A typical stress corrosion cracking case caused by alkaline embrittlement
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A storage tank for storing 30% sodium hydroxide, with a diameter of 1.22 meters and a height of 1.4 meters, made of carbon steel. A jacket is welded around the exterior of the storage tank, used to circulate hot water at 30-50°C in order to maintain the temperature of the sodium hydroxide solution. In the north, where winter temperatures are low, some factories switch to introducing steam at 0.2 MPa into the jacket. It was soon discovered that the bottom of the tank was leaking. Upon inspection, radial cracks were found around the alkali liquid outlet pipe at the bottom of the tank. If steam is introduced from the upper pipe opening, cracks will quickly form at the weld between the jacket and the cylinder. This type of corrosion damage is typical alkali embrittlement. That is, the stress corrosion cracking that occurs in stressed metal materials in sodium hydroxide solution. Carbon steel is a type of metal material that is prone to alkaline embrittlement. Through numerous experiments and the analysis of practical production experience, the sodium hydroxide concentration and temperature range at which carbon steel suffers from alkaline embrittlement have been determined. For a 30% NaOH solution, when the temperature exceeds 55°C, it can cause alkali embrittlement in carbon steel. In this case, no corrosion issues occurred when hot water at 30–50°C was used. With steam flowing through, the temperature at the steam inlet is very high, far exceeding the 55°C threshold. And the area near the steam inlet, whether at the top or bottom, is precisely where welding stress concentration occurs. It’s not surprising at all that the storage tank suffered from alkali embrittlement. To prevent alkali embrittlement in storage tanks, improvements should be made to the equipment design in order to achieve as uniform a temperature distribution as possible and avoid localized overheating; in particular, areas with higher temperatures should be kept away from zones where welding stress concentrations occur. For example, some factories switched from jacket heating to coil heating, and no more alkali embrittlement failures occurred. Of course, eliminating welding stress is also effective.
The welds of carbon steel equipment can be heat-treated.
Measures to prevent alkali embrittlement: Improve the equipment design to ensure as uniform a temperature distribution as possible and avoid localized overheating. In particular, areas with higher temperatures should be avoided where welding stress concentration occurs. Replacing the jacket heating with coil heating allows for more effective control of the heating area, preventing excessive local temperatures. Eliminating welding stress: Welding stress is eliminated through appropriate welding and heat treatment processes, thereby reducing the risk of alkali embrittlement. Control the concentration and temperature of sodium hydroxide: Try to reduce the concentration of sodium hydroxide in order to minimize the risk of alkali embrittlement. Strictly control the temperature of the heating medium to ensure it does not exceed the critical temperature for alkali embrittlement. Use corrosion-resistant materials: If conditions permit, consider using corrosion-resistant materials such as stainless steel to replace carbon steel in order to improve the corrosion resistance of storage tanks. Strengthen monitoring and maintenance: Regularly inspect and maintain the storage tanks to identify and address potential corrosion issues promptly. Monitor the concentration and temperature of sodium hydroxide in the storage tank to ensure they remain within safe limits.
I’ve learned it.* I learned about alkali embrittlement corrosion for the first time. We usually use coiled tubes; we didn’t pay attention to this. It seems that a high-temperature alarm is needed for the alkali tank temperature indication.