The authoritative public forum
  for Metal Finishing since 1989
-----

Did we "over-passivate" our tubing?

2003

Dear Sir,

My company have recently passivated our WFI loop. The tubing we used is of SS 316L, electropolished to 0.8micron. The passivation medium we used is Nitric Acid 40%. After passivating it for 20 minutes, we suspect the tubings are over passivated. I wish to know what are the consequences of over passivation? Will it damage the internal surface thus promoting the growth of bioflim? What other consequences we may suffer as a result.

Thanking you in anticipation.

Mark McGreggor
Material Engineer - Ulu Klang, Selangor, Malaysia

---

What are you seeing that you think that the tube is "over-passivated"? Are you seeing some etching? This can happen with nitric, but it sounds like you used a good oxidizing mixture and it should have worked fine.

You cannot "over-passivate" the 316L stainless. The more passive it is, the better corrosion resistance. You CAN, however have a real problem if you etched the surface, as this WILL make it more susceptible to biofilm growth because of the rough and porous surface.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2003

Dear Mr. Kremer,

Foremost, thank you for your response. As you've mentioned, the consequence of using too strong concentration(Nitric Acid) would be over etching of the SS tubing surface thus promoting biofilm growth as a result of damaged surface.

Thanks again.

Mark McGreggor
- U. Klang, Malaysia
2004

---

Very common misconception - stronger nitric IS NOT more aggressive on stainless; just the opposite is true! Fuming nitric can be shipped in straight iron kegs but the less concentrated the acid, the more aggressive it becomes on steel/iron. This is why 400 series stainless steel, that is alloys more prone to nitric attack, is typically passivated in 40% nitric whereas 300 series are treated in MORE AGGRESSIVE (albeit weaker) 25% nitric!

Milt Stevenson, Jr.
Syracuse, New York
2004