which chemical is used for passivation of s.s equipment?
5% v/v nitric acid is used for passivation.
Method 1- Nitric Acid
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.
Method 2- Citric Acid
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.
Method- 3- Dichromate
Dichromate is a very strong oxidizer that is sometimes added to nitric acid solutions to improve the oxidation of chromium. Dichromate is toxic and is not permitted in industrial effluent so food plants stay away from using it.
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