Stainless steel rusts with ferrous alloys

Contrary to popular belief, stainless steel is not fundamentally stainless. Decisive are the alloying partners and their contact with the steel surface. When ferrous stainless ferritic metals are present, stainless steel is rusting. Depending on the proportion and oxidation behavior, superficial rust or deep corrosion may occur.

Oxide layers and chemical refinement

Of the approximately 1200 types of stainless steel that are produced, about twenty percent are corrosion-resistant or rust-free. If iron oxide can form in the stainless steel due to participating alloying partners, a sealing of the stainless steel is possible. In this way, additional stainless or corrosion-protected stainless steels are produced.

The most typical protective measures to prevent the formation of iron oxide and thus rust are electromagnetic and chemical processing methods. When the stainless steel is burnished, the ferritic alloy is immersed in an acid bath. The surface closes by chemical reaction and forms an oxygen-insensitive protective layer.

When anodising stainless steel, the electromagnetic reaction of special molecules of the existing alloying partners is used. The supplied voltage causes a controlled oxidation, which leads to an oxide layer several nanometers thick. A similar principle is based on the galvanizing of stainless steel, which uses the specific molecular charges of the zinc molecules.

Pitting and rusting

Even though the surfaces of stainless steel always remain stainless, both methods have the risk of pitting, which can occur during drilling or milling. As the protective oxide layer is broken, the inside of the stainless steel reacts with oxygen unprotected. Existing ferrous alloy partners will rust the interfaces and drill holes. This can only be avoided by using completely iron-free alloys.

Frequently, tarnished stainless steel is mixed up with corroded metal. That may be the case, but it does not have to be. Surface haze must be checked by hand. If a change results from mechanical action such as polishing, it usually involves the removal of rust or a film of dirt, often consisting of grease.

Steel grades and corrosion behavior

Stainless steels are divided into material number groups, which are united by the same characteristics. Common stainless steels that can rust are:

  • No. 1.4301, also known as V2A steel
  • No. 1.4571 & 1.4404, also known as V4A steel

A common corrosion-free and stainless steel is No. 1.4116, also known as knife steel. To ensure ferromagnetic inductive ability, iron, nickel and cobalt can be used in the alloy. Special manufacturing processes also produce magnetizable chromium dioxide, which has the distinct advantage of preventing corrosion at the same time. A chromium content of over 10.5 percent in the alloy ensures that stainless steel does not rust.

Tips & Tricks

If you use stainless steel that requires induction, the magnetism required always requires iron-containing ferritic alloy components. For stainless pots and pans, only a surface treatment by anodizing or burnishing can be present. Any damage to the seal will cause the stainless steel to rust.

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