What is the relationship between corrosion resistance and passivation of stainless steel plates?

The corrosion resistance of stainless steel plates is closely related to the "passivation film" formed on their surface. Passivation refers to the formation of a dense oxide film on the surface of stainless steel naturally or through chemical treatment. This oxide film is mainly composed of chromium oxide (Cr₂O₃), which can effectively prevent moisture, oxygen and other corrosive substances from reacting with the stainless steel surface. Therefore, the passivation film plays a vital role in improving the corrosion resistance of stainless steel.

The passivation film is a very thin and hard protective layer, usually only a few nanometers thick. The film is mainly formed by the combination of chromium and oxygen, which can cover the surface of stainless steel and protect it from reacting with corrosive substances in the environment (such as water, oxygen in the air, chloride ions, etc.). Under most common environmental conditions, this passivation film is self-repairing, and even if the surface is damaged, it can recover quickly and continue to play a protective role.

The role of passivation can be understood from the following aspects:

The passivation film can effectively prevent the occurrence of oxidation and corrosion reactions, so that stainless steel has good corrosion resistance in a variety of environments (such as acidic, alkaline, marine climate, etc.). Even in environments with high chloride ion content, the passivation layer can still act as a barrier to prevent accelerated corrosion.

If the surface of the passivation film is damaged, the exposed chromium will react with oxygen to regenerate the passivation film. This self-healing property allows stainless steel to maintain good corrosion resistance for a long time, especially in harsh working environments.

The passivation film of stainless steel can be enhanced through chemical treatment (such as pickling and passivation treatment), thereby further improving its corrosion resistance. Passivation treatment usually involves the following steps:

Pickling is a process that removes the oxide layer, iron filings and impurities on the surface of stainless steel through an acidic solution (such as nitric acid or hydrofluoric acid). After removing these impurities, a more uniform and dense passivation film can be formed on the surface of stainless steel.

After pickling, the surface of stainless steel is usually treated with a passivation solution (such as a solution containing nitric acid) to help form a more stable and durable passivation film and improve the corrosion resistance of stainless steel.

Different types of stainless steel (such as 304, 316, 430, etc.) also show different corrosion resistance after passivation. Austenitic stainless steels such as 304 and 316, because they contain a higher proportion of chromium and nickel, can form a stronger passive film and have better corrosion resistance. However, 430 stainless steel (ferritic type) has a relatively poor corrosion resistance due to its low chromium content.

The effectiveness of the passivation film is particularly important in chloride-containing environments. Chloride ions (such as chlorine in seawater) can destroy the passivation film on the surface of stainless steel, causing local corrosion and pitting. Therefore, highly corrosion-resistant alloy materials such as 316 stainless steel, because they contain elements such as molybdenum (Mo), can better resist the erosion of chloride ions.

Stainless steel that has been passivated can maintain strong corrosion resistance during long-term use. However, if the passivation film on the surface of stainless steel is physically damaged or chemically attacked, the passivation film may lose its protective effect, making the stainless steel more susceptible to corrosion.

Passivation is a key step in improving the corrosion resistance of stainless steel plates. With proper passivation treatment, stainless steel can maintain long-term corrosion resistance in various harsh environments. The presence of the passivation film enables the stainless steel surface to have self-repairing ability and can effectively isolate corrosive substances. Therefore, it is crucial to the long-term service life of stainless steel.

However, the passivation film is not a panacea, especially in extreme environments (such as high concentrations of chloride ions, strong acids or high temperatures), local corrosion may still occur. Therefore, choosing the right material and surface treatment process, as well as reasonable maintenance and care, can ensure the best performance of stainless steel plates.