Zinc-rich Coatings - How They Work and How to Check If They'll Work

Zinc-rich paints are a commonly used method of corrosion control.  The basic premise is that these paints provide sacrificial cathodic protection to the steel substrate by using zinc particles as the pigment within the coating.  For this protection to occur zinc particles must touch each other and the steel substrate providing electrical contact; as particles are consumed the protection is reduced until all the zinc is consumed or electrical continuity is lost.  Anecdotally, the optimum zinc loading is 90% (or greater) by weight, providing a film with sufficient zinc particles and contact to provide corrosion protection.  In an atmospheric environment there are additional factors that can affect performance, such as:  periodic wetting and re-wetting of the surface, moisture coverage, topcoating, etc.  This project investigated the performance of zinc-rich coatings on steel surface with varying loadings (percentage by weight) of zinc through electrochemical and atmospheric testing.  Methods to quantify zinc loading and performance were also investigated to ascertain film quality and ability to provide sacrificial cathodic protection.  These results are being used to better understand the mechanisms by which zinc-rich coatings provide corrosion protection, factors that may impact that protection and methods to perform Quality Assurance (QA) testing on the as-cured film.  The results to date are summarized in this paper.