What makes stainless steel stainless: passive layer, steel groups, designations

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

Chromium. From about 10.5 % chromium, the steel spontaneously forms an extremely thin layer of chromium oxide in air, the passive layer. It is only a few nanometres thick and invisible, yet it protects the steel against many media. If it is damaged, it grows back by itself as long as oxygen reaches the surface.

Nickel, molybdenum, nitrogen, titanium and niobium further improve corrosion resistance and determine microstructure and strength. Stainless steels are standardised in EN 10088.

For welding, this leads to the most important principle of this series: the weld is only as resistant as its surface. Anything that disturbs the passive layer or lowers the chromium content at the surface makes the steel vulnerable at that point.

When the passive layer cannot form, or cannot form completely. This happens when oxygen is lacking or when too little chromium is available at the surface.

• No oxygen: In narrow gaps, under washers, under deposits or in stagnant liquid, the passive layer does not renew itself. If a corrosive medium penetrates, crevice corrosion begins.
• Foreign iron: Grinding dust and particles of carbon steel, for example from tools, brushes or supports, rust themselves and prevent the passive layer from forming underneath. The result is rust spots on stainless steel, known as extraneous rust.
• Temper colours and scale: The coloured oxide layers next to the weld are thicker than the passive layer but not protective, and the surface underneath is depleted of chromium. They must be removed, see Temper colours, pickling, passivation.
• Chromium carbides: If a steel with too much carbon remains too long in the range of about 450 to 850 °C, the carbon binds chromium at the grain boundaries. Next to them, the chromium content falls below the necessary limit and the steel becomes susceptible to intergranular corrosion.
• Chlorides: Salt water, road salt and chloride-containing cleaning agents attack the passive layer locally. Pitting corrosion results, and with warm media and tensile stress also stress corrosion cracking.

By microstructure, a distinction is made between austenitic, ferritic, austenitic-ferritic (duplex) and martensitic steels. Welded components almost always use austenitic steels, increasingly also duplex and ferritic grades.

The microstructure that forms in the weld metal depends on the ratio of the ferrite-forming elements (chromium, molybdenum, silicon, niobium) to the austenite-forming ones (nickel, carbon, manganese, nitrogen). It can be estimated with the Schaeffler diagram; more on this in Microstructure, delta ferrite and hot cracking and in the calculator for the Schaeffler, DeLong and WRC diagrams.

Under EN 10088, a steel has a material number such as 1.4404 and a steel name such as X2CrNiMo17-12-2. The steel name reveals the composition: X stands for high-alloy, the 2 for 0.02 % carbon, followed by the alloying elements and their contents in per cent, here around 17 % chromium, 12 % nickel and 2 % molybdenum.

In international trade, the American AISI designations are common. They correspond only approximately to the European grades; the permitted analyses differ in detail. For material selection and the welding procedure specification, the designation in the order or the inspection certificate always applies.

An “L” in the AISI designation (low carbon) stands for a maximum of 0.03 % carbon. “Ti” means titanium-stabilised.

Austenitic steel conducts heat only about a third as well as carbon steel and expands around a third more. Heat builds up at the weld, the component distorts more and temper colours extend further.

• Thermal conductivity: at room temperature around 15 W/(m·K) for austenitic steels, around 25 for ferritic, compared with about 50 for unalloyed carbon steel.
• Thermal expansion: around 16 · 10⁻⁶ per kelvin for austenite, 10 to 13 for ferritic and duplex steels, 12 for carbon steel. Ferritic grades therefore distort about as little as carbon steel.
• Electrical resistance: three to four times as high as for carbon steel. In MIG/MAG welding, the wire stickout heats up more, which affects the settings.

In addition, there are material-specific risks: hot cracks in fully austenitic weld metal, grain coarsening in ferritic steels, hardening cracks in martensitic steels and brittle precipitates such as the sigma phase if duplex steel stays too hot for too long. How to control the heat is shown in Heat input and interpass temperature.

Guide values according to ISER information sheet 823 (2019), Table 1. The values of individual grades differ.

