PMI Testing: XRF vs OES for Alloy Verification

Why PMI exists

Alloys are mixed up in practice: a bundle is re-tagged wrongly, two similar-looking bars share a rack, a repair weld uses the wrong filler. Because most alloy steels and stainless grades look identical, the only defence is to test the metal itself. That verification is PMI — positive material identification. In the process industries, API Recommended Practice 578 sets out how systematic PMI programmes are run; the same logic applies anywhere alloy identity is safety- or cost-critical.

XRF: fast sorting by alloying elements

A handheld XRF (X-ray fluorescence) analyser excites the surface with X-rays and reads the characteristic radiation of the elements present. A result takes seconds, leaves no mark, and is excellent at the distinctions that depend on the major alloying elements:

• 304 vs 316 (separated by molybdenum),
• stainless families and nickel alloys sorted by Cr/Ni/Mo content,
• checking that a Cr-Mo steel sits in the right family (for example 5% Cr vs 9% Cr grades).

The limitation is physics: XRF cannot measure carbon (or the other lightest elements) in steel. It therefore cannot distinguish 316 from 316L, 304 from 304L, or any pair of grades that differ only in carbon — such as 4130 and 4140. Coatings, scale and contamination also distort readings, so the test spot must reach clean metal.

Spark OES: the carbon-capable method

Optical emission spectrometry sparks the sample and analyses the light from the vaporized metal. Mobile and laboratory OES instruments measure carbon as well as the alloying elements, with laboratory-grade accuracy. The price of that completeness is a small spark burn on the surface, a need for argon supply and a reasonably flat, clean test area. Bench OES is the standard chemistry instrument in mill laboratories; portable spark OES is the field method when carbon decides the grade.

A third technology, handheld LIBS (laser-induced breakdown spectroscopy), can also measure carbon and is increasingly used where L-grade verification is routine.

	Handheld XRF	Spark OES
Measures carbon	No	Yes
Speed	Seconds	Typically under a minute
Surface effect	None	Small spark burn
Best role	High-volume sorting and screening	Definitive grade verification

Typical use cases

• Receiving inspection of stainless and high-alloy components, forgings and pipe.
• Verification of Cr-Mo pressure-part grades (P11/P22/P91 families), where both carbon and alloy content matter.
• Sorting mixed or unmarked stock before it enters production.
• Pre-weld confirmation of base metal and review after repairs.

PMI and certificates are complements

PMI proves the identity of the piece in your hand; the mill certificate proves the tested properties of its heat. Neither replaces the other. A sound programme ties both together — PMI result recorded against heat number, instruments calibrated with certified reference materials, and test coverage scaled to how critical the part is.