Passivation is carried out for a new equipment installation as well as for modified equipment, where it helps in removing surface contamination (especially at welds).
The passivation is achieved through treatments with Citric acid, Nitric acid and Phosphoric acid etc. This strong oxidizing agent strengthens the steel surface. The acid depletes the steel surface of acid soluble species, leaving behind a highly reactive chromium oxide layer (compounded oxide form).
Before performing passivation take 25-50% by volume of water in the storage tank. (Water quality –chlorine content shall be less than 5ppm), then circulate the water for 30 minutes. Drain the water from the storage tank by opening the drain valve and system by lowest point valves.
Passivation Method
- The equipment/system is normally treated with 3% Nitric acid Solution at ambient temperature.
- The treatment is done through Re-circulation / Holding of the passivating agent.
Note:
1. Concentration 3% Nitric Acid Solution
If Nitric Acid concentration is to maintain 3%, solutions have to hold in the equipment or calculate for the following minimum period at the temperatures indicated.
25°C = 3 ½ hrs
50°C = 2 hrs
65°C= 1 ¾ hrs
80°C= 1 hrs
2. Concentration 5% Nitric Acid Solution
If Nitric Acid concentration is to maintain 5%, solutions have to hold in the equipment or calculate for the following minimum period at the temperatures indicated.
25°C = 2 hrs
50°C = 1 ½ hrs
65°C = 1 hrs
80°C = ¾ hrs
3. Concentration 10-15% Nitric Acid Solution
If Nitric Acid concentration is to maintain 10-15%, solutions have to hold in the equipment or calculate for the following minimum period at the temperatures indicated
25°C = 1 hrs
50°C = ¾ hrs
65°C = ½ hrs
80°C = ¼ hrs
Final Cleaning
- Take 25-50% by volume of water in the storage tank. (Water quality –chlorine content shall be less than 5ppm), circulate the water for the period of 30 minutes.
- Drain the water in the storage tank by opening the drain valve and system by the lowest point valves.
- Repeat the above procedure till we observe the Conductivity 1.5 µs/cm. This will end the cleaning cycle.
Water Quality Test during Flushing
- Check Conductivity and HNO3 concentration of supply line water and return line water.
- pH of the water should be between 5 to 7.
- Same pH to be maintaining for feed and drain water.