A stainless steel pipe and a carbon steel pipe painted silver look the same. A 316L flange and a 304 flange look the same. That is why buyers insist on PMI testing — positive material identification — before they accept alloy material. For exporters, offering PMI is now a basic expectation in oil and gas, chemical and power projects.
What is PMI testing?
PMI is the quick, usually non-destructive analysis of a metal's chemical composition to confirm that it is the grade claimed. Instead of trusting the marking or the certificate alone, the inspector measures key alloying elements — chromium, nickel, molybdenum, manganese, copper, titanium, niobium — directly on the piece.
PMI does not replace the mill certificate. It verifies that the item in hand matches it.
The two main PMI methods
1. XRF (X-ray fluorescence)
A handheld XRF analyser fires X-rays at the surface. Each element emits fluorescent X-rays at characteristic energies, which the instrument reads to calculate composition.
Advantages:
- truly non-destructive — no marks on the surface,
- fast: a reading in a few seconds,
- portable and easy to use on site, in stockyards and in containers.
Limitations:
- cannot measure carbon and struggles with other light elements,
- needs a clean, bare-metal surface,
- cannot distinguish grades that differ only in carbon, such as 316 vs 316L or 304 vs 304H.
2. OES (optical emission spectroscopy)
A mobile spark OES unit creates a small electrical spark on the surface, vaporising a tiny amount of metal. The light emitted is analysed for composition.
Advantages:
- measures carbon, as well as sulphur, phosphorus and other light elements,
- distinguishes L and H grades and carbon steel grades,
- laboratory-level accuracy.
Limitations:
- leaves a small burn mark that may need grinding,
- heavier equipment, slower per reading,
- requires argon and more operator skill.
XRF vs OES at a glance
| Point | XRF | OES |
|---|---|---|
| Carbon measurement | No | Yes |
| Surface damage | None | Small burn spot |
| Speed | Seconds | About a minute |
| Portability | Handheld | Mobile trolley |
| Best for | Stainless, nickel alloys, sorting | Carbon steel, L/H grades, low alloy |
When do buyers require PMI?
PMI is common in:
- refineries, petrochemical and LNG projects,
- offshore and subsea work,
- power plants (P11, P22, P91 alloy steels),
- chemical plants handling acids and chlorides,
- nickel alloy and duplex orders, where substitution risk is high.
Industry guidance such as API RP 578 (material verification for alloy piping) and ASTM E1476 (guide for metals identification and sorting) is often referenced in specifications. Many project specs demand 100% PMI — every pipe, fitting, flange and weld — for alloy materials.
How to prepare for PMI
- Clean the test spot. Remove paint, coating, oil and scale. Grind lightly if needed.
- Mark items clearly so every piece can be matched to a heat number. See our heat number traceability guide.
- Lay out material so inspectors can reach every item — especially in crates.
- Keep certificates ready to compare readings against specified ranges.
- Check instrument calibration and keep reference standards available.
Reporting PMI results
A good PMI report includes:
- report number, date, location and inspector name,
- instrument make, model, serial number and calibration date,
- item description, size, heat number and quantity tested,
- measured values of key elements,
- specified range and acceptance result (accept/reject),
- signature of the inspector and, if present, third-party witness.
Attach the PMI report to your EN 10204 3.1 certificate pack. If the buyer requires a third-party witness, see EN 10204 3.1 vs 3.2 and our third-party inspection guide.
Marking after PMI
Many buyers want each tested item marked — for example with a "PMI OK" sticker, paint dot or stamp — so PMI status is visible on site. Agree the marking method before inspection, and use chloride-free markers on stainless steel.
Cost of PMI
PMI by your own analyser costs only time. Third-party PMI is usually charged per day or per piece. Include the cost in your quotation as a separate line or "at actuals" so there are no surprises. QuoteMitra helps you build priced quotations with clear terms, including inspection lines like these.
Frequently asked questions
Can PMI detect 316L vs 316?
Not with XRF, because the difference is carbon content. OES can measure carbon and distinguish them.
Is PMI destructive?
XRF is non-destructive. OES leaves a small spark mark, which is usually acceptable or can be lightly ground.
Does PMI replace the mill certificate?
No. PMI verifies alloying elements on the actual item, while the certificate provides full chemistry and mechanical test results from the mill.
How accurate is handheld XRF?
For alloying elements in stainless steels and nickel alloys, modern analysers are accurate enough to identify grades reliably when the surface is clean and the instrument is calibrated.
Key takeaways
- PMI confirms the actual grade of each piece, not just the paperwork.
- XRF is fast and non-destructive but cannot read carbon.
- OES measures carbon and separates L and H grades.
- Prepare surfaces, traceability and certificates before the inspector arrives.
Offering PMI proactively tells buyers you have nothing to hide — one of the strongest trust signals a supplier can send.
