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Stain problem after passivation by HNO3

(-----) 2006

My company have process of passivation the steel by HNO3.
The process is as below:
Pre-degease (Resolve 152) => degrease(Resolve 152 & ultrasonic)=>HCL=>passivation(HNO3) => Neutralising(NaOH)=>Air spray =>oven

I found "brown stain problem on surface after passivation. I can't find the cause of problem.
Please suggest for me.

Thank you very much,

- Ayutthaya, Thailand

First of two simultaneous responses -- 2006

Are you using demineralized water? I suggest you run a sample using distilled water for chemical batching AND rinsing. Rinse VERY well. Measure the rinse water effluent conductivity to ensure it approaches (within 1-2 microsiemens) the original rinse water conductivity. Rinsing with pure water is paramount. Good luck.

Joseph Lockrem
- Indianapolis, Indiana, USA

Second of two simultaneous responses -- 2006

The brown stain is probably rust. Is this a stainless steel? Which alloy?
ASTM A380 [link is to the practice at TechStreet] is a guide to cleaning, descaling, passivation (and avoiding iron contamination) of stainless steels. HCl treatment isn't recommended; it can etch stainless steels. What is the HNO3 concentration, and when is it changed (at what Fe level or color)?
Rather than using a neutralizing rinse (which may leave rust-inducing salt residues), simply rinse in flowing tap water, with final rinsing in de-ionized water. Heating the DI water aids drying.

Ken Vlach [dec]
- Goleta, California

contributor of the year Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.


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