges by knocking them again the rack and against other objects.
- Fine knives should be carefully stored in their own block, or special vinyl "roll" produced by Wusthof for this purpose.
Source: Wüsthof brochure, vintage ~1994. (I still have it, and can scan it in if required.)
I avoided putting them in the dishwasher, until I had a partner who refused to treat them with such respect. (To quote Wüsthof, it was an "outrage"!) For domestic harmony reasons, I got over it.
Chef's Choice notes that the environment in a dishwasher would cause metals of different electronegativity to react (working principle of a battery), and that the force of the water may knock knifes against other objects. I.e. it proposes at least two specific risks. However, for both mechanisms, the effect depends on other objects. A knife in a dishwasher on its own would be safe.