Passivation of Stainless Steel in Pharmaceuticals


Passivation Basics:

The term Passivation originates from the word “Passive”. Passivation is a natural phenomenon in which an inert layer is formed on the surface of stainless steel alloy, when it is exposed to air or oxidizing atmosphere.
Passivation can also be achieved using chemical agents which are basically acid solutions. The later reacts with stainless steel and removes exogenous iron or iron compounds without significantly affecting the quality of steel. Passivation results in formation of a very thin protective layer, which acts as a barrier to corrosion formation. 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. These strong oxidizing agents 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). The thickness of this layer ranges from 0.5 – 5.0 nm. The chrome to iron ratio measured in atomic percentage, within the chromium oxide should be at least one is to two.
Before performing passivation it is important to ensure that the steel surface are free from surface contamination. Therefore to expedite passivation there is a prerequisite to perform a pre-treatment or a de-greasing step

This pre-treatment step is required to achieve a uniform, non-defective passive layer. This step focuses on removal of organic/inorganic soil and free iron metallic contaminant or corrosive products from the steel surface. A treatment with 0.2M NaOH or a detergent(water soluble) is given for a period of 6 to 8 hrs to remove surface contaminants. Following this, a rinsing is performed with water to neutralize cleaning agent and as well as to remove dissolved contaminants.

The equipment/system is treated with 2% of Nitric acid for 12-15 hrs. at ambient temperature, or  at 50 degrees centigrade for 6 – 8 hours. The treatment is done through re-circulation of the passivating agent. The re-circulation velocity should be maintained at minimum of 1.5 meters/second. Subsequent to completion of passivation, rinsing with water is done to remove dissolved contaminants and at the same time it facilitates in neutralization of Nitric acid.
Verification for Effective Passivation: 
The effectiveness of applied passivation procedure must be evaluated to confirm as to whether the surface contamination has been removed and an inert protective layer is formed on the steel surface. Various tests are conducted to assess the quality of passivation, which includes water Immersion Test, High Humidity test, Copper Sulfate Test etc. Other tests like XPS (X-ray Photoelectron Spectroscopy) analysis the oxidation states of element while AES (Auger Electron Spectroscopy) assesses the chrome/iron ratio on the metal surface. These destructive type of tests which are basically applied to weld coupons only.