The stainless characteristic of stainless steel is due to the special passivation protective film on the surface of the steel plate. First, the corrosion resistance mechanism of stainless steel, that is, the theory of passivation film, is briefly introduced.

The so-called passive film is a thin film mainly composed of Cr2O3 on the surface of stainless steel. Due to the existence of this film, the corrosion of the stainless steel substrate in various media is hindered, and this phenomenon is called passivation. There are two cases for the formation of this passivation film. One is that stainless steel itself has the ability of self-passivation, and this self-passivation ability is accelerated with the increase of chromium content. Another broad formation condition is that stainless steel forms a passivation film in the process of being corroded in various aqueous solutions (electrolytes), which hinders corrosion.

Generally, the corrosion types of stainless steel are divided into two categories: uniform corrosion and local corrosion. With the popularization of stainless steel in people’s lives, a new corrosion type – “rust” is derived.

1. Uniform corrosion
Uniform corrosion means that all the exposed metal surfaces in the corrosive environment undergo electrochemical or chemical reactions and are uniformly corroded. This kind of corrosion can also measure the speed of its progress, predict the degree of corrosion in the future, set the safety factor, and set the service life of the material, so it is the least dangerous corrosion among many types of corrosion. Generally, the degree of corrosion of uniform corrosion is in accordance with It is measured by how much the weight and thickness are reduced. Except for special environments, the uniform corrosion rate of stainless steel is extremely low, the service life is long, and the maintenance cost is low.

If it is required to maintain a mirror surface or equipment with precise dimensions during use, stainless steel of grade 1-3 should be selected; equipment that requires long-term leak-proof or required service life should choose grade 2-5; for easy maintenance or long service life The equipment can be made of grade 4-7 stainless steel. It is generally not selected for the annual corrosion rate exceeding 1mm.

2. Local corrosion
Localized corrosion refers to a form of corrosion in which the matrix of steel is rapidly corroded at specific locations under the action of corrosive media. This corrosion poses a great threat to equipment, so stainless steel must be selected correctly according to the media conditions. The main types of localized corrosion are: intergranular corrosion, pitting corrosion, stress corrosion, rust and so on.

● Intergranular corrosion

Intergranular corrosion mostly occurs in acidic media such as medium-concentration sulfuric acid, high-concentration nitric acid, and organic acids. The form of corrosion is that the grain boundaries of the stainless steel matrix are subjected to accelerated corrosion. The reason for this corrosion is the lack of chromium at the grain boundaries.

In order to prevent the depletion of chromium at the grain boundary and improve the resistance to intergranular corrosion, there are two main methods: one is to reduce the ultra-low carbon stainless steel with a carbon content of ≤ 0.03% in the steel; the other is to add titanium or niobium to the steel.

● Pitting corrosion

Pitting corrosion is a very dangerous local corrosion, which mostly occurs in aqueous solutions containing chlorine, bromine, iodine, etc., resulting in small holes and then rapid corrosion. degree of corrosion.

There are two main measures to improve the pitting resistance. One is to improve the local pitting resistance and reduce the inclusions in the steel, especially the sulfur content; the second is the pitting resistance of the steel matrix, which affects the corrosion resistance of the matrix. The alloying elements are mainly chromium, molybdenum and nitrogen.

● crevice corrosion

The main reason for crevice corrosion is that there are gaps in the equipment, such as riveting, gaskets, or dead ends in the equipment. Because the medium does not flow in these places, the chloride ions are concentrated and the corrosion is accelerated.

In order to prevent crevice corrosion, the design with crevices should be avoided first, or the crevices should be opened; secondly, the ability to resist crevice corrosion should be improved, in which the influence of alloying elements is the same as that of pitting corrosion.

● Stress corrosion

The appearance of stress corrosion is dendritic corrosion along the vertical direction of the thickness of the equipment, causing the equipment to crack. In addition to the medium conditions, the conditions for stress corrosion are directly related to the tensile stress generated by the equipment during the manufacturing process. The main equipment where this corrosion occurs are heat exchangers, coolers, steam generators, blowers, dryers and boilers.

Measures to improve the stress corrosion resistance of stainless steel: one is to improve the stress corrosion resistance index △Ni; the other is to heat the equipment to eliminate residual stress.

3. Corrosion
The corrosion resistance of stainless steel is a new corrosion resistance index proposed in recent years due to the wide application of stainless steel as a decorative material. Stainless steel as a building plate, pipe and other materials should be decorative and aesthetic at the same time. The factors affecting the rust resistance of stainless steel are exactly the same as those of pitting corrosion resistance, which mainly depend on the rust resistance of the matrix and the content of rust sources (inclusions).
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