Corrosion is a process that causes the deterioration of metals due to interaction with the surrounding environment.
This phenomenon can manifest in differents forms depending on several factors. In the case of stainless steel, corrosion can be prevented by choosing the most suitable material for each situation.
Below are the main types of corrosion.
PITTING OR CREVICE CORROSION
Pitting corrosion involves the formation of holes on the metal’s surface. These holes can be concealed and tiny, but also deep enough that they compromise the integrity of the material.
Crevice corrosion, on the other hand, is about the corrosion in cracks and all the narrow gaps where corrosive agents can stagnate. For example, this phenomenon is common in bolted joints, beneath rubber or plastic gaskets, in the gaps within two materials fixed together, and in structures exposed to constant humidity.
To oppose these types of corrosion it is important to use molybdenum-containing steel, such as austenitic steel grades: A4 (Aisi 316), A5 (material 1.4571) and A8 (material 1.4529)
STRESS CORROSION CRACKING (SCC)
Stress corrosion cracking (SCC) is the deterioration of the metal caused by the mechanical stress to which the piece is subjected combined with a corrosive environment.
This is also a subtle phenomenon because often the damage is noticed only when it is too late. SCC leads to the formation of tiny cracks in the metal, the accumulation of which can result in a sudden break.
The most common austenitic grades are susceptible to SCC, as they are very sensitive to mechanical stress. For this reason, Duplex steels are recommended. Duplex combines the austenitic structure with the ferritic one, providing corrosion resistance properties similar to austenitic grades while offering superior mechanical properties.
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Standard Duplex (1.4462) in fact has better corrosion resistance characteristics than the austenitic grades up to A5.
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Super Duplex (1.4410) is comparable in this sense to the super austenitic A8 (1.4529).
In case of stress corrosion cracking, grade A8 (1.4529) can also be used.
INTERGRANULAR CORROSION
Intergranular corrosion is the type of corrosion that occurs at the most microscopic level, as it attacks the grains (set of molecules) that compose the metal.
Steel becomes more vulnerable when subjected to hot working or high temperatures. It is also more frequent in materials with a higher carbon concentration.
Therefore, the best materials are:
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“Low carbon” austenitic grades: A2L (AISI 304L) and A4L (AISI 316L)
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Titanium or niobium stabilized grades: A5 (AISI 316 Ti or 1.4571)
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A8 (1.4529)
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All Duplex grades