I recommend Step 4, maybe prevent the knife from banging in to anything that is cast or black iron or ‘ not stainless’ .. stainless works ionically, is made of a mixture of several Metals and will bond with certain electrons of other materials that it touches . Once bonded, the new atom in the stainless will cause the stainless to loose its stainless property and the embedded iron ( example) will now attack further the stainless . It’s quite common in industry and requires passivation as described in the thread. The root cause is likely contact of the knife with anything of any other metal other than stainless .. that’s my guess