GENERAL PROCEDURE FOR PASSIVATION -

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 What is Passivation and What are major Guidelines? Here discuss the 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% of 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.

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NOTE:- If Nitric acid concentration is to maintain 3%, solutions have to hold in the equipment or calculate for the following minimum period at the 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 until we observe the conductivity of 1.5 µs/cm.
This will end the cleaning cycle.