Phosphoric acid transfer through SS 304 L pipeline 2

[dkbheda]

Dear Sirs,

We want to transfer phosphoric acid (70 to 80% strength) through 12" dia stainless steel SS 304L grade pipeline (5km long)@450 tons/hour at 4 to 5 bar pressure.There will be such two transfers of about 2000 tons parcels each in a month.This pipeline will be immediately pigged with suitable pig & washed with water after each transfer is over.I would like to know the compatibility of the pipeline grade with the product & any adverse effect,corossion in the line,effect on pipeline life if any.

Thanks,

dkb

 

[Metalguy]

The Carpenter SS data book I have indicates that you won't have a pitting problem, but intergranular corrosion/stress cracks are possible. Hopefully McGuire or someone with actual experience with your acid will respond.

Make sure your rinsing water is very low in Cl-you'll probably have concentration of whatever's in the water.

 

[mcguire]

You have to get up early to respond before Metalguy. Buon giorno! Anyway, phosphoric acid in this concentration will cause 304L to be active rather than passive. So you will get general corrosion rather than pitting. The rate will be less than 0.1mm/year. 316L is the recommended grade for phosphoric, but the low exposure time should keep your pipeline safe. The presence of abrasives, chlorides or fluorides or increasing temperatures could drastically affect this recommendation. Let me know if that's the case.

 

[kenvlach]

What are the impurity concentrations? Depending upon the original rock phosphate and method of acid production, phosphoric acid may contain fluoride and sulfate (sometimes given as sulfuric acid), which are detrimental as noted above.

Due to the limited use, the main corrosion problem would be residue left after rinsing. As mcguire said, phosphoric acid leaves the SS in a non-passive state. Concentrated phosphoric acid is syrupy and difficult to rinse. I recommend using warm water (120-140^oF), and if feasible, following that with a 5-10% nitric acid solution (for passivation) and additional rinsing. This is similar to the procedure we use following electropolishing of SS. You can store and re-use the nitric solution several times.

 

[mcguire]

Good advice. Adding chloride or fluoride will double the corrosion rate for each 1000ppm added.