The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Passivation with low strength Nitric Acid

I have recently been involved in the installation of a biopharm tube installation using 316L EP materials. The cleaning and passivation was carried out at ambient temp with 5% NaOH solution for 30 mins and pumped at turbulent flow rates. The wash was rinsed and the tubing was passivated with HNO3 at 8% for 30mins and turbulent flow rates. All tubing passed a Ferrocyanide Free Iron Test. Is this test sufficient to indicate successful passivation? If not what would be the easiest/simplest way of ensuring adequate passivation?

Larry Cislowski
- Sydney NSW Australia
2001

---

There are devices to measure the surface of stainless steel on passivation. The cyanide test is to check the presence of "free iron", which can be best removed by pickling. For example in HF/HNO3 solution. The passivation will be after rinsing with demineralized-water on air. But can be fastened by using HNO3 solution (we use at least 18%).

Kind regards,

Thomas van Os
Maassluis, Holland, Netherlands
2001

Ferroxyl is a very demanding test. If you honestly passed it, especially in the welded area, you should have very little problem. Granted, a high HNO3 solution, especially one with hex chromium will provide a lot faster passivation than just letting it set. It will passivate just setting there, open to the air. Blanket statements can not be made, especially since you did not indicate its final use.

James Watts
- Navarre, Florida
2001

---

2001

Larry:

316L is a very good grade of stainless that will usually pass any passivation test without any treatment. The weld areas is a different story. These must be passivated to get good corrosion resistance. The ferrocyanide test is a good test, but will tell you only if there is free iron on the surface, it will not give you a measure of HOW corrosion resistant the surface is.

The 316L itself should be passivated to get the BEST corrosion resistance. Passivation raises the chromium content of the surface layer of the steel, giving optimum corrosion resistance.

8% nitric acid is NOT a passivation solution, however. You need to use 20% or more; or use a citric acid solution at 5-10% concentration. There are commercial citric products that will give you what you need without the hazards of nitric or nitric/dichromate (which contains hexavalent chromium).

Contact us if we can help.

Regards,

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois