Why do stainless steel metal hoses used for oil transportation degrade over time?
Release time:
2021-07-05
The corrosion cracking of stainless steel metal hoses is caused by the sensitivity of the alloy (material) to specific media (environmental factors), such as the use of austenitic stainless steel in chloride-containing solutions. Additionally, certain tensile stresses—both external stresses and stresses arising from phase transformations and residual stresses—are involved.
If these three basic conditions are met, stainless steel metallic hoses may be at risk of stress corrosion cracking. When stress corrosion cracking occurs, there is no obvious uniform corrosion, and even very few corrosion products are present—sometimes making it difficult to detect with the naked eye. Therefore, stress corrosion is an extremely dangerous hazard. The process of stress corrosion can be divided into three stages.
In the first stage of stainless steel metal hoses, there is a latent period. During this stage, due to localized corrosion processes and tensile stress, crack nuclei—fine pits—form at the sites on the stainless steel that are most susceptible to corrosion.
The second stage of the stainless steel metal hose is the NT stage of corrosion cracking. When cracks nucleate, they expand under the combined action of the corrosive medium and tensile stresses in the metal, resulting in elongated cracks.
In the three-stage process of stainless steel metal hoses, due to the convergence of localized tensile stresses, cracks expand sensitively and can cause serious damage within a short period of time.
The stress corrosion cracking of stainless steel metal hoses is influenced by the characteristics of the medium, temperature, stress levels, as well as the composition and microstructure of the steel. The effects of these factors are described below. Generally speaking, as the concentration of chloride ions increases, the time to onset of stress corrosion cracking in chromium-nickel stainless steels shortens. Magnesium chloride is generally considered more prone to stress corrosion cracking; among different chlorides, the corrosive effect decreases in the following order: Mg²⁺, Fe³⁺, Ca²⁺, Na⁺, and Li⁺ ions.
Effect of Medium Temperature on Stainless Steel Metal Hoses: It is generally believed that higher temperatures tend to increase the likelihood of stress corrosion. However, at excessively high temperatures, certain steels may exhibit critical cracking. According to statistics, the chromium-nickel stress-corrosion temperature range for austenitic stainless steels lies between 50°C and 300°C.
Effects of Alloying Elements and Stress: As can be seen, increasing the nickel content in steel and silicon alloys enhances the resistance of austenitic stainless steels to stress-corrosion cracking in magnesium chloride solutions. Conversely, elements such as nitrogen and phosphorus in steel reduce corrosion resistance. Stress-corrosion cracking of stainless steels in concentrated chloride media.
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