Discussing the alkali embrittlement problem of carbon steel caused by sodium hydroxide

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Let’s discuss the issue of alkali embrittlement of carbon steel caused by sodium hydroxide. The acetylene purification unit in a vinylon factory uses storage tanks for 30% sodium hydroxide; these tanks have a diameter of 1.22 meters and a height of 1.4 meters, and they are 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 through the upper pipe opening, cracks will quickly form at the weld between the jacket and the cylinder. Analysis: 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 alkali embrittlement. Through numerous experiments and the accumulation of experience from practical production, 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 failure. Improvement: The relationship between alkaline embrittlement and temperature was published as early as the early 1950s and cited in many corrosion-related publications; yet for decades, many carbon steel components continued to suffer from alkaline embrittlement failure, specifically under the environmental conditions of the \"fracture zone\". Why repeat the same mistakes? In this case, to prevent alkali embrittlement in the storage tanks, improvements should be made to the design of the equipment to ensure as even a temperature distribution as possible, thereby avoiding localized overheating; in particular, areas with higher temperatures should be kept away from zones where welding stress concentrations occur. For example, some factories changed from jacket heating to coil heating, and alkali embrittlement failure no longer occurred. Of course, eliminating welding stress is also effective.