Positive Material Identification (PMI) in NDT – Complete Guide

What is PMI?

Positive Material Identification (PMI) is a Non-Destructive Testing (NDT) and inspection method used to verify the chemical composition of metals and alloys.
It ensures that the correct material has been used as per design specifications, preventing material mix-ups that could lead to failures in critical industries.

Principle of PMI

PMI works by analyzing the chemical elements present in a material using advanced techniques:

• When materials are exposed to X-rays or spark excitation, they emit characteristic signals.

• These signals are detected and analyzed to identify the type and concentration of elements.

• This confirms whether the material grade is correct (e.g., SS304 vs SS316).

Techniques of PMI

1. X-Ray Fluorescence (XRF):

   • Uses X-rays to excite atoms.

   • Identifies elements based on emitted fluorescent X-rays.

   • Portable and quick.

   • Cannot detect very light elements (C, Be, B).

2. Optical Emission Spectroscopy (OES):

   • Uses an electrical spark to excite atoms.

   • Measures light spectrum to identify elements.

   • Can detect light elements (including Carbon, Phosphorus, Sulfur).

   • More accurate for carbon steels and alloy steels.

Step-by-Step Procedure

1. Surface Preparation – Clean the test area (remove coatings, rust, dirt).

2. Instrument Setup – Select XRF or OES instrument as required.

3. Measurement – Place the probe or spark head on the material.

4. Analysis – Instrument detects and analyzes chemical composition.

5. Result & Verification – Compare with specifications/standards.

6. Reporting – Document findings for traceability.

Applications of PMI

• Oil & Gas: Verification of pipelines, valves, fittings.

• Power Plants: Turbines, boilers, heat exchangers.

• Aerospace: Aircraft components requiring precise alloys.

• Petrochemical Industry: Pressure vessels, storage tanks.

• Fabrication & Construction: Material verification before welding/assembly.

Advantages

• Portable and quick (on-site analysis).

• Prevents costly material mix-ups.

• Ensures safety and compliance with standards.

• Detects alloy grades accurately.

• Suitable for 100% inspection in critical components.

Limitations

• XRF cannot detect light elements like carbon (so not suitable for differentiating low-carbon vs high-carbon steels).

• OES requires surface preparation and may leave small burn marks.

• Equipment is expensive and requires skilled operators.

Standards & Codes

• ASTM E1476 – Standard guide for PMI.

• API RP 578 – Material verification program for new & existing alloy piping systems.

• ASME Section II – Material specifications.

✅ Summary:
Positive Material Identification (PMI) is a critical NDT technique to confirm the right material is being used in critical industries. By ensuring alloy grades match specifications, PMI prevents failures, improves safety, and maintains regulatory compliance.