Purpose and method of passivation of stainless steel pipes for water supply

Usually, before stainless steel pipes are used as water supply pipes, there will be a passivation treatment process during the processing process. The purpose is to generate a uniform chromium oxide protective film on the smooth inner wall surface of the stainless steel pipe, so as to resist the impact of high-temperature liquids on the stainless steel surface. Possible intergranular corrosion.
One of the commonly used passivation treatment formulas is to use chemical water and chemical nitric acid to prepare an 8% acid solution. Cycle at a temperature of 49 to 52°C for 60 minutes before discharging. The passivation effect of 8% nitric acid is mainly due to the action of oxidant. The surface of the stainless steel pipe is oxidized into chromium oxide under the action of nitric acid, so that the purpose of passivation can be achieved. Usually stainless steel manufacturers do not do passivation, it is done by processors. For passivated water pipes, stainless steel pipes that are polished both inside and outside must be used, and the inner surface of the pipe must be passivated.
The entire passivation process of stainless steel pipelines includes cleaning, passivation and disinfection. The main processes are deionized water circulation pre-rinsing, alkali circulation cleaning, chemical water flushing, passivation, chemical water flushing again, discharge, and steam disinfection.
First, perform chemical water circulation flushing. Prepare a liquid storage tank and a water pump in advance, and connect them to the pipes that need to be passivated to form a circulation path. Inject enough normal temperature deionized water into the liquid storage tank, and use the water pump to circulate. After 15 minutes, open the drain valve and discharge while circulating. It is best to install another flow meter.
Next is lye cleaning. Prepare sodium hydroxide chemical reagent in advance, add hot water to prepare 1% lye, and then use a pump to circulate it for no less than 30 minutes, and then discharge it.
The next step is to rinse. After adding the melted water to the liquid storage tank, start the water pump and open the drainage valve to discharge until the resistivity of the water at each outlet point is consistent with the resistivity of the water in the tank, and the discharge time is greater than 30 minutes.
The following is the passivation operation. You can use chemical water and chemical nitric acid to prepare an acid solution with a concentration of 8%, and circulate it for 60 minutes at a temperature of 49°C-52°C before discharging it; or use 3% hydrofluoric acid or 20% nitric acid. , 77% dissolved water is prepared into a solution, the temperature of the solution is controlled at 25-35°C, circulated for 10-20 minutes, and then discharged, and then the stainless steel pipes are flushed with high-pressure steam to remove dirt.
Regarding flushing, it is divided into initial flushing and post-rinsing. The former uses normal temperature deionized water for more than 5 minutes. The latter requires that the resistivity of the inlet and outlet ionized water must be consistent.