Introduction to Surface NDT Methods
Eddy current testing and magnetic particle testing are essential for detecting surface and near-surface defects. Both methods are widely used in the steel industry for quality control and inspection applications.
Testing Principle Comparison
Eddy Current Testing (ECT)
Eddy currents are induced electrical currents generated in conductive materials by an alternating magnetic field. Discontinuities affect the flow of these currents, indicating defects.
Magnetic Particle Testing (MT)
Magnetic particles are applied to a magnetized surface. Defects create magnetic flux leakage that attracts and holds particles, forming visible indications.
Detection Capability Comparison
| Aspect | Eddy Current Testing | Magnetic Particle Testing |
|---|---|---|
| Defect Detection | ||
| Surface cracks | Excellent | Excellent |
| Near-surface defects | Good (to ~6mm) | Limited (~2mm) |
| Subsurface defects | Poor | Not detected |
| Orientation sensitivity | Directional | Perpendicular to field |
Applicable Materials
| Material | Eddy Current | Magnetic Particle |
|---|---|---|
| Carbon Steel | Yes (conductive) | Yes (ferromagnetic) |
| Stainless Steel (Austenitic) | Yes (conductive) | No (non-magnetic) |
| Stainless Steel (Martensitic) | Yes | Yes |
| Alloy Steel | Yes | Yes (ferromagnetic) |
| Aluminum | Yes | No (non-magnetic) |
| Copper | Yes | No |
Detection Depth Comparison
| Method | Surface Detection | Near-Surface | Subsurface |
|---|---|---|---|
| ECT (Standard) | ✓ Excellent | ✓ Good (1-3mm) | ✗ Limited |
| ECT (Low Frequency) | ✓ Good | ✓ Excellent (3-6mm) | ✗ Poor |
| MT (Wet) | ✓ Excellent | ✓ Fair (0.5-1mm) | ✗ None |
| MT (Dry) | ✓ Excellent | ✓ Fair (1-2mm) | ✗ None |
Test Parameters
Eddy Current Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Frequency | 1 kHz – 1 MHz | Higher = surface only |
| Probe Types | Absolute, differential, reflection | For different applications |
| Scan Speed | 25-150 mm/s | Visual inspection speed |
| Coverage | 100% with overlapping scans | Standard requirement |
Magnetic Particle Parameters
| Parameter | Typical Value | Notes |
|---|---|---|
| Magnetizing Force | 800-4,000 A/m | Per ASTM E709 |
| Particle Size | 1-50 μm | Wet: 1-10 μm |
| Application | Continuous or residual | Continuous preferred |
Equipment Comparison
| Aspect | Eddy Current | Magnetic Particle |
|---|---|---|
| Equipment Types | ||
| Portable | Instrument with probe | Yoke, prod, or coil |
| Automated | Multi-probe systems | Conveyor systems |
| Consumables | ||
| Type | Couplant (optional) | Particles, inks, liquid |
| Cost | Low | Moderate |
Advantages and Limitations
Eddy Current Advantages
- Works on all conductive materials
- Quantitative measurements possible
- No demagnetization required
- Can measure coating thickness
- Fast scanning possible
- Digital records achievable
Eddy Current Limitations
- Limited to conductive materials
- Interpretation requires skill
- Depth limitation
- Surface preparation needed
Magnetic Particle Advantages
- Simple to perform
- Visual indication of defects
- Works on ferromagnetic materials
- Minimal surface preparation
- Large areas covered quickly
Magnetic Particle Limitations
- Ferromagnetic materials only
- Demagnetization may be needed
- Coating may interfere
- Subjective interpretation
- Environmental concerns (particles)
Application Scenarios
Eddy Current Testing Applications:
- Weld inspection (surface)
- Tube inspection (heat exchangers)
- Coating thickness measurement
- Crack detection on conductive surfaces
- Stainless steel component inspection
- Aluminum alloy inspection
Magnetic Particle Testing Applications:
- Weld inspection (carbon/low-alloy steel)
- Forgings and castings inspection
- Pipeline girth weld inspection
- Heavy equipment inspection
- Field inspection of steel structures
- Maintenance inspection
Standards Compliance
| Standard | Eddy Current | Magnetic Particle |
|---|---|---|
| ASTM | E426, E1001, E1316 | E709, E1444, E1316 |
| ASME | Section V Article 8 | Section V Article 7 |
| API | 1104 (as allowed) | 1104, 650 |
| ISO | 15549 | 9934 |
NDT Capabilities on CREATEEL Pipe Orders
Our quality control labs perform both electromagnetic (eddy current) and magnetic particle testing inline and as final inspection. Test type, calibration standard, and acceptance level can be specified on your order — and results are recorded on the MTC.
| Test | Standard | Detects | Coverage at CREATEEL |
|---|---|---|---|
| Eddy Current (ET) | EN 10246-2 / ISO 9302, ASTM E309, API 5L E.2 | Weld seam & body defects — laps, seams, inclusions, holes | Inline 100% on all ERW/HFW tube; notched reference per L2/L3 levels |
| Magnetic Particle (MT) | EN 10246-7 / ISO 9304, ASTM E709, ASME V Art.7 | Surface & near-surface cracks — transverse/longitudinal | Pipe ends, weld seams, forged bar & machined parts on request |
| Ultrasonic (UT) | EN 10246-3 / ISO 9303, API 5L E.4, ASTM A388 | Internal defects, wall thickness, laminations | 100% weld UT on PSL2 LSAW; body UT on seamless ≥100 mm |
| Hydrostatic | Per product standard (API 5L 9.5, ASTM A530) | Through-wall leaks, pressure integrity | 100% on every pipe — test pressure and duration on MTC |
Questions We Get from Customers
Q1: Can eddy current detect subsurface defects?
A: Eddy current can detect near-surface defects (1-6mm depending on frequency), but has limited capability for deeper subsurface defects compared to ultrasonic testing.
Q2: Why can’t magnetic particle testing be used on stainless steel?
A: Austenitic stainless steel is non-magnetic and cannot be magnetized sufficiently for magnetic particle testing. Eddy current is the appropriate method for stainless steel inspection.
Q3: What is the maximum coating thickness for MT?
A: Generally, coatings should be ≤ 0.05mm (2 mils) for effective MT. Thicker coatings may prevent particle accumulation at defect locations.
Q4: Is demagnetization required after MT?
A: Demagnetization may be required for components where residual magnetism could affect subsequent processing, electronic equipment, or magnetic particle inspection of other areas.
Q5: How do I choose between ECT and MT?
A: Use MT for ferromagnetic materials (carbon/low-alloy steels) when surface and near-surface detection is needed. Use ECT for non-magnetic materials (stainless, aluminum) or when coating thickness measurement is also required.
Need a Custom Quote for Your Project?
At CREATEEL International Limited, we supply steel pipes and related products to global buyers with full traceability, EN 10204 3.1/3.2 Mill Test Certificates, and third-party inspection support (SGS, BV, TUV). Send us your specification — grade, standard, size, quantity, and destination port — and we will respond with a competitive quotation within 24 hours.
