ASTM A967 passivation - Nitric treaments in specific

[prdave00]

A machine shop with internal passivization capabilities is telling me that the Nitric #4 treatment is "less aggressive", than say the Nitric #2 treatment for example, given the higher concentration of nitric acid and therefore lower concentration of oxygen in the solution. Any truth to this? We're trying to preserve laser marked surface on a 400 series stainless part and the shop indicated this was the best strategy versus using the Nitric #2 or #3 treatments we originally stipulated.

Also can anyone tell me the benefit of sodium dichromate in the Nitric #1 treatment and whether there is any risk of sodium dichromate contamination on the part even when using the recommended neutralization and post-cleaning steps?

My apologies if this is the wrong forum. If so please point me to the right one.

 

[prdave00]

Never mind I got my answers from Carpenter's technical article "Passivating and Electropolishing Stainless Steel Parts" where it states: [blue]"...adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface.... Another option is to increase the concentration of nitric acid to 50% by vol. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack. It should be understood here that the 50% nitric acid concentration is more oxidizing and therefore less aggressive to stainless steels than 20% nitric acid."[/blue]

Link here:

http://www.cartech.com/techarticles.aspx?id=1456