49315

Maximum time limit on nitric acid passivation [Indiana] 

July 11, 2008

We use nitric acid passivation for stainless steels and titanium parts. ASTM standards specify only a minimum time requirements for immersion in 40% nitric acid. does that mean there is no limit on how long one can keep the parts in nitric acid? I would like to know from experts in this forum if there is any maximum time limit for passivation of stainless steel and titanium alloys in 40% nitric acid at room temperature as well as at 40-50oC.
your help is highly appreciated.
Thanks.

Devendra Gorhe
Engineer - Goshen, Indiana, USA

From a practical standpoint, cycle times longer than 20 to 30 minutes are a waste of both time and chemistry. Parts for passivation should be free of all dirt, oil and rust before they enter the acid and occasionally agitated if they are nesting/touching. Was there a particular reason for wanting to stay in longer? I've seen platers try to use a passivation tank to clean or descale parts with terrible results.

Jim Conner
- Dallas, TX

Nitric solutions are able to etch the surface of stainless steel. If the surface finish is important to maintain or there is a strict size tolerance, extra time in the bath would likely be undesirable. But you can see for yourself, take one of your parts, maybe a scraped part if you have one, and leave it in your passivation bath for extra time and see what it looks like afterwards.

Also note that the same is not true of citric acid passivation baths, extra time in those will not adversely affect parts.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA