Why nitric acid was used for passivation
When stainless steel is newly made it is cleaned of oils and greases used in the fabrication process, and acid, usually nitric, is used to remove free iron from the surface. Slowly and naturally a passive layer develops on the surface of the steel as the chromium at the surface reacts with oxygen in the air to produce chromium oxide. The term passive refers to steel now being ‘unreactive’.
4Cr + 3O2 → 2Cr2O3
This layer is invisible and only a few molecules thick but it provides a barrier to prevent oxygen and moisture from reaching the iron underneath. If oxygen got to the iron, the iron would oxidize or rust, producing an unwanted yellow to orange-red color and over the long term, weakening the steel.
Most Common Method – Nitric Acid
The most commonly used chemical method to passivate a stainless steel surface is to apply nitric acid. Nitric is a strong mineral acid so it can quickly dissolve all iron compounds and other trace metals that are on the surface. Nitric acid is also a strong oxidizer so it can generate the chromium oxide layer at
the same time. Even though nitric acid is a strong chemical, high temperatures and extended
times are used to ensure the reaction is effective and complete. The application condition ranges are:
Time: 20 minutes to 2 hours
Temp: up to 70°C (160°F)
Conc: 20 to 50% by volume nitric acid
Note: Under these conditions nitric acid is very aggressive on gaskets and it may be necessary to replace them after a passivation procedure.
Citric acid is also sometimes used for passivating stainless steel. It is an acid that can effectively remove iron and its compounds from surfaces.
Temp: ambient temp. to moderate heating
Conc: 4 to 10% by weight
This method is being promoted because citric acid is safer to use than nitric acid, is biodegradable, produces fewer effluent concerns and is also used as a food ingredient. Citric acid does an excellent job of removing iron from surfaces, which is the first step of traditional passivation. It is not an oxidizer and so it cannot oxidize chromium which is the second step of classic passivation. It therefore cannot build up the protective layer so this process depends on natural air oxidation. Citric acid is mostly used on small parts that will not be used in aggressive chemical or physical environments.
Which guidelines please specify
There are no specific Regulatory guidelines on passivation procedures. Regulatory guidelines like WHO says that passivation should be done as per written prosedure as follows,
“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.”
Please refer following references,
WHO - Good manufacturing practices: Water for pharmaceutical use, May 2020 (Draft) Working document QAS/20.842
Banes PH. Passivation: understanding and performing procedures on austenitic stainless-steel
870 systems. Pharmaceutical Engineering, 1990: 41.
Tverberg JC, Kerber SJ. Effect of nitric acid passivation on the surface composition of
891 mechanically polished type 316 L sanitary tube. European Journal of Parenteral Sciences,
892 1998, 3: 117–124
Sir i say that any guidelines for nitric acid con
I have referred the above procedures of passivation which are generally followed by the industry. (These are industry practices).
thank you sir for share most valuable information
You are always welcome.