The short answer
After CNC on stainless, **passivation** is a chemistry step that removes free iron and helps restore a chromium-rich surface—it is not a decorative coating and it is not automatically included on every stainless RFQ. Specify it when the drawing, end-use environment or a named specification (such as ASTM A967 or AMS 2700 language on the print) requires a verified post-machine treatment. Silence on **whether passivation is required**, **which method/spec** and **what happens after bead blast or weld** is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the stainless kg price alone.
This guide is for buyers packaging stainless bracket and housing RFQs who are deciding whether to add passivation after machining. Illustrations use an **Example part** only—not a customer medical device. Related notes live in [passivation vs electropolishing on stainless](https://3dwk.com/resources/passivation-vs-electropolishing-stainless), [stainless 304 vs 316 vs 17-4PH](https://3dwk.com/resources/stainless-304-vs-316-vs-17-4ph), and [bead blast vs tumble finish CNC RFQ](https://3dwk.com/resources/bead-blast-vs-tumble-finish-cnc-rfq); here we focus on **when to specify passivation after CNC** so estimators do not guess a finish route.
What passivation changes on a quote
Passivation (typically nitric or citric chemistry under a named spec) is a batch chemical process plus handling, masking questions and, when required, test coupons or certification language the buyer must request explicitly. It does not add a plated thickness stack the way zinc or electroless nickel does—see [Does plating thickness eat CNC tolerances?](https://3dwk.com/resources/plating-thickness-cnc-tolerance-stack)—but it does add a process step, rinse/dry control and any mask of surfaces that must stay untreated. Process pages for medical stainless ([CNC 3-axis SS 316 medical](https://3dwk.com/en/cnc-3axis/ss-316/medical.html), [CNC 5-axis SS 316 medical](https://3dwk.com/en/cnc-5axis/ss-316/medical.html), [CNC turning SS 316 medical](https://3dwk.com/en/cnc-turning/ss-316/medical.html)) assume the RFQ says whether passivation follows machining when lots ship into regulated or wet environments.
Do not bury “passivate if needed” inside a notes cloud. Put **PASSIVATE AFTER CNC PER ASTM A967 CITRIC (OR THE SPEC ON YOUR PRINT)** or **NO PASSIVATION — AS-MACHINED STAINLESS** on the PDF and mirror it in RFQ text. Background: [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
Grade, blast and sequence
304, 316 and 17-4PH do not share identical passivation habits. 17-4PH condition (H900 vs H1025) belongs on the same RFQ as any chemistry step—see [17-4PH H900 vs H1025](https://3dwk.com/resources/17-4ph-h900-vs-h1025-what-buyers-choose) and [tolerances CNC 17-4PH](https://3dwk.com/resources/tolerances-cnc-17-4ph-what-buyers-specify). If you bead-blast or tumble before passivation, say so: blast media can embed or smear iron, which is exactly what passivation is meant to address, and the sequence **machine → (optional blast) → passivate** should be written, not assumed. Welded assemblies need a note on whether passivation is after weld cleanup.
Stainless 316 bar $/kg planning bands sit above commodity aluminum under **rates.v1.1** material tables; the passivation adder is usually a process/handling line, not a material-cost story.
When you can skip it
You can omit passivation when the print family does not require it, the part stays in a dry/benign environment, and the buyer accepts as-machined stainless with only a cleaning note. Some prototype lots ship as-machined for fit checks and add passivation only on production—say which revision this RFQ is. Do not assume “stainless = always passivate”; unused chemistry steps still consume quote attention when the note is ambiguous.
Electropolish is a different process (metal removal plus brightening) and should not be used as a silent substitute; compare [passivation vs electropolishing on stainless](https://3dwk.com/resources/passivation-vs-electropolishing-stainless) if appearance or micro-deburr is the real goal.
Tolerance and inspection traps
Calling “passivate to medical grade” without a spec, asking for certs you have not named, or requiring passivation after laser mark without saying whether the mark is before or after chemistry, all force **needs_manual**. Default non-critical envelope to ISO 2768-m; passivation itself is not a dimensional callout, but any electropolish substitute would be. Related: [laser marking before vs after anodize](https://3dwk.com/resources/laser-marking-before-vs-after-anodize) for the sibling habit of naming mark stage (here: mark vs passivate sequence), and [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Illustration 1 shows an **Example part** stainless bracket with a labeled post-CNC passivation note versus a silent “clean stainless” cloud.
Example part walkthrough
**Example part:** Stainless 316 bracket ~90×40×8 mm, as-milled faces, two clearance slots, ISO 2768-m general. With STEP + **PASSIVATE AFTER CNC PER ASTM A967 CITRIC; NO ELECTROPOLISH; BEAD BLAST OPTIONAL BEFORE PASSIVATE — CALL OUT IF USED**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “stainless, medical finish” often lands DFM notes or **needs_manual**—without inventing any customer PO or certificate.
Illustration 2 contrasts a vague RFQ (“passivate if needed / medical grade”) versus a labeled spec-and-sequence pack and the clearer estimate path.
What widens bands fastest
Unnamed “medical passivation,” mixing passivate with electropolish on the same line, blast-after-passivate with no redo note, and cert language that the RFQ never names. Quantity breaks help unit minutes but do not erase chemistry ambiguity—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost).
What to do next
Decide **passivate or as-machined**, name the specification and sequence (blast/weld/mark), then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the chemistry questions rather than forcing a fake firm price. This page does not claim a factory certificate—only that naming the spec keeps the quote path honest.
Common questions
Does every CNC stainless part need passivation?
No. Specify it when the print, environment or named spec requires it. As-machined stainless is a valid RFQ choice when you say so.
Is passivation the same as electropolishing?
No. Passivation is a chemical cleanup of free iron; electropolish removes metal and changes appearance/edges. Pick one primary process per surface family.
Where do I put the spec name?
On the drawing notes and in the RFQ text (for example ASTM A967 or AMS 2700 language that your print actually uses). “Passivate to medical” is not a spec.
What if I also bead-blast?
Write the order: usually machine → blast → passivate. Silent order leaves iron-smear risk and a wide **rates.v1.1** band.