Stainless steel: what it is and what it is made of
Stainless steel is an iron-based alloy containing at least 10.5% chromium. In air, chromium forms a thin oxide film, and it shields the metal from oxygen and moisture.
The film wears away, gets scratched and restores itself again as long as the steel has a reserve of chromium. This also explains why AISI 304 rusts in some conditions and lasts for decades in others.
Below are the definition, the composition and structure, the classes of steel, and an analysis of a common question: why AISI 304 rusts even though the steel is called stainless.
Definition
Stainless steel is not a separate metal and not a "special iron" but a group of alloy steels. Their base is iron, and chromium is what makes them stainless. The colloquial word "stainless" and the technical term "corrosion-resistant steel" mean the same thing.
The key features by which a steel is classified as stainless:
- chromium — at least 10.5% by mass;
- carbon is usually low, especially in grades that are welded;
- corrosion protection rests on a passive film, not on a coating.
Grades and the chemical composition of corrosion-resistant steels are set by ГОСТ 5632; for sheet, pipes and long products, separate standards govern dimensions and tolerances. A standard number tells you what exactly that standard regulates, not the quality of a particular batch. At acceptance, what is checked is the certificate for the batch, not the name of the material in the specification.
The designation of stainless steel in a specification is a trap of its own. A single line may contain AISI 304, 08Х18Н10 or simply "stainless steel". These are different ways of naming a similar material, and confusion begins when substituting one entry for another is considered equivalent by default. Formally the compositions are close, but a batch is confirmed by analysis, not by a coincidence of digits in the name. That is why, in an order, the grade is written the way it is stated in the supplier's certificate.
Not every stainless steel behaves the same way when heated. Corrosion-resistant grades withstand a chemical medium, heat-resistant ones withstand high temperatures and scale. Grades such as 310S combine both properties, but these are already special solutions rather than mass-produced steel products.
What stainless steel is not matters too. It is not a coating. Zinc, paint or a galvanic layer lie on the surface and work as long as the layer is intact. In stainless steel, protection is built into the material itself, so a cut edge, grinding and a scratch are not a verdict for it but a working situation. That is exactly why stainless steel is used both for cookware and for heat exchangers and fasteners for damp rooms.
A separate distinction is the one from aluminium. Aluminium also becomes covered with its own oxide film and does not rust in the everyday sense, but it is a different metal: lighter, softer, with different thermal conductivity and a different welding technique. In assemblies where stainless steel contacts aluminium or galvanised steel, the galvanic couple is monitored and an insulating gasket is fitted.
For purchasing, a simple conclusion follows from this: first the environment is determined, then the grade, and only then the size and the surface. The same pipe in 304 and 316L looks identical but behaves differently in a chloride environment.
| Material | Base | How it protects itself | Magnetic | Weldability |
|---|---|---|---|---|
| Stainless steel | iron and chromium | passive film throughout the whole volume | austenite weakly, ferrite and martensite strongly | good with the right technique |
| Aluminium | aluminium | its own oxide film | no | requires its own technique and weld protection |
| Galvanised steel | carbon steel | zinc layer on the surface | yes | zinc burns off, ventilation is needed |
| Carbon steel | iron and carbon | paint or another coating | yes | good |
Composition and structure
The composition of stainless steel is iron plus alloying elements, and each element solves its own task. Chromium is responsible for the passive film, that is, for the very property of "not rusting". Nickel turns the steel into an austenitic structure and adds toughness. Molybdenum increases resistance to chlorides. Titanium and niobium bind carbon so that it does not precipitate along grain boundaries after welding. Manganese partially replaces nickel in economy grades, while carbon adds strength but worsens weldability and resistance to intergranular corrosion.
| Element | What it is responsible for | Where the effect is visible |
|---|---|---|
| Chromium | passive film | overall corrosion resistance |
| Nickel | austenitic structure, toughness | ductility, deep drawing, operation at low temperatures |
| Molybdenum | resistance to chlorides | brines, marine environment, chemicals |
| Titanium, niobium | bind carbon | welded structures without heat treatment |
| Manganese | partially replaces nickel | economy grades of the 200 series |
| Carbon | strength | the lower it is, the better the weldability |
Nickel is the second most important element after chromium. It expands the austenitic region and makes the structure stable at ordinary temperatures. This is exactly why austenitic grades are ductile, bend and weld well, and after annealing hardly respond to a magnet. Economy grades of the 200 series replace part of the nickel with manganese: this lowers the cost but does not improve resistance.
