1. What is non-rusting? Does it really not rust? Just as aluminum alloy is an alloy based on aluminum, stainless steel is actually an alloy, but it is an alloy based on iron. The alloying elements of stainless steel include chromium, nickel, carbon, titanium, etc. Among them, the addition of chromium is the key to stainless steel not rusting. This is determined by the characteristics of the chromium element. In stainless steel, the chromium content is at least about 10.5%. It can form a protective self-healing oxide film on the surface of stainless steel, which is why stainless steel is corrosion-resistant. The so-called self-healing means that if the protective film is damaged, it will re-form the oxide film in an aerobic environment and continue to provide anti-corrosion protection. So many times, when we choose surface treatment, we often use chrome plating or galvanization, especially for martensitic stainless steel. Of course, in practical applications, galvanizing is often chosen because it is cheaper. The world produces 12 million tons of zinc every year, half of which is used for galvanizing. This is not the same as carbon steel or low alloy steel, the principle of using a surface coating for corrosion protection, because after the surface coating of this type of steel is scratched, the corrosion resistance is greatly weakened, or even gone. So, is stainless steel really rustless? Of course not, there are conditions for stainless steel not to rust. Stainless steel is corrosion resistant under most normal conditions, but not rust free in all environments. For example, at high temperature, in an environment containing 50% sulfuric acid and hydrochloric acid, severe corrosion will occur, causing the protective film on the metal surface to rupture. Of course, we can also improve the corrosion resistance by adding other alloy elements, for example, molybdenum and chromium increase the resistance to chloride penetration, and nickel can improve the corrosion resistance in some acidic environments. 2. Several types of stainless steel corrosion There are many types of corrosion in stainless steel, such as pitting corrosion, crevice corrosion, stress corrosion, electrical corrosion, contact corrosion, etc. Here I will only talk about a few corrosions that are prone to occur in general environments. (1) General corrosion General corrosion usually dissolves materials evenly, such as when stainless steel is used to contain strong acids in chemical plants. (2) Pitting corrosion Pitting corrosion is the corrosion that slowly spreads outward from a point. In a strong sulfuric acid environment, in a high-concentration chloride environment, such as seawater sodium chloride environment, or in a medium-high temperature acidic environment, pitting corrosion is prone to occur, causing air and water leakage in pipelines, joints, etc. Once pitting corrosion starts, it is easy to continue to develop because there is a potential difference between the pitted part and the uncorroded part. In many cases, 316 and 317 stainless steels can be used for enhanced resistance to pitting corrosion. In addition, the surface with roughness less than 0.5um has better resistance to pitting corrosion. (3) Crevice corrosion Crevice corrosion is caused by local differences in oxygen concentration in crevices. Crevice corrosion is associated with deposits in the crevices of metal surfaces, gaskets, joints, etc., where small amounts of liquid can collect and stagnate, inducing crevice corrosion. Crevice corrosion has been reported to be caused by wood, plastic, rubber, glass, and living organisms, and once it starts in a crevice, it develops rapidly, especially in chloride environments. Therefore, molybdenum-containing stainless steels, such as 316, 317, are usually used to minimize crevice corrosion. Of course, the best solution is to eliminate the gap design. (4) Stress corrosion Stress corrosion is caused by the joint action of tensile stress and corrosion. Many alloys are known to undergo stress corrosion cracking, such as brass in ammonia, carbon steel in nitrate solutions, aluminum alloys in seawater, etc. Stainless steel is susceptible to stress corrosion cracking in chloride environments. At present, the mechanism of stress corrosion is not fully understood, but methods to reduce stress corrosion have been found through experiments and experience. For example, using duplex stainless steel 2205, the stress corrosion resistance can be improved because of its excellent resistance to chloride stress corrosion cracking.