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topic 19833

Rust preventative with Phosphoric Acid

A discussion started in 2003 but continuing through 2017


Q. Is there or can you make a rust preventative containing phosphoric acid that will protect steel tubing for an extended period of time. I have been told that the acid reacts with iron oxides to create a zinc (?) phosphate like coating. Is this true.

John M Kish
Vari-Wall - Columbiana, Ohio


A. Hi John. Phosphoric acid is an important ingredient in iron phosphate, but it is not the only one, so straight phosphoric acid is a marginal substitute for a good iron phosphate process. Even zinc phosphate is not intended to remain unpainted, and iron phosphate is not as good as zinc phosphate, and phosphoric acid is not as good as a properly designed iron phosphate -- so you are pretty far from protecting steel tubing for "an extended period of time" :-( Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


A. Phosphoric acid-based solutions are commonly used on steel for cleaning and temporary protection from rust. These form a light iron phosphate coating of limited rust resistance; it is primarily a base for painting or other rust preventative. The solution treatments include

1) Acidic cleaning (phosphoric acid, organic solvents, acid-stable detergent & wetting agents), primarily as pretreatment & rust removal prior to painting: MIL-C-10578. This specification covers five types of concentrated phosphoric acid corrosion removing and metal conditioning compound which, when diluted with water and applied to metal surfaces, will remove rust from ferrous metal surfaces and provide a slight etching action for ferrous and nonferrous metals to promote the adhesion of paint or corrosion preventives,

2) Resin-acid wash primers (phosphoric acid, polyvinylbutyral resin and zinc chromate pigment) per DOD-P-15328 [link by ed. to spec at TechStreet] and MIL-C-8514, and

3) Combination cleaning/phosphating of steel surfaces (including cases where some surfaces are Zn or Al) prior to painting. Federal spec. Fed TT-C-490 [link by ed. to spec at TechStreet] (several solutions) includes both iron and zinc phosphate coatings, which usually is painted but can be given an oil or wax coating.

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


RFQ: We are interested to buy Phosphoric Acid. Please reply

Mahtab Uddin
- Dhaka, Bangladesh
^- Sorry, this RFQ is outdated
     View Current RFQs

October 3, 2017

Q. What harm is there in letting the product completely dry before covering in oil?

David Boyden
- Milton Vermont USA

October 2017

A. Hi David. I would not think there is any harm in letting phosphoric acid dry before covering in oil. Did someone say it was harmful? Is so, please tell about it and provide particulars about the situation you're inquiring about. Thanks!


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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