Why does stainless steel rust on machined parts, and is it a material, passivation or handling problem?

How do you tell whether the stainless part itself is corroding or something on it is rusting?

Wipe a spot with a mild acid stainless cleaner or a nonabrasive pad and look at the metal underneath with a magnifier. If the stain lifts off and the surface below is smooth, the rust was iron sitting on the part; if a pit remains, the stainless itself has been attacked, and the next question is what started the pit.

Stainless steel is iron alloyed with at least 10.5% chromium, and its corrosion resistance depends on a passive oxide film that only forms properly on a clean surface free of organic and metallic impurities, as Breznay and Varbai at the Budapest University of Technology and Economics note in their study of iron contamination on stainless plates (Acta Materialia Transylvanica, 2023). The same paper points out that corrosion caused by iron contamination is often confused with corrosion of the stainless steel itself, and that it can range from a slight brownish discoloration to surface pitting. In their exposure tests the loose rust wiped off easily, but the number and depth of pits under it grew over the two months of the test.

That distinction matters for the disposition. Surface iron caught early is a cleanliness defect: the parts can usually be cleaned, passivated and retested. Once pits have formed, cleaning removes the stain but not the pit, whatever started it: iron left on the part long enough, a sulfide site, or chlorides. A passivation step will not make a sulfide-rich grade or an overly aggressive environment safe, so pitted parts need a decision on the pits themselves, not just a rework. Record where the spots sit on each part (machined faces, end faces, threads, crevices, contact lines with packaging), because the pattern is the fastest clue to the cause.

How do you diagnose the cause at receiving inspection before rejecting the lot?

Work from the cheapest check to the most specific: map the spot pattern, wipe and magnify, screen with a magnet, check the certificates, then run one of the ASTM A967 verification tests on sample parts. Most lots can be dispositioned in one pass without a lab.

1. Quarantine and photograph. Tag the lot, photograph spots on several parts with a scale in the frame, and note whether every part is affected or only some trays.
2. Map the pattern. Spots on every machined face suggest free iron or a missing passivation step; spots along the bar axis or at end faces suggest a sulfide grade; spots only at contact points suggest packaging or handling.
3. Wipe and magnify. Clean a few spots with a nonabrasive pad and a mild acid stainless cleaner. A smooth surface underneath points to surface iron; pits mean the base metal has been attacked, either at sulfide sites, by chlorides, or under iron that sat on the part long enough to start pitting. The spot pattern from step 2 usually tells these apart.
4. Magnet screen. Annealed 304 and 316 are essentially nonmagnetic, although cold work can make them slightly magnetic. A strong pull on a part that should be austenitic is reason to suspect a 400 series or carbon steel mix-up, not proof.
5. Check the paperwork. Compare the mill certificate grade and sulfur content with the drawing, and ask for the passivation certificate and the lot test result if passivation was specified. A certificate that names the specification but no test is a gap, not proof.
6. Test samples. ASTM A967/A967M-25 lists seven qualitative practices, from Practice A (water immersion) to Practice G (boiling water immersion), intended to confirm that contaminant iron has been removed; the BSSA passivation article describes the main ones as tests for the effects of residual iron contamination. Copper sulfate (Practice D) is fast, but it works by plating copper where free iron is present, so run it on samples rather than on parts to be shipped. It is aimed at austenitic, precipitation-hardening and higher-chromium ferritic grades and is generally not used on lower-chromium 400 series grades, where it tends to fail parts falsely; water immersion or high humidity is the safer choice there.
7. Confirm the grade. If the mapping points to the wrong material, run PMI on sample parts against the drawing grade before any rework.

Keep the evidence. If the lot goes back to the supplier, the photos, the spot map and the test result let the supplier find the step that failed instead of simply recleaning parts and shipping them again.

What should the drawing, RFQ and purchase order say so rust spots do not recur?

Name the exact grade and say whether free-machining substitutes are allowed, name the passivation specification with the treatment and the verification test, and state packaging that keeps carbon steel and chlorides off the parts. A note that only says “passivate” leaves the treatment, the test and the evidence to the supplier.

• Material. For example, “MATERIAL: 304 STAINLESS STEEL BAR PER ASTM A276. FREE-MACHINING GRADES (303, 416) NOT PERMITTED.” If 303 is acceptable, say so and say where it may be used.
• Passivation. For example, “PASSIVATE PER ASTM A967/A967M, CITRIC OR NITRIC TREATMENT AS SUITED TO THE GRADE. VERIFY EACH LOT BY PRACTICE A (WATER IMMERSION) OR PRACTICE B (HIGH HUMIDITY). SUPPLY CERTIFICATE OF CONFORMANCE WITH TREATMENT, TEST AND RESULT.” The passivation guide covers choosing the treatment.
• Welds. “REMOVE HEAT TINT BY PICKLING PER ASTM A380 BEFORE PASSIVATION.”
• Handling and packing. “NO CONTACT WITH CARBON STEEL TOOLS, RACKS OR BINS AFTER PASSIVATION. PACK DRY IN CLEAN POLYETHYLENE WITH DESICCANT.” This matters most for ocean freight, where condensation and salt air reach the parts.
• Material verification. Ask for the mill certificate with heat number and, where a mix-up would be costly, PMI on a sample from each lot.
• Service environment. One line such as “SERVICE: OUTDOOR COASTAL” or “FOOD WASHDOWN WITH CHLORINATED CLEANERS” lets the supplier flag a grade that will pit.