Quick Answer
An INOX-rated (contaminant-free) cutting or grinding disc is one whose formulation holds iron, sulfur and chlorine below 0.1% combined (Fe + S + Cl < 0.1%), so the wheel cannot seed rust or pitting in a stainless steel workpiece (KB: Stainless Steel Finishing; Thin Cut-Off Wheel). It is a chemistry specification set by the manufacturer, not a safety certification, and it is the difference between a disc that is genuinely safe for stainless work and one that merely says "for stainless" on the sticker. If the label or technical data sheet does not state a Fe+S+Cl limit, treat the disc as a carbon-steel disc.
The Fe+S+Cl rule
Stainless steel resists corrosion because of a thin, self-healing chromium-oxide passive film on its surface. That film survives cutting and grinding only if the abrasive leaves nothing behind that attacks it. Three elements do exactly that, which is why the INOX specification names all three (KB: Stainless Steel Finishing):
| Contaminant | INOX limit | What it does to stainless |
|---|---|---|
| Iron (Fe) | < 0.1% Fe + S + Cl combined | Embeds free iron particles in the surface; they rust as orange freckles ("rouging") the first time the part gets wet. The most common contamination source. |
| Sulfur (S) | < 0.1% Fe + S + Cl combined | Residues attack the passive chromium-oxide layer and promote pitting. |
| Chlorine (Cl) | < 0.1% Fe + S + Cl combined | Drives pitting and stress-corrosion cracking, a particular risk on austenitic grades such as 304 and 316. |
The figure comes up consistently across manufacturer technical guides: contaminant-free bonded wheels for stainless carry "less than 0.1% of chlorine, iron, and sulfur" (Weiler cutting-wheel guide, via KB: Cutting-Off Technique; MSC Industrial, via KB: Stainless Steel Finishing Technique). The British Stainless Steel Association puts the failure mode bluntly: "as soon as any of this contamination becomes wet, rust stains will result" (BSSA, via KB: Stainless Steel Finishing Technique).
One honesty note, because spec sheets disagree at the margin: most sources state under 0.1%, while at least one manufacturer page prints under 0.01% — likely a typo, but the safe habit is to read the exact figure off the specific product's technical data sheet rather than off a blog, ours included (KB: Stainless Steel Finishing).
Why an ordinary disc contaminates stainless
This is not marketing theatre; the contamination is built into general-purpose wheels. Ordinary bonded and coated abrasives contain iron-bearing fillers and binders that leave residue on the cut and cause "after-rust" on stainless (KB: Stainless Steel Finishing). Worse, several classic active fillers — the additives that decompose under grinding heat to lubricate the cut — are made from precisely the elements the INOX rule bans: pyrite (FeS₂) releases sulfur as it breaks down, lithopone (ZnS + BaSO₄) decomposes to sulfur compounds, and lead chloride (PbCl₂) appears in some filler systems (KB: Filler and Active Filler Systems). A contaminant-free wheel is therefore a genuinely different formulation, not the same wheel with a different label.
The second contamination path is cross-use. A disc, flap disc or wire brush that has touched carbon steel carries microscopic ferrous particles, and those get pressed into the stainless surface on the next job. That is why the trade rule is one-directional: a stainless-dedicated disc may later be demoted to carbon steel, but a disc that has ground carbon steel must never touch stainless again (KB: Stainless Steel Finishing; Cut-Off Wheel Sub-Forms).
What INOX does not mean
It is not a safety certification. The standards stamped on a bonded disc — EN 12413 in Europe, ANSI B7.1 in North America — govern burst-speed margins, marking and shelf life. They do not define the contamination chemistry; the Fe+S+Cl limit is a manufacturer specification layered on top (EN 12413; ANSI B7.1; KB: Stainless Steel Finishing). A disc can pass every burst test oSa runs and still rust your stainless. We cover the safety side separately in our guide to EN 12413, oSa and ANSI B7.1.
It is not the same as "for stainless." Labeling a product for stainless does not make it contaminant-free — look for the explicit INOX or contaminant-free claim, ideally with the number attached (MSC Industrial, via KB: Stainless Steel Finishing Technique).
It is not heat protection. The Fe+S+Cl rule says nothing about how cool the disc cuts, and heat is the other way grinding ruins stainless: visible heat tint starts around 290 °C on type 304 and signals a chromium-depleted layer forming beneath the oxide (KB: Heat Tint). Cool cutting comes from grain choice and technique — zirconia and ceramic alumina grain, light pressure, multiple shallow passes — which we walk through in the best abrasives for stainless steel (KB: Stainless Steel Finishing Technique; Cutting-Off Technique).
