Article
Positive material identification, or PMI, is a quick check that a piece of metal is the alloy it is supposed to be. It is not a full chemical analysis, and it does not replace the mill certificate. What it catches is the error that a certificate cannot: the wrong piece of material, correctly certified, ending up in the wrong place.
Why it matters
Alloy steels and stainless steels often look identical to carbon steel once they are painted or dirty. A single carbon steel elbow installed in a chrome-moly steam line, or a 304 fitting in a 316L system, can fail in service long before the rest of the line. Refineries learnt this the hard way, which is why API RP 578 sets out a material verification programme for alloy piping. PMI is the practical tool behind it.
How the analysers work
X-ray fluorescence (XRF)
A handheld XRF analyser shines X-rays at the surface and measures the characteristic X-rays each element gives back. It is fast, non-destructive and leaves no mark. It identifies chromium, nickel, molybdenum and most other alloying elements reliably. It cannot measure carbon, so it cannot tell 316 from 316L, or confirm the carbon content of carbon steel.
Optical emission spectroscopy (OES)
A mobile OES analyser strikes a small spark on the surface and reads the light the vaporised metal gives off. It can measure carbon, so it can distinguish L grades and confirm carbon steel. It leaves a small burn mark, needs a cleaned surface and is slower than XRF.
What PMI can and cannot do
| Question | XRF | OES |
|---|---|---|
| Is this stainless steel or carbon steel? | Yes | Yes |
| Is this 304 or 316? | Yes (molybdenum) | Yes |
| Is this 316 or 316L? | No | Yes |
| Is this P11 or P22? | Yes (chromium, molybdenum) | Yes |
| Does it meet every limit in the standard? | No | Not reliably |
PMI verifies the grade. It does not certify the material to its standard; the mill test certificate does that.
When to specify PMI
- Alloy steel piping (P11, P22, P91) and stainless steel in critical service.
- Mixed orders where carbon and alloy material travel together.
- Where the client or the project specification requires a verification programme.
- After fabrication, on welds, to check that the correct filler was used.
What to put on the enquiry
Whether PMI is required, on what percentage of items (100% or a sample), by which method, and whether it must be witnessed or reported on a separate certificate. We can arrange PMI before dispatch; say so at the enquiry stage. See also reading a mill test certificate.
Frequently asked questions
What does PMI stand for?
Positive Material Identification – a non-destructive check of alloy composition, usually by XRF or OES.
Can XRF detect carbon?
No. Handheld XRF cannot measure carbon, so OES is used where carbon content (for example L grades) must be verified.
Is PMI required on every piece?
It depends on the project specification; critical alloy systems often require 100% PMI.
Key takeaways
- PMI confirms the grade without damaging the item.
- XRF is fast but cannot measure carbon; OES can.
- Request PMI at enquiry stage so it is planned before dispatch.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.