Sometimes when a metal material (stainless steel tube) is corroded, it will produce a corrosive product that is highly viscous and easily attached. This viscous and highly adherent product adheres to the surface of the metal material and can serve as a protective material. Protected from further corrosion, this effect is a passivation, and the passivated metal material has a very low rate of corrosion in a particular environment.
In other environments, it is possible to produce quite serious corrosion, which is called passivation. For example, for nitric acid, as the concentration of nitric acid increases, the oxidizing property increases, and accordingly the progress of corrosion should be accelerated, but when the concentration reaches 65%, it does not corrode. This is because nitric acid, which is a strong oxidizing agent, forms an inert film on the surface of the iron, and this film becomes a passivation film. This state becomes such that the metal has been passivated.
The property of easily forming passivation is that the metal originally has. The metal elements that are easy to form passivation are mainly iron, chromium, nickel, and molybdenum. Since the stainless steel tube contains enough elements such as chromium, nickel, molybdenum and the like, the passivation film is extremely easy to form and is a more stable and strong substance, so the stainless steel seamless tube has superior corrosion resistance.
The corrosion resistance of the passivated stainless steel tube depends not only on the content of chromium, nickel, molybdenum, but also the chloride and oxygen content and temperature in the environment. The passivation state is achieved under certain conditions, depending on whether the relative absolute values of the various passivation factors exceed the factors that impede passivation. The passivation film on the surface of the stainless steel seamless pipe is mainly immersed in the oxyacid solution of the pickling tank.
Although the corrosion state of the stainless steel tube is very small under certain circumstances, it may be quite severe in other cases. In contrast, stainless steel tubes, which typically exhibit a passivated state, are often active in a non-passivated state.
Some elements can break through the passivation film, causing the surface of the passivation film to be corroded. For example, a stainless steel tube encounters chloride ions in seawater, and the passivation film can be broken down to cause pitting corrosion. Therefore, users of the use of passivated stainless steel seamless pipes should pay special attention to pitting corrosion, stress corrosion cracking, sensitization, and oxygen depletion corrosion.