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topic 19833

Rust preventation with Phosphoric Acid

A discussion started in 2003 but continuing through 2019


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 one 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; and iron phosphate is not as good as zinc phosphate; and even zinc phosphate is not intended to remain unpainted ... 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
Striving to live Aloha


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 [link is to free spec at Defense Logistics Agency,]. 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
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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. But on the other hand, flash rusting of non-oiled surfaces can happen pretty quick too. Thanks!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

February 19, 2019

Q. I am cleaning up the bolts and things on my old pickup. I want to protect the parts, would a phosphate or based solution work to control the corrosion. When I clean and paint the parts they still seem to rust.

Daniel Turner
- Monticello, New York, USA

February 2019

A. Hi Daniel. Every little bit helps. Getting them really clean with a strong alkaline detergent, phosphating them with Ospho [paid link to product info at Amazon] or other Rust Converter [paid link to product info at Amazon], two coats of paint. Good luck


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

March 20, 2019

Q. I am cleaning/restoring a metal lathe. I had removed all the rust but this winter, due to improper storage of acid nearby, it got a good surface rust again.

Anyway, I am using a hardware bought metal cleaner (phosphoric acid) which has always given me good results.

However, many of the lathe's parts will not be painted (obviously), I would like to know how to remove the white-ish iron phosphate on the surfaces. I know that a wire brush can do a lot, but this is time consuming and there are a lot of nooks and crannies ...

I want to bring the metal back to a clean shiny state before oiling it. Just oiling it does not do it. The metal keeps a bit of a "yellow" color.

Also, any suggestions on the best rust "oil" to keep the rust off would be appreciated. Remember those are sliding surfaces for the most part.

Thanks a lot. Great site!

Francis Brabant
Verger bio d'Oka - Oka, Québec, Canada

April 18, 2019

A. Try 20 gms paraffin wax dissolved in 1 lit petroleum. You can use WD 40 too. Hope it helps and good luck!

Goran Budija
- Zagreb , Croatia is possible thanks to our supporting advertisers, including this shop:none


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