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Chemical Cleaning Stainless Pipe Using Nitric Acid

Dear Sir:

I am in the process of chemical cleaning newly installed 316 stain less steel pipe lines, these lines will be used for hydrogen peroxide service at a steel mill that produces primarily stainless steel products. The lines will first circulated with a ten percent solution of sodium hydroxide and heated to 150 F-the water used for mixing will all be demineralized. The sodium hydroxide will circulated for four hours and flushed with demineralized water. When P.H. is 7 a 1% nitric acid solution will be circulated for two hours and flushed with demineralized water. When water test at a P.H. of 7 the lines will be blown dry using dry nitrogen.

My question is 1.) do you feel this a good cleaning process for stainless steel piping being used for hydrogen peroxide service? 2. Do you think it would be wise to use an inhibitor with the Nitric Acid ? If so what do you recommend?

Any comments and or opinion would be appreciated.

Thanks,

Richard C Puhl
- Kent, Ohio,


 

A small caveat: Keep in mind chromium is attacked by strong warm alkaline solutions, so with prolonged contact you may preferentially etch some of the chromium from the stainless. The net effect is the contact surfaces will turn streaky/brown from the ferric oxide which will appear after you enrich the surface with iron in this manner. However, the iron oxide is susceptible to most acids and the acid rinse may, in fact, restore the iron/chrome balance and leave the surface with the metal oxides which make stainless, well, stainless. If you are merely looking to passivate the system, I would think the H2O2 you plan to use in the system will be more than sufficient to keep the stainless surface well oxidized.

Are are you trying to clean or remove some contaminant from the system? If so, is the contamination on the ID of the tubing soluble in detergents? Maybe an ionic surfactant rinse followed by some distilled water would be sufficient. A good commercial product for this might be LF2100 or Micro-90 from International Products--both low foam, high surfactant cleaners that do not contain silicates. Dale

Dale Woika
Surface Conversion Sciences - Bellefonte, Pennsylvania


 

Richard, cleaning stainless steel piping with a 10% solution of caustic soda at 150 F and for four hours seems like a hammer and tongs approach to cleaning. You could use a commercially available alkaline cleaner designed for recirculation at lower concentration and a shorter cleaning time and get better results. I'm not an expert on cleaning prior to peroxide service but I suspect the removal of all organic materials is critical. The surfactants in the commercial alkaline cleaner will significant improve the organics removal compared to caustic alone.

Rinsing with a 1% solution of nitric acid will remove any traces of alkalinity or hard water deposits left by the alkaline cleaning. It will not passivate any cut, abraded or welded stainless steel surfaces though. A higher concentration of nitric acid or a formulated passivation material is needed to do that.

Roy Nuss
Trevose, Pennsylvania, USA


March 4, 2009

If this is newly installed stainless pipe, and it is your desire to improve the chrome content of the ID, thereby improving resistance to corrosion, then you should only use a degreasing solvent sufficient to remove any oils or preservatives that may be present from the manufacturing process. Then we recommend chelant circulation to improve the surface chrome content by reducing the iron content.

R.G. Head
- Houston, Texas


May 15, 2009

Dear Roy

What percentage of nitric acid solution do you recommend for passivating stainless steel as referring to the answer you supplied above?

Stefan Ferreira
- Midrand South Africa

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