Many people believe that stainless steel will not rust. In fact, everything has a life span. Stainless steel is also a kind of metal, which will be oxidized and corroded in nature, causing rust; The only difference is that the material or protection degree is different, which can delay the probability and time of rusting to the greatest extent. However, in many cases, stainless steel will also be “rusted”. Here are the main culprits of stainless steel rust:
1、 Presence of chloride ions in the service environment
Chloride ions mainly exist in purified liquids and gases. The use of stainless steel in the presence of chloride ions will accelerate its corrosion, even exceeding that of ordinary low-carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry.
2、 Without solution treatment
Solution treatment: refers to the heat treatment process that heats the alloy to the high-temperature single-phase area and maintains the constant temperature, so that the surplus phase can be fully dissolved in the solid solution and then rapidly cooled to obtain the supersaturated solid solution. Its purpose is to fully dissolve various phases in the alloy, strengthen solid solution, improve toughness and corrosion resistance, eliminate stress, and soften, so as to continue processing or molding. If the alloy elements are not dissolved in the matrix material, the alloy content in the matrix structure is low and the corrosion resistance is poor.
3、 Natural intergranular corrosion
Under the action of a corrosive medium, a corrosion phenomenon between grains of stainless steel is called intergranular corrosion. When the stainless steel with intergranular corrosion is subjected to stress, it will fracture along the grain boundary and the strength will almost disappear. This is the most dangerous form of stainless steel damage.
Stainless steel will not rust because the content of chromium (Cr) and nickel (Ni) has reached a certain standard. These two alloys are the key to rust prevention in stainless steel. Generally, stainless steel produced by small factories will rust, mainly because of the low nickel content (the cost price of nickel largely reflects the benchmark price of stainless steel); If the nickel content reaches the standard required by various stainless steel, no matter how you cut it, it will not rust.
4、 Mechanical stress corrosion
When stainless steel materials are made into products, they need to go through such processes as stamping, shearing, punching, stretching, grinding, shearing, bending, planing, thermal cutting, or welding. Because of the external stress loading, the machined parts and their edges will be damaged by their own stress to a large extent, which will lead to the intergranular transformation; Therefore, it destroys its corrosion resistance and speeds up the rusting process.
5、 Damage to surface passivation protective layer
Passivation is a method to change the metal surface into a state that is not easy to be oxidized, thus delaying the corrosion rate of the metal. Passivation is due to the interaction between metal and oxidizing substances. During the interaction, a very thin, dense, well-covered, and firmly adsorbed passivation film is formed on the metal surface. Once this “protective film” is damaged by external forces (mainly large scratches), the stainless steel is like a body without resistance, which is easy to react with the outside world to rust and corrosion.