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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 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 is that the former protects the underlying surface by galvanic protection, whereas the zinc in the epoxy gets encapsulated with epoxy binder and hence to an extent behaves like a barrier coating sacrificing the all-important properties of zinc metal. Also, epoxy zinc requires higher dosage of pigment in the dry film to afford close to the protection offered by IOZ primer. In essence, they are not an apple to apple comparison of products, but when overcoated, both perform almost identically in many ways.

From Marco Antonio Alvarado Meneses of Sherwin Williams Perú on August 23, 2010:
According to SSPC Paint 20, zinc rich coatings can be classified in two groups: Type I (IOZ - Inorganic) and Type II (OZ - Organic). These coatings form a film that provides galvanic protection of the underlying steel. A high concentration of zinc particles in the film will provide the necessary conductivity for galvanic protection. This high zinc loading contributes to the film’s porosity and its poor internal cohesion. Inorganic zinc-rich silicate coatings frequently do not bond well to each other, and it is safest to repair them using an organic zinc-rich coating. When topcoating inorganic zinc-rich films, small bubbles may form in the wet topcoat from the escape of air or solvent vapors trapped in the porous binder. Many painters attempt to minimize this problem by applying a mistcoat (a thin, quick coat) and allowing it to dry before applying a full topcoat. Because of topcoating problems and good performances without topcoating in a variety of services, it is often best not to topcoat inorganic zinc-rich coatings. IOZ are brittle and may crack if applied too thick; thus, they are generally applied at less than 5 mils [125 m] dry film thickness, although some products can successfully be applied at greater thicknesses. IOZ are normally specified for new building because, within the parameters of zinc silicates, the solvent-borne ethyl silicates have been found to be more generally tolerant than the waterborne alkali metal zinc silicates. Zinc silicates give the best corrosion protection (especially alone),and demonstrate adhesion to SSPC SP 10, chemical resistance, heat resistance, abrasion resistance, welding, and cutting properties. While zinc silicate is a typical “new building” coating, organic zinc rich (OZ) is more of a maintenance primer. The epoxy is easier to apply in higher film thickness without cracking and can be applied with conventional airless spray, while alkali silicates normally need special equipment. Organic zinc-rich coatings are not as electrically conductive as inorganic zinc-rich coatings; and thus, they have a lower level of galvanic protection. Organic zinc-rich coatings do not require as high a level of blast-cleaned steel surface as do inorganic zinc coatings, and they are easier to topcoat. The zinc in both generic types is attacked by acid or alkali. See some advantages/Limitations of IOZ & OZ. Inorganic Zinc Rich - IOZ: Advantages • Can be low in VOCs • Excellent abrasion resistance • Excellent heat resistance • Good atmospheric durability • Useful as shop primer • Fast-drying • Can be used untopcoated Limitations • Needs very clean, blasted surface • Requires skilled applicator, agitated coating • Difficult to topcoat • Attacked by acid and alkali • High initial cost Organic Zinc Rich - OZ: Advantages • Can be low in VOCs • Good atmospheric durability • Relatively easily topcoated • Moderate surface preparation Limitations • Requires skilled applicator • Constant agitation necessary • Unsuitable for acid or alkali • High initial cost • Requires a topcoat

From Adam Backhaut of Diamond Vogel Paints on August 18, 2010:
Epoxies, by nature, have a slower (cross-link) cure time. Both systems offer good corrosion resistance, but epoxy seems to be the most proven vehicle for corrosion control due to excellent adhesion to bare metal. Inorganic zinc-rich coatings are corrosion-resistant primers for iron and steel incorporating zinc dust pigment in an inorganic silicate vehicle. Inorganic zinc-rich coatings require very good surface preparation; are highly abrasion-resistant; have good dry heat resistance; have good resistance to immersion in hydrocarbon products, solvents, fresh water, and pH-neutral aqueous solutions. These coatings are also resistant to exposure in atmospheric environments, damp, humid environments, and chemical environments where the pH ranges between 5 and 9. Organic zinc-rich coatings are corrosion-resistant organic coating materials formulated by combining finely-divided zinc metal and organic resin. Organic zinc-rich coatings often are used for touch-up and repair of defects and damaged areas in inorganic zinc-rich coatings because the organic binder provides better adhesion to bare metal than another coat of inorganic zinc-rich. Organic zinc-rich coatings are more tolerant of surface preparation deficiencies than inorganic zinc-rich coatings. (

From remko tas of Futuro SRL on August 17, 2010:
Ethyl silicate zinc-rich can handle higher temperatures (up to 500ªC) than epoxy zinc-rich (150ªC). Epoxy zinc-rich tends to be more expensive.

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

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