Passivation is a chemical immersion surface modification process that involves no additional metal plating and alters neither the part’s dimensions nor its surface luster; instead, it forms an ultra-thin, dense protective oxide film (1–5 nm) on the metal surface, primarily addressing the issue of stainless steel being prone to rusting after CNC machining.
During CNC turning, milling, drilling and tapping processes, hard cutting tools, carbon steel fixtures and iron-containing cutting fluid press free iron particles into the stainless steel surface, destroying the original chromium-rich passive film and causing rust spots in humid environments.
Passivation solution dissolves free iron on the surface and promotes the substrate chromium element to generate a dense Cr₂O₃ passive film, isolating moisture and chloride ions to restore the anti-corrosion performance of stainless steel.
Reagent: Nitric acid + minor hydrofluoric acid system
Advantages: Thorough iron removal, optimal salt spray performance; ideal for martensitic stainless steel 410/420 and high-strength 17-4PH.
Disadvantages: Strong corrosivity, strict environmental control, high waste liquid treatment cost.
Application: Aerospace, hydraulic and marine high-corrosion resistant stainless steel CNC parts.
Reagent: Food-grade chromium-free citric acid passivation solution
Advantages: Low corrosivity, full environmental compliance, heavy-metal-free, compliant with EU RoHS and FDA food & medical standards.
Disadvantages: Slightly lower salt spray performance than nitric acid process under standard immersion time; performance can be matched by extending immersion duration.
Application: 304/316 medical devices, food processing equipment and consumer electronic precision CNC parts.
Applied to parts with heavy oxide scale or welding discoloration: Pickling for oxide removal → Synchronous passivation. Precision milled CNC parts with smooth surface generally adopt independent passivation without pickling.
Degreasing: Alkaline cleaning or ultrasonic cleaning to thoroughly remove cutting fluids and anti-rust oils (oil contamination can cause localized failure of the passivation film)
Multi-stage pure water rinsing: To wash away alkaline solution and prevent contamination of the passivation bath
Activation (optional): Brief immersion in dilute nitric acid to open the surface metal lattice and enhance film adhesion
Passivation immersion
Multi-stage pure water overflow rinsing: To eliminate acid residue (residual acid can lead to subsequent pitting corrosion)
Low-temperature hot air drying: Drying at 60–80°C to prevent water spots
| Process | Film Thickness | Dimensional Change | Appearance | Primary Applications |
|---|---|---|---|---|
| Passivation | 1~5nm | No change | Retains natural metal color | Stainless steel corrosion resistance, precision parts, medical & food applications |
| Electropolishing | Micron-scale | Minimal material removal | High-gloss mirror finish | High-purity fluid system components, medical/aesthetic device parts |
| Anodizing | 5~25μm | Increases thickness | Dyeable | Wear-resistant and insulating (for aluminum alloys) |
| Blackening (Oxidation) | 1~3μm | Negligible dimensional change | Matte black | Low-cost rust prevention for carbon steel |
| Nickel/Chrome Plating | 5~50μm | Increased thickness | Bright finish | Wear-resistant decorative coating; sacrificial corrosion protection |
Localized rust spots; failure to meet salt spray test standards
Grayish surface appearance and color inconsistency
Loss of thread precision
Design recommendations: For CNC parts, avoid extremely small blind holes and deep, narrow gaps to ensure the flow of passivation fluid and thorough rinsing.
Medical device implants, food-grade fluid valve bodies, aerospace stainless steel fasteners, semiconductor vacuum chambers, marine seawater fittings, precision instrument stainless steel bushings, hydraulic valve blocks, and stainless steel automation fixtures.
