Problem Solving Forum

August 17 - August 22, 2010

What are the significant differences in performance between an inorganic zinc primer (ethyl silicate) and an organic zinc primer (epoxy), given the same application and exposure conditions?

Selected Answers

From Vyacheslav Volosiuk of Polymerprotection Ltd. on March 10, 2011:

     The Michigan DOT has b more

     The Michigan DOT has been the most influential proponent of multi-coat shop systems. In the early 1980s, the Michigan DOT began requiring their three-coat system of inorganic zinc/epoxy/polyurethane to be applied in a controlled shop environment. This lead to handling-damage problems associated with fabricators’, haulers’ and erectors’ unfamiliarity with handling finish-painted steel and topcoated inorganic zinc that could be dry to the touch but not necessarily cured hard. The remedy for excessive handling damage selected by Michigan was to change the paint system to epoxy zinc rich/ epoxy/polyurethane. This system is more resistant to handling damage and is not likely to be applied too quickly. Michigan’s decision to change from the inorganic zinc primer to the organic zinc primer was based on similar results of accelerated laboratory corrosion testing of both systems. The fallacy here is that the inorganic zinc-primed steel provides substantially improved corrosion resistance when it is allowed to weather two months or more before topcoating. This condition was not present for the accelerated test panels where each coat was applied back-to-back in the laboratory. This difference in inorganic zinc system performance is well documented by NASA and some coating suppliers. This change in coating procedure has had profound effects on costs of fabricated steel. In order to achieve a Class B surface for slip-critical connections, faying surfaces are primed with inorganic zinc. These surfaces must dry and then be masked from application of the coating system. This effectively adds a fourth coat in the shop and then requires additional field coating of the connection plates and fasteners, normally done during final paint touch-up. Field touch- up should not require spot blasting and full system application but rather, a spot prime of epoxy mastic or similar high-performance surface-tolerant product, followed by a spot application of the finish coat.     

     A glossy polyurethane finish can be difficult to tie in uniformly and invariably will not look as good as its full-coat application. The quality of the field-applied topcoats over the inorganic zinc has little bearing on the long-term corrosion resistance of the system. Providing for their application in a better painting environment while eliminating the weathering of the inorganic zinc primer, or replacing it with an organic zinc primer, results in lower corrosion resistance. The most important coating, the inorganic zinc primer is still best applied in a controlled shop environment. It is damage- resistant, has a Class B surface rating for slip-critical connections, maintains its corrosion protection for many years and does not have a finite recoat “window.” Many states have adopted the Michigan system in their new bridge construction specifications. This is understandable because Michigan had, and perhaps still has, the most comprehensive testing program for evaluating performance of coating systems in the development of their qualified systems list. The Michigan DOT materials laboratory has done a great service to our industry with its technical findings. Invariably, there are circumstances where finish coating in the fabrication shop is prudent. It is important, however, to balance the costs and benefits of this approach and understand the history of this practice before making a wholesale policy decision. In summary, with a multi-coat shop system, corrosion resistance is reduced from that of a shop-applied inorganic zinc/field-applied topcoat system; fabrication costs are increased substantially; field coating costs are not completely eliminated because of the need for touch-up; and aesthetics may be compromised because of the difficulty in blending and matching glossy topcoats during field touch-up.

From richard d souza of stoncor middle east llc on August 24, 2010:
The greatest difference between IOZ and epoxy zinc more

From Marco Antonio Alvarado Meneses of Sherwin Williams Perú on August 23, 2010:
According to SSPC Paint 20, zinc rich coatings can more

From Adam Backhaut of Diamond Vogel Paints on August 18, 2010:
Epoxies, by nature, have a slower (cross-link more

From remko tas of Futuro SRL on August 17, 2010:
Ethyl silicate zinc-rich can handle higher tempera more

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Tagged categories: Zinc-rich (inorganic); Zinc-rich (organic)