The inspection of stainless steel seamless pipes before their installation in high-purity gas pipeline systems aims to verify that the pipe material meets the quality standards, the surface cleanliness complies with requirements, and there are no defect risks. This is to prevent unqualified pipes from entering the installation process and causing gas pollution or leakage. The inspection needs to cover five core dimensions: material verification, appearance and size inspection, surface treatment quality inspection, non-destructive testing, and cleanliness inspection. The specific contents are as follows: 

I. Material Verification: Ensure that the grade and composition meet the design requirements 

Verification of Material Certificate 

Verify the material list provided by the pipe material supplier (Mill Test Report, MTR), and confirm that the stainless steel grade (such as 304L, 316L), chemical composition (carbon, chromium, nickel, molybdenum content), and mechanical properties (tensile strength, yield strength, elongation) comply with the corresponding standards (such as ASTM A269, GB/T 14976). 

Low-carbon stainless steel (304L/316L) should be carefully checked to ensure that the carbon content is no more than 0.03%, in order to prevent intergranular corrosion. 

On-site material re-inspection 

Samples of the batch of pipes are taken for testing. The surface chemical composition is detected using a spectrometer (portable XRF) to ensure the correct brand name and avoid the mixing of pipes of different brands. For special stainless steels (such as 316Ti), the titanium content needs to be additionally tested. 

II. Appearance and Dimension Accuracy Inspection: Ensuring Installation Compatibility 

Appearance defect inspection 

Examine the outer surface of the pipe with the naked eye or using a magnifying glass. There should be no cracks, folds, scratches, pockmarks, oxide scale, rust, or other defects. The inner wall must be smooth and clean, without burrs, pits, or foreign residue. 

If there are minor scratches (with a depth of ≤ 5% of the wall thickness), they can be repaired by polishing; for severely defective pipes, they should be directly removed. 

Measurement of size parameters 

Using tools such as calipers, micrometers, and wall thickness gauges, the following key dimensions are to be measured. The deviations must comply with the standard requirements. 

Outer diameter and wall thickness: The deviation should be ≤ ±0.1mm (for cold-drawn tubes), ensuring compatibility with fittings such as joints and valves; 

Roundness: The roundness deviation of the pipe cross-section should be ≤ 0.2mm to avoid uneven time gaps during the assembly and welding process. 

Straightness: The deviation of straightness within each meter length should be ≤ 1mm to prevent the pipeline from deforming after installation. 

III. Surface Treatment Quality Inspection: Core Guarantee of High Purity and Cleanliness 

The stainless steel pipes used for high-purity gas pipelines are usually treated with acid washing and passivation (AP) or electrolytic polishing (EP). The effectiveness of the surface treatment needs to be verified: 

Detection of the integrity of the passivation film 

Using the blue dot method: Drop a solution of potassium ferrocyanide on the surface of the pipe. If no blue spots appear within 3 minutes, it indicates that the passivation film is dense and intact; if blue spots appear, it means that the passivation film has defects and needs to be re-passivated. 

For high-end scenarios, electrochemical impedance spectroscopy (EIS) can be used to measure the thickness of the passivation film (typically requiring a thickness of ≥ 50 nm). 

Inspection of inner wall smoothness 

Measure the Ra value of the inner wall using a surface roughness tester: 

Pickling and passivation pipe: Ra ≤ 0.8 μm; 

Electrolytic polishing pipe: Ra ≤ 0.2 μm (The Ra value for electronic-grade high-purity gas pipelines should be ≤ 0.1 μm). 

Surface contamination detection 

Dip a lint-free cloth in high-purity alcohol or acetone, and wipe the inner wall of the pipe. Observe whether the lint-free cloth has any stains or metal particles. Alternatively, you can use an ion chromatograph to detect the residual metal ions in the wiping solution to ensure it meets the requirements of the high-purity system. 

IV. Non-destructive Testing: Identifying Internal Hidden Defects 

For metallurgical defects in seamless pipes (such as pores, shrinkage cavities, and interlayers), sampling non-destructive testing is required: 

Eddy Current Testing (ET) 

It is suitable for detecting surface and near-surface defects of pipe materials. It is a routine inspection item for seamless pipes upon leaving the factory. Before installation, key parts can be re-inspected to ensure there are no cracks, inclusions, or other issues. 

Ultrasonic Testing (UT) 

For pipes with a wall thickness of 3mm or more, ultrasonic testing can be used to detect internal defects, ensuring that the internal structure of the pipe is dense and free from potential problems such as layering and shrinkage cavities. 

Notes for Attention   

During the inspection process, it is necessary to avoid damaging the passivation film on the surface of the pipe material; after the inspection, the pipe ports need to be sealed to prevent impurities from entering. 

V. Packaging and Cleanliness Inspection: Preventing Pollution During Transportation and Storage 

Packaging integrity inspection 

Check whether the factory packaging of the pipe materials is vacuum-sealed packaging or packaging filled with high-purity nitrogen gas for protection. If the packaging is damaged, the pipe materials need to be re-cleaned and p assivated before being tested. 

Residual detection in the pipe 

Randomly select the pipe materials, remove the end sealing, use high-purity nitrogen gas to blow the inner wall, collect the blown-off gas, and detect the particle content (the number of particles ≥ 0.5 μm should be ≤ 100 per m³) using a particle counter; at the same time, measure the humidity inside the pipe to ensure that the relative humidity is ≤ 30%, to prevent the inner wall from getting damp and rusting. 

VI. Criteria for Determining Compliance of Tests 

Only when all the following conditions are met can the pipe materials proceed to the installation stage: 

The material, size and mechanical properties all meet the design standards. 

The appearance is free from defects and the surface treatment meets the standard. 

Non-destructive testing reveals no hidden defects. 

The cleanliness and humidity inside the pipe meet the requirements for the transportation of high-purity gases.