Positive Material Identification (PMI) for Nickel Alloy Scrap Lots

What Is Positive Material Identification?

Positive material identification is the verification, with a recorded result, that a material's chemistry matches its declared grade. "Positive" means the identity is confirmed by measurement rather than assumed from paperwork, markings or appearance.

In practice, a PMI check answers three questions about a scrap lot:

• Is each tested piece the grade the seller declared?
• Were enough pieces tested, chosen fairly, to speak for the whole lot?
• Is the record specific enough for someone else to check it?

A measurement without a record is just sorting. A record without measurements is just a declaration. PMI needs both.

Where PMI Comes from, and Why the Scrap Trade Uses It

PMI grew up in refineries, petrochemical plants and power stations, where a single wrong alloy in a pressure system can fail in service. The reference document there is API RP 578, Guidelines for a Material Verification Program for New and Existing Assets, whose third edition was published in 2018. ASTM E1476 is the general guide to metals identification, grade verification and sorting. Both are listed on the testing and identification standards page.

The scrap trade borrowed the practice because it faces the same problem in reverse. A plant needs to know the pipe going in is the right alloy; a melter needs to know the scrap going into the furnace is the right alloy. One wrong grade in a nickel alloy heat can put the melt out of specification, and the cost of that far exceeds the cost of testing. Much nickel alloy scrap also comes from the same plants that run PMI programmes, so the habit of testing and recording is already familiar on both sides. The oil and gas scrap page and chemical plant scrap page describe those sources.

Which Methods Count as PMI?

For trading nickel alloy scrap, PMI means a quantitative method that reports element percentages: handheld XRF, spark OES or, increasingly, handheld LIBS. ASTM E1476 also describes qualitative methods such as chemical spot tests, spark testing, eddy current and thermoelectric sorting, but those can only show that a piece behaves like a reference, not name its grade.

Which quantitative method a lot needs depends on the element that decides its grade; XRF vs OES vs lab analysis sets out the choice, and XRF nickel alloy identification covers how to get a trustworthy handheld reading.

How Is PMI Carried Out on a Scrap Lot?

PMI on scrap follows a fixed sequence: segregate, verify the instrument, prepare and test, decide, then record. Skipping a step is where most disputes begin.

Two steps deserve emphasis. Instrument verification means reading a reference sample of known grade before testing begins, and again during long sessions, so that drift does not go unnoticed. Marking means that every tested piece carries something, paint, a tag or a stencil, that ties it to its line in the record. Otherwise a piece tested on Monday cannot be matched to its reading on Friday.

How Many Pieces Should Be Tested?

Test every piece where pieces are large and few, and test a planned sample plus a laboratory composite where pieces are small and many. The aim is the same in both cases: a sample that represents the whole lot rather than the convenient top layer.

For a mixed plant clear-out of unknown history, the rule is simply every piece. The mixed and unsorted scrap page explains why such lots are valued only after they have been sorted.

What Should a PMI Report Record?

A useful PMI report records enough for a stranger to repeat the check: who tested what, with which instrument, how pieces were chosen and what each reading was. A report that only says "625 OK" proves nothing.

Each line in the record should tie back to a physical piece. A simple tag code does the job:

Using the UNS number rather than a trade name avoids confusion between similar names; how UNS numbers work explains the codes. The PMI inspection checklist sets these fields out as a form.

Accept, Reject or Hold: What Happens to Each Piece?

Every tested piece should leave the check with one of three decisions, and the rule for each should be agreed before testing starts.

A rejected piece is not worthless. A piece of C-276 found in a 625 lot is still Hastelloy; it simply belongs in a different lot. Keeping rejects tagged and separate protects the accepted lot and preserves the value of the rest.

PMI Is Not a Mill Certificate or a Laboratory Certificate

A PMI report confirms grade identity for trade; it does not certify material for service and it is not a certified chemical analysis. Buyers and sellers sometimes treat the three documents as interchangeable, and that causes problems.

For fabrication offcuts that still carry heat numbers, a matching certificate and a PMI check together make the strongest case; see reading an EN 10204 3.1 mill certificate. For remelting, a melter may still want laboratory chemistry on a composite even with a full PMI record.

Common PMI Mistakes on Scrap Lots

Most weak PMI records fail for the same few reasons, and nearly all of them are about procedure rather than the instrument.

• Testing only the convenient pieces. The pieces on top of a bundle or at the mouth of a drum are the easiest to reach and the least likely to show a problem. A sampling plan written before testing prevents this.
• Recording the grade name only. If the record holds "625" but no molybdenum, niobium or iron readings, nobody can tell later whether the analyser was right or simply named its nearest library match.
• Testing through scale or coatings. Furnace parts and coated blades read as their surface. A ground spot, noted in the record, shows the reading came from base metal.
• Losing the link between piece and reading. Untagged pieces, or tags that wash off in the monsoon, break the chain. Paint markers and stamped tags survive storage better than chalk.
• Ignoring the light elements. A clean XRF record for "Monel K-500" says nothing about the aluminium that defines it unless a light-element method was used. The record should state the method so the buyer knows what was and was not measured.
• Re-mixing after testing. Accepted and rejected pieces put back on the same pallet undo the whole exercise.

How PMI Prevents and Settles Disputes in Scrap Trading

A PMI record settles a dispute only if it is specific enough for a third party to check: readings per piece, the instrument, photographs and a retained sample. Most disputes come down to one question, whether the material was what the contract said, and a vague record cannot answer it.

• Agree the method first. State in the contract whether PMI is by XRF, OES or both, on every piece or a sample, and where it happens.
• Agree who witnesses. For larger and export lots, buyers often appoint an independent inspection company to witness PMI, weighing and loading, and agree that its results govern.
• Keep evidence. Photographs of tagged pieces and a sealed retained sample let a referee check the claim months later.

The guide to choosing a scrap buyer covers how buyers should explain their own testing, and the scrap preparation guide covers getting a lot ready for inspection.

Preparing a Lot for PMI as a Seller

The easiest PMI check is on a lot that was segregated at source. Keep grades apart at the point of generation, remove attachments and coatings where you can, and keep any certificates and markings with the material. Then a check confirms what you already know instead of discovering what you did not.

When you send us an enquiry, we ask for whatever grade evidence you have: PMI readings, certificates and photographs. We'll review your enquiry on that basis, whether the lot is Inconel 625 scrap, Hastelloy C-276 scrap or 316 stainless scrap. Our quality page explains how we describe scrap by grade and where independent testing fits.

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