Zinc-Rich Coatings

Question:

I read an article some time back about the relative merits of zinc-rich epoxy coatings vs. inorganic zinc coatings. I can no longer find the article and am hoping you can help. The gist of the article was that zinc-rich coatings, which do not have the zinc flake overlap of the inorganic zincs, performed equally well due to a chemical reaction of some sort. Can you confirm this? W.M.

Answer:

For the record, zinc-rich coatings are used in extreme corrosion service conditions. They are applied to blast-cleaned surfaces. The inorganic zinc-rich primers use vehicles consisting of waterborne resins containing silicates, phosphates and titanates. Organic zinc-rich primers use vehicles consisting of a number of organic resins, the most important of which is epoxy.

I cannot confirm the zinc flake overlap condition in either the inorganic or organic zinc-rich coatings. On the other hand, mica flakes are used in more conventional coatings to reduce their moisture vapor transmission rate by increasing the mean free path through the coating film. I am not aware of this happening in zinc-rich coatings, which use zinc dust.

My reference source is the Steel Structures Painting Council (SSPC) "Systems and Specifications, Steel Structures Painting Manual." It describes both inorganic and organic zinc-rich coatings as being heavily pigmented with metallic zinc. The coating is described as being electrically conductive and capable of providing long-term protection by a barrier mechanism. Further protection is provided by galvanic reaction of the zinc, which is sacrificial in the corrosion reaction. Zinc corrosion products tend to stop corrosive attack of the base metal by blocking any breaks in the zinc-rich coating film. These are the chemical reactions that occur.

Without going into details, the choice of an inorganic or organic zinc-rich coating depends on the length and type of service conditions the coating film will see. Both types are most often topcoated. SSPC Paint Specification No. 20 covers Zinc-Rich Primers (Type I, "Inorganic," and Type II, "Organic." In the description section, the pigmentation is described as zinc dust as per ASTM-D 520 "Zinc Dust (Metallic Zinc Powder)." During the check of the ASTM specification, I could not find any reference to zinc flake.