How to detect the quality of passivation film on sanitary stainless steel pipes

Core conclusion: To assess the quality of the passivation film on sanitary-grade stainless steel pipes, it is necessary to focus on four key indicators: compactness, integrity, corrosion resistance, and purity of composition. The assessment can be divided into two categories: "routine rapid testing" and "precise laboratory testing", and the appropriate method should be selected based on the on-site screening or compliance verification scenario. 

I. Routine Rapid Testing (On-site / Acceptance Scenarios, Simple Operation)

1. Appearance and Surface Condition Inspection

Principle: The basic integrity of the passivation film is judged through visual and tactile inspection. The absence of defects is a prerequisite for quality compliance.

Operation: Illuminate the pipe surface with white light and observe for rust spots, color spots, scratches, and residual oxide scale; wipe with a clean white cloth and check for black dust or stains.

Judgment: A uniform and bright surface without rust or obvious processing marks, and no contamination on the white cloth after wiping, is considered qualified.

2. Water Drop Contact Angle Test (Density Screening)

Principle: The denser the passivation film, the stronger the surface hydrophobicity, and the larger the water drop contact angle.

Operation: Use an injector to drop 1-2 drops of deionized water on the pipe surface and measure the angle between the water drop and the surface with a contact angle meter (or a simple protractor).

Judgment: A contact angle of ≥85° is qualified, and ≥100° indicates excellent density (artificial passivation films usually meet this standard, while natural films are often <70°).

3. Blue Spot Test (Rapid Verification of Chromium Content)

Principle: Cr³⁺ in the passivation film reacts with the blue spot reagent (potassium ferricyanide + nitric acid) to form blue precipitate. The absence of blue spots indicates sufficient chromium content and intact film.

Operation: Apply the reagent to the pipe surface and let it stand for 5-10 minutes, then observe for blue spots.

Judgment: No blue spots within 5 minutes is qualified; the presence of blue spots indicates insufficient passivation and chromium deficiency in the film.

4. Rapid Salt Spray Test (Initial Screening of Corrosion Resistance)

Principle: Simulate a humid corrosion environment to accelerate the inspection of the protective ability of the passivation film.

Operation: Place the sample in a simple salt spray chamber and spray with a 5% sodium chloride solution at room temperature for 24-48 hours.

Judgment: No red rust or pitting within 48 hours, with only slight color difference, is qualified; obvious rust indicates film failure. 

II. Precise Laboratory Testing (Compliance / High-Requirement Scenarios, Data Support)

1. X-ray Photoelectron Spectroscopy (XPS) Testing (Composition + Thickness)

Principle: By using X-rays to excite surface elements, the chemical composition and thickness of the passivation film are analyzed.

Core Indicators: Cr/O atomic ratio ≥ 1.3 (indicating sufficient Cr₂O₃ content), film thickness 5-15nm (standard range for artificial passivation).

Applicable Scenarios: Compliance audits in the pharmaceutical and food industries, requiring reports on composition and thickness testing.

2. Polarization Curve Testing (Precise Corrosion Resistance Assessment)

Principle: Through electrochemical methods, the corrosion current and pitting potential of the passivation film are measured to quantify its protective ability.

Core Indicators: Pitting potential ≥ 0.5V (vs SCE), corrosion current density ≤ 1×10⁻⁶A/cm².

Judgment: The higher the pitting potential, the stronger the corrosion resistance (typically ≥ 0.6V for 316L artificial passivation films).

3. Ellipsometry (Precise Thickness Measurement)

Principle: Utilizing the polarization reflection characteristics of light to precisely measure the thickness of nanoscale passivation films.

Advantages: Accuracy up to ±0.1nm, more suitable for thickness measurement alone compared to XPS, avoiding interference from composition analysis.

Applicable Scenarios: High-end medical and semiconductor industries with strict thickness requirements.

4. Microbial Adhesion Testing (Hygiene Verification)

Principle: Simulating usage scenarios to test the ability of the passivation film surface to support microbial growth.

Operation: Immerse the sample in sterile nutrient solution, inoculate common bacteria such as E. coli, and count after 24 hours of incubation.

Judgment: ≤ 10CFU/cm² is qualified, and ≤ 1CFU/cm² is required for high-end hygiene scenarios. 

III. Testing Priorities and Scenario Matching

On-site Acceptance: Prioritize "Appearance Inspection + Blue Dot Test + Water Drop Contact Angle" for a quick screening within 30 minutes.

Conventional Compliance: Add "48-hour Salt Spray Test + XPS Composition Analysis" to meet the basic requirements of the food and pharmaceutical industries.

High-end Scenarios (Vaccines, Injection Water): Complete "XPS + Polarization Curve + Ellipsometry + Microbial Adhesion Test" for comprehensive quality verification.