Stainless does not mean rustproof – three steps to corrosion resistance

Rule of thumb

Three steps: the German word *Edelstahl* means a steel of special purity – stainless steel has at least 10.5 % chromium – and it is resistant only in the environment it was chosen for. *Stainless* means low-staining, not rust-free.

Where does the rule come from?

From the complaint call: "You sold us stainless steel, and now it is rusting." Behind it are two confusions that everyday language merges. The first: in German, *Edelstahl* has become the consumer word for stainless steel, although the steel standard uses it for something else. The second: "rustproof" sounds like an unconditional promise – whereas the English term *stainless steel* describes the same material more cautiously, as low-staining steel. The rule breaks the word down into the three steps that really stand behind it.

What is technically correct?

Step 1 – special steel (Edelstahl). According to the definition of steel, *Edelstahl* is any alloy or non-alloy steel of special purity, above all low in sulphur and phosphorus. This includes tool steels and quenched-and-tempered steels, which rust like any structural steel. The word says nothing about chromium and nothing about corrosion.

Step 2 – stainless. What the piping trade means is stainless steel (in German collectively "Edelstahl Rostfrei"): at least 10.5 % chromium, which forms a dense passive layer of chromium oxide on the surface. It is the barrier against the surrounding medium and heals itself under oxygen when damaged. More chromium, plus nickel and molybdenum, make the layer more resistant – that is the difference between 1.4301 and 1.4404, between 1.4404 and duplex.

Step 3 – resistant only in the matching environment. The passive layer holds as long as nothing breaks through it. What breaks through it is known: chlorides from sea air, de-icing salt, chlorinated water or brine baths (pitting), narrow crevices under compression and push-fit joints (crevice corrosion), chlorides plus tensile stress plus heat (stress corrosion cracking), sulphur dioxide in industrial air, dried-on deposits without cleaning, cleaners containing hydrochloric acid – and the medium in the line itself. Water alone does not attack; concentration in wet-dry cycles does. Whether a stainless steel is resistant in an environment can therefore only be described together with the material: for that there are the corrosion resistance classes (CRF/CRC to EN 1993-1-4 for components outdoors), the media lists of the system manufacturers and the resistance tables of the material manufacturers.

Where does the rule end?

• It does not say which stainless steel is right where – the designer decides that by environment, medium, temperature and cleaning.
• Rust stains are not always rust of the stainless steel: foreign rust from steel chips, flash rust from the surroundings and temper colours from welding sit on the surface and can be removed; pitting sits in the material.
• "The magnet sticks" does not mean "not stainless": ferritic stainless steels (1.4520, 1.4521) are magnetic and still stainless.
• The rule applies to steel. Copper, brass and gunmetal have their own forms of corrosion (dezincification, erosion) and their own rules.

What does the standard say?

The classification of steels by purity grade (special steel) is in EN 10020; the stainless steels with their material numbers in EN 10088; the choice of material for components outdoors by corrosion resistance class in EN 1993-1-4. For piping, the manufacturer's media and system approval applies. None of these standards knows "rustproof" as a property – they know material numbers and conditions.

DIN/EN versus ASME/ASTM

The same thing, a more honest word: *stainless steel* – low-staining steel – and for resistance, data sheets speak of *corrosion resistance* under stated conditions, never of *rustproof*. The ASTM types (304, 316, 444) are the same material families as 1.4301, 1.4401 and 1.4521; the environment decides there as here.

Worked example

A railing made of 1.4301 on a road with winter gritting shows brown dots on the underside of the handrail after two winters. Not a material defect: de-icing salt spray dries where no rain rinses it off, the chlorides concentrate, the passive layer breaks through locally – the beginning of pitting. Two ways: 1.4404 or duplex for this environment, or regular washing of the surfaces not exposed to rain. The same railing five metres further on, freely exposed to rain, stays bright.

Zickwolff reference

We supply stainless steel by material number and certificate – as the mill certifies it. Whether it is resistant in your environment is determined by the designer; we tell you which materials we have from stock and on request, and pass on the manufacturers' approvals. The material profiles (austenitic, ferritic) and the page on mixed installation and corrosion show where the differences lie.

Checked today

Traffic light: true (class A). The three steps follow the definition of steel, the definition of stainless steels and the corrosion data sheets of the Informationsstelle Edelstahl Rostfrei.

Further reading

• Materials compared: DIN, EN designations and ASTM by product form – Materials overview with the profiles
• Austenitic stainless steels 1.4301, 1.4307, 1.4401, 1.4404, 1.4571: which one for which line – and how to recognise it – 1.4301, 1.4307, 1.4401, 1.4404, 1.4571
• Ferritic stainless steels 1.4520 and 1.4521: nickel-free, magnetic, in system tubes – and how to recognise them – 1.4520, 1.4521: magnetic and stainless
• Mixed installation and corrosion: flow rule, material pairings, drinking water – Flow rule and chlorides in the installation
• Store stainless pipes dry, separately and without film – why a pipe can start corroding in the warehouse – the sister rule for the warehouse