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Stainless steel head is usually good corrosion resistance, but in some cases, its corrosion resistance may decline over time, the following from the destruction of the passivation film, the long-term role of environmental factors, their own materials and processing defects to be analyzed:
Destruction of passivation film
Natural loss: stainless steel has corrosion resistance because of its surface has a dense layer of passivation film. However, in long-term use, even in a normal environment, the passivation film may be due to a slight chemical reaction with the medium, tiny friction and other reasons gradually suffered loss, the protective effect of the passivation film will gradually weaken, and then make the stainless steel size of the corrosion resistance has decreased.
Mechanical damage: In the process of installation, use, stainless steel size head may be subject to mechanical collision, friction, etc., resulting in passivation film damage. For example, in the pipeline construction, tools on the size of the head surface scraping, or pipeline fluid carrying solid particles on the size of the head of the inner wall of the scouring, may damage the passivation film, so that the internal metal is exposed to the external environment, accelerating the corrosion process.
Chemical substances: If the stainless steel head long-term contact with strong oxidizing or reducing chemical substances, may be chemically reacted with the passivation film, resulting in the destruction of the passivation film. Such as long-term in the environment containing a high concentration of chloride ions, chloride ions will penetrate the passivation film, and the internal metal reaction, the formation of pitting and other localized corrosion, reducing corrosion resistance.
Long-term effect of environmental factors
Temperature and humidity: high temperature and high humidity environment will accelerate the corrosion process of metal. At high temperatures, the activity of metal atoms increases, the rate of chemical reaction accelerates; and high humidity environment will form a water film on the metal surface, providing electrolyte for electrochemical corrosion, thus accelerating the corrosion of stainless steel head, so that its corrosion resistance decreases.
Acidity and alkalinity: If the acidity and alkalinity of the environment in which the stainless steel head is located changes beyond its tolerance range, it will also affect the corrosion resistance. In a strongly acidic or alkaline environment, the passivation film on the surface of the stainless steel may be dissolved, and the metal substrate is in direct contact with the acid and alkali substances, and corrosion reactions occur.
Material and processing defects
Uneven material: In the production process of stainless steel, if the composition is uneven, or the content of impurities is high, a microscopic potential difference will be formed within the metal, which will trigger electrochemical corrosion in long-term use, resulting in a decrease in corrosion resistance.
Processing stress: In the processing of stainless steel size head, such as forging, bending, welding and other processes, may produce stress concentrations within the material. These stresses will make the metal lattice distortion, reduce the stability of the material, in the long-term use process, it is easy to trigger stress corrosion cracking and other problems, reduce corrosion resistance.