Talking about alkali embrittlement at high temperatures and high concentrations
Thread Content
A few years ago, there were some disputes with the design team regarding the selection of liquid alkali storage tanks and certain reactors that use liquid alkali as a primary reactant. The designers believe that at high alkali concentrations (30–50 wt%) and high temperatures (100–130°C), severe alkali embrittlement occurs in both carbon steel and ordinary stainless steels; even 316L can only last for 3–4 years under such conditions. In reality, there are examples of both ordinary Q235 and 304 stainless steel being used, whether in storage tanks or reaction vessels, with no problems occurring over a period of more than 5 years. And no equipment has actually been scrapped due to alkali embrittlement within 5 years. I would like to ask fellow maritime professionals, especially those involved in production management, equipment management, and chemical engineering design, to share their views and experiences. Thank you!
The 32-alkali and 50-alkali tanks in our factory are both made of 304 material, and the temperature there is relatively low, at 40-50 degrees; no problems have been encountered, so the situation is fairly good. By regulating the control of the medium temperature, I believe that good results will be achieved whether carbon steel or 304 stainless steel is used.
The temperature gradient has the greatest impact. In devices like alkali liquid evaporators, the temperature in the first stage is above 120 degrees. To date, no instance has been found of such devices experiencing alkali embrittlement or leaks after several years of use, regardless of the type of stainless steel used; this problem still hasn’t been resolved
•Sodium hydroxide can cause stress corrosion cracking that leads to intergranular fracture. For moderate temperatures and concentrations, carbon steel is recommended as the base material for construction. •Below 120°C, austenitic stainless steels may sometimes be specified (above this temperature, these alloys are more susceptible to alkaline stress corrosion cracking). However, alkalis are often contaminated with chloride ions, and due to the availability of other alternative materials, austenitic stainless steel is generally avoided in alkaline environments. For highly demanding environments, nickel alloys such as Nickel Alloy 400 are the preferred choice. •The current NACE hydrogen-sodium hydroxide service curve is typically used to determine the critical values for operational limits; this curve indicates the preferred materials at different temperatures and concentrations.
•Sodium hydroxide can cause stress corrosion cracking that leads to intergranular fracture. For moderate temperatures and concentrations, carbon steel is recommended as the base material for construction. •Below 120°C, austenitic stainless steels may sometimes be specified (above this temperature, these alloys are more susceptible to alkaline stress corrosion cracking). However, alkalis are often contaminated with chloride ions, and due to the availability of other alternative materials, austenitic stainless steel is generally avoided in alkaline environments. For highly demanding environments, nickel alloys such as Nickel Alloy 400 are the preferred choice. •The current NACE hydrogen-sodium hydroxide service curve is typically used to determine the critical values for operational limits; this curve indicates the preferred materials at different temperatures and concentrations.
What the people upstairs said is all good. http://bbs.hcbbs.com/forum.php?mod=redirect&goto=findpost&ptid=1508864&pid=17086438 Could the issue in post 20# of this thread be discussed in more detail to clarify it? Thank you