Stainless steel is steel with at least 10.5% chromium, which forms a thin, self-repairing film of chromium oxide that stops the metal from rusting. Three families matter: austenitic (18% chromium and 8% nickel, non-magnetic and tough), ferritic (12-17% chromium with no nickel, magnetic and cheaper) and martensitic (12-14% chromium with 0.1-1% carbon, hardenable for knives and valves). Keep the carbon low, or the film fails along the grain boundaries.
Part of Advanced Alloys
Stainless steel is an iron alloy with at least 10.5% chromium whose corrosion resistance comes from a thin passive oxide film on the surface rather than from the bulk metal.
The Surface Film Does The Protecting
The metal beneath the film is no more noble than ordinary steel. Stainless Steel Production describes the film as microscopically thin chromium oxide, transparent, adherent, and self-healing: if scratched, it re-forms within hours in air. Two things follow.
- It needs oxygen to heal. In a tight crevice, under a deposit, or in a stagnant joint where air cannot reach, the film cannot re-form and the steel can corrode there even though the same steel is untouched in the open.
- It can be broken chemically. Chloride, the ion in salt, attacks the film locally and causes pits. Stainless is not salt-proof.
The film also depends on enough chromium being present at the surface. That is the root of sensitisation, below.
Three Families, Sorted By Crystal Structure
| Family | Composition | Structure | Magnetic | Hardens by quenching | Uses |
|---|---|---|---|---|---|
| Austenitic | 18% Cr, 8% Ni | Face-centred cubic | No | No | Cookware, surgical tools, chemical tanks |
| Ferritic | 12-17% Cr, no Ni | Body-centred cubic | Yes | No | Decorative trim, automotive exhaust |
| Martensitic | 12-14% Cr, 0.1-1% C | Body-centred, hardened by quench | Yes | Yes | Knives, bearings, turbine blades |
The composition column is from the Advanced Materials article. The structure column explains the rest. Only martensitic grades have enough carbon to form martensite on quenching, so only they can be hardened like a tool steel. Austenitic grades are held in their soft high-temperature form by nickel (see Austenitic Steel), and ferritic grades have no phase change to exploit (see Ferritic Steel).
Carbon Is The Enemy Of The Film
If carbon is too high, chromium and carbon combine into chromium carbides on the grain boundaries as the steel cools. The production article names carbon above 0.12% as the cause, keeps the target below 0.08%, and calls this sensitisation, the most common failure in amateur stainless steel. The carbide takes chromium from the metal next to the boundary. That strip drops below 10.5% chromium and can no longer form the film, so the steel corrodes along the boundaries. See Grain Boundaries.
Welding is the common trigger, because the metal beside a weld sits for a while in the temperature range where carbides form. The production article advises low heat input in the heat-affected zone.
Choosing A Grade
- Knife or valve that must hold an edge or a seat: martensitic.
- Food, medical or chemical equipment where nickel is available: austenitic.
- No nickel, moderate corrosion duty: ferritic. The production article’s recommendation is to start with ferritic grades because they need only chromium and tolerate less precise carbon control.
- Unknown salvage: sort it with a magnet first. A magnet separates austenitic from the other two, but not ferritic from martensitic.
Welding, grinding or cutting stainless gives off toxic fume
Chromium and nickel fume from hot stainless steel is harmful to breathe, and the effect is worse in confined spaces. Work outdoors or with a strong draught pulling fume away from your face, and do not weld or grind stainless indoors without ventilation.
Common Mistakes
- Touching stainless with carbon-steel tools — iron particles embed in the surface and rust spots follow. Use separate files and brushes.
- Assuming “stainless” means “never corrodes” — it resists corrosion in air, food and many chemicals but not hot chloride or a starved crevice.
- Quenching austenitic or ferritic grades for hardness — it does not work.
What’s Next
- Learn next: Austenitic Steel — why nickel changes the whole behaviour of the alloy
- Combine with: Stainless Steel Production — how the metal is made and which furnace work it takes