TMR Stainless
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Stainless Steel Information


What is Stainless Steel?

 

Stainless steels are iron-chromium alloys with a minimum of 10.5% chromium. When the steel contains at least that much chromium, a thin, transparent, protective passive film forms on the surface. The passive film forms spontaneously as a result of the reaction between the chromium in the steel and the oxygen in the air. The passive film is a barrier between the steel and the environment and prevents corrosion attack of the underlying steel as long as the passive film remains intact. This protective passive film is quite thin, on the order of 10 to 20 atoms thick, and its actual composition depends on the alloying elements in the stainless steel and the environment to which it is exposed. For example, the passive film on a piece of Type 304 stainless is not quite the same when it is exposed to air and when it is exposed to potable water.

Role of Alloying Elements

 

Chromium is the essential ingredient in stainless steels. As the chromium content is increased above 10.5%, the passive film becomes stronger and is able to resist more aggressive environments, particularly those containing chlorides. High chromium also helps the passive film to heal itself more rapidly if it is disrupted, for example, by scratching of the surface.

 

At elevated temperatures the chromium reacts with the oxygen in the air to form a thick, visible oxide layer. The color and thickness of the chromium oxide will depend on the temperature and time of exposure. The chromium oxide layer is visible and is thick enough to be scraped off and measured. This is quite unlike the thin, transparent, protective, passive film.

 

Various alloying elements are added to stainless steels to improve corrosion performance in specific environments, or to modify or improve the mechanical, or physical properties of the stainless steel. There are several hundred different stainless steels, all formulated to provide a specific combination of corrosion resistance, weldability and mechanical properties.

 

Nickel is the most common alloying element in stainless steels. It changes the crystal structure of the steel from ferritic to austenitic. The austenitic structure has improved ductility, formability and weldability. Nickel also improves corrosion resistance in reducing environments such as sulfuric acid. The most common stainless steel is Type 304 which has about 18% Cr and 9% Ni.

 

Molybdenum is added to stainless steels to improve their resistance to pitting and crevice corrosion in chloride-containing environments. Type 316 is the most common Mo stainless steel. Its nominal composition is 17%Cr-10%Ni-2%Mo.

 

Since about 1970, nitrogen has been an important alloying addition to stainless steels. The high performance austenitic stainless steels and the second-generation duplex stainless steels all contain a deliberate addition of 0.10 to 0.50% nitrogen. For these stainless steels, nitrogen improves pitting and crevice corrosion resistance, makes them stronger, and retards the formation of sigma phase during welding. In duplex stainless steels, nitrogen promotes the re-formation of austenite at higher temperatures and helps maintain an acceptable austenite-ferrite phase balance in the as-welded condition. Precipitation of intermetallic phases such as sigma reduce the toughness and corrosion resistance of stainless steels.

 

Additions of copper to a stainless steel increase the corrosion resistance in reducing environments such as sulfuric acid. Alloy 20 and 904L are examples of stainless steels with deliberate copper additions.

 

Other alloying additions are used to enhance specific properties. For example, sulfur is added to the free machining stainless steel, Type 303, for improved chip breaking during turning operations. Aluminum and silicon additions improve the oxidation resistance of other stainless steels.

 

Stainless Steel Corrosion

When stainless steels are selected properly, fabricated correctly and maintained adequately, they will perform without attack indefinitely. This is the case in the vast majority of applications. However, if the environment becomes overly aggressive, for example due to a process change or a process upset, the passive film may be overwhelmed, usually on a very localized basis, and no longer be a protective barrier for the stainless steel underneath. Corrosion will then occur. For stainless steels, there are several forms of possible attack including: pitting, crevice corrosion, stress corrosion cracking, galvanic corrosion, and intergranular corrosion.