Carbon in stainless steel is not only about strength. When heated in the dangerous range, it combines with chromium and forms carbides along grain boundaries. The boundaries become depleted in chromium, and intergranular corrosion starts there. Hence two technologies of counteraction: reduce carbon to hundredths of a percent, as in 304L and 316L, or introduce titanium and niobium, which intercept carbon before chromium does.
The structure of the steel depends on which elements predominate and how the steel has been processed. Austenitic grades are weakly magnetic after annealing, and after cold deformation they are pulled by a magnet noticeably more strongly. Ferritic and martensitic grades are always magnetic. That is why a magnet test says nothing about quality: it shows only the type of structure and the processing history of the part.
The density of stainless steel is about 7.9 g/cm³, that is, close to that of carbon steel. The mass of a part is calculated as volume multiplied by density, so switching from carbon steel to stainless steel hardly changes the weight of a structure. The difference is in behaviour, not in mass.
Classes of steel
Three designation systems circulate in trade and technical documentation. The American AISI gives a series number, the European EN a number of the form 1.4301, and the Russian ГОСТ encodes the composition directly in the name. An AISI grade does not tell you about the composition, so at purchase a cross-reference table is checked, and at acceptance the analysis of the batch is checked.
| System | Example | What it shows |
|---|---|---|
| AISI | 201, 304, 316L, 430 | series number and composition group |
| EN | 1.4372, 1.4301, 1.4404 | full composition under the European standard |
| ГОСТ | 12Х18Н10Т, 03Х17Н14М2 | the composition is encoded in the grade itself |
Decoding a ГОСТ grade is simple: 12Х18Н10Т reads as 0.12% carbon, 18% chromium, 10% nickel and titanium. The letter Т means titanium, М — molybdenum, Н — nickel, Д — copper, Г — manganese, Б — niobium. Thus 12Х18Н10Т turns out to be close to AISI 321, and 03Х17Н14М2 to AISI 316L.
The logic of the AISI series is simple. The 200 series is economical, with manganese instead of part of the nickel. The 300 series is austenitic, the most widely traded: 304 for ordinary conditions, 316 with molybdenum for chlorides and acids, 321 with titanium for elevated temperatures. The 400 series is ferritic and martensitic: 430 for decoration and household use, 410 and 420 for cutting tools and fasteners.
When choosing, it is useful to keep in mind not the number but the environment. Dry, without chlorides and without food requirements — 201 and 430 are enough. Water, food, damp rooms — 304. Brines, marine atmosphere, chemicals, washing with active chlorine — 316L. This way the grade stops being an abstraction and becomes a consequence of the conditions.
By structure and purpose, grades are divided into four groups.
| Group | Examples | Properties | Typical application |
|---|---|---|---|
| Austenitic | 304, 316L, 321, 201 | ductile, weakly magnetic, weldable | pipes, sheet, cookware, equipment |
| Ferritic | 430 | magnetic, cheaper, weld worse | decoration, household appliances, cladding |
| Martensitic | 410, 420 | magnetic, hardenable | knives, fasteners, tools |
| Duplex | 2205 | strong, resistant to chlorides | chemicals, marine environment, apparatus |
Separately stand the heat-resistant grades, which withstand high temperatures, and super-austenitic ones such as 904L for aggressive chemicals. The exchange catalogue trades four widely used grades — AISI 201, 304, 316L and 430 — and this set is enough for most tasks: from decoration to food equipment.
The standards divide the areas of responsibility: ГОСТ 5632 describes grades and composition, ГОСТ 5582 and 7350 — sheet, ГОСТ 9941 and 9940 — pipes, ГОСТ 2590 and 2591 — round bar. A reference to a specific standard in an order removes a dispute about tolerances before shipment.
What makes it stainless — and why AISI 304 rusts
What makes steel stainless is a passive film of chromium oxide. It forms by itself in air, has a thickness of fractions of a micron and instantly heals a fresh scratch if oxygen is nearby. As long as the film is intact, the metal beneath it does not interact with water and air. As soon as the film stops restoring itself, corrosion begins — and that is when people ask why the stainless steel has rusted.
