Several Culprits of Stainless Rust
Many people think that stainless steel will not rust. In fact, everything has a life span. Stainless steel is also a kind of metal. Metal is oxidized and corroded in nature, causing rust. The only difference is the difference in material or degree of protection, which can delay rust to the greatest extent. The odds and time. However, in many cases, stainless steel will also be “rusted”. The following is a description of the major culprits of stainless rust:
1. The presence of chloride ions in the operating environment
Chloride ions mainly exist in purified liquids and gases. The use of stainless steel in the presence of chloride ions will speed up the degree of 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 in which the alloy is heated to a high temperature single-phase zone and maintained at a constant temperature so that the excess phase is fully dissolved in the solid solution and then quickly cooled to obtain a supersaturated solid solution. Its purpose is to fully dissolve the various phases in the alloy, strengthen the solid solution, and improve the toughness and corrosion resistance, eliminate stress and softening, so as to continue processing or molding. If the alloying elements are not dissolved into the matrix material, the alloy content of the matrix structure will be below, and the corrosion resistance will be poor.
3. Natural intergranular corrosion
Under the action of corrosive medium, stainless steel produces a kind of corrosion phenomenon between crystal grains called intergranular corrosion. Stainless steel that produces intergranular corrosion, when subjected to stress, will break along the grain boundary and almost completely lose its strength. This is the most dangerous form of damage to stainless steel.
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 whether steel can prevent rust. Generally, stainless steel produced by small factories will still rust. The main reason is that the nickel content is too low (the cost price of nickel largely reflects the benchmark price of stainless steel); if the nickel content meets the required standards for all types of stainless steel Above, no matter how you cut it, it won’t rust.
4. Mechanical stress change corrosion
When stainless steel products are made into products, they need to be stamped, sheared, punched, stretched, ground, sheared, bent, sliced, thermally cut, or welded; these will be affected by external stress loading. The processed part and its edge cause a large degree of damage to its own stress, causing its intergranular phase change; thereby destroying its own anti-corrosion performance and speeding up the process of rusting.
5. Destruction of the surface passivation protective layer
Passivation is a method to transform the metal surface into a state that is not easy to be oxidized and to delay the corrosion rate of the metal. Passivation is due to the action of metal and oxidizing substances, which produces a very thin, dense, and good-covering passivation film on the metal surface during the action. Once this layer of “protective film” is destroyed by external forces (mainly large-area scratches), stainless steel is like a body with no resistance, and it is easy to oxidize and rust and corrode with the outside world.
Terms & Definition:
Supersaturated solid solution: refers to a solid solution in which the amount of dissolved solute at a given temperature is greater than the solubility when it is in an equilibrium state at that temperature and is a solid solution in a metastable state.