Q. I am reviewing my passivation process with nitric acid per ASTM 967 using nitric 2 (126.96.36.199), and am not sure how to mix or check the solution. I also want to be sure I am neutralizing correctly. Any information will be helpful. —A.B.
A. The ASTM nitric 2 (188.8.131.52) concentration is recommended to be in the range of 20 to 45 volume percent. Since this is a volume percentage, you will want to have a total of 20 to 45 gal of nitric acid in a total process tank volume of 100 gal. Note that you will need to calculate the percentage based on your starting concentration of nitric acid. Commercially available nitric acid will vary in concentration, with the highest concentration typically about 69-70 percent. In my experience, the 20-45 volume percent will be made up with a 70-percent concentration of nitric acid. Lower concentrations would need to be adjusted by adding more acid, proportional to the lower percentage of the concentrate.
The ASTM standard refers to using simple rinsing to neutralize the acid on the stainless surface. If additional neutralization is required, additional rinsing can be done in a dilute alkaline tank (sodium hydroxide) followed by more rinsing.