Rust preventative 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 Kish
Vari-Wall - Columbiana, Ohio
A. Well, the first problem is that even a well-applied high quality iron phosphate will not protect steel for very long at all if not immediately topped with paint or powder coating.
Phosphoric acid is an important ingredient in iron phosphates, but it is not the only one, and straight phosphoric acid is a poor substitute for a good iron phosphate process. There are many reputable suppliers of iron phosphate processes listed in finishing.com's Chemicals directory.
Ted Mooney, P.E. RET
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 is 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-490E [link is 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.
- Goleta, California
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