You may have the right drawings, a valid material certificate and an approved welding procedure, yet still face one important question: is the component actually made from the specified alloy? Mixed stock, replacement parts and unclear markings can make paperwork alone insufficient.
Positive Material Identification, commonly known as PMI, answers that question by checking the alloy composition of the component itself. This guide explains what PMI confirms, which technique suits the job and when full testing is more appropriate than sampling.
Have questions about PMI? Call 01795 432 634 or 01302 203597, or email info@standwinspections.co.uk.
PMI confirms what the component is made from; other NDT methods confirm how well it has been joined. Reliable weld assurance needs both questions answered.
What does PMI verify?
Positive material identification verifies the elemental composition of metal on site without cutting out a sample or damaging the component. It can be applied to parent metal, fittings, valves, pipework and weld metal.
Our positive material identification service supports checks during supply, fabrication, installation and repair. PMI is especially useful where markings are missing, certificates cannot be matched to individual items or similar-looking alloys are stored together.
Why material grade matters to weld integrity
PMI does not look for cracks, lack of fusion or porosity. It addresses a different question: do the materials have the chemistry needed for their intended corrosion resistance, strength, temperature performance and weldability? A sound-looking weld can still connect the wrong materials.
It can help prevent:
- The wrong pipe grade being installed from mixed stock
- Counterfeit or incorrectly certified material entering the supply chain
- Mixed alloys increasing galvanic-corrosion risk
- Specification drift as teams, suppliers and batches change
- Filler metal that does not meet the approved requirement
Choosing XRF or carbon-sensitive analysis
Handheld XRF is widely used to verify the composition of welds and parent materials because it is fast, portable and non-destructive. X-ray fluorescence measures alloying elements and is well suited to stainless steels, nickel alloys and many duplex grades.
However, standard handheld XRF does not measure carbon reliably. Where carbon content or carbon equivalent separates steel grades, Laser-Induced Breakdown Spectroscopy, or another suitable optical method, may be required. LIBS uses a focused laser pulse and removes a minute amount of surface material, so it may leave a small mark.
Agree the method before attendance. Coatings, scale, contamination, access and test-spot size can affect test quality.
When should you specify 100% PMI?
There is no universal testing percentage for every welding project. The PMI scope should reflect the governing code, client specification, material-verification plan and consequences of an incorrect alloy being installed.
A 100% scope is more defensible for safety-critical alloy systems, unfamiliar suppliers, mixed stock or broken traceability. Sampling may suit controlled batches where certificates are linked to items and the specification permits it.
An important consideration is that “10% PMI” means little unless the population is defined. Ten per cent per heat, batch, component type or delivery provides very different assurance. The plan should state how items are selected and what happens after a failed result.
How standards and certification fit together
API RP 578 provides guidance for material verification programmes covering new and existing alloy assets. ASME B31.3 covers process piping materials, fabrication, examination, inspection and testing, while project documents define the applicable PMI scope. On projects governed by ASME B31.3, the required level of PMI should be established from the purchaser’s specification and the project quality plan.
BS EN 10204 defines inspection documents for metallic products. A 3.1 certificate supports traceability, but PMI verifies the physical item. The two controls complement rather than replace each other.
Our wider non-destructive weld testing services allow PMI to sit within a coordinated plan. MPI can identify surface-breaking flaws in ferromagnetic materials, while ultrasonic or radiographic testing can assess internal weld conditions. PMI verifies chemistry, so no single method proves complete weld integrity.
Plan pmi around the decision required
A useful instruction identifies the grades, test locations, technique, acceptance basis, testing percentage, reporting format and action for non-conforming results. It should clarify whether the aim is alloy sorting, grade verification or carbon analysis.
S.T & W Inspections provides on-site and off-site PMI alongside weld inspection, procedures and qualification support. We are ISO 9001 certified, Constructionline approved and operate from Sittingbourne and Doncaster, supporting projects across the UK.
Material certainty strengthens the inspection plan
Documentation, appearance and workmanship cannot prove alloy chemistry. The right scope combines traceability, suitable analysis and complementary NDT.
Specify the decision first, then choose XRF, carbon-sensitive analysis, sampling or 100% verification to provide proportionate evidence.
Discuss your material-verification scope
For help defining an appropriate PMI and weld-inspection plan, call 01795 432 634 or 01302 203597, or email info@standwinspections.co.uk.
Frequently asked questions
Is PMI testing non-destructive?
Yes. Handheld XRF normally leaves the component unchanged. LIBS or optical techniques used for carbon analysis may leave a very small surface mark.
Can XRF distinguish every stainless-steel grade?
No. XRF identifies many stainless and nickel-alloy grades, but some low-carbon distinctions need another technique.
Does a 3.1 certificate remove the need for PMI?
No. A certificate records supplied material information, while PMI checks the actual component. Risk, traceability and specification determine whether both are required.
Should weld metal be tested separately?
Often, yes. Where filler-metal compliance matters, specify separate readings for parent material and weld metal, subject to access and test-spot size.
Can PMI replace ultrasonic or radiographic inspection?
No. PMI verifies chemistry. Other NDT methods assess different flaw types, so they perform complementary roles.
