Passivation
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Passivation is a process that treats metals to form a protective oxide layer on the surface to make the metal less prone to corrosion. It involves cleaning the metal surface, then submerging it in a passivation solution like nitric acid, which removes any free iron or other contaminants and forms a chromium oxide layer. The key variables in passivation include time, temperature, acid concentration, and the type of metal. Effectiveness is tested using methods like the free iron test to ensure all free iron is removed and the protective layer is formed properly. At processing sites, systems undergo cleaning, passivation treatment, and testing before being put back into use.
Passivation
- 1. PASSIVATION Ranjeet Kumar 18 Jul 08
- 2. Definition & Mechanism Passivated means de-activated. Two very common meanings refer to the (A) treatment of stainless steel in nitric acid or other materials to make it less prone to rusting, and (B) The application of a chromate conversion coating to zinc or cadmium plating Passivation is described as the treating of a metal with a mild oxidant (such as nitric acid) to remove surface iron or iron compounds by dissolution. This action forms a protective passive film (of chromium oxide) on the surface of the metal. The trace iron left behind from machining and fabrication can provide sites for corrosion if left untreated.
- 3. What is undesired in Process Pipes & Why ? free iron oxide scale rust iron particles metal chips or other nonvolatile deposits Exogenous materials in process lines might adversely affect the metallurgical or sanitary condition stability of the surface, the mechanical operation of a part, contaminate the process fluid
- 4. Steps of Passivation cleaning the surface with solvents or an alkaline solution to remove organic or metallic residues the parts are placed into passivation solutions. The choice for this solution is usually citric acid/ nitric acid.
- 5. Range of Key Variables Time – 20 – 120 minutes Temp –15 – 80 °C Conc – 20 – 50% of Nitric Acid by volume Key variables are determined by the Types of steel e.g. 316L would have different conditions than 317L. Nature of the fluid to be used – Industries (Pharma/ Food/ Fine Chemicals) Solid content Corrosive nature Temperature Cleaning action
- 6. Conformity Test for Passivation To check the effectiveness of the passivation wrt the removal of free iron and other exogenous matter – Water Immersion Test. High Humidity Test – 24 Hr Salt Spray Test – 2 Hr salt spray Copper Sulfate Test – The sample is placed into a copper sulfate solution. Any remaining iron will show up as pink spots. Potassium ferricyanide-Nitric Acid Test – Free Iron Test – Ferroxyl Test – Very Sensitive.
- 7. Free Iron Test Highly sensitive test Used when traces of free Iron or Iron Oxide is objectionable Preparation of Solution – Distilled Water – 94% (w/w) Nitric Acid – 3% (w/w) Potassium Ferricyanide – 3% (w/w) Use atomizer spray or filter paper and the solution on testing surface. The metal turns blue when iron is present. Freshly prepared solution is suggested for the test as this solution has limited performance-life. Not recommended for use on products destined for use in Food, & Pharmaceuticals.
- 8. Activities at Site Preparing different loops for passivation Removing diaphragms of valves, filter cartridges, Pressure hold test – using compressed air Flushing with ambient water Degreasing – 6 hrs with alkaline solution – cleaning of lines Passivation – 2 hrs – maintaining a pH of 3.5 for 1 st hr to ensure dissolution of iron particles & a pH of 4.5 for 2 nd hr to stabilize the formation of protective chromium oxide layer. Test piece investigation using Free Iron Test. Flushing with WFI – to remove all chemicals from the subjected systems – final conductivity of water coming out of the loop should match with conductivity of WFI. Reassembling original diaphragms of the valves. Leak test.
- 9. References http://www.delstar.com/passivating_information.html www.sciencedirect.com www.techstreet.com http://en.wikipedia.org/wiki/Passivation www.freepatentsonline.com ASME BPE-2007