Passivation vs electropolish — when each is right for stainless steel

§ 01 / WHAT

What passivation does

Stainless steel's corrosion resistance comes from a thin chromium-oxide layer on the surface. Machining, grinding, welding, and handling can damage this layer — leaving free iron at the surface. Free iron rusts.

Passivation restores the chromium-oxide layer by:

Cleaning the surface (alkaline wash removes oils)
Acid immersion (nitric acid per A967 Type II, or citric acid per A967 Type VIII)
Water rinse
Natural oxide formation in air

The result: a fully-passive surface with maximum corrosion resistance. No material is removed (typically <0.5μm loss). Dimensional tolerances are unaffected. Surface finish unchanged.

Passivation is the default treatment for most machined stainless parts — especially after welding, grinding, or cutting operations that can contaminate the surface.

§ 02 / WHAT

What electropolish does

Electropolish is more aggressive: the part is immersed in an acid electrolyte and connected as the anode. Electrical current causes surface material to dissolve preferentially at peaks — smoothing the surface and leaving a near-mirror finish.

Typical material removal: 10-40 μm. This is significant:

Dimensions change (outside features shrink by 10-40 μm, inside features grow)
Surface roughness decreases substantially (Ra drops by 30-50% typically)
Micro-peaks are removed entirely
Surface becomes smoother and more cleanable than passivation achieves
Takes longer than passivation (30-60 min vs 10-15 min)

Use electropolish for: pharmaceutical, food-processing equipment, semiconductor fabrication, medical instruments. Anywhere cleanability and low bacterial attachment matter more than dimensional precision.

§ 03 / SIDE-BY-SIDE

Side-by-side comparison

Aspect	Passivation	Electropolish
Process type	Chemical (acid immersion)	Electrochemical (anodic dissolution)
Material removed	< 0.5 μm	10-40 μm
Dimensional impact	None (negligible)	Significant — must be designed in
Surface finish change	None	Dramatic improvement (Ra 50% reduction)
Corrosion resistance gain	Restores baseline	Restores baseline + smoother surface extends life
Cleanability	Good	Excellent (bacteria don't attach to smooth surfaces)
Time required	10-15 minutes	30-60 minutes
Cost (typical)	+5-15% of part cost	+20-40% of part cost
Process standard	ASTM A967	ASTM B912

§ 04 / WHEN

When to specify which

Specify passivation when:

Standard corrosion-resistant stainless parts (most industrial)
After welding (required to restore weld-zone passivity)
After grinding, polishing, or heavy machining
Medical devices where cleanability matters but surface roughness does not
Food-contact parts in standard applications

Specify electropolish when:

Pharmaceutical WFI (water for injection) systems
Semiconductor process equipment (wet bench, chemical delivery)
Implantable medical devices needing low bacterial attachment
Food-processing with validated cleanability (CIP systems)
Hygienic valves and fittings for biotech
When Ra < 0.8 μm is required on complex geometry that can't be polished mechanically

Design for electropolish

Electropolish removes 10-40 μm of material. Critical dimensions should be designed 20-40 μm oversized (external) or undersized (internal) to account for this. Specify the target final dimension AFTER electropolish on the drawing.

§ 05 / COMMON

Common FAQ

Is passivation required on stainless parts?

Not universally, but highly recommended. Machined, welded, or polished stainless surfaces can have embedded iron contamination that rusts. Passivation is cheap (10-15% cost addition) and always improves corrosion performance. Default to passivation unless there's a specific reason not to.

Can I electropolish after welding?

Yes — electropolish is often specified after welding for pharmaceutical and biotech applications. The material removal levels the weld bead and removes surface contamination from the weld zone.

What about stainless 17-4 PH or 15-5 PH?

Precipitation-hardened stainless steels can be passivated but not electropolished cleanly — the grain structure of PH steels causes non-uniform material removal. For PH stainless, passivation only; if electropolish is required, switch to austenitic grade (304, 316).

Does electropolish affect welds differently?

Welds have different surface chemistry (higher residual stress, some oxide formation, possible sensitization). Electropolish affects welds similarly to base metal, but may weld defects that were masked by original surface texture. For critical welded parts, inspect before electropolish.

READY WHEN YOU ARE

Stainless parts needing passivation or electropolish?

Email [email protected]. We handle both processes in-house on smaller parts; larger assemblies go through qualified partners with ASTM certifications on request.

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