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Publication Number: N/A
Date: November 1995
FHWA Bridge Coatings Technical Note: Zinc-Rich Bridge Coatings
From: Special Projects and Engineering Division, Office of Engineering Research and Development
Topic: Zinc-Rich Bridge Coatings
Description: Zinc-rich coatings for steel bridges fall into two general categories; inorganic zinc and organic zinc. Both types of coatings are usually used as direct-to-metal primer coats in a multicoat paint system. Inorganic zinc coatings consist of zinc metal powder mixed into an inorganic silicate paint binder. This binder can be either solventborne (ethyl silicate) or waterborne (alkali silicate). The concentration of zinc powder in the mixed coating is >80% by weight for the best performing inorganic zinc paints. Organic zinc coatings contain zinc metal pigment mixed into an organic paint resin such as epoxy or urethane. The high zinc metal concentration in these coatings creates high electrical conductivity which allows the primer to provide sacrificial protection to any areas of exposed underlying steel. This sacrificial protection is the most desirable feature of these coatings; therefore, formulation of the coating to maintain conductivity after application and long-term exposure is critical. Zinc-rich coatings for steel are covered by AASHTO M300.
Cost Impact: Information from paint manufacturers indicates an average material cost of approximately $0.33 per square foot of painted steel for a coating system with a zinc-rich primer, an epoxy intermediate, and a polyurethane topcoat. This represents an increase in paint material costs of approximately 30% over a typical epoxy mastic/ polyurethane type barrier coat system (at approximately $0.25 per square foot). However, since current typical bridge repainting jobs with full removal of the existing system by blasting now cost between $5 and $12 per square foot, the increase in paint material cost by specifying the more durable zinc-rich system represents an impact to the cost of the job of less than 2%.
Performance Experience: Recent and ongoing FHWA-sponsored test programs have found that coating systems employing zinc-rich primers have performed very well as a generic class, even in harsh marine and salt-rich environments when applied over blast-cleaned steel (SSPC SP-10 or SP-5). Three-coat systems such as solvent borne inorganic zinc or organic zinc primer/ epoxy intermediate/ urethane topcoat, have shown performance far superior to all other conventional coatings (without zinc-rich primers) tested in parallel to-date under harsh exposure conditions. The primary performance difference between zinc-rich coating systems and systems based on barrier or inhibitive protection is the resistance to disbondment and underfilm corrosion at holidays or defects in paint films and at corners and edges of structural steel members. Performance of inorganic zinc-rich coating systems has been particularly good in various test programs.1, 2, 3
Organic zinc-rich systems have also performed well over blast-cleaned surfaces, as long as the zinc pigment concentration in the formulation has been sufficient to ensure conductivity of the primer.4, 5 Many formulations are marketed as organic "zinc-rich" with low levels of zinc pigment. These formulations would not be expected to perform better than a similar barrier coating system without the zinc pigment.
Waterborne inorganic zinc coatings have gained popularity in recent years. These coatings can perform very well when properly applied and cured. These coatings also contain zero VOC - an attractive feature for fabrication shops with tight point-source emission regulation. Recent experience with these coatings has been mixed. Various laboratory and field exposures have shown excellent performance, whereas other laboratory, shop, and field applications have produced very early rust-bloom and topcoat disbondment problems. These failures are related to the complex curing mechanism and application techniques for waterborne inorganic zincs and are currently under investigation by researchers at FHWA.
Summary of Supporting Data: In marine atmospheric exposure testing, inorganic zinc or organic zinc/ epoxy/ urethane type coating systems showed excellent performance after 7 years over near-white blast cleaned (SSPC SP10) steel. 6
On a steel bridge in Central New Jersey used for testing 47 various coating systems, 10 of the 14 systems tested with zinc-rich primers scored 8 or better, and 13 of 14 scored 7 or better after eight years of service.7 (ASTM D610 "8" = 0.1% rust; "7" = 0.3% rust)
In the FHWA-sponsored "PACE" study, the coatings with zinc-rich primers performed best compared to other generic types of coatings evaluated under similar conditions. This study showed the benefits of zinc-rich primer coating systems in various environments (marine, industrial). 8
Recommendation: The test results and field experiences detailed above demonstrate the merits of coatings systems employing zinc-rich primers for steel bridges, particularly in salt-rich environments. These coatings have gained popularity and are currently widely used in new construction due to their excellent long-term corrosion control performance. In harsh environments research to-date has shown no suitable substitute for the corrosion protection provided by zinc-rich primers at the site of coating defects or structure edges. In the past, zinc-rich coatings (particularly inorganic zinc) were formulated at high VOC levels and have the reputation of being difficult to apply properly. While proper application of zinc-rich coatings is nominally more difficult than the very forgiving alkyd paints of the past, these coatings provide significant performance advantages in harsh exposures. Coating systems employing zinc-rich primers are widely available in formulations which meet all forthcoming environmental regulations, provide excellent long-term corrosion control performance, and can be easily applied with the proper equipment and know-how.
The use of waterborne inorganic zincs should be continued only with proper equipment and under appropriate environmental conditions. Conditions of low air flow and high humidity during application and cure must be avoided. This is particularly true of field applications where environmental conditions during curing are uncontrollable. In shop environments where these coatings have been successfully applied on a regular basis, this coating can be used to provide excellent performance with greatly reduced VOC emission levels.
TRT Terms: Metal bridges; Zinc coatings