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Corrosion

Even stainless steels get corrosion. Only in the case of stainless steels, corrosion may not create the visible rust seen on the surface of common steels. So the corrosion effects of stainless steels can be sudden and devastating. Corrosion of stainless steels occurs in many ways

Microarrays

Micro-corrosion or corrosion by acupuncture or pitting occurs when stainless steel is exposed to an environment lacking oxygen or in an environment where other ions compete with oxygen as an oxidizing agent. Thus, e.g. When a stainless steel is exposed to chloride solutions, the protective layer of Cr2O3 is destroyed by the Cl-anions, resulting in microscopic recesses on the surface of the steel. The recesses may develop into cracks which, under some relatively low stress, grow at high speed with devastating effects

Microarrays are often seen in cavities or welds of stainless steel parts. In this case, it is a matter of cavity erosion, cavernous erosion or corrosion of corrosion. Cavity erosion can be intense even at a relatively low temperature.

Pericrystalline erosion


Perrystalline corrosion of stainless steel as observed in the metallographic microscope.

Intergranual corrosion occurs when stainless steel is heated to form chromium (Fe, Cr) 7C3, and so on) around the alloy crystals. These carbides replace chromium oxide and thus steel loses its protection. Pericrystalline corrosion depends on the carbon content of the alloy. Stainless steels with 0.06% C get peristaltic corrosion within 2 minutes at 700 ° C; .On the contrary, stainless steels with 0.02% C do not get perrystalline corrosion. Crystalline corrosion is observed after welding of stainless steels due to local overheating of the alloy.

Perrystalline corrosion can be reversed by heating the alloy to 1000 ° C, dissolving chromium carbides, and quenching ("paint"). Stainless steels which contain titanium, niobium or tantalum exhibit high resistance to perrystalline corrosion

Corrosion with mechanical stress

Corrosion cracking or stress corrosion cracking is a complex phenomenon observed when stainless steel is under mechanical stress in a corrosive environment, such as, for example, In chloride solutions. Cold-rolled steels are more sensitive to corrosion with mechanical stress, due to residual stresses. With annealing, residual stresses disappear and steel recovers its corrosion resistance

Mechanical stress corrosion is associated with the creation (or simply the presence) of structural flaws in the alloy crystalline matrix. These imperfections are spread out to the surface of the alloy resulting in the local wear of the protective oxide Cr2O3, cracking and ultimate failure of the alloy.

The austenitic stainless steels AISI-SAE 304 and AISI-SAE 316 easily fail due to mechanical stress in mixtures containing minimal mg / L Cl- when the temperature exceeds 50 ° C. Austenitic stainless steels with a high molybdenum (> 6%) or nickel content, ferritic and biphasic stainless steels show better corrosion resistance with mechanical stress.

Electrochemical erosion

Electrochemical or galvanic corrosion occurs when two different metals are in contact with one another. Then a local galvanic element is created resulting in the erosion of the most electroplating metal. If, for instance, a few common steel beads are found on the stainless steel surface, they will begin to oxidise due to electrochemical corrosion, and then corrosion can extend to the surface of the stainless steel. This is also the case for contact corrosion. Contact corrosion is prevented by suitable cleaning of the alloy with nitric or hydrofluoric acid.

Other forms of erosion

ηStainless steel can suffer other forms of corrosion, such as rouging when in contact with excess water, mechanical erosion (sulphide ergot corrosion) when in contact with hydrogen sulphide (H2S) at a temperature of 60-100 ° C C, etc


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