Stainless Steel Corrosion
The main reason for the existence of the stainless steels is their resistance to corrosion. Chromium is the main alloying element, and the steel should contain at least 11 %. Chromium is a reactive element, but it and its alloys passivate and exhibit excellent resistance to many environments. Higher Cr contents may be necessary for to improve the corrosion resistance of stainless steels in more aggressive media. Among other alloying elements, Nickel is the most important : it is added to control the alloy microstructure and to improve the corrosion resistance in acidic or caustic media. The addition of other elements such as Molybdenum, Tungsten, Copper, Silicon, Titanium, Niobium, Nitrogen , … enables a wide range of properties to be obtained.
All stainless steels contain some carbon. Even though it is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more, modern melting processes allow to reduce the content below 0.015 or even 0.010%, providing stainless steels with superior resistance to intergranular corrosion. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for resisting corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.
Nitrogen is often added in modern stainless steels (especially in duplex grades or in highly alloyed austenitics); this element provides high mechanical properties, better resistance to pitting initiation and more generally improves the corrosion resistance thanks to a better microstructural stability.
From a microstructural point of view, alloying elements may be classified as either Austenite formers (Nickel, Nitrogen, Carbon, …) or Ferrite formers (Chromium, Molybdenum, …). Depending upon the ratio between austenite former elements and ferrite former elements, the resulting microstructure may be Ferritic (Body-Centered-Cubic crystal) or Austenitic (Face-Centered-Cubic crystal). The Martensitic structure (Close-Packed Hexagonal crystal) is the result of the transformation of austenite to martensite during cooling or during strong cold working. The Duplex microstructure is a mixture of Austenite and Ferrite (generally about 50/50%) which is the result of an accurate balance between Austenite and Ferrite former alloying elements.
A large number of stainless steels are available. Their corrosion resistance, mechanical properties, and cost vary over a broad range. For this reason, it is important to specify the exact stainless steel desired for a given application. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. The new interactive software CorrIntel™ covers the topic of the Corrosion Resistance and Metallurgy of Stainless Steels and Nickel base Alloys.
There are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex. The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are fully magnetic; duplex stainless steels are partly magnetic.
The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility.
Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications.