Since there are people that are still questioning the claim of leftover particles affecting corrosion resistance I will provide some extra context. I am not saying that heat treatment or other factors are not involved, simply providing more information on this part of the concern over corrosion resistance.
One thing that I often see repeated is that they are worried about steel particles left over from grinding other steels. That would indeed be worse. But even particles left from the same steel are still bad for corrosion. Ground particles are very small, and a lot of them oxidize (sparks). Any free iron particles will make corrosion happen more easily. They act as high surface area initiation sites. There is a very common practice called "passivation" which uses a weak acid, usually nitric or citric acid, to remove free iron particles. Passivation happens with stainless steel spontaneously; stainless steel exposed to oxygen forms a passive chromium oxide layer at the surface which is what prevents rust. However, free iron particles prevent this from happening which is why the acid treatment is called "passivation." There are a lot of articles online where you can read about passivation.
Earlier in the thread I said I couldn't say for sure if free iron particles would lead to patina rather than rust. Several people opined in the thread that free particles would lead to obvious points where rust formed rather than patina. I don't know for sure if that is true. The free iron particles would presumably be very small, since the bigger ones should have been removed in cleaning. But as I said I haven't done experiments in this area and I may not have the knowledge to do so effectively. I am not a corrosion expert.
Passivation is not common among knife companies. I have never heard of any companies doing it though that doesn't mean none are. I would be curious to hear about any that have done it or at least tried it. Knife steels are not generally known as being very corrosion resistant so maybe even the weak acid could give potential issues like a patina forming during the process.
