Cracks appeared in the alkali tank due to improper operation
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In the north, where winter temperatures are low, some factories modify the alkali tanks by introducing steam (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. What is the reason for this?
The temperature at which carbon steel suffers from alkali embrittlement corrosion is around 50°C (with an alkali concentration of about 30%), and steam heating (jacketing) can easily reach this temperature. Residual stresses exist in the welds and heat-affected zones, and alkaline embrittlement corrosion can exacerbate stress corrosion cracking; therefore, stress-relief heat treatment should be applied to the weld areas.
This type of corrosion damage is typical alkali embrittlement, that is, 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 alkali embrittlement have been determined. As can be seen from the figure on the right, for a 30% NaOH solution, carbon steel may suffer from alkali embrittlement when the temperature exceeds 55°C. In this case, no corrosion issues occurred when hot water at 30–50°C was used. When steam is introduced, the temperature at the steam inlet is very high, far exceeding the threshold of 55°C. 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 failure. The relationship shown in this graph was published as early as the early 1950s and has been cited in many works on corrosion; yet for decades, many carbon steel components have suffered from alkali embrittlement, clearly under the environmental conditions of the \"fracture zone\". Why repeat the same mistakes? In this case, to prevent alkali embrittlement in the storage tank, improvements should be made to the equipment’s structure to ensure as uniform a temperature distribution as possible and avoid localized overheating; in particular, areas with higher temperatures should be kept away from locations where welding stress concentrations occur. For example, some factories switched from jacket heating to coil heating, and alkali embrittlement damage no longer occurred. Of course, eliminating welding stress is also effective.
Is the tank material compatible with the welding electrode?
Check the tank material and welding rod material.
The pipe material is 304 stainless steel, with steam tracing for heating; recently, there have been three leakage points within a distance of less than ten meters, and they are not located at the welds. What could be the cause?
If stainless steel pipes are used intermittently, although they are not in use for a short period of time, there is still residual liquid inside the pipes. The residual liquid still contains corrosive agents, which will deposit at the defects on the pipe surface, causing corrosion, especially pitting corrosion.