Positive Material Identification PMI

Positive Material Identification

Positive material identification confirms what an alloy actually is, rather than what a drawing, a stamp or a certificate says it is. On safety-critical work that distinction is the whole point: material mix-ups are common, rarely visible, and only become obvious when something fails in service.

We carry out PMI by XRF and OES, on site across the UK and in our Cumbria laboratory, and we will tell you which of the two your job actually needs — because the difference between them decides whether a result means anything.

XRF — fast, non-destructive, and blind to carbon

X-ray fluorescence directs X-rays at the surface. Electrons are ejected from inner shells, electrons from outer shells drop in to fill the vacancy, and the energy released comes back as a secondary X-ray at a wavelength characteristic of that element. The detector counts what returns and reports elemental composition.

It takes seconds, leaves no mark, needs no consumables and works on components in place. For confirming that a spool is the alloy the drawing says, it is the right tool.

What it cannot do is measure light elements. Their fluorescence is too low in energy and gets absorbed before it reaches the detector. In practice handheld XRF cannot measure carbon, and also struggles with boron, beryllium, lithium and nitrogen.

That means XRF cannot separate 304 from 304L, or 316 from 316L. Those pairs differ only in carbon content, and everything XRF can see is effectively identical. An instrument presented with either will match the fingerprint and report a result; some display the pair honestly, others display whichever grade sits first in their library. A report claiming an L grade on XRF alone is stating more than the measurement supports, and ours never will — where the result cannot distinguish them, we report the pair.

Why the carbon grade is worth the extra step

Carbon governs sensitisation. Hold austenitic stainless in roughly the 450–850°C range — which happens in the heat-affected zone every time it is welded — and carbon migrates to the grain boundaries to form chromium carbides. That strips chromium from the metal immediately around them, and chromium is what makes the passive film work.

The result is a grain boundary network with no corrosion resistance, preferentially attacked in service. That is intergranular corrosion, and in chloride-bearing conditions it is a starting point for stress corrosion cracking. L grades exist to limit it.

So a component specified 316L, installed in chloride service, and actually made of 316 is not a paperwork discrepancy. It is a different service life in the application where the difference was the reason for the specification.

OES — when you need the carbon number

Optical emission spectroscopy vaporises a small amount of the surface with an electrical arc or spark and analyses the emitted light. It measures carbon, and portable OES brings that capability to site.

The trade-off is that it is slightly destructive, leaving a small burn mark a few millimetres across, and it needs more surface preparation. On a finished, coated or cosmetically critical surface that matters; on a pipe spool it does not.

Laboratory analysis of a removed sample gives the most complete result, including trace elements, where the component can be sampled and the timescale allows.

Which one your job needs

The sensible pattern on most scopes is XRF to screen everything, with OES or laboratory analysis on the subset where carbon carries consequence. You need the carbon number when:

• the specification calls for an L grade and the service environment is the reason;
• the component will be welded and then run in corrosive or elevated-temperature service;
• you are investigating a corrosion failure and need to know whether sensitisation was a factor;
• a code, client specification or regulator requires full chemical composition rather than elemental verification;
• you are distinguishing carbon steel grades, where carbon is the defining property.

Where XRF is decisive

Having been clear about the limitation, it is worth being equally clear that XRF settles most questions outright. Separating 304 from 316 is trivial — 316 carries roughly 2–3% molybdenum and 304 essentially none. Distinguishing carbon steel from any stainless is immediate on chromium alone. Nickel alloys, duplex versus austenitic, titanium, copper alloys and brasses all read clearly.

Real-world material mix-ups are almost always between alloy families rather than between a grade and its L variant, and XRF catches those instantly and at scale.

What changes the answer

Surface condition. XRF reads microns deep. Coatings, plating, oxide scale, weld spatter and grinding smear all read as the material. A reading taken through paint is a reading of the paint, so preparation is part of the test rather than preparation for it.

Weld metal is not parent metal. Filler is frequently over-alloyed by design. A reading across a weld gives a composition matching neither the parent nor the filler specification. Readings are taken on parent material clear of the weld unless the weld metal is deliberately the subject.

Reading time. Short readings screen; longer counts verify, particularly for the lighter detectable elements. An operator under time pressure taking one-second readings is producing screening data, and the report should say so.

Calibration. Instruments are verified against certified reference materials representative of the alloys being tested, at a defined frequency, and the checks are recorded. A PMI report with no evidence of instrument verification is an assertion rather than a measurement.

It is a radiation-emitting device. Handheld XRF is subject to IRR17, with the risk assessment, appointed persons and operator training that implies. Not an onerous regime for these instruments, but a real one.

Where PMI gets specified

Alloy verification on incoming material and at goods-in, before anything is fabricated. Material traceability through fabrication, so a certificate can be tied to a component rather than a delivery. Verification after a shutdown, confirming that what went back in is what came out. Waste routing and free-release decisions on decommissioning scopes, where material identity determines the disposal route. Failure investigation, where the first question is usually whether the material was what it was meant to be. And positive verification on safety-critical systems where a substitution would not be visible any other way.

What our report contains

The instrument and its calibration status. The surface preparation. The reading duration or mode. The measured elemental composition with actual figures rather than only a grade name. The reference standard the result was matched against. And, explicitly, a statement that carbon was not measured where XRF was the method — with the grade pair reported where the result cannot separate them.

That last item is what distinguishes a useful report from a reassuring one. We would rather hand you a result with its limitation stated than a clean-looking certificate that quietly overstates what the instrument did, because the second kind only becomes a problem years later in a corrosion investigation, when somebody asks how the grade was determined.

On site or in the laboratory

Most PMI is done where the material is. Our technicians work across the UK from bases in Cumbria, Durham and Southampton, and on decommissioning scopes PMI usually runs ahead of everything else — you cannot select an ultrasonic technique without knowing the material, and "it should be 316" is not a basis for a procedure.

Where laboratory analysis is needed, samples go to our own facility rather than a third party, which keeps the chain of custody in one quality system alongside the destructive testing and metallurgical work.

If your scope involves L grades in corrosive service, talk to us about which components genuinely need the carbon number and which do not. It is usually fewer than people expect, and knowing which is which is most of the saving.