"Phosphoric acid treatment of steel to prevent corrosion?"

Question:

"I am interested in finding information which substantiates the claim that H3PO4 (Phosphoric acid) will provide excellent corrosion protection to fresh steel surfaces prior to application of cold galvanising coatings. I have been told the acid (about 30%) strength in water can be applied to either cleaned, oil free fresh steel or steel which has had all surface rust removed.The trick is to allow 3-4hrs min before painting. Is there any truth in this? How does it work? Would it be suitable for marine applications."

Answer:

Its a little more complicated than slapping on some 30% phosphoric acid. There are two ways that phosphoric acid is used to prepare steel surfaces. The one you are referring to is used to clean the steel of rust, etc. in preparation for painting (or cold galvanizing). It is commonly used with a water based solvent to concurrently remove oils, grease, etc. It removes rust and provides a slight etch as an anchor pattern for coating. The second use of phosphoric acid is in phosphate conversion coatings. These are phosphate salts, with some excess acid and accelerators, detergents, etc. that are applied to steel to act as a base for coatings, to improve corrosion protection in mild environments. They form metal salt crystals on the surface and are used extensively in the automotive industry as a base for painting. If you need additional information, send me your fax number and I'll slip you a few pages.

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