Whether AISI 304 rusts or not is a question not about the grade but about the environment. In ordinary water, in air and in neutral solutions, 304 lasts for decades. Spots appear under specific conditions:
- chlorides — seawater, brines, de-icing agents, household chemicals with active chlorine;
- stagnant water and poor access of oxygen, when the film cannot restore itself;
- contamination of the surface with particles of ordinary steel after cutting or grinding with the same tool;
- a weld seam without cleaning and without pickling, where scale and a chromium-depleted layer remain;
- prolonged heating in the dangerous range, because of which carbon binds chromium along grain boundaries.
The corrosion rate also depends on whether a more active metal is nearby. Stainless steel paired with carbon steel or aluminium works as the cathode, and the less noble metal is destroyed faster. Where contact is unavoidable, a gasket is fitted, the joint is painted, or the metals are separated into different assemblies.
The condition of the surface influences it no less than the grade. A ground and polished surface retains moisture and dirt less, while scale after welding, on the contrary, creates stagnant zones and a chromium-depleted layer. After welding, the seam is cleaned and pickled to restore the passive film.
For such environments, 316L with molybdenum is taken, rather than trying to "reinforce" 304 with thickness. A rusty spot on 304 is a signal that the environment is more aggressive than assumed, or that the technology has been violated. More on the causes and the treatment — in the analysis why stainless steel rusts.
A magnetic check does not help here. Austenitic 304 hardly responds to a magnet after annealing, but after bending or drawing it is pulled by a magnet more strongly while remaining the same grade. Ferritic 430 is always magnetic and at the same time works honestly in decoration. A magnet is about structure, not about resistance.
Frequently asked questions
Does AISI 304 rust or not
Under ordinary conditions it does not rust: drinking water, air, neutral media. Spots appear in a chloride environment, with stagnant water, after contamination with ordinary steel or with a poorly cleaned seam. For such conditions 316L is needed, not 304.
Why the stainless steel has rusted
There are usually three causes: an aggressive environment with chlorides, a violated technology after welding or cutting, contact with carbon steel without insulation. First the source is determined, then the grade, the surface or the operating conditions are changed.
What stainless steel consists of
Of iron and alloying elements. The main one is chromium, at least 10.5%. Then, according to the task, come nickel, molybdenum, titanium, manganese, and also carbon, whose amount is kept low.
What kind of metal stainless steel is
It is steel, that is, an iron-based alloy. In industry classification, steel is assigned to ferrous metals, although chromium, nickel and molybdenum are themselves non-ferrous metals. More details — in the article stainless steel: ferrous or non-ferrous metal.
What is the melting point of stainless steel
The exact value depends on the grade: alloying elements shift the melting range relative to pure iron. For a specific grade, the figure is taken from a reference book or the batch certificate. In welding, what matters more is not the melting point but the crystallisation range and the susceptibility to hot cracking.
How stainless steel differs from aluminium
In its base and its properties. Stainless steel is heavier, stronger, welds better with familiar methods and holds its shape when heated. Aluminium is lighter and softer, but requires its own welding technique and weld protection. In contact with each other without insulation, both materials corrode faster.
How stainless steel differs from galvanised steel
Galvanised steel is carbon steel with a layer of zinc. As long as the layer is intact, it is protected, but a cut edge and a scratch expose the base. Stainless steel is protected throughout its whole volume, so it serves in a damp environment without a coating. On the other hand, galvanised steel is cheaper where dry conditions are sufficient.
Can stainless steel be welded with a semi-automatic welder
It can, but not with ordinary wire and not in carbon dioxide. Stainless steel welding wire and a suitable shielding gas are needed, more often an argon-based mixture. For thin metal and critical seams, argon-arc welding is more often used.
What to do next
If the composition and the class of the steel are clear, the next step is to choose a grade for the environment and place an order. The GCX exchange trades stainless steel products of grades AISI 201, 304, 316L and 430: round and profile pipe, sheet, round bar, angle. An order specifies the grade, the size, the surface and the shipment week, and exactly the metal that you have chosen goes into the trade.