What Is PMI Testing?
Positive Material Identification (PMI) is a non-destructive testing method that verifies the chemical composition of metallic materials. A handheld analyzer reads the alloy content of pipes, fittings, flanges, valves, and welds to confirm that the correct material grade was installed. PMI catches mix-ups that mill test certificates alone cannot prevent.
| Aspect | Details |
|---|---|
| Purpose | Verify alloy composition matches specification |
| Primary methods | X-Ray Fluorescence (XRF), Optical Emission Spectroscopy (OES) |
| Test duration | 2-10 seconds per reading (XRF) |
| Elements detected (XRF) | Cr, Ni, Mo, Cu, Ti, Nb, V, Mn, Si, W, Co (not carbon) |
| Elements detected (OES) | All elements including carbon |
| Accuracy | Typically +/- 0.1-0.5% for major elements |
| Surface prep | Remove paint, scale, and coatings from test area |
| Standards | ASTM E1476 (XRF), ASTM E1086 (OES), API RP 578 |
| Governing spec | API RP 578 (Material Verification Program for New and Existing Alloy Piping Systems) |
| Result | Pass/fail with grade identification and elemental percentages |
XRF vs OES Methods
X-Ray Fluorescence (XRF) is the dominant field method. A handheld gun emits X-rays that excite atoms in the sample surface. The returned fluorescent X-rays have energies unique to each element, allowing identification and quantification. XRF is fast, portable, and non-destructive. Its limitation: it cannot measure carbon content, which means it cannot distinguish between standard and low-carbon grades (e.g., 316 vs 316L) without supplementary OES testing.
Optical Emission Spectroscopy (OES) uses an electrical spark to vaporize a small area of the sample. The emitted light spectrum identifies all elements including carbon. OES can distinguish 304 from 304L and 316 from 316L. It leaves a small burn mark (approximately 3-5 mm) on the surface, which may require grinding and NDE on pressure-containing components.
| Feature | XRF | OES |
|---|---|---|
| Carbon detection | No | Yes |
| Portability | Fully portable | Semi-portable (larger unit) |
| Surface damage | None | Small spark burn |
| Test speed | 2-10 seconds | 10-30 seconds |
| Distinguish L grades | No | Yes |
| Cost per test | Lower | Higher |
When PMI Is Required
PMI is mandated or recommended in these situations:
- ASME B31.3 Chapter IX (sour service piping): PMI on all alloy components
- API RP 578: Material verification for alloy piping in refineries and chemical plants
- NACE MR0175 compliance verification: confirms alloy composition meets sour service requirements
- Client project specifications: Many EPC contractors require 100% PMI on all alloy steel and stainless steel components
- Receiving inspection: Verifies incoming materials at warehouse or job site before installation
What PMI Catches
Material mix-ups are more common than most engineers assume. Typical errors PMI detects:
- Carbon steel installed where stainless steel (F316) was specified
- 304 stainless where 316 was required (missing molybdenum)
- Standard chrome steel where chrome-moly (F11/F22) was specified
- Wrong alloy in weld filler metal deposits
- Material substitution by suppliers (intentional or unintentional)
A single material mix-up in a high-temperature or corrosive service line can cause in-service failure. PMI is one of the most cost-effective quality tools available, complementing NDT, hydrostatic testing, and documentation review. The test confirms that cast or forged components match the piping material specification.
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