ABSTRACT
Zinc rich primer coatings, both organic and inorganic, are extensively used in highly corrosive environments and they are part of a high performance coating system in the Protective Coatings Industry. During the 60's and the 70's, zinc rich epoxy primers dominated the market. Later, zinc ethyl silicate primers took over mainly due to their higher potential to corrosion protection. Nowadays however, new developments in zinc epoxy primers are setting the two categories of Zinc rich primers on the same level in terms of anticorrosion performance while adding the advantages of Epoxy based primers. Some of these advantages of zinc epoxies over to zinc silicates are the less demanding curing conditions (epoxies will cure at low humidity), the easier overcoating and the less demanding preparation of the substrate prior to application. In addition Zinc epoxies are typically formulated with high loads of zinc dust, and show better mechanic behavior, meaning that over thickness is less problematic during application, avoiding the common failure by mud cracking of zinc silicates that can fail by mud cracking.
The protecting mechanism of zinc rich coatings is believed mainly to be based on galvanic protection provided by the zinc dust in the paint. When a coating system containing a zinc rich primer is exposed to a corrosive environment, rust creep and blistering are amongst the most important failure mechanisms to be considered. Many accelerated exposures will not, within their exposure time, show the visual defects on intact coated surfaces. Therefore, the development of efficient anti-corrosive primers is mainly based on the behavior of the coatings when an artificial damage, i.e. score, has been made on the surface of the protected steel. Many prequalification tests (e.g. ISO 129441-6, ISO 203402, NORSOK M5013 Rev.6) are based amongst others on rust creep and blistering as well as detachment from scores. According to ISO 12944 a zinc rich coating contains more than 80% zinc by weight in the dry film.
The increasing demands on the performance of zinc rich primers (low rust creep and better mechanical properties) has focused attention on maximizing the utilization of the zinc dust in the paint. In addition to this, the recent developments in the market prices of zinc dust suggest opportunities for new developments in this area. As we show in this study, it is now possible to achieve the same level of corrosion protection with an organic binder containing similar level of zinc dust as its inorganic equivalent. Previously Zinc rich epoxy coatings required higher levels of zinc dust to achieve the same level of corrosion protection as their inorganic zinc silicate counterparts.
