Corrosion resistance of stainless steels – All you need to know
The term corrosion (lat.: corrodere = disintegrate, erode) refers to the reaction of metal materials with their environment which may result in change and impairment of the metal. The reaction may be caused, for example, as a result of contact with oxygen (formation of rust), moisture or various salts. Corrosion resistance thus defines the ability of a metal to resist external influences and to remain inert in conjunction with other elements.
Corrosion-resistant stainless steels
Corrosion-resistant stainless steels are also referred to as chemical-resistant, rust-resistant, acid-resistant and heat-resistant steels and are alloyed stainless steels. The corrosion resistance is primarily dependent on the alloy elements, but the surface and structural conditions play a key part in the resistance of steel to external influences.
Alloy elements of corrosion-resistant stainless steels
Non-rusting corrosion-resistant stainless steels receive their special properties by alloying with chromium which, in conjunction with oxygen, forms a dense, firm and transparent protective layer made of chromium oxide. The chromium oxide layer is only a few nanometers in thickness yet still effectively protects the steel from corrosion. This is not the case with non-alloyed or low-alloyed steels. The passive layer is able to automatically renew itself in the event of damages like scratches, if there is sufficient oxygen and time for regeneration. A proportion of at least 10.5% chromium is necessary to make a steel corrosion-resistant. The element chrome is thus one of the main alloying elements. Other alloying elements are, in particular, nickel which makes steel more resistant to many acids if a quantity upwards of 10% is included. When even greater demands are made of the materials, molybdenum, in particular, is added to the alloy. These individual procedures are more complex than can be explained at this time. If no passive layer is present on the steel, electro-chemical elements may form, especially in conjunction with moisture, which may attack the steel.
Structural conditions of corrosion-resistant stainless steels
For high corrosion-resistance, it is important to ensure that the stainless steel has as stable and homogeneous structure as possible. The corrosion-resistant steels are divided into four main groups in accordance with their structural states.
Ferritic stainless steels
Ferritic steels with a proportion of 11% to 13% Cr are considered slow to rust because they only have a low corrosion resistance. Others with around 17% Cr have a higher corrosion resistance, which may be further improved by the addition of around 1% Mo.
Martensitic stainless steels
The martensitic steels with a proportion of around 12-18% Cr and carbon content as of approximately 0.2% and higher obtain higher mechanical strength properties thanks to their hardness than the remaining stainless steels. The corrosion resistance is acceptable, but not sufficient for applications such as the food industry or medical technology.
Austenitic stainless steels
Austenitic steels are alloyed with around 18% chrome and at least 10% nickel. Their key feature is their high rust and acid resistance, which can be further improved with increased Cr and Ni content.
Duplex stainless steels
Duplex steels have an austenitic-ferritic structure and have greater strengths than austenitic steels. The good mechanical properties and the corrosion-stability has resulted in austenite being replaced by duplex steel in many applications.
Applications of corrosion-resistant stainless steels
As has already been mentioned, there are a variety of corrosion-resistant steels and thus also a variety of potential applications. Rust-free steels are available as rolled, forged or cast materials. Applications can be found in automotive engineering, the food processing industry, medical technology, the tap fitting industry, fixture technology and many more sectors.
Hardening corrosion-resistant stainless steel with BORINOX®
BORINOX® process for stainless steel hardening can be used to harden duplex steels, martensitic steels capable of precipitation-hardening, austenitic steels, as well as nickel-based alloys without reducing the corrosion-resistance of the material. The surface hardness is increased, according to steel type, from 280 HV to 1.700 HV. Austenitic steel is particularly well-suited for treatment with BORINOX®.
The material and the selection of suitable upstream manufacturing processes play a key role in the corrosion-resistance of each application. Our materials specialists would be happy to provide qualified consultation as required.
