Is stainless steel the same as stainless steel?

Is all stainless steel rustproof?

Stainless steel is a steel of particular purity. The requirements for this are regulated in the EN 10020 standard. Stainless steel is often associated with corrosion-resistant properties. Strictly speaking, however, these properties only apply to the subgroup of stainless steels. If stainless steel lacks chromium, it will rust just like conventional steel.

A stainless steel only becomes a rust-resistant steel once a certain amount of chromium is present in the steel. The correct designation for this type of steel is then “stainless steels” or NiRoSta for short.

The minimum requirements for stainless steels according to standard EN 10088 are as follows

• At least 10.5% chromium content
• Maximum 1.2% carbon

Why can stainless steel rust? What makes stainless steel so resistant to rust?

From 10.5% chromium content in the steel, the so-called passive layer is formed. The passive layer is only a few atomic layers thick and protects the material from environmental influences. The passive layer is also known as the barrier layer. It forms independently under the influence of oxygen (air or water) and protects the material from corrosion. The barrier layer is self-healing. If it is damaged/scratched, it forms again by itself (under the influence of oxygen).

This functional principle of the passive layer is the same for all stainless steels. The composition (alloying additions) of the steel has a significant influence on the resistance of this passive layer.

What stainless steels are available?

After chromium, nickel is the second most important alloying addition. Nickel does not directly inhibit rusting. It has a supporting effect. The corrosion-resistant passive layer forms faster and becomes more compact with a comparable proportion of chromium.

From a metallurgical point of view, stainless steels are divided into chromium steel and chromium-nickel steel.

The most important alloy components of stainless steel

The alloying additions give the stainless steel different properties.

Chrome

• Is responsible for the corrosion resistance of stainless steels and the formation of the protective passive layer.
• The higher the chromium content, the more corrosion-resistant the steel (almost linear ratio)
• Increases the yield strength and tensile strength

Nickel

• Makes the passive layer more compact
• The resistance to aqueous media and acids is significantly increased
• Increases the polishing/polishing ability
• Promotes rapid independent restoration of the passive layer

Molybdenum

• Increases resistance to sulphuric acid and formic acid
• Increases resistance to pitting and crevice corrosion on contact with media containing chlorine
• Increases the yield strength and tensile strength

Titanium

• Stabilizes the steel at higher application temperatures (300-400 degrees Celsius)
• Prevents chromium depletion when exposed to heat (e.g. welding)
• Reduces notched impact strength as well as ductility and machinability

Manganese

• Partly performs a similar function to nickel
• Is cheaper than nickel and can be used as a nickel substitute to a certain extent

Other alloy surcharges can be:

• Aluminum
• Copper
• Cobalt
• Carbon
• Niobium
• Sulphur
• Silicon
• Sticksoff
• Vanadium
• Tungsten

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