Types of Stainless Steel Corrosion – Corrosion Resistance of Stainless Steel Series

2022.7.28

In many industrial applications, stainless steel can provide great corrosion resistance, this is the reason why stainless steel does not rust. In addition to mechanical failure, a serious form of corrosion of stainless steel is local corrosion. These failure cases caused by local corrosion account for almost half of the total. In fact, many failures can be avoided through reasonable material selection. What are common types of corrosion in stainless steel? Let’s learn about the corrosion resistance of different stainless steel grades. 

Types of Stainless Steel Corrosion

– Stress corrosion cracking: is a general term that refers to the mutual failure of stressed alloys due to the expansion of severe cracks in corrosive environments. Stress corrosion cracking has brittle fracture morphology, but it may also occur in materials with high toughness. The necessary condition for stress corrosion cracking is the existence of tensile stress (residual stress, applied stress, or both) and a specific corrosion medium. The formation and expansion of the pattern are roughly perpendicular to the direction of tensile stress. The stress value causing stress corrosion cracking is much smaller than the stress value required for material fracture in the absence of a corrosive medium. On the micro level, the crack passing through the grain is called a transgranular crack, and the crack extending along the grain boundary is called an intergranular crack. When the stress corrosion crack extends to a depth, the material will be broken according to the normal crack (in ductile materials, it is usually through the polymerization of micro defects). Therefore, the cross-section of parts that fail due to stress corrosion cracking will contain the characteristic area of stress corrosion cracking and the area associated with the polymerization of micro defects.

– Pitting corrosion: is a form of local corrosion that leads to corrosion.

– Intergranular corrosion: intergranular boundaries are the boundaries of disordered dislocation between grains with different crystallographic orientations. Therefore, they are the segregation of various solute elements or metal compounds in a favorable area for precipitation. It is not surprising that in some corrosive media, the intergranular boundary may be corroded first. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in specific corrosive media.

– Crevice corrosion: it is a form of local corrosion, which may occur in the crevice where the solution stagnates or on the shielded surface. Such a gap can be formed at the junction of metal and metal or metal and non-metal, for example, at the junction with rivets, bolts, gaskets, valve seats, loose surface sediments, and marine organisms.

– General corrosion: a term used to describe corrosion that occurs on the entire alloy surface in a relatively uniform manner. When comprehensive corrosion occurs, the village material gradually becomes thinner due to corrosion, and even the material corrosion failure. Stainless steel may show overall corrosion in strong acid and alkali. The failure caused by total corrosion is not very worrying, because this kind of corrosion can usually be predicted by a simple immersion test or by consulting the literature on corrosion.

Corrosion Resistance of Stainless Steel Series

Ferritic stainless steel (chromium steel, such as 400 series)

Stainless steel, which is mainly ferritic in service, has a body-centered cubic crystal structure with a CR mass fraction of 11% – 30%. Compared with austenitic stainless steel, this kind of steel generally does not contain Ni, and sometimes contains a small amount of Mo, Ti, Nb, and other elements. The application of Argon Oxygen Decarburization and Vacuum Oxygen Decarburization can greatly reduce the elements such as C and N. It has the advantages of Ni saving, low price, good stress corrosion resistance, and good oxidation resistance. It is mostly used in environments resistant to atmospheric, steam, water vapor, and oxidizing acid corrosion, It has certain corrosion resistance in dilute nitric acid and weak organic acid at room temperature, so this kind of steel is widely used. However, it is not resistant to reducing acid and other media corrosion. The general ferritic stainless steel has some disadvantages, such as poor impact toughness, poor plasticity and corrosion resistance after welding, sensitivity to intergranular corrosion, poor pitting corrosion resistance, and more, which limits its use.

 

Austenitic stainless steel (chromium-nickel steel, such as 300 series and 200 series)

Austenitic stainless steel is widely used in various pressure components of pressure vessels because of its non-magnetic properties, high strength, toughness, and ductility in a wide temperature range, easy cold processing such as rolling and pressing, and resistance to oxidation medium corrosion. Because austenitic stainless steel has a face-centered cubic structure, it does not undergo phase transformation and is easy to weld. Austenitic stainless steel generally has no cold brittle transition temperature, so it is often used as low-temperature steel.

 

18-8 series solid solution stainless steel has good corrosion resistance in oxidizing acid, atmosphere, water, steam, and other media, and the intergranular corrosion resistance of low carbon or stabilized element Ti or Nb is better, and the pitting corrosion resistance of Mo is better. Those containing Mo, Cu, and other elements can also resist the corrosion of nonoxidizing acids such as dilute sulfuric acid and phosphoric acid and organic acids such as formic acid and acetic acid. However, the pitting corrosion resistance is poor, and stress corrosion cracking is easy to occur in halide solution. Creep and sensitization will occur after long-term use at temperatures above 500 ℃ -600 ℃.