Leak in stainless steel pipe
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The material of the equipment is S30408; the dimensions are 159*6, and it uses 14926 type fluid pipes. The media used are compressed air and nitrogen, with a working pressure of 0.1–0.6 MPa. The installation took place in May and June 2023 within trenches, with all welds being performed using TIG welding. After installation, pressure testing was carried out to ensure there were no leaks. During the summer, due to rain, water accumulated in the trenches, submerging the pipes; however, the quality of this water was not tested. After the leaks were detected this time, the water in the trenches was analyzed, and its pH was found to be 7.6. This month, leaks were identified in three pipes, with the leak points located either at the welds or in the base metal itself； Of the two images above, the first one shows the leakage site from the outside, while the second one shows it from the inside. The orange-yellow color is unclear as to what causes it. Please analyze and see what causes this kind of leakage
Based on the information you provided, the leakage problem in the stainless steel pipes may be caused by the following reasons: 1. Stress corrosion cracking (SCC): The S30408 material may experience stress corrosion cracking under certain conditions (such as in the presence of chloride ions). If there is water accumulation in the trench, especially water containing chloride ions, it can cause localized corrosion on the surface of stainless steel, leading to cracking, particularly in the stress concentration areas around the welds. 2. Welding quality issues: Even if no leaks are detected during pressure testing after welding, subsequent operational conditions such as pressure fluctuations within the pipes, temperature changes, or mechanical vibrations can cause the existing minor welding defects to gradually expand and turn into leakage points. 3. Material defects: If stainless steel contains defects during the manufacturing process, such as inclusions or microcracks, these defects may gradually expand under operating conditions to become sources of leakage. 4. Corrosion: Although the pH of the water you measured is 7.6, indicating a weakly alkaline environment, other corrosive substances present in the water (such as sulfides, oxides, etc.) may cause pitting or uniform corrosion in stainless steel pipes. The orange-yellow color may be caused by corrosion products (such as rust) or other deposits, which usually indicates that corrosion is occurring. If such a color change occurs on the surface of stainless steel, it may be due to prolonged exposure to a corrosive medium, resulting in chemical or electrochemical corrosion. In response to this situation, you can take the following steps for further analysis: Conduct a thorough visual inspection as well as non-destructive testing (such as ultrasonic testing, magnetic particle testing, etc.) on the pipeline section at the leakage site, in order to determine the exact location and size of the leak. - Test the chemical composition and material properties of the materials near the leakage point to determine whether there are any material defects. - Analyze the detailed chemical composition of the water in the trench to determine whether it contains highly corrosive substances. - Check for external forces acting on the area around the leak site, such as mechanical impact, compression, or other forms of external damage. - Consider whether the welding process meets the standards, whether proper heat treatment was carried out after welding, and whether necessary stress-relief measures have been taken. After following these steps, a clearer understanding of the cause of the leak should be possible, allowing for targeted repairs and preventive measures to be taken. .
The base material, right? I encountered this years ago in a shipyard: the stainless steel pipe used for compressed air had holes in its material, which prevented the pressure from being maintained ； In the dead of night, I walked along the piping system on the ship, listening for sounds, and finally located the leak.
The potential energy of electric welding materials differs from that of stainless steel materials, creating a battery – could that be the reason?
Electrochemical corrosion caused by impurity defects in the base material
Pitting has occurred; the pressure testing and water purging were not thorough enough.
It depends on your industry – is the air in the factory area contaminated with chlorides? If chlorides are present, combined with poor pipe surface quality (roughness), it could be intergranular corrosion. If not, the pipe may have defects during manufacturing (defects in the pipe billet or those that occur during threading), or issues such as material inclusions.
Is it the side, bottom, or top?
It is possible to determine whether corrosion started from the inside or from the outside by examining the accumulation of corrosion products. Generally, such local defects are caused by the quality of the base material; if acid washing and passivation are carried out before use, then such defects will quickly spread throughout the material during use
This is a horizontal tube, I think there is liquid stored inside this section; you can see a distinct horizontal corrosion surface at the bottom of the orange-colored part