Actually, Cr depletion causes sensitization, not the other way around. Rusting is caused by chromium depletion.  By itself, chromium depletion is not a problem.  The only issue is the resulting corrosion.  single instruction to watch for carbon contamination from grinding   Every mechanical treatment damages the top layer of components by contamination with ferric matter, change in the structure of the layers, development of stresses and reduction of chromium content.

Heat treatment like annealing and welding leads to scaling and discoloration. These not only deteriorate the appearance of the component but especially its oxidizes which are chemically less resistant. A continuous passive layer of chromic oxide cannot form here. Corrosive strain on insufficiently passivated stainless steel leads to the forms of corrosion typical for this type of material:

• Pitting Corrosion
• Crevice Corrosion
• Stress Corrosion Cracking
• Inter-crystalline Corrosion
  

Corrosion Resulting from contact with Foreign Metals. Disruption of the passive film by chemicals, mechanical action, embedded iron particles, or oxygen starvation can readily occur in the workshop or during fabrication. Surface free iron particles, dust, grit and iron oxide contaminants arise from handling, fabrication/forming, welding, grinding, machining, paint and crayon marks, polishing, tumbling and workshop cross
Contamination These contaminants penetrate the passive film, absorb and generate chlorides, ferric chloride or produce inorganic chlorides by the decomposition of organic compounds such as PVC and marking materials.

• Loss of Production
• Repair & Maintenance of Parts
• Inventory Redundant Equipment
• Replacement of Equipment

If a Stainless Steel surface is scratched then more Chromium is exposed which reacts with oxygen allowing the passive layer to reform but if, however, a particle of carbon steel is embedded in the scratch then the passive layer cannot reform and corrosion will occur when the metal is welded or exposed to a corrosive environment. While welding or heat treatment is carried out complex oxides are produced which can vary in colour from straw to black and these oxides must be completely removed to allow the passive layer to reform.

This is best done chemically by SPR - 3 - AN with pickling and passivation as mechanical methods such as grinding or blasting may not.All this one says is that if you grind or sand blast stainless steel you remove the passive layer.  No surprise there.  Heat treatment and welding causes scale..
All stainless steels have a majority of Iron in them. Hence why they are called Steels. When stainless alloys are processed it is common for particles of free iron to end up on the surface.

If the piece is immersed in salt water, the tiny iron particles rust and hence cause galvanic corrosion of the metal around them.This is even more common on welds. When the weld area goes from liquid to solid, iron particles often leach to the surface.