5 replies to this topic

[Dave2dExtreme]

Is anyone familiar with the use of Phosphoric Acid (85%) for rust removal and metal protection? Phosphoric acid was recommended to be used on a Seamless Carbon Steel Economizer coil in order to clean the rust and provide metal protection by its manufacturer however the concentration was not supplied. 85% is readily available, is it ok to use this on the metal surface? If anyone has any literature or insight on this please point me in the right direction! Thanks!

[kkala]

Wet process phosphoric acid is quite corrosive due to dissolved F; it is suitable for fertilizers, not for contacting steel surfaces. Usual concentration for handling is 54% P2O5, that is 75% H3PO4 (w/w).
Having no experience on thermal process phosphoric acid (from P burning), I guess this can be used for cleaning /rust removal of metal surfaces. See http://www.fenghaich...template/p1.htm, also in wikipedia, "phosphoric acid.
Impurities in wet phosphoric acid (mainly F) affect corrosive behavior. Better to ask for specifications of phosphoric acid to be used. Fenghaichem acid used for intended purpose seems quite pure.
"Phosphoric acid" by A V Slack (M Dekker, 1968) is a "classical" reference to phosphoric acid, devoting a whole chapter to corrosion and suitable stainless steels. I cannot remember whether its use as rust remover is mentioned there though, and to what extent.

[Dave2dExtreme]

^^^Thanks alot for your response, I will query about the concentration from the vendor, however the 85% is what is readily available in my location. This method of preservation seems to be an old method that was used.

[breizh]

I experienced issues with phosphoric acid 85% , it's very corrosif at high temp .At that time the material was SS 316L . High concentration of phosphoric acid means risk of crystallization , the reason why we were using a coil to warm up the product .
hope this helps
Breizh

[Dave2dExtreme]

^^^ Thanks for your response! Actually it is going to be used at ambient temperature because this Economizer coil is being stored and not going into operation anytime soon.....

[kkala]

...I will query about the concentration from the vendor, however the 85% is what is readily available in my location. This method of preservation seems to be an old method that was used.

Impurities of concentrated phosphoric acid affect corrosive behavior too. Thermal process phosphoric acid is quite pure, so of limited corrosion effect.
You can ask for acid specs (concentration, impurities) as well as recommended contact time with economiser steel grade; to either economiser supplier and / or to local H3PO4 supplier.
Following supports that phosphoric acid 85% w/w is used for the intended purpose; conditions applicable may be the issue.
1. Fenghaichem says (at their site) that their phosphoric acid can be used for: electroplating (thus cleaning surface from dirt and oils), polishing (assumed more or less same action as before), phosphate industry (this is not understood, phosphates+H2SO4-- corrosive wet process H3PO4).
2. Wikipedia (Phosphoric acid, paragraph:chemical reagent) mentions the use of 85% H3PO4 as rust converter. It also notes that it gets corrosive, if diluted.
I imagine the dirt is on the external tube surface, that is on stack gases side. So you have to place the coil in a bath of acid. Ask whether you can use brushes to remove the dirt, or you should merely leave the coil in the bath for X time. You can see the result "in situ".
It would be useful for us to know method eventually applied and result, for our knowledge.