Optically Activated Pigmented (OAP) coating systems have become a U.S. Navy requirement in marine ballast tank coating applications due to their ability to ensure correct application of epoxy linings. In steel and concrete water storage tanks, the use of this same technology can aid in eliminating holidays, identifying pinholes, and reducing premature failures caused by poor film thickness on edges, angles, or areas that are difficult to inspect. Epoxies that meet the ANSI/NSF Standard 61 for coatings for potable water service are available with OAP technology. This article will discuss common coating failures seen in water storage tanks and how these failures can be addressed by the use of OAP coatings designed for potable water service.
Corrosion In Water Storage Tanks
A common cause of premature coating failures in water storage tanks is uncoated, exposed substrate or exposed zinc-rich primer that goes undetected during the inspection process. These holidays, pinholes, or insufficient film thicknesses occur not only in the inside wet areas of a water storage tank, but also in the areas above the intended water level of the vessel itself. These potential coating failure mechanisms can be broken into several distinct categories, which could all be prevented by the use of OAP technology. Pinholes and holidays, insufficient thickness and improper coverage of stripe-coated areas are three of the failure mechanisms.
Pinholes and Holidays
Basic corrosion theory tells us that any pinhole, holiday, or discontinuity exposing ferrous metal substrates will result in pinpoint rusting, undercutting of the coating around the pinhole and, ultimately, rapid coating failure and corrosion of the steel. These pinholes or discontinuities can occur for many reasons and at any level in the painting process. Holiday detection is designed to identify pinholes, thus allowing the applicator to address the uncoated areas. Specifications typically require holiday detection, often following the NACE RP0188 recommended practice guidelines, but these guidelines can be difficult to follow in an elevated water tank where one undetected holiday or discontinuity can result in considerable coating failure when immersed in potable water.
Failures resulting from pinholes and holidays are amplified if the pinhole is present over one of the zinc-rich primers commonly used in today’s fabrication of water storage tanks. Zinc is a highly reactive metal, and, when the pinhole is exposed to water, it produces zinc hydroxide and hydrogen gas. These blisters and coating failures formed by the reaction process are frequently located in areas that are difficult to reach for coating application and that can be difficult areas to inspect and perform using “traditional” (NACE RP0188) holiday detection testing