Differences Between Eddy Current Testing, Radiographic Testing, and Solution Passivation for Stainless Steel Pipes

In modern industries such as shipbuilding, oil & gas, and chemical processing, stainless steel pipe testing plays a critical role in ensuring safety and long-term performance. Testing not only verifies compliance with international standards but also guarantees that the pipes can withstand corrosion, high temperatures, and pressure.

When it comes to ensuring both quality and performance of stainless steel pipes, three processes often come into discussion: eddy current testing (ET), radiographic testing (RT), and solution passivation. While ET and RT are non-destructive testing (NDT) methods designed to detect defects, solution passivation is a heat treatment and surface treatment process aimed at improving corrosion resistance and restoring mechanical properties.

At Ganyeah Group, we integrate these processes within our production and quality assurance system to deliver stainless steel pipes that not only meet but exceed international standards for safety, durability, and corrosion resistance.

Stainless steel pipes after quality inspection

1. Eddy Current Testing (ET) – Surface and Near-Surface Defect Detection

Principle:
ET is based on electromagnetic induction. A coil carrying alternating current induces eddy currents in the pipe surface. Any discontinuity, such as cracks, scratches, or wall thinning, disturbs the eddy current flow, which can be detected as impedance changes.

Applications:

• Detecting surface or near-surface defects (typically ≤20% of wall thickness).
• Ideal for thin-wall stainless steel seamless pipes during continuous production.

Advantages:

• High-speed, real-time inspection.
• No couplant required (non-contact or light-contact method).
• Highly sensitive to micro-cracks.

Limitations:

• Limited to surface and near-surface flaws.
• Sensitive to pipe surface conditions (scale, oil, or oxide layers).

2. Radiographic Testing (RT) – Internal Defect Inspection

Principle:
RT uses X-rays or gamma rays to penetrate stainless steel. Since defects (voids, porosity, inclusions) have lower density than sound metal, they appear as distinct contrasts on film or digital imaging systems.

Applications:

• Detecting internal flaws and weld defects such as lack of fusion, porosity, and slag inclusions.
• Widely applied in thick-wall pipes and critical welded joints for oil, gas, chemical, and nuclear pipelines.

Advantages:

• Provides clear images of defect size, shape, and position.
• Effective for deep-wall inspections.
• Permanent records can be stored for traceability.

Limitations:

• Radiation hazards require strict safety measures.
• Slower and more costly compared to ET.
• Less sensitive to planar cracks aligned with the radiation beam.

3. Solution Passivation – Performance Enhancement

Principle:
This process combines solution annealing and passivation.

• Solution Annealing: Stainless steel pipes are heated to 1050–1150°C, held, then rapidly quenched to dissolve harmful carbides and prevent chromium depletion at grain boundaries.
• Passivation: Pipes are treated in acid solutions (e.g., nitric or citric acid) to form a dense chromium oxide film, enhancing corrosion resistance.

Applications:

• Commonly applied to 304, 316, and other austenitic stainless steel pipes.
• Essential for food-grade, pharmaceutical, and chemical piping systems.

Advantages:

• Improves corrosion resistance at the root level.
• Restores ductility and toughness after cold working.
• Produces a clean, passive surface layer for hygienic use.

Limitations:

• Energy-intensive heat treatment process.
• Cannot detect or repair existing structural defects.

4. Key Differences at a Glance

5. Combined Application in Production

At Ganyeah Group, these three processes are not seen as alternatives but as complementary measures:

• Solution Passivation first to ensure mechanical strength and corrosion resistance.
• Eddy Current Testing for fast and reliable surface inspection.
• Radiographic Testing for critical welded sections requiring deep defect detection.

By integrating advanced stainless steel pipe testing and treatment processes, Ganyeah Group guarantees pipes that meet stringent standards for safety, hygiene, and long-term reliability.