Why do stainless steel metal hoses used for oil transportation degrade over time?
Release time:
2021-09-28
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. Certain tensile stresses—both external stresses and internal stresses arising from phase transformations and residual stresses—are also involved.
If a stainless steel metal hose meets these three basic conditions, it may be at risk of stress corrosion. When stress-corrosion cracking occurs, there is no obvious uniform corrosion, and even the amount of corrosion products is minimal—sometimes so few that they are difficult to detect with the naked eye. Therefore, stress corrosion poses a highly dangerous hazard. The process of stress corrosion can be divided into three stages.
The first stage is the incubation period. During this stage, due to localized corrosion processes and tensile stress, the crack nucleus—the site where stainless steel becomes susceptible to corrosion—is formed.
Second, development—the NT corrosion stage. As cracks nucleate, they expand under the combined action of the corrosive medium and tensile stress in the metal, forming slender cracks.
In the third stage, due to the convergence of local tensile stress, the crack will undergo sensitive propagation, causing 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. In general, magnesium chloride is more prone to stress corrosion cracking, and the corrosive effects of different chlorides decrease in the following order: Mg²⁺, Fe³⁺, Ca²⁺, Na⁺, and Li⁺ ions.
The effect of temperature on stainless steel metal hoses: It is generally believed that higher temperatures 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.
The Influence of Alloy Elements and Stress on Stainless Steel Metal Hoses: As can be seen, increasing the nickel alloy element in steel and silicon can enhance the resistance of austenitic stainless steel to stress corrosion cracking in magnesium chloride solutions, whereas elements such as nitrogen and phosphorus in steel tend to reduce corrosion resistance. Stress corrosion cracking of stainless steel in concentrated chloride media.
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