The film is the alloy's whole defense
"Stainless" is a surface property: chromium in the alloy forms a nanometres-thin oxide film that isolates the metal. Everything that matters about stainless corrosion behavior follows from whether that film is intact. In the galvanic series, the same alloy appears twice — passive near the noble end, active far below — because a stainless surface with a locally destroyed film behaves like a different, much less noble metal at that spot.
What fabrication does to the film
Welding leaves heat tint (chromium-depleted oxide), grinding and handling embed free iron from carbon-steel tools, and shop dirt shields spots from oxygen. Each is a future initiation site for pitting and staining. Post-fabrication treatment — cleaning, pickling where heat tint must go, then passivating per the specification's cited practice — removes the contamination and lets the film reform uniformly. Verification tests for free iron exist and belong in the QC plan of any stainless fabrication destined for aggressive service.
The practical rule
Segregate stainless fabrication from carbon steel work — tools, brushes, blast media, even grinding sparks across the shop — and treat passivation as a specified, verified step, not a property the alloy brings from the mill regardless of what fabrication did to it.