Water is of great importance in pharmaceutical & food industries because every activity in pharma is performed with water either purified or water for injection.
The water in pharmaceutical industries is not stored and supplied like other facilities; rather it is stored in stainless steel storage tanks & supplied by loop circulation systems.
MOC of Storage Tanks & Loop System
MOC is an abbreviation of material of construction & stainless steel 316L grade is used for the construction of water storage tanks & Loop circulation system.
What Is Passivation?
Passivation is the chemical process where a chemical treatment is done for stainless steel to increase its corrosion resistance.
Science Of Passivation
To completely understand the science of passivation let’s have a look over the composition of stainless steel.
Stainless steel is an alloy & is composed of Iron, Nickel & Chromium.Chromium contribute about 10% of the composition & it is the chromium which gives resistance to corrosion in stainless steel.
The composition ratio varies in stainless steel according to layers & at the surface there is a ratio of 1.5 to 1 between chromium & iron.
At surface chromium reacts with oxygen and results in formation of an inert layer which is known as passive surface or passive film.In other words it is the natural passivation.
How Passivation Works?
Passivation results in removal of free iron from the surface & ensures high chromium level on the surface which results in passive film formation by reacting with oxygen.
The ratio between chromium & iron from 1.5 to 1 or higher is considered good to give resistance against corrosion.
When We Need Passivation
As we discussed in above portion Passivation or passive film naturally exist in all chromium excess alloys but still we need passivation of new stainless equipment like storage vessel,loop circulation systems before usage because of following,
- The fabrication, welding or finishing of stainless steel equipment may result in change in natural ability to resist corrosion due to contaminants mainly iron oxide,debris & tramp iron.
Advantages Of Passivation
There are following benefits of passivation
- The process of passivation results in enhanced corrosion resistance.
- Passivation process removes contaminants from the surface.
- Equipment or utility life is enhanced.
- Maintenance cost is reduced by timely passivation.
- Product quality is enhanced due to reduced risk of contamination.
Which Chemical Is Used For Passivation?
The chemical process of passivation is done by using an acid treatment & usually the following are used.
- Nitric Acid
- Citric Acid
Nitric acid usage is high in routine practices for passivation.
How To Perform Passivation of a Water System?
For passivation of water storage vessels & loop circulation system following method is used.
- Prepare 7 to 10% of nitric acid solution according to the procedure defined by the quality control department.
- Add sufficient quantity of 7 to 10% nitric acid solution in the water storage vessel.
- The quantity of nitric acid solution may vary depending upon the length & end user points of the Loop circulation system.
- Close all the valves at the user end and circulate the nitric acid solution for at least 2 hrs.
- Discard the nitric acid solution after 2 hrs by opening all the end user valves & discharging valves of the storage vessel.
- Again close all the valves & add freshly prepared purified water in a storage vessel & circulate it for about 1 hr.
- After 1 hr take a sample of this water & check its pH & conductivity & discard it as mentioned above for nitric acid.
- Again fill the water storage vessel with purified water & circulate it till 30 to 60 min & again check its pH & conductivity.The pH & conductivity of this water must be equal to the water when it was circulated.
- If results are in range then passivation is done & our system is ready to use.