How to Verify 316 Stainless Steel on a Filter Housing (PMI)
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The reliable way to verify 316 stainless steel on a filter housing is a positive material identification (PMI) reading taken with a handheld X-ray fluorescence (XRF) analyzer on the finished housing: molybdenum around 2 to 3% means 316, molybdenum near zero means 304. A magnet proves nothing, because both grades are non-magnetic when annealed and slightly magnetic after forming. A molybdenum spot test is a cheap screen that darkens on 316 and stays clear on 304, but it also reacts to other molybdenum grades. Telling 316 from 316L needs the mill certificate or a spark (OES) test, since XRF cannot read carbon.
- Magnet: separates ferritic and duplex steels from the 300 series. It cannot separate 304 from 316.
- Molybdenum spot test: a drop of reagent darkens within a few minutes on 316. Compare against a known 304 and a known 316.
- Handheld XRF (PMI): reads chromium, nickel and molybdenum in 20 to 60 seconds, leaves no mark. The standard answer.
- OES or mill certificate: the only way to confirm the carbon limit that makes 316L.
This is the verification chapter of our stainless steel filter housing buyer's guide. If you are still deciding which grade you need, start with 304 vs 316 for filter housings.
Why verify the grade of a filter housing at all?
Because 304 and 316 look, weigh and polish the same, and the price gap between them is exactly the kind of margin a careless supplier recovers by shipping the cheaper grade. The difference does not show until chlorides, brine or an aggressive chemical start pitting the shell months later. A housing is also several parts: shell, lid, basket or tube sheet, nozzles, legs, and bought-in valves and fittings. The grade statement on a quotation normally covers the housing body, so verification should ask about the body first and the valves and fasteners separately.
Can a magnet tell 304 from 316?
No. Both are austenitic grades, and in the annealed condition austenitic stainless steels have a relative permeability of about 1, which is to say non-magnetic. Forming changes that: cold-drawn sumps, spun heads and bent brackets develop a weak magnetic response, and 304 does so more readily than 316. The Australian Stainless Steel Development Association notes that cold-worked austenitics "can develop a magnetic response," while higher-nickel grades such as 316 show a much lower one (ASSDA, testing for grade confirmation). So a magnet that sticks weakly to a formed sump tells you the sump was formed, not which grade it is. What a magnet does do is catch a different substitution: ferritic and duplex grades, and plain carbon steel, are strongly magnetic, so a housing that grabs a magnet firmly is not a 300-series housing at all.
What does the molybdenum spot test show?
The spot test is the cheapest real check. A drop of a molybdenum reagent, sold under names such as Moly Drop or Decapoli 304/316, is placed on a cleaned area of the steel and on known samples of 304 and 316 at the same temperature. If the drop darkens within about 2 to 4 minutes, the steel contains significant molybdenum (Atlas Steels, qualitative sorting tests). Three limits to keep in mind. A positive result does not by itself prove 316: 317, 904L, 2205 duplex and other molybdenum grades react too, and some heats of 304 with around 0.5% molybdenum react slightly, which is why the known samples matter. The test marks the surface, so use an inconspicuous spot, such as under a leg pad or inside a drain boss, and wash the reagent off at once. And the result is yes or no on molybdenum, not a percentage.
How does a handheld XRF analyzer verify 316 stainless steel?
A handheld XRF analyzer fires X-rays at the surface and reads the characteristic energies that come back, giving a percentage for each heavier element: chromium, nickel, molybdenum, manganese, copper and others. On a clean, smooth spot a reading takes 20 to 60 seconds, leaves no mark, and can be saved and downloaded as a record. For a 316 housing you expect roughly 16 to 18% chromium, 10 to 14% nickel and 2 to 3% molybdenum; for 304, similar chromium and nickel and no significant molybdenum. The method is standardized as ASTM E1476, and API RP 578 describes how a material verification program is run around it (Projectmaterials, what is PMI testing).
Two things XRF cannot do. It cannot read light elements, so it does not see carbon or nitrogen, which means it cannot separate 316 from 316L. And it reads the surface only, so a reading taken through heavy oxide, paint or a thick polishing residue can be wrong; the spot should be cleaned first. Because PMI is a per-item check, it is the test to ask for on your actual housing, not on a sample from the same batch.