Because they remain resistant to intergranular corrosion even after welding. With a maximum of 0.03 % carbon, hardly any chromium carbides form in the heat-affected zone. Alternatively, titanium or niobium binds the carbon before it can bind chromium.

Steels with more than 0.03 % carbon, such as 1.4301, can become susceptible in the heat-affected zone at wall thicknesses above about 6 mm without post-weld heat treatment, depending on the corrosive load. With thin sheet, the weld cools so quickly that this rarely matters.

Carbon can also come from the welding process: shielding gases with a lot of CO₂ release carbon to the weld metal and can thus make it susceptible. This is why MAG welding of stainless steels uses only a few per cent of CO₂ or O₂, see Shielding gases.

Resistance to pitting corrosion is estimated using the pitting resistance equivalent number (PREN). The higher it is, the more chloride the steel can tolerate.

For duplex steels, the calculation is: PREN = % Cr + 3.3 × % Mo + 16 × % N. Molybdenum therefore counts more than three times. For simple austenitic steels, nitrogen is usually left out; for high-alloy austenites it counts with a factor of 30.

• 1.4301: around 18 to 20
• 1.4404: around 24
• 1.4362 (lean duplex): around 25
• 1.4462 (duplex): around 35
• 1.4410 (super duplex): around 42; from 40 upwards it is called super duplex

Guide values for duplex steels according to DVS 0946, Table 1; austenites calculated from the nominal analysis. The PREN is a comparative value for the base material. The weld is often less resistant, especially with temper colours. This is why the filler metal usually contains slightly more alloy than the base material.

In construction, EN 1993-1-4 assigns the steels to corrosion resistance classes CRC I to V according to the environment (chlorides, sulphur dioxide, cleaning). There, this also applies to the welds, provided they have been properly post-treated.

As a rule of thumb, the filler metal is chosen to match the base material or slightly higher alloyed. For 1.4301 and 1.4307, this is 308LSi; for 1.4404 and 1.4571, 316LSi. Joints with carbon steel are welded with an over-alloyed filler, usually 309LSi.

• 308LSi (ML 19 9 LSi): for 1.4301, 1.4307, 1.4541
• 316LSi (ML 19 12 3 LSi): for 1.4401, 1.4404, 1.4571
• 318Si (ML 19 12 3 NbSi): when a stabilised weld metal is required
• 2209 (ML 22 9 3 NL): for duplex steels such as 1.4462
• 309LSi (ML 23 12 LSi) and 307 (ML 18 8 Mn): dissimilar joints with carbon steel and buffer layers

The selection in detail is explained in Choosing the filler metal. All wires: stainless steel welding wire; assignment to the base materials: material selection stainless.

Is stainless steel magnetic?

Austenitic steels such as 1.4301 are not magnetic in the annealed condition. Cold-worked areas and the weld metal can be slightly magnetic, the latter because of its intended delta ferrite content. Ferritic, martensitic and duplex steels are magnetic. A magnet therefore cannot tell you whether a steel is stainless.

What is the difference between 304 and 316?

316-type steels such as 1.4404 and 1.4571 contain around 2 % molybdenum and are therefore more resistant to chlorides, for example near the sea, in swimming pools and in the food industry. 304 (1.4301) is sufficient for many indoor applications.

Why is my stainless steel weld rusting?

Usually because of temper colours that were not removed, foreign iron from tools that were also used on carbon steel, or a filler metal with too little alloy. The causes and how to remedy them are shown in Weld defects and corrosion damage.

Do I need to preheat stainless steel?

Not austenitic steels; preheating tends to do harm. For duplex steels it can make sense with large wall thicknesses; for martensitic steels it is necessary. Ferritic steels are welded with as little heat as possible.

The series from the material via process, gas and root protection to the tested weld. For an overview, see Welding of stainless steel.

1. Microstructure, delta ferrite and hot cracking
2. Choosing the filler metal
3. Processes at a glance
4. Shielding gases
5. Heat input and interpass temperature
6. Joint preparation and tack welding
7. Root protection and purging
8. Temper colours, pickling, passivation
9. Duplex and super duplex
10. Joining stainless to carbon steel
11. Weld defects and corrosion damage
12. Testing, health and safety