Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Rust preventation with Phosphoric Acid
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 KishVari-Wall - Columbiana, Ohio
2003
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, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003
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, dla.mil]. 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 and MIL-C-8514 [from DLA] , and
3) Combination cleaning/phosphating of steel surfaces (including cases where some surfaces are Zn or Al) prior to painting. Federal spec. TT-C-490 [from DLA] (several solutions) includes both iron and zinc phosphate coatings, which usually is painted but can be given an oil or wax coating.
Ken Vlach [deceased]- Goleta, California
Finishing.com 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, and we at finishing.com, continue to benefit from.
2003
RFQ: We are interested to buy Phosphoric Acid. Please reply
Mahtab Uddin- Dhaka, Bangladesh
2004
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
Q. What harm is there in letting the product completely dry before covering in oil?
David Boyden- Milton Vermont USA
October 3, 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!
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2017
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 19, 2019
A. Hi Daniel. Every little bit helps. Getting them really clean with a strong alkaline detergent, phosphating them with Ospho [adv: Ospho on eBay or Amazon [affil links] or other rust converter ⇦this on eBay or Amazon [affil links] , two coats of paint. Good luck
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 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!
Verger bio d'Oka - Oka, Québec, Canada
March 20, 2019
A. Try 20 gms paraffin wax dissolved in 1 lit petroleum. You can use WD-40 ⇦this on eBay or Amazon [affil links] too. Hope it helps and good luck!
Goran Budija- Zagreb , Croatia
April 18, 2019
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