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Extraneous rust and how it forms

Because of their constituent parts, austenitic stainless steels such as A2 and A4 fall under the category of "active" corrosion protection.

These high-grade
stainless steels must contain at least 16 % chrome (Cr) and are resistant to oxidising corrosive agents. Increasing the Cr content and if necessary other alloy components such as nickel (Ni), molybdenum (Mo), titanium (Ti) and niobium (Nb) improves resistance to corrosion. These additives also affect the mechanical properties. Depending on use, this may have to be noted. Other alloy components are only added to improve the mechanical properties, e.g. nitrogen (N), or the chip-removing process, e.g. sulphur (S).

The fasteners may experience a certain degree of magnetisability during cold working. Austenitic stainless steels are not however generally magnetic. But the resistance to corrosion is not affected by this. The level of magnetisation produced by cold work hardening may even extend to the steel part sticking permanently to a magnet.

In practice it should be noted that a whole series of different types of corrosion may arise. The most common forms of corrosion for high-grade stainless steel are shown in the diagram below and detailed underneath:

Diagram of the most common types of corrosion in screw connections


When particles of a carbon steel ("normal steel") adhere to a stainless steel surface, this produces extraneous rust on the surface of the stainless steel which turns into rust under the action of oxygen. If these areas are not cleaned or removed, this rust can cause electro- chemical localised corrosion in austenitic stainless steel.



Extraneous rust is produced for example by:


      using tools which have previously been used with carbon steel.

      sparks when working with an angle grinder or grinding dust or during welding.

      objects that rust coming into contact with a stainless steel surface.

      water containing rust dripping onto a stainless steel surface.