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Does phosphoric acid on steel leave a protective layer?

We service compressed gas cylinders. The insides are bare steel. Various products are sold that are supposed to offer short term rust prevention after the tanks have been cleaned or hydrotested however some of these are of questionable safety.

One manufacturer says they use a anti-rust coating variously described as "metal oxide" and "ferrous phosphate". We are wondering whether this is just the coating left by a phosphoric acid rinse or cleaner, or if other chemicals must be added to produce a protective phosphate layer.

K. Morley
Fire Prevention - Elyria, Ohio
2005

Yes You can use only phosphoric acid,concentration must be under 2%(rinsing must be omitted),but protection is weak(compared to proprietary solutions).You must oil,wax or lacquer it!

Good luck!

Goran Budija
- Cerovski vrh Croatia
2005

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2005

THE PHOSPHORIC ACID IS NOT ENOUGH TO MAKE A GOOD PHOPHATING BECAUSE THE PHOSPHORIC ACID IS A TRIBASIC HYDRATE NEED AS EXAMPLE COUSTIC SODA TO REACT TO PRODUCE SODIUM PHOSPHATE WHICH REACT WITH THE STEEL.THIS SYSTEM MUST BE ACCELERATED WITH OXIDIZING AGENT TO FAST THE OXIDATION OF FERROUS TO FERRIC PHOSPHATE.
DEGREASING MATERIALS MUST BE INCLUDED IN THE FORMULA CHELATING AGENT AND RUST INHIBITOR.
I AM SORRY FOR MY LONG ANSWER BUT THE PROCESS NOT SO EASY AS YOU THINK MAY I NEED 1000 PAPERS TO DISCUSS THE CHEMISTRY AND THE FACTORS OF THE PROCESS.
BUT YOU HAVE NOW A GOOD GUIDE LINE TO START YOUR RESEARCH
GOOD LUCK

Ahmed Fouad Serag Eldin
Ein Shams - Cairo - Egypt