GENERAL PROCEDURE FOR PASSIVATION

Spread the love

Passivation is a non-electrolytic process to removes free iron from the surface and forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistance due to lack of iron to react with the atmosphere by using nitric or citric acid. We already discussed about What is Passivation and What are major Guidelines? Here discuss procedure of Passivation.

INITIAL CLEANING FOR PASSIVATION

All welding flux and weld spatter to be removed by initial cleaning.

  • Take 25-50% by volume of water in the storage tank (Water quality- chlorine content should be less than 5 ppm), 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

The composition of the passivation bath shall as be as follows

  • 1% concentrated nitric acid
  • 99% water with not more than 1 ppm chloride.
  • 1.5 times the hold up volume of the distribution piping and 1.5 times the volume of the equipment in the loop (except tanks) or 25% volume of the storage tanks whichever is higher.
  • Tanks, pipes and other equipment have to be filled with the solution. The solution shall be held in the equipment or calculate for the following minimum period at the temperature indicated
35° C 15 hours
50 °C 8 hours
65 °C 4 hours
80 °C 2.5 hours
  • Stop the circulation
  • Drain the acid solution in the storage tank by opening the drain valve and system by opening the lowest point valves.
  • Flush the system with water (of the water quality- chlorine content less than 1 ppm) till the whole acid solution  is flushed out.

NOTE:- If Nitric acid concentration is to maintain 3%, solutions have to hold in the equipment or calculate for the following minimum period at temperature indicated.

35°C 3.5 hours
50°C 2 hours
65°C 1.75 hours
80°C 1 hours
  • 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.
35°C 2 hours
50°C 1.5 hours
65°C  1 hours
80°C 45 minutes
  • If nitric acid concentration is to maintain 10 % solutions have to hold in the equipment or calculate for the following minimum period at the temperatures indicated.

FINAL CLEANING AFTER PASSIVATION

  • Take 25-50% by volume of water in the storage tank. (Water quality- chlorine content shall be less than 5 ppm), 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 AFTER PASSIVATION

  • PH of the water should be between 5 to 7. Same PH to be maintaining for feed and drain water

PROCEDURE FOR % OF ACID PREPARATION:

Formula:                      N1V1 = N2V2

Where

N1 Actual (original) concentration of acid
V1 Volume of acid required
N2 Concentration to be prepared
V2 Volume to be prepared in tank

Leave a Reply

Your email address will not be published. Required fields are marked *

Copy
error: Content is protected !!