Seeking advice on corrosion of welds in carbon steel pipes
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Our factory uses 2\" carbon steel pipes to transport alkali solutions and wastewater containing such solutions; the alkali solution is sodium hydroxide at a concentration of 32%. Currently, we are experiencing a problem where many of the welded areas are leaking alkali solution, which appears black in color. What causes this? Will the welds of carbon steel corrode?
The leakage at the weld seam is likely due to stress corrosion cracking. Was post-weld heat treatment performed on the pipeline after welding? What is the operating temperature of the medium? One can refer to the requirements regarding alkali corrosion specified in SH/T3059. If the temperature is not high, carbon steel treated through heat treatment will suffice; if the temperature is high, it may be necessary to use a higher-grade material. Of course, plastic-lined pipes can also be considered
Stress corrosion crack. PWHT is required
…Alkali embrittlement: Corrosivity is proportional to temperature and concentration
What is heat treatment? Could the poster share this information or tell me where I can find it?
It was carried out by China’s 9th Chemical Construction Company at that time; I’m not sure if heat treatment was done or not But it’s been suppressed too much!
I really don’t know if post-weld heat treatment was carried out; the ambient temperature was extremely low at that time, and many of the pipes were prefabricated by a chemical engineering company. Our current operating conditions are such that the pipeline is kept at 50°C through electric heating, and it needs to be flushed with CN at around 20°C after each discharge, which is when leaks are detected. Can carbon steel welds withstand the stresses resulting from such temperature changes after welding heat treatment? Thank you, thank you for your guidance.
After stress relief through heat treatment, the weld becomes as resistant to stress corrosion as the base material; if the base material is unaffected, then the weld will also be unaffected. If heat treatment is not required in the design, the contractor will not carry it out
Based on the curve graph upstairs, I checked the actual operating temperature of the pipeline, which is around 50°C. The concentration of the alkaline solution is also below 5%, so heat treatment is not necessary according to the design specifications. But if there is a leak in the welds, it might be due to issues with the construction quality The problem is that it’s been suppressed too much.
Usually, we are more cautious when doing this; even at lower concentrations, heat treatment is applied, because you can’t be sure that local concentration won’t increase