5-axis CNC milling drawing: Al 7075-T6 block with pockets, toolpath and dimensions

Passivation

Remove free iron. Enhance corrosion resistance. ASTM A967.

Q: Why is passivation necessary on stainless?

Machining, welding, and handling leave free iron on stainless surfaces. This iron rusts quickly, even on 316L stainless. The rust doesn't penetrate the part but appears as surface staining — problematic for cosmetic, medical, and food applications. Passivation dissolves this surface iron and enables the chromium oxide passive layer to fully form. Without passivation, stainless parts may exhibit "tea staining" or visible surface rust within days.

Q: Nitric vs citric acid — which to specify?

Both comply with ASTM A967. Nitric: traditional, fastest acting, well-understood, slight fume concerns. Citric: newer, environmentally friendlier, safer for workers, slightly slower. For general industrial: nitric is standard and cheaper. For pharmaceutical, food, or environmentally-certified operations: citric is preferred. Technical result equivalent — both remove free iron and enhance passive layer.

Q: How is passivation verified?

Multiple tests per ASTM A967: (1) Water break test — water sheets evenly on properly passivated surface. (2) Copper sulfate test — no copper deposition on properly passivated surface. (3) High-humidity exposure — no rust after 24 hours at 95% humidity. (4) Salt spray test — 2 hours ASTM B117 without rust. We provide documentation with method used and test results on request.

Q: Passivation vs electropolishing — do I need both?

Electropolishing includes passivation as part of its process — electropolished parts don't need separate passivation. Passivation without electropolishing leaves the mechanical finish intact but enhances corrosion resistance. For maximum corrosion resistance and cleanliness (pharmaceutical, BPE, critical medical), specify electropolish. For standard corrosion enhancement, passivation alone is adequate and cheaper.

Q: Is passivation always needed?

For critical applications (medical, pharma, food, aerospace, marine), yes — specify explicitly. For general industrial stainless in indoor dry environments, maybe not — the thin surface iron layer may never rust under benign conditions. When in doubt, passivate — the cost is low and the insurance against field rust is high.

Q: Lead times?

Passivation is a 1-day process. Added to overall lead time without significant impact. For urgent orders, same-day passivation is typical after CNC completion. Rack processing enables high-volume passivation — 100+ parts in single bath cycle. Documentation packages (method used, test results) add 0–1 day.

Stainless steel passivation removes surface contamination (free iron from tooling, embedded particles) and enhances the naturally-forming chromium oxide passive layer. Standard for medical, food, aerospace, and corrosion-critical stainless applications.

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Technical plate for Remove free iron. Enhance corrosion resistance. ASTM A967.

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Passivation services

3-7 daysTypical lead time

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01 · What it is

How Passivation works.

Stainless steel is corrosion-resistant because a chromium oxide "passive layer" forms on the surface, protecting the metal from further corrosion. During manufacturing (machining, grinding, handling), this passive layer can be disrupted: tool steel tooling leaves embedded iron particles, heat affects the layer during welding, grinding smears surface material. Parts with damaged passive layer will rust — even though they're technically stainless steel.

Passivation is a chemical treatment that: (1) dissolves free iron and other contaminants on the surface, (2) allows the chromium oxide passive layer to fully reform and stabilize, (3) provides documented verification that the surface meets corrosion-resistance requirements.

Two primary passivation chemistries: nitric acid (traditional, per ASTM A967 Nitric 1–5, most common), and citric acid (newer, environmentally friendlier, per ASTM A967 Citric 1–4). Both achieve similar results; citric is preferred when worker safety and environmental factors matter.

02 · Specifications

Capability specs.

American standard for passivation of stainless steel parts

Aerospace passivation per AMS 2700 — Nitric 1–5 or Citric 1–4 methods

Typical bath temperatures depending on acid concentration and steel grade

Varies by method. Longer for heavier contamination

Eliminates embedded iron from machining, grinding, forming operations

Water break test, copper sulfate test, salt spray (per method)

All common stainless grades. 300-series and precipitation-hardening grades

Passivation included with stainless parts at no additional charge

03 · Applications

Where Passivation excels.

Medical devices

Surgical instruments, implants — per ASTM F86 medical passivation requirements

Food processing

Sanitary food contact surfaces — 3-A Sanitary Standards compliance

Pharmaceutical

BPE-compliant surface treatment before final qualification

Aerospace

Per AMS 2700 for aerospace stainless components — flight-qualified passivation

Marine hardware

Enhanced salt-water corrosion resistance for marine stainless applications

Chemical processing

Equipment for chemical service — critical chromium oxide layer protection

Semiconductor

Passivated 316L for semiconductor vacuum chambers and process equipment

Precision instruments

Scientific instrument stainless components — measurement reliability

High-volume production

Rack passivation for batch processing of production parts

04 · When not to use it

Not suitable for:

Every process has its limits. Being honest about where Passivation isn\'t the right answer saves time and money.

Non-stainless materials — carbon steel, aluminum, titanium not passivated this way
Parts with significant carbon steel embedded (e.g., press-fit with carbon steel pins)
Very severely contaminated parts — may need pickling first, then passivation
Parts where surface material removal would exceed tolerance — passivation removes very little
As a substitute for electropolishing — passivation enhances but doesn't polish

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FAQ

Remove free iron — questions

Why is passivation necessary on stainless?

Machining, welding, and handling leave free iron on stainless surfaces. This iron rusts quickly, even on 316L stainless. The rust doesn't penetrate the part but appears as surface staining — problematic for cosmetic, medical, and food applications. Passivation dissolves this surface iron and enables the chromium oxide passive layer to fully form. Without passivation, stainless parts may exhibit "tea staining" or visible surface rust within days.

Nitric vs citric acid — which to specify?

Both comply with ASTM A967. Nitric: traditional, fastest acting, well-understood, slight fume concerns. Citric: newer, environmentally friendlier, safer for workers, slightly slower. For general industrial: nitric is standard and cheaper. For pharmaceutical, food, or environmentally-certified operations: citric is preferred. Technical result equivalent — both remove free iron and enhance passive layer.

How is passivation verified?

Multiple tests per ASTM A967: (1) Water break test — water sheets evenly on properly passivated surface. (2) Copper sulfate test — no copper deposition on properly passivated surface. (3) High-humidity exposure — no rust after 24 hours at 95% humidity. (4) Salt spray test — 2 hours ASTM B117 without rust. We provide documentation with method used and test results on request.

Passivation vs electropolishing — do I need both?

Electropolishing includes passivation as part of its process — electropolished parts don't need separate passivation. Passivation without electropolishing leaves the mechanical finish intact but enhances corrosion resistance. For maximum corrosion resistance and cleanliness (pharmaceutical, BPE, critical medical), specify electropolish. For standard corrosion enhancement, passivation alone is adequate and cheaper.

Is passivation always needed?

For critical applications (medical, pharma, food, aerospace, marine), yes — specify explicitly. For general industrial stainless in indoor dry environments, maybe not — the thin surface iron layer may never rust under benign conditions. When in doubt, passivate — the cost is low and the insurance against field rust is high.

Lead times?

Passivation is a 1-day process. Added to overall lead time without significant impact. For urgent orders, same-day passivation is typical after CNC completion. Rack processing enables high-volume passivation — 100+ parts in single bath cycle. Documentation packages (method used, test results) add 0–1 day.

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