Is a pipe with a medium concentration of 4% NaOH, made of 316L stainless steel and with a diameter of 2”, considered a pressure pipe?

Based on the information you provided, 4% NaOH is a corrosive substance. Furthermore, the 316L stainless steel material exhibits a relatively low corrosion rate in the presence of NaOH. However, in accordance with **relevant regulations and standards, this pipeline still falls under the category of pressure pipelines, and procedures for design, manufacturing, installation, inspection, and operation management must be followed in line with relevant standards. Therefore, stress analysis is required for this pipeline to verify its reliability and safety. .

I checked the List of Hazardous Chemicals; NaOH is included in this list only when its concentration is above 30%. Therefore, NaOH with a concentration lower than that should not be considered a corrosive substance, and thus it does not fall under the category of pressure pipelines, right?

Thank you for the additional information provided. According to the Regulations on the Catalogue of Hazardous Chemicals, NaOH is classified as corrosive, but only NaOH with a concentration greater than 30% is listed in this catalogue. For NaOH concentrations below 30%, although it may have a certain degree of corrosivity, its hazard is relatively low. Therefore, if you use NaOH with a concentration of less than 30% as the medium, such a pipeline may not necessarily be considered a pressure pipeline; nevertheless, a safety assessment and appropriate material selection are still required to ensure its safe and reliable operation. .

There is a saying that when the pH value is less than 2 or greater than 13, the corrosivity reaches the levels considered acceptable for pressure pipelines. A 4% NaOH solution (1 mol/l) has a pH of around 14; it is highly corrosive and should be considered a pressure pipeline. TSG_D0001-2009 Code for Safety Supervision of Pressure Piping – Industrial Piping. Corrosivity refers to: In the context of pressure piping, corrosive fluids are those that cause visible necrosis upon contact with skin within 4 hours, or those that result in a corrosion rate of more than 6.25 mm/year for 20-grade steel at 55°C. GB 30000.19-2013 Code for Classification and Labelling of Chemicals – Part 19: Skin Corrosion/Irritation. Corrosivity is classified into 1A, 1B, and 1C. Sodium hydroxide solution causes skin corrosion/irritation and is classified as Category 1A. A concentration of 4% NaOH (1 mol/l) is not specified; it might fall under Category 1B or 1C, or it could still meet the criteria for skin corrosion/irritation. In the \"Technical Regulations for Industrial Pipeline Safety (Second Draft for Comment)\", corrosionivity refers to corrosive media in pipelines as those metal-corroding substances that fall under Category 1 of GB 30000.17; in other words, chemical media whose corrosion rate on steel or aluminum surfaces exceeds 6.25 mm per year at a test temperature of 55°C. No requirements are anymore for skin corrosion/irritation. How to determine whether this standard has been implemented is another new challenge.

When the pH is between 4 and 10, corrosion is controlled by a diffusion process, and the corrosion rate has little relation to pH. When the pH is below 4, the oxide film is dissolved, allowing the metal surface to come into contact with the acidic solution; this results in two types of depolarization: oxygen depolarization: O2 + 4H+ + 4e → 2H2O, and hydrogen depolarization: 2H+ + 2e → H2. As a result, electrochemical corrosion intensifies and the corrosion rate increases. At pH values between 10 and 13, the pH on the surface of carbon steel increases; the critical concentration for oxygen passivation drops to 6 ppm, leading to the formation of r-Fe2O3 and a reduction in the rate of passivated corrosion. When pH > 13, the passivation film is dissolved, forming soluble complexes such as sodium ferrate (NaFeO2) and sodium ferrous oxide (Na2FeO2), and the corrosion rate increases again. What are the two pH values corresponding to a corrosion rate greater than 6.25 mm/y (per year)?