Q: We are having some debate as to passivisation techniques, so I was hoping you could shed some light on what is best practise. In particular, could you clarify the different methods of passivisation in terms of chemicals used (and at what concentrations), time, temperature, and whether or not rinsing is desirable?
A: There is no definitive answer to this question but the main objective to passivation of stainless steel tanks and vessels is to eliminate tramp iron deposits and establish a layer of protective chromium oxide to resist corrosion. I have asked two experts with a lot of experience in this area, Joe Dirkson of EcoLab and Ashton Lewis of the Paul Mueller Company to comment.
Joe's response is based on the traditional method of passivation involving a strong oxidizing Nitric acid solution:
Stainless steel derives its corrosion resistance from a very thin layer of chromium oxide that forms at the metal surface when exposed to oxygen in the air. The chromium oxide film can form in air if the stainless steel is clean and dry. So the passivation process is really about making sure that all contaminants are removed from the surface, then allowing the surface to dry, so that the passive layer can form.
New vessels and piping may have a variety of soils, including polishing oil, EP (extreme pressure) additive oils, adhesive film and tramp iron (polishing dust). All of these soils must be completely removed.
The following steps are recommended for effective passivation. High chemical concentrations are required - consult the Safety Data Sheet prior to chemical use. Be sure to wear appropriate PPE.
1. Inspect the metal surfaces for any soil, film, deposits or extraneous material. Use a flashlight. A digital camera is helpful to document the surface appearance.
2. Check with the equipment supplier to determine if the manufacturer/supplier has any special restrictions or cautions about passivation – want to be sure to not invalidate the equipment manufacturer’s warranty.
3. Assuming a CIP system is available, rinse the tank with water. Note, avoid water that has a high (greater than about 50-75 ppm) chloride ion content. High chloride content can lead to pitting corrosion.
4. Wash the tank with a strong caustic solution. Some caustic products are specially formulated for washing new tanks – check with your chemical supplier about specific products. Circulate for one hour at 140-150F.
5. Rinse the tank with water until neutral. Observe sheeting action during the rinse. If the tank is satisfactorily clean, the rinse water should sheet over the surface. Any streaking or spots indicates incomplete cleaning. Burst rinsing is recommended.
6. Repeat the caustic clean as necessary.
7. Wipe the surface with a soft white cloth. Inspect for any particles or brown film. Brown film can be either tramp iron dust (polishing dust) or oil. Polishing dust can be confirmed by holding a lab magnet onto the dry cloth – iron particles will adhere to the magnet. .
8. To remove tramp iron dust, first the surface should be washed with a cationic detergent to minimize static charge. Apply a cationic detergent by foam or CIP. Brush wash if necessary. Rinse.
9. Circulate a strong nitric acid solution for one hour at 140-150F. A concentration of 10-20% active nitric acid by weight is recommended to dissolve all tramp iron. Note, nitric acid is significantly more effective than citric acid in terms of dissolving tramp iron. Excercise extreme caution with this strong acid solution!
10. Rinse. Repeat the white cloth test to validate that all tramp iron has been removed. Repeat the acid wash as necessary.
11. Allow the surface to dry completely. Dry time can vary depending on temperature, and air exposure. For pipes and surfaces that are not readily exposed to air, 72-hour dry time is recommended.
12. Prior to equipment startup, run a normal CIP cycle and sanitize.
Ashton's response is based on a less aggressive solution of Citric acid incorporated with EDTA:
The old stand by method is to use hot, highly concentrated nitric acid. Nitric is a strong oxidizing acid capable of dissolving iron from the surface of stainless steel alloys. Since it is a strong oxidizer it also actively forms a very thin layer of chromium oxide; this is the "passive" layer that protects stainless steel alloys from corrosion.
The problems with this method are numerous: this can be dangerous to personnel, it can damage elastomers and it is problematic for disposal to municipal wastewater systems. I was on a start-up of a pasteurizer at a large rum producer's facility and an outside chemical contractor came in to passivate the equipment we installed. They insisted on using 50% nitric acid heated to 160F for the operation. I was extremely nervous for the entire operation. This person then insisted on dumping the solution after each use and this turned out to cause some problems with the plant's effluent treatment system. We have pharmaceutical customers who use this method and they change out ALL of the elastomers in their system after passivation. In a large process module this is an expensive and time-consuming endeavor.
An alternate method is to use a citric acid solution that has been pH adjusted and enriched with EDTA. Citric acid forms a complex with free iron and is an effective and safe method to perform the first step of passivation. At Mueller, we use the citric acid method because the nitric acid method is simply too dangerous. Years ago I was discussing this topic with someone at a major global brewer and learned that they also have an approved citric acid method. Talk to your chemical supplier about options using a citric acid/EDTA solution.
With any new cleaning/passivation method it is very important to test the methodology on a small sample of equipment to verify that the procedure does not damage equipment. I do not mean to suggest that 50% hot nitric used on Type 304 stainless needs to be tested before use, but if a solution with some unknown additives is applied to "stainless steel" with unknown alloy composition the user should really do some testing before exposing a whole new tank to the method. I have heard some unpleasant stories about foreign steel with unknown composition being damaged during cleaning.