Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2004

Zinc Phosphate Conversion Coatings

Q. Is it appropriate to use both a zinc phosphate conversion coating (with a non-chromate seal) and an inorganic zinc primer to prepare cold rolled steel sheet for subsequent top coating with a primer and an aliphatic isocyanate polyurethane top coat? I have read that the zinc phosphate conversion coatings create an electrically neutral, non-conducting barrier layer, in addition to a surface with better adhesion for the subsequent paint. If so, would that not make the inorganic zinc primer useless, since there would be no conduction path to the steel substrate?

A. Thanks for your email. Everything you wrote about zinc phosphate is correct, and yes, it is an excellent method for preparing cold rolled steel prior to the application of an epoxy primer and a polyurethane topcoat. You final conclusion is incorrect. Zinc phosphate is not intended to provide "cathodic protection" to the substrate, such as a metallic zinc coating does. Galvanizing, zinc plating and zinc-rich primers provide cathodic protection and they MUST be able to conduct an electrical current so that they can function. I think you are confusing zinc phosphate inorganic conversion coatings with zinc-rich primers. They are very different creatures.

Zinc and iron phosphates are deposited electrolytically to form an inhibitive layer over the metal. They "passivate" the substrate so that, in the presence of moisture and oxygen, galvanic corrosion cannot take place. You can think of zinc and iron phosphates as functioning like an insulating blanket, preventing iron ions (Fe2+) from dissolving into the surrounding electrolyte. Zinc-rich primers (organic or inorganic) are intended to corrode in place of the expensive substrate, in this case, steel. Hence zinc is called a "sacrificial" anode in the corrosion cell.

Another major advantage of phosphate coatings is that they provide an excellent adhesion foundation for paints and coatings. Zinc phosphates are generally considered to provide better protection than iron phosphate, but the latter is often quite adequate for its intended purpose.

I hope this answers your question.

Q. Thank you so much for taking the time to try to answer my questions. I do understand that the various phosphating processes function by passivating the surface. I was concerned, because I read an article on Products Finishing Online (www.pfonline.com) entitled "Preparing Steel for Organic Coatings", by Brad Gruss of Pretreatment & Processes, Inc. (http://www.pfonline.com/articles/pfd0001.html). In this article, the author states in the section on phosphating that "Phosphating develops a non-conducting layer between the base metal and coating..." This was the first time I had ever heard this, and if true I could not understand how a zinc primer would function, since it would not have a conduction path to the steel substrate. Have you heard of this? Can you comment on the use of zinc phosphate conversion coats with zinc primers? Thank you in advance for your time.

A. Brad is correct. He should know his stuff as he has been in the business of phosphating for much of his life having worked for Fremont Chemicals. The fact is that zinc-rich primers should not be applied over phosphated surfaces for the very reason that you mentioned, namely that the zinc dust particles cannot make electrical contact with the steel substrate. Zinc-rich primers are always applied directly over virgin steel surfaces that have usually been abrasive blast-cleaned to remove mill scale.

Zinc chromate, zinc molybdate, strontium chromate and similar primers can and should be applied over phosphated surfaces. All three are inhibitive primers that do not contain metallic particles. Zinc chromate and the others are not conductive, but when they dissolve (they have low solubilities) in the presence of moisture, they form passive corrosion resistant films over the substrate.

In summary, zinc rich primers contain metallic zinc particles, and prevent corrosion by means of "cathodic protection". They are always applied directly over steel to maximize the electrical conductivity between the two metals. This is very different from inhibitive primers, such as zinc chromate, which forms an inhibitive "blanket" over the metal preventing iron ions (Fe2+) from dissolving. They are usually applied over pretreated, conversion-coated surfaces, such as iron or zinc phosphates. Here the conversion coating provides both corrosion resistance and adhesion between the substrate and the organic primer.