By Mike Bauer
A two-component, moisture cured urethane, zinc-rich primer can extend the life of interior and exterior coating systems for steel potable water tanks and has been certified in accordance with the requirements of ANSI/NSF Standard 61 for potable water contact.
Because the 91-H2O primer from Tnemec can be applied to both interiors and exteriors of water tanks, it eliminates the need for two primers and allows for faster application and reduced costs. Since its release in 1997, more than 75,000 gallons of the primer has been used on some 800 water tanks nationwide.
91-H2O contains 83.0 percent zinc dust by weight in the cured film. The zinc dust meets the requirements of the American Society for Testing and Materials (ASTM) D520 Zinc Dust Pigment and contains less than 0.002 percent lead by weight. It has 2.67 lbs./gal. (320 grams/liter) VOC content and a volume solids of 63.0 percent, making it VOC-compliant for both shop and field applications.
Unlike most zinc-rich primers, this product cures quickly at surface temperatures down to 35° F and an accelerator can be added when even faster dry times are desired. It has extended maximum recoat time if it is dry and clean when topcoated, and provides both galvanic and barrier protection of steel substrates.
Application Properties
91-H2O can be applied with a variety of conventional application equipment including airless spray, air spray, brush and roller. It also exhibits "dry fall" properties, which is advantageous when spray-applied to tank exteriors.
Extensive laboratory tests have been conducted including salt fog, adhesion, recoat adhesion studies, potable water immersion, galvanic protection and seawater immersion. Following are the test results:
Adhesion
The long-term performance of any coating system is directly related to adhesion of the primer to the substrate. Two separate adhesion studies were conducted on the Tnemec primer. ASTM D 3359 measures shear adhesion. An adhesion rating scale of 0 to 5 is used with complete loss of adhesion rated at 0B and no loss of adhesion rated at 5B. 91-H2O obtained a 5B rating. ASTM D 4541 measures tensile adhesion in pounds per square inch (psi). As a general rule, a rating of 500 psi or greater represents excellent tensile adhesion. The primer achieved a rating of 1000 psi, which is the maximum applied force capability for the instrument used.
Salt Spray
ASTM B 117 is an accelerated corrosion test where test panels are coated, scribed to bare steel and subjected to a continuous spray mist (fog) of 5 percent sodium chloride solution at 95° F. Following exposure, panels are rated for amount of corrosion on plane surfaces, rust undercutting at the scribe, blistering and other signs of failure. At 2.5 mils DFT topcoated with one coat of two-component epoxy at 4.0 to 6.0 mils DFT, the primer survived 10,900 hours with no rusting on plane surfaces, no rust undercutting at the scribe and no other signs of failure.
To put these results in perspective, two coats of two-component epoxy applied direct to abrasive blast cleaned steel at 4.0 to 6.0 mils DFT per coat will generally show signs of failure within 3,000 to 4,000 hours of exposure.
Potable Water Immersion
Steel test panels were abrasive blast cleaned (SSPC-SP10), coated with one coat of zinc-rich urethane primer at 2.5 mils DFT and one coat of two-component epoxy at 4.0 to 6.0 mils DFT. No signs of failure were evident after seven years immersion in tap water.
Sea Water Immersion
Abrasive blast cleaned steel test panels were coated with one coat of 91-H2O at 2.5 mils DFT and two coats of a Tnemec two-component epoxy at 4.0 to 6.0 mils per coat. There was no rusting or any other signs of failure after two years immersion in seawater.
Re-coat Time/Immersion (Tank Interior)
Currently, two-component epoxy systems are the most frequently specified for the interior of potable water tanks. For new tanks, the two-component epoxy prime coat is usually applied in the fabrication shop then topcoated in the field after tank erection. Two-component epoxies chalk readily when exposed to sunlight. As a general rule, this can result in decreased intercoat adhesion if shop-applied primers are exterior-exposed for extended periods prior to topcoating in the field.
This maximum recoat time generally ranges between two weeks and two months depending on the formulation and exposure conditions. If too much time has elapsed, scarification of the epoxy primer is required to achieve optimum intercoat adhesion for immersion service. Recoat Time/Immersion tests conducted on the 91-H2O two-component epoxy system demonstrated that the primer can be exposed to direct sunlight for at least 12 months without the need for scarification prior to topcoating.
Re-coat Time/Adhesion (Tank Exterior)
This test was conducted to determine if the primer had a maximum recoat time when used for tank exterior systems. Primed panels were exposed to direct sunlight for various lengths of time up to six months. One set of primed panels was topcoated with a Tnemec two-component epoxy and the other set was topcoated directly with Endura-Shield. The panels were then subjected to 10 freeze/thaw cycles to provide external stress (expansion/contraction) to the coating systems. Test results demonstrate that 91-H2O can be exposed to direct sunlight for at least 12 months without the need for scarification prior to topcoating with selected Tnemec exterior tank systems.
Service Life Projections
The service life of coating systems for steel substrates depends on many variables including generic type, degree of surface preparation, quality of application, film thickness, exposure conditions and maintenance schedules.
When properly applied, the most popular water tank coating systems in use today typically provide 14 to 18 years of service life (corrosion protection) with little or no maintenance. An organic zinc-rich primer typically extends the service life of two-component epoxy systems and two-component epoxy/aliphatic urethane systems by 40 percent to 50 percent, depending on the exposure environment. Using 45 percent as the projected average increase in service life, then 20 to 26 years corrosion protection for steel water tanks with little or no maintenance is not unrealistic. Even longer service life is feasible if proper maintenance programs are implemented.
Conclusions
Zinc-rich primers have had a long history of success in corrosion protection of steel structures such as bridges and other structures exposed to corrosive environments. When used in conjunction with high-performance coatings such as two-component epoxies and/or aliphatic urethanes, zinc-rich primer coating systems provide both barrier-type and sacrificial protection of steel substrates.
ANSI/NSF Std. 61-certified 91-H2O adds another dimension to long-term corrosion protection for the interior and exterior of steel potable water storage tanks. It cures quickly and can be topcoated the same day at surface temperatures down to 35° F (2° C). It is easy to apply, exhibits "dry fall" properties and has extended maximum re-coat time. These properties make this product an excellent choice for priming both sides of steel plate in water tank fabrication shops and for subsequent use in the field after tank erection.
About the Author: Mike Bauer is Senior Vice President, Technical and Regulatory Affairs at Tnemec. A graduate of Central Missouri State University, Bauer has been in the coatings industry for 32 years and is a past president of the Kansas City Society for Coatings Technology.
