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Stainless Steel - Sorting and Identification Tests

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These tests are intended for rapid, inexpensive and usually non-destructive and on-site sorting of grades of stainless steel. They are particularly useful for sorting products when, for example, bars of grades 304 and 303 have been accidentally stored together, or grade 304 and 316 sheet offcuts mixed.

These tests are extremely useful, but it is important to realise that they have limitations; they cannot sort one heat from another of the same grade, and there is no easy way of sorting certain grades from each other. For instance, it is not possible to readily sort 304 from 321, 316 from 316L or 304 from 304L. The Molybdenum spot test therefore indicates that a piece of steel contains Mo, but does not alone indicate 316. In the absence of other knowledge the steel could be 316L, 2205 or 904L etc.

The simple tests described may assist in grade identification and product sorting. Other, more complex tests can also be carried out; these can involve several chemical reagents, hardness tests or checking response to heat treatment. In most cases, however, if these simple tests are not sufficient to identify the product it is best to have a full spectrometric analysis carried out by a competent laboratory.

The need for these sorting tests can be reduced if original product identification is retained. Product colour codes, tags and stickers and stamped or stencilled Heat/Grade/Specification markings should be retained as much as possible.


Table 1. Tests for the identification and sorting of stainless steel


What Can Be Sorted



Magnetic Response

Austenitic (300 Series) stainless steels from other steels. All other steels are attracted to a magnet, including the ferritic, duplex, martensitic and precipitation hardening stainless steels. The only other non-magnetic steels are the austenitic manganese steels (eg “P8”).

Note response, if any, when a permanent magnet is brought close to the steel.

Some austenitic grades, particularly 304, are attracted when cold worked, eg by bending, forming or rolling. Stress relieving at cherry-red heat will remove this response due to cold work. This stress relief may sensitise the steel and should not be performed on an item which is later to be used in a corrosive environment. A full anneal is acceptable, however.

Nitric Acid Reaction

Stainless steels from non-stainless steels.

1. Place a piece of the steel in strong nitric acid (20% to 50%) at room temperature, or a drop of the acid on a cleaned surface of the steel.
2. Test standard samples in the same way, ie stainless and non-stainless steel samples.
3. Non-stainless steels will quickly be attacked, a pungent brown fume is produced. Stainless steels are not affected. Compare result with standards.
4. Wash samples thoroughly afterwards.

Wear safety glasses. Strong nitric acid attacks skin and is very corrosive. Handle carefully. Use minimum quantities. Wash off immediately if skin contact occurs. Do not breathe brown fume.

Molybdenum Spot Test


Stainless steels which contain significant Molybdenum from those which do not. The most common use is to sort 404 from 316, but the following grades also contain sufficient Mo to give a positive response to this test: 316, 316L, 317, 317L, 444, 904L, 2205, "6-Mo" grades, 4565S and all “super duplex” grades (e.g. S32760 / Zeron 100 / S32750 / 2507 / S32550 / Alloy 255 / S32520 / UR52N+). Other similar grades with deliberate Molybdenum additions will also respond.

1. Clean the steel surface; use abrasive paper, and if necessary degrease and dry.
2. Use "Decapoli 304/316" solution – shake well, then place one drop on the steel.
3. Place similar drops on standard 304 and 316 samples.
4. Darkening of the yellow drop in 2 to 4 minutes indicates significant Mo. Compare with indications on standard samples.
5. Wash or wipe samples clean.

Avoid contact of test solution on skin, and particularly eyes. Wash off immediately if contacted. Reliable results only obtained if samples all the same temperature and freshly cleaned. Avoid very low sample temperatures. Some Heats of "Mo-free" stainless steels, such as 304, contain enough Mo to give a slight reaction. Standard comparison samples must be used.

Sulphur Spot Test (S)

Steels (stainless and plain carbon) containing at least 0.1% Sulphur, ie free machining grades. (eg S1214, S12L14, 303, 416, 430F), from non-free machining steels. Ugima 303 contains high sulphur (the same as standard Type 303) so will give a positive reaction, but Ugima 304 and Ugima 316 have the same low sulphur contents as their standard (non-Ugima) equivalents, so will not give positive reactions.

1. Clean the steel surface; use abrasive paper, and if necessary degrease. A flat area is preferred.
2. Prepare standard samples in the same way, eg known CS1020 and S1214, or 304 and 303.
3. Soak photographic paper in 3% sulphuric acid for about 3 minutes.
4. Press the prepared steel surfaces on the face of the photographic paper for 5 sec.
5. A dark brown stain indicates significant sulphur. Compare with indications from standard samples.
6. Wash samples thoroughly.

Wear safety glasses. Avoid contact of acid with skin and eyes. Wash immediately if contacted. Reliable results depend on good contact with the paper, and consistent time of contact. Standard comparison samples must be tested in conjunction with the unknown samples.



Source: Atlas Steels Australia


For more information on this source please visit Atlas Steels Australia


Date Added: Feb 7, 2002 | Updated: Jun 11, 2013
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