Spotted 304L SS

[fredE]

I have a pickling tank made of solution annealed 304L SS and contains a solution of 25-38% nitric acid and 3% hydrofluoric acid. Acid temp can be up to 95 deg F. One panel of the tank started showing leopard spots 2-3" in diameter after 2 years of service with severe corrosion. The other two panels do not have this problem. XRF has shown 304SS, 301SS and 321SS at various locations at the tank. My metallugist said the spots are from incorrect annealing. Are the spots and severe corrosion caused by incorrect annealing? How is it possible for XRF to show three different alloys in one panel? Any help is appreciated.

 

[unclesyd]

I would check the XRF. I don't think you will find three metals in one sheet. Annealing doesn't change the chemical composition of the metal only the crystalline form of the elements. 304L will readily corrode in this environment. In fact I am a little surprised you are getting the life you state.
What you are seeing is corroded areas where they may be some selective dissolution of the metal and or residuals from the pickled materials trapped in the corroded areas.

What do the welds and heat affected zone look like?

 

[fredE]

The welds are severely corroded. They are usually the first part of the tank to reach the minimum requirements.

The corroded spots are also in a uniform pattern on the sheet. We have not seen corrosion like this before. It is usually uniform along the horizontal. What else could cause selective corrosion?

 

[unclesyd]

It is not usual to have islands of corrosion in your environment especially with the HF present. Also a lot depends on the concentration of metallic ions in the bath at any time. Sometimes when you have materials in close proximity of the tank walls for any period of time you you could create cells where the local temperature would increase with much higher corrosion rates iat this spot.

I don't see any material problems as this is a highly corrosive media. Depending on what you are doing, any decrease in HF concentration would be very helpful.

 

[EdStainless]

The variations in alloy grades is due to selective attack. The real culprit isn't the anneal, but the cleaning before and after.
What about the heating method and agitation? Can your work ever contact the walls?
I would never use 304 for a pickle tank. 2 years is great. Build a carbon steel frame for a new palstic tank and move on.

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Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

Cleaning the welds & then annealing the entire tank will help.

A pickling inhibitor may help bigtime, but difficult to find one for nitric acid -- it oxidizes the usual inhibitors used in HCl or sulfuric. Henkel doesn't have a Rodine® inhibitor for nitric, nor does Oakite Chemetall have a Gardobond®.
AARVEE Chemicals has INHIBITOR-AVHY for nitric-HF pickling of stainless; says to add to the HF solution before adding the nitric. "effectively inhibits up to 80^oC."

http://www.aarveechemicals.com/Pickling_acid.htm

An inhibitor also prevents overpickling (etching) of parts, reduces acid consumption and prevents hydrogen embrittlement of susceptible alloys.

Another approach is anodic protection, keeping the tank passive using a trickle charger. The cathode should be graphite.

But, I've always used plastic or plastic-lined tanks.

 

[kenvlach]

The aforementioned AAR VEE CHEMICALS is in Bangalore,INDIA.

Another inhibitor is "Nevamine n f155: Corrosion inhibitor against nitric & hydrofloric acid."

http://www.navdeep.com/ci.htm

(Mumbai, India)

"A review of steel pickling and acid regeneration an environmental contribution," by Wolfgang F. Kladnig, International Journal of Materials and Product Technology,
Volume 19, Number 6, Pages: 550-561 (2003).
the abstract mentions hydrofluoric and nitric acid pickling, so maybe useful.

http://inderscience.metapress.com/(...al,17,34;linkingpublicationresults,1:110879,1

Any inhibitor for nitric & hydrofluoric acid solutions available in the US?