Frequency of passivation of purified water loop system & WFI water loop system.
There is no simple rule that says when a piece of equipment must be passivated.
The need will vary according to how the equipment is being used and whether the
surface has been damaged. Some companies will choose to passivate processing
equipment once per year as a scheduled maintenance procedure. Other companies
will do it more frequently because they are processing materials that are aggressive
on the stainless steel. Aggressive materials are those that contain high chloride levels
and are acidic. Plants that use raw water that has a naturally high chloride level may have
to passivate more frequently since the chloride will disrupt the protective layer.
Pharmaceutical companies that use ultra pure water for injection are known to
passivate 4 times per year because the high purity water itself is hard on the surface layer.
PW, HPW and WFI are highly corrosive in nature. Many times companies will passivate when they notice iron deposits forming on the stainless steel and it is not coming from the water. There are test kits available from chemical supply firms that will test for free surface iron. If a high level is found, it could be time to passivate.
Passivation should be considered after initial installation and after significant modification in accordance with a documented procedure defining the solution to be used, its concentration, the temperature and contact time.
Reference: Good manufacturing practices: water for pharmaceutical use- May 2020
Above referred are WHO guidelines (Only last paragraph about passivation).