Your stainless was probably supplied with a common 'mill finish' - the surface is rough, has many mill rolling marks, not polished.
Also, possibly the material was processed with plain steel tools, jigs and fixtures with the plain carbon steel contaminating the surface of the stainless steel.
For rust prevention and corrosion retardation, 300 series stainless has to be supplied in at least a 300 to 180 grit finish to remove processing scars and surface defects of manufacture by the mill followed by or purchased from the mill already 'sanded'/ground; then surface then can be 'passivated' to remove the remaining surface IRON. Simple passivation should only be done with a surface ground to at least that mill supplied 180 grit finish. For best corrosion resistance (rusting, etc.) is to grind all welds flat and planar to the surfaces followed by grinding, then finely sand (down to 600 grit) followed by power-buffing to a mechanical mirror bright surface to remove all scratches and mechanical blemishes, followed by electropolishing.
If you take 'any' 300 series stainless steel that is not mill or shop finished to the above, it will surface rust, especially on boats when in contact with sea water. Thats why 99.9% of the stainless steel you see on 'boats' is 'very shiny' - its highly polished to prevent rust and corrosion.
when the pipes were new, they were actually shiny. Definately not sanded with 180. I see electro polishing unfortunately also includes emerging in a special bath.