Pay attention to the conditions for alkali embrittlement when carbon steel comes into contact with NaOH

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This post was last edited by Wang Wei2 on 2020-4-6 at 09:56. Note the conditions under which carbon steel becomes brittle in contact with NaOH. The tank used for storing 30% sodium hydroxide has a diameter of 1.2 meters and a height of 1.4 meters; it is 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 supplying 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. Comment: 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 at the steam inlet? Whether it’s the upper or lower inlet, that area 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 the figure was published as early as the early 1950s and cited in many works on corrosion; yet for decades, many carbon steel components have continued to suffer from alkali embrittlement, notably 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 uniform a temperature distribution as possible, 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 damage no longer occurred. Of course, eliminating welding stress is also effective.