Seeking passivation for small lot of 316 stainless steel parts

We have small (3inch round challace shape) 316 stainless steel parts we want to have passivated. We machine them out of bar stock on a CNC machine. We wash them to remove the cutting oils. We have small batches -- 25 to 50 pieces each month. We notice some "rust" on returned product from the field. Our customers clean the area where are parts are with strong cleaning solutions. We do not see the rust when the parts are passivated.

WE ARE LOOKING FOR COMPANY TO PASSIVATE THESE SMALL LOTS. The parts can be shipped UPS to the job shop. They must be packed in such a way as not to damage each other during shipment. They do not get painted. The stainless finish is a positive marketing concept.