I want to step back and explain for the readers who don't know what we are talking about.
In machining and grinding of stainless objects, some particles of iron may get trapped in the surface grooves and pits. If you look at the surface of freshly machined or 100 grit ground stainless steel under high magnification, it will look like a moon scape from a sci-fi movie. Deep ravines and high mountains. Lots of stuff can get wedged and trapped in those topographical features.
When grinding iron and carbon steel, there is grinding dust everywhere, and some gets stuck in the grit on grinding belts. All this contains lots of free iron just looking for a hook-up. When you work with stainless in the same place as the carbon steel was worked on, some of these micro-particles of carbon steel and iron get stuck in the stainless steel surface. As you refine the sanding, polishing, and buffing of the blade, you can push the mountains down over the ravines and completely trap the iron. We occasionally see this show up as tiny rust spots all over a freshly ground stainless blade. In most all cases it is because we just ground a carbon blade on the same 36 grit belt. In our case, it really isn't much of a worry, as we will be doing HT and further sanding, which will help get rid of most of the trapped iron. In aero-space industry and medical devices, the tiny traces of iron and microscopic rust spots are a concern. They can cause spots to show up on surfaces, and affect test results. They are even a structural concern in the most high tech cases.
To remove them they passivate the steel. This is done by either exposure to oxygen to make iron oxide or by dissolving the free iron and removing it all together. Nitric acid and other acids are used as well as many powerful chemicals. The result is no free iron on or near the surface to worry about. This makes the stainless steel as stainless as it can get.
The passivated surface is thus covered with a thin coating of oxides that protects it and makes it more inert. In knifemaking, we call this surface patina.
In the home knife shop, we usually have a few things that would work to passivate stainless steel, FC being one of the most readily available. The FC would oxidize and/or remove the loose iron grains.
However, with stainless steel knife blades, patina isn't usually wanted or necessary. Also, properly heat treated stainless steel is usually more than stainless enough for our purposes. Knives will be sharpened and used, so the surfaces will constantly be exposed to abrasion and other effects. The surface is normally left as bright steel in finishing. Any free iron will slowly combine with atmospheric oxygen and exposure to acids in use and self-passivate.
Thus, for stainless steel knives, passivation isn't necessary, or an issue. Avoid cross contamination of your stainless by cleaning up the bench and dunk tanks, and use separate belts if you really worry about it, but in my feelings, this subject isn't a worry for knifemakers.