How to read the label on an INOX disc
EN 12413 requires 12 marking elements on every bonded disc, and element 7 is the one this post is about: the intended use / material, marked as steel, INOX/stainless, stone and so on (EN 12413; KB: EN 12413 Bonded Abrasive Safety Standard). The elements worth checking before a stainless job:
| Marking element | Example | What it tells you |
|---|---|---|
| Intended use / material | INOX / stainless | The material rating — the subject of this post |
| Specification code | A 30 S BF | Grain (A = aluminum oxide) · grit 30 · hardness S · BF = fibre-reinforced resin bond |
| Max operating speed | 80 m/s and the RPM figure | Must meet or exceed your grinder's speed |
| Expiry date | MM/YYYY | Resin-bond wheels are capped at 3 years from manufacture |
| Conformity + cert marks | EN 12413, oSa/MPA | The safety layer, separate from the contamination claim |
(EN 12413 marking set via KB: EN 12413 Bonded Abrasive Safety Standard.) The Fe+S+Cl figure itself usually lives on the technical data sheet rather than the disc face, so for critical work, pull the TDS rather than trusting the sticker — the Fe+S+Cl figure is what tells you a disc in our cut-off wheel range is genuinely stainless-suitable.
Chemistry is only half the rule: shop discipline
An INOX disc in a dirty workflow still produces rusty stainless. The discipline that makes the chemistry count (KB: Stainless Steel Finishing; Stainless Steel Finishing Technique):
- Dedicate and segregate. Keep a separate, clearly colour-coded set of discs, flap discs, brushes and pads that only ever touch stainless.
- One-way traffic. Stainless tools may be demoted to carbon steel later; carbon-steel tools never get promoted to stainless.
- Control the environment. Keep stainless off carbon-steel work tables and out of the spark stream from mild-steel grinding; use non-metallic supports.
- Wire brushes too. Use a stainless-only wire brush — never one that has touched carbon steel.
- Cut cool. Thin 0.040" / 1.0 mm wheels are favoured on stainless because the narrow kerf removes about a third of the material a 1/8" wheel does, with correspondingly less heat (KB: Thin Cut-Off Wheel). Sizing and form are covered in how to choose a cut-off wheel.
Where does this level of care actually pay? Food-processing, medical, nuclear and other corrosion-sensitive work are the classic cases, and the same contaminant-free logic applies to aluminum (KB: Thin Cut-Off Wheel). For a decorative No.4 or No.8 finish after the cut, follow the grit ladder in our stainless finishing guide. Fabricators outfitting a stainless-only bench can start with our metal fabrication essentials.
Verifying the work: the ferroxyl test
If you suspect a part picked up free iron, there is a standardized check: the ferroxyl test of ASTM A967 / ASTM A380 §7.3.4. A swab of nitric acid and potassium ferricyanide solution turns blue within about 15 seconds wherever free iron is embedded (BSSA, via KB: Stainless Steel Finishing Technique). A 15-second swab on a test coupon tells you whether your "stainless" abrasive is actually leaving iron behind before you commit a full batch. If contamination is found, passivation per ASTM A967 (nitric or citric) strips the free iron and rebuilds the passive layer (KB: Stainless Steel Finishing Technique).
FAQ
What does INOX mean on a cutting disc?
It means the disc is formulated for stainless steel: iron, sulfur and chlorine are held below 0.1% combined, so the wheel does not deposit contaminants that rust or pit the workpiece (KB: Stainless Steel Finishing; Thin Cut-Off Wheel).
Is an INOX disc the same as a disc labelled "for stainless steel"?
No. "For stainless" is marketing wording; contaminant-free is a formulation claim. A product can be labelled for stainless without meeting the Fe+S+Cl limit — look for the explicit INOX/contaminant-free statement, ideally on the technical data sheet (MSC Industrial, via KB: Stainless Steel Finishing Technique).
Can I use an INOX disc on regular carbon steel?
Yes — the chemistry does no harm on carbon steel. But once a disc has ground carbon steel it carries ferrous debris and must never be used on stainless again. The demotion only works in one direction (KB: Stainless Steel Finishing).
Why do iron, sulfur and chlorine matter on stainless?
Embedded free iron rusts on the surface as soon as it gets wet, while sulfur and chlorine residues attack the chromium-oxide passive layer, promoting pitting and stress-corrosion cracking — especially on austenitic grades like 304 and 316 (KB: Stainless Steel Finishing).
How do I check whether iron got embedded in my stainless part?
Run the ferroxyl test (ASTM A380 §7.3.4 / ASTM A967): a nitric acid + potassium ferricyanide swab turns blue in about 15 seconds where free iron is present. Passivation per ASTM A967 removes it and restores the passive layer (KB: Stainless Steel Finishing Technique).
Does EN 12413 certify that a disc is contaminant-free?
No. EN 12413 (like ANSI B7.1) is a safety standard covering burst speed, marking and shelf life. It requires the disc to state its intended material — which is where the INOX marking appears — but the under-0.1% Fe+S+Cl chemistry is a manufacturer specification, verified on the product's own data sheet (EN 12413; KB: EN 12413 Bonded Abrasive Safety Standard; Stainless Steel Finishing).