When do you need OES or a mill certificate?
Only when the question is 316 or 316L, or when a code or specification demands a full composition. A portable optical emission spectrometer, often called a spark tester, vaporizes a tiny spot with an electric arc and reads all elements including carbon; it needs a flat area of about 20 mm and leaves a small burn mark that may need dressing. ASSDA notes it is the portable method that can distinguish low-carbon from standard-carbon grades, and that laboratory analysis, with a separate combustion test for carbon, is slower but more complete. For most buyers the practical route is the mill test certificate for the sheet, which states the carbon, and which ASSDA calls sufficient "unless there is reason to doubt" it. 316 vs 316L for filter housings explains when the L matters.
Verification methods compared
| Method | What it tells you | What it cannot tell you | Marks the housing? |
|---|---|---|---|
| Magnet | Whether the steel is 300 series (weak or no pull) or ferritic, duplex or carbon steel (strong pull) | 304 from 316; both respond weakly after forming | No |
| Molybdenum spot test | Whether molybdenum is present: 316 darkens, 304 does not | 316 from other molybdenum grades; no percentage; needs known samples | Yes, small stain |
| Handheld XRF (PMI) | Chromium, nickel, molybdenum and other heavy elements as percentages; 304 vs 316 in seconds | Carbon or nitrogen, so not 316 vs 316L; reads the surface only | No |
| Spark OES | Full composition including carbon; 316 vs 316L | Nitrogen; less accurate than a laboratory | Yes, burn spot |
| Mill certificate | The sheet's certified composition including carbon | Whether that sheet is the one in your housing, unless traceability is kept | No |
How to verify 316 stainless steel on a filter housing: a practical sequence
- Put it on the order. Write "housing body 316, PMI reading on the finished housing before shipment" on the request. A manufacturer who objects is telling you something.
- Ask for the reading with the shipping photos. A photo of the analyzer screen against the housing, with the chromium, nickel and molybdenum percentages visible, is enough to file with the order.
- Check on arrival. A magnet to rule out a ferritic or carbon steel substitution, then a spot test in a hidden place, or an XRF reading if you have access to a unit or a local inspection service.
- Test the right parts. The shell and lid are the housing body. Nozzles and flanges are usually the same grade but are worth a second spot. Drain valves, gauges and fasteners are separate items with their own materials, which should be listed on the quotation.
- For 316L, ask for paper. The mill certificate for the sheet, or an OES reading, is the only check that covers the carbon limit.
What we do about it
Our 304 and 316 housings can be verified by spectrometer, and we can take the reading on your actual housing before it ships and send it with the pre-shipment photos; ask for it on the order. The reading covers the housing body; valve, gasket and fastener materials are listed separately on the quotation. Custom vessels are built to an approved drawing and delivered to its dimensions, with a minimum order of one piece. Standard sizes are in stainless steel filter housings, and anything built to your drawing starts on the custom bag and multi-cartridge housing page.
FAQ
Can a magnet tell whether my filter housing is 316?
No. 304 and 316 are both non-magnetic when annealed and both become weakly magnetic after forming, so a weak pull on a sump or head says nothing about the grade. A strong pull means the steel is not a 300-series grade at all.
What XRF reading should a 316 housing show?
Roughly 16 to 18% chromium, 10 to 14% nickel and 2 to 3% molybdenum. A 304 housing shows similar chromium and nickel with molybdenum near zero. The exact figures vary within those ranges from heat to heat.
Can XRF or PMI tell 316 from 316L?
No. XRF does not read carbon, which is the only difference between the two. Ask for the mill certificate of the sheet or an OES test if the L matters for your application.
Does the molybdenum spot test damage the housing?
It leaves a small stain where the drop sat. Test on a hidden surface, wash the reagent off immediately, and do not use it on a polished sanitary surface that will be in product contact.
Should I test every housing or just one from the batch?
PMI is a per-item test by definition; batch testing only proves the batch, not your unit. For a single housing, ask for the reading on that housing. For a larger order, ask for a reading per housing or per heat of sheet, with the certificates to match.
Want the reading before it ships?
Tell us the grade you need and ask for the PMI reading on the order. We confirm the grade on the quotation and drawing, take the reading on your housing, and deliver to the approved drawing. Start at custom bag and multi-cartridge filter housings or request a quote.