In the application of stainless steel seamless pipes in high-purity gas pipelines, the core of surface treatment quality inspection is to verify the integrity of the passivation film, the smoothness of the inner wall, and the cleanliness of the surface. The specific methods are divided into physical testing and on-site rapid testing. The following is a structured and detailed description: 

I. Passivation Film integrity Testing (Core indicators, Preventing Corrosion and ion leaching) 

The passivation film (Cr₂O₃) is crucial for the corrosion resistance of stainless steel. Its density and integrity need to be tested. Common methods include: 

Blue dot method (on-site rapid testing) 

Principle: Iron ions react with potassium ferrocyanide to form a blue precipitate. If there are defects in the passivation film, the exposed iron substrate reacts with the reagent to produce a color change; if the passivation film is intact, the reagent cannot penetrate and no blue spots will appear. 

Operation steps

① Prepare the detection solution: Mix 5g of potassium ferricyanide + 10mL of concentrated hydrochloric acid + 100mL of distilled water evenly; 

② Use a dust-free cloth to wipe the surface of the pipe, remove the oil stains, and then add the test solution. 

③ Observe for 3 to 5 minutes. If there are no blue spots on the surface, it indicates that the passivation film is intact; if blue spots appear, the passivation process needs to be repeated. 

Applicable scenarios: On-site sampling inspection, rapid assessment of passivation effect, suitable for acid pickling passivation (AP) pipe testing. 

Electrochemical impedance spectroscopy (EIS, precise laboratory testing) 

Principle: By applying a small alternating voltage, the impedance value of the passivation film is measured. The higher the impedance, the denser and thicker the passivation film is. 

Operation steps: Place the pipe material as the working electrode, immerse it in the electrolyte solution (such as 3.5% NaCl solution), connect the electrochemical workstation, test the impedance spectrum and analyze the data. 

Criteria for determination: The impedance value of the passivation film on high-purity gas stainless steel tubes usually needs to be ≥ 10⁶ Ω・cm², and the film thickness needs to be ≥ 50 nm. 

Applicable scenarios: Factory inspection of electronic-grade and ultra-high-purity gas pipelines, for precise assessment of the performance of the passivation film. 

Copper sulfate drop test (auxiliary detection) 

Principle: When copper sulfate solution reacts with an unpassivated iron substrate, red elemental copper is produced. There is no reaction when the passivation film is intact. 

Operation: Add 5% copper sulfate solution to the surface of the pipe, observe for 10 minutes. If no red spots are observed, it is considered qualified. 

Note: This method has a lower sensitivity than the blue dot method and is mainly used for rough screening. 

II. Inspection of Inner Wall Smoothness (Reducing particle retention and ensuring gas cleanliness) 

The degree of smoothness is measured by the surface roughness Ra value. The detection methods include contact type and non-contact type: 

Surface roughness tester inspection (contact type, precise measurement) 

Principle: By using a probe to glide over the inner wall of the pipe, the fluctuating data of the surface's unevenness is recorded, and the Ra (Arithmetic Mean Deviation of Profile) value is calculated. 

Operation steps

① Cut the pipe sample to ensure that the end face is flat and the inner wall is free of impurities. 

② Place the probe of the roughness meter lightly against the inner wall, and measure 3 to 5 points along the axial and circumferential directions respectively. 

③ Criteria for determination: The Ra value of the acid pickling and passivation tubes should be ≤ 0.8 μm; for the electrolytic polishing (EP) tubes, it should be ≤ 0.2 μm, and for electronic-grade pipes, it should be ≤ 0.1 μm. 

Note: Before measurement, the inner wall should be cleaned with high-purity alcohol to avoid impurities affecting the accuracy of the probe. 

Laser confocal microscopy detection (non-contact, visual analysis) 

Principle: By using laser scanning to scan the inner wall surface, a three-dimensional contour image is generated, allowing for an intuitive observation of the surface flatness and microscopic defects. 

Applicable scenarios: Quality verification of high-end electrolytic polishing tubes, capable of simultaneously analyzing surface microstructures (such as scratches, pits). 

III. Surface Cleanliness Inspection (Preventing Impurities from Polluting High-Purity Gases) 

The cleanliness needs to be tested for three indicators: particle residue, metal ion residue, and organic matter residue. The methods are as follows: 

Granularity measurement (key indicator) 

Dry blowing method 

① Blow the inner wall of the pipe with high-purity nitrogen (purity ≥99.999%) at a fixed flow rate for no less than 30 minutes. 

② Use a particle counter to collect the particles in the purging gas, and count the number of particles with a size of ≥0.5 μm and ≥5 μm. 

③ Criteria for determination: The ultra-high purity gas pipeline must have a particle size of no more than 0.5 μm and no more than 100 particles per cubic meter, and there must be no large particles with a size of ≥ 5 μm. 

Wet wiping method 

① Use a lint-free cloth dipped in high-purity deionized water to wipe the inner wall of the pipe. 

② Place the dust-free cloth that has been wiped in deionized water for ultrasonic cleaning, and use a particle counter to measure the number of particles in the cleaning solution. 

Detection of metal ion residues (Laboratory test) 

Principle: Using an inductively coupled plasma mass spectrometer (ICP-MS) to detect residual metal ions such as iron, copper, and zinc on the surface, avoiding ion dissolution and contamination of the gas. 

Operation: Soak the inner wall of the pipe in a high-purity nitric acid solution. After extracting the soaking solution, analyze it using ICP-MS. The ion concentration should be ≤ 10 ppb (one part per billion). 

Applicable scenario: Factory acceptance inspection of high-purity pipelines used in semiconductors and biomedicine. 

Detection of Organic Residues 

Gas Chromatography - Mass Spectrometry (GC-MS) analysis 

① Wipe the inner wall with high-purity methanol or acetone and collect the wiping liquid. 

② The organic substances (such as oil stains and residual solvents) in the wiping solution shall be analyzed by GC-MS. The total organic content should be ≤ 50 ppb. 

On-site rapid testing: Scan the surface of the pipe with an organic volatile compound (VOC) detector. If no obvious VOC signal is detected, the pipe is considered合格. 

IV. Key Points for Testing Different Surface Treatment